Goals

On completing this module, students are expected to be able to:

• describe how energy is absorbed, stored, and moved around in Earth's climate system;
• distinguish how the amount of energy stored determines the temperature;
• interpret the importance of feedback mechanisms that make our climate system sensitive to forcings, but also provide a stabilizing influence;
• infer how temperature responds to changes in solar input, albedo, and greenhouse gas concentrations;
• evaluate how simple (i.e., STELLA) models can be used to make projections of climate variables.

Learning Outcomes

After completing this module, students should be able to answer the following questions:

• What are heat and thermal energy?
• What are the different types of electromagnetic radiation?
• What is blackbody radiation and what is the significance of the Stefan-Boltzmann law?
• What is emissivity and what is its significance?
• What is albedo and what are albedo values for different materials?
• What is the solar constant and how is it measured?
• What is insolation and what are its geographic and annual distributions?
• What does sunspot history look like and how is it related to solar intensity?
• What are the relative heat capacities of different materials?
• What is the greenhouse effect and what are the different greenhouse gasses?
• What are the basic energy flows in the atmosphere?
• What is positive and negative feedback and what are examples of each?
• What are the energy budgets of different latitudes?
• How is heat transferred in the atmosphere?
• How is heat transferred in the oceans?
• What is the Global Conveyor Belt and what is its significance?

Below is an overview of your assignments for this module. The list is intended to prepare you for the module and help you to plan your time.

Assignments

1. Lab 3: Climate Modeling
2. Submit Module 3 Lab 3 (Graded).
3. Take Module 3 Quiz.
4. Yellowdig Entry and Reply

We begin with a quick glimpse of the global climate — and then we’ll try to understand why it looks this way. But first, what does climate mean? In the simplest sense, it is the average weather of a region — the average temperature, rainfall, air pressure, humidity, cloud cover, wind direction, and wind speed. This means that climate is not the same as weather; weather implies a very short-term description of the atmospheric conditions, and it tends to change in a complex manner over short time scales, making it notoriously difficult to predict. In contrast, the climate is less variable — it smoothens out the variability of the short-term weather. This course is about climate, how it is changing, and what that means for our future; as we move through this class, you should remind yourself periodically that we are not talking about the weather — our time frame is much longer.

So, let’s have a look at the climate as expressed by temperature:

The average near-surface air temperature (sea surface temperature over the oceans) of the Earth for the period from 1961-1990.

Click for a text description of the World Surface Temperatures map.

This image is a world map showing the annual mean temperature across the globe, measured in both degrees Fahrenheit and Celsius. The map uses a color gradient to represent temperature variations, with colder regions in blue and warmer regions in red.

• Map Type: World map
• Measurement: Annual mean temperature
• Color Scale (bottom of the map):
  • Range: -40°F (-40°C) to 80°F (30°C)
  • Colors: Dark blue (-40°F/-40°C) to dark red (80°F/30°C), with purple, green, yellow, and orange in between
• Regions with Notable Temperatures:
  • Coldest (dark blue, -40°F/-40°C to 0°F/-18°C):
    • Polar regions (Arctic and Antarctic)
    • Northern Canada, Greenland, and Siberia
  • Cool (green to light blue, 0°F/-18°C to 40°F/4°C):
    • Northern Europe, parts of Russia, and the northern U.S.
  • Moderate (yellow to orange, 40°F/4°C to 60°F/15°C):
    • Southern Europe, the central U.S., and parts of China
  • Warm (red, 60°F/15°C to 80°F/30°C):
    • Most of Africa, South America, India, Southeast Asia, and Australia
    • Parts of the Middle East and Central America

The map illustrates the global distribution of annual mean temperatures, with the coldest temperatures in polar regions and the warmest in equatorial and tropical areas.

Credit: Annual Average Temperature Map by Robert Rohde from Wikimedia, licensed under CC BY-SA 3.0

As you can see, the equatorial regions are the warmest, and the poles are the coldest, with Antarctica being noticeably colder than the Arctic. The temperature varies more within the continents than the oceans, and there is a pronounced northward extension of warm water in the North Atlantic.

The global climate system is like a big machine receiving, moving, storing, transferring, and releasing heat or thermal energy. The machine consists of the oceans, the atmosphere, the land surface, and the biota on land and in the oceans; in short, it consists of everything at the Earth’s surface. The average state of this system — the global climate — is represented most simply by the pattern of temperatures and precipitation at the surface.

In order to really understand this complex machine, we will have to understand something about its parts, but we also need to begin with some fundamental ideas about energy, heat, and temperature, including the source of the energy for the climate system — the sun.

Useful Terms and Definitions Related to the Energy of the Climate System

Energy

In the broadest terms, energy is a quantity that has the ability to produce change in a physical system; it includes all kinds of kinetic energy (energy of motion) and potential energy (energy based on the body's position) and is measured in joules. One joule represents the amount of energy needed to exert a force of one Newton over a meter; so 1 Joule = 1Nm.

Power

Energy expended over a period of time is a measure of power, and in the context of climate, power is expressed in terms of Watts (1 Watt = 1 joule per second). This is also called a heat flux — the rate of energy flow.

Heat

This is simply the thermal energy of a body, measured in joules. Think of this as the average kinetic energy (vibrations) of the atoms of a material.

Heat Flux Density

This is a measure of how concentrated the energy flow is and is given in units of Watts per square meter.

Temperature

This is obviously closely related to heat, but it is the average kinetic energy within some body. Materials can be the same temperature, but they may have different amounts of thermal energy — for instance, a volume of water has much more thermal energy than a similar volume of air at the same temperature. Remember that there are 3 temperature scales: Fahrenheit, Celsius, and Kelvin. We’ll use Celsius and Kelvin, which have the same scale, just offset so that 0°C = 273°K.

We begin with a very simple analog model for our planet’s climate (figure below) in which solar energy enters the system, is absorbed (some will have been reflected), stored (some will have been transformed or put to work), and then released back into outer space. The amount of energy stored determines the temperature of the planet. The balance between the incoming energy and the outgoing energy determines whether the planet becomes cooler, warmer, or stays the same. Notice the little arrow connecting the box to the Energy Out flow — this means that the amount of energy released by the planet depends on how hot it is; when it is hotter, it releases, or emits, more energy and when it is cooler, it emits less energy. What this does is to drive this system to a state where the energy out matches the energy in — then, the temperature (energy stored) is constant. This energy balance, sometimes called radiative equilibrium, is at the heart of all climate models.

Systems diagram for a simple energy flow system. Energy is added to a body (a reservoir in systems language), is stored by the body, and then leaves the body. The amount of energy stored determines the temperature, which in turn controls how much energy is released. This relationship between the energy out and the energy stored makes a negative feedback mechanism that tends to drive the system to a steady state where the energy in and the energy out are equal, and thus the temperature is constant.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Now, let’s consider the connection between this idea of an energy flow system to the actual Earth. As shown in the figure below, this system includes the atmosphere, the oceans, volcanoes, plants, ice, mountains, and even people — it is intimately connected to the whole planet. We will get to some of these other components of the climate system later, but to begin with, we will focus on just the energy flows — the yellow and red arrows shown below.

Global Climate System

Click for a text description

The image is a labeled diagram titled "The Global Climate System," illustrating various processes and components that influence Earth's climate. It depicts interactions between the atmosphere, land, ocean, and Earth's interior, with numbered annotations and a key explaining the symbols used.

• Overall Structure:
  • The diagram is a cross-sectional view of Earth, showing the atmosphere, ocean, land (continental and oceanic crust), and the underlying lithospheric mantle and asthenosphere.
  • The asthenosphere is depicted in red at the bottom, indicating its semi-fluid nature.
• Atmospheric Components:
  • 1: Short-wavelength (SW) solar radiation (yellow wavy arrows) enters the atmosphere from the top.
  • 2: Some solar radiation is reflected back into space (yellow wavy arrows pointing upward).
  • 3: Long-wavelength (LW) radiation (red wavy arrows) is emitted from the Earth's surface upward.
  • 4: Clouds reflect solar radiation back into space (yellow arrows) and trap long-wavelength radiation (red arrows).
  • 5: Some solar radiation is absorbed by the atmosphere (yellow arrows curving within the atmosphere).
  • 7: Clouds release precipitation (blue arrows pointing downward), contributing to the water cycle.
  • 8: Evaporation from the ocean surface (blue arrows pointing upward) adds water vapor to the atmosphere.
  • 9: Evaporation from land surfaces (blue arrows pointing upward) also contributes to atmospheric moisture.
• Land and Ocean Components:
  • 6: Ice on land reflects solar radiation (yellow arrows bouncing off ice).
  • 10: Ocean currents (black arrows) show the movement of water within the ocean.
  • 11: Transfer of CO₂ between the ocean and atmosphere (green arrows) indicates carbon exchange.
  • 15: Ice melts, contributing to the water cycle (blue arrows from ice to ocean).
  • 16: Runoff from land to ocean (blue arrows) shows the movement of water.
  • 17: Human activities, depicted as factories and vehicles on land, release CO₂ into the atmosphere (green arrows).
• Geological Components:
  • 12: A volcano on the continental crust releases CO₂ (green arrows) and ash (gray cloud) into the atmosphere.
  • 13: Volcanic eruptions emit particles and gases (gray cloud) that can influence climate.
  • 14: Weathering of rocks on the continental crust (green arrows) removes CO₂ from the atmosphere.
• Earth's Interior:
  • The diagram shows the continental crust and oceanic crust as part of tectonic plates.
  • The lithospheric mantle (part of the plate) is labeled beneath both the continental and oceanic crust.
  • Black arrows indicate the movement of plates, with a divergent boundary at the oceanic crust where new crust is formed (red area).
  • The asthenosphere beneath the lithospheric mantle is shown in red, indicating its role in plate movement.
• Key (Bottom of Diagram):
  • Long-wavelength (LW) radiation: Red wavy arrows.
  • Short-wavelength (SW) solar radiation: Yellow wavy arrows.
  • Transfer of CO₂: Green arrows.
  • Movement of water: Blue arrows.
  • Movement of plates: Black arrows.

The diagram visually represents the complex interactions within the global climate system, highlighting the roles of solar radiation, the carbon cycle, the water cycle, geological processes, and human activities in shaping Earth's climate.

Credit: Penn State Department of Geosciences, Modeling Earth's Climate System with STELLA

Numbers in the figure refer to the following key:

1. Incoming short-wavelength solar radiation
2. Reflected short-wavelength solar radiation
3. Emission of long-wavelength radiation (heat) from surface
4. Absorption of heat by greenhouse gases and emission of heat from the atmosphere back to the surface (the greenhouse effect)
5. Emission of surface heat not absorbed by the atmosphere
6. Evaporation cools the surface, adds water to the atmosphere
7. Condensation of water vapor releases heat to the atmosphere, precipitation returns water to the surface
8. Evapotranspiration by plants cools the surface
9. Chemical weathering of rocks consumes atmospheric CO₂
10. Oceans store and transfer thermal energy
11. Sedimentation of organic material and limestone (CaCO₃) transfers carbon to sediment on the ocean floor
12. Melting and metamorphism of sediments sends carbon back to surface
13. Emission of CO₂ from volcanoes
14. Emission of CO₂ from burning fossil fuels
15. Cold oceans absorb atmospheric CO₂
16. Warm oceans release CO₂ to the atmosphere
17. Photosynthesis and respiration of plants and soil exchange CO₂ between the atmosphere and biosphere

The figure above includes some new words and concepts, including short-wavelength and long-wavelength radiation, that will make sense if we devote a bit of time to a review of some topics related to energy.

Brief Review of Electromagnetic Radiation

The energy we are concerned with here comes in the form of electromagnetic radiation, so it will help us to review some aspects of this form of energy. Electromagnetic (EM) radiation comes in a spectrum of waves, each consisting of an electrical and a magnetic oscillation of particles called photons; this spectrum is shown in the figure below:

The electromagnetic (EM) spectrum, showing wave types and corresponding wavelengths, with the detail of the more familiar visible light portion of the spectrum. The Sun emits energy in the ultraviolet to near infrared, while the Earth emits energy entirely in the infrared. Also shown are the temperatures of objects whose peak energy emission is associated with the corresponding wavelengths, according to Wien’s Law.

Click for a text description of elecromagnetic spectrum image.

The image is a diagram titled "The Electromagnetic Spectrum," illustrating the range of electromagnetic radiation types, their wavelengths, and their relationship to the temperature of objects emitting them. The diagram is structured vertically, with various segments representing different types of radiation, their wavelengths, and associated temperatures.

• Title: "The Electromagnetic Spectrum" is written at the top.
• Main Structure: The diagram is a vertical bar divided into segments, each representing a type of electromagnetic radiation, with wavelengths and temperatures labeled on the sides.
• Wavelength Scale (Right Side):
  • Wavelengths are shown in a logarithmic scale, ranging from 0.01 nm (nanometers) at the top to 1000 m (meters) at the bottom.
  • Specific wavelength ranges are marked:
    • 0.01 nm (10^-11 m) for gamma rays
    • 1 nm (10^-9 m) for X-rays
    • 10 nm for ultraviolet (UV)
    • 100 nm to 1000 nm (10^-6 m) for visible light (400 nm to 700 nm highlighted in a color gradient from purple to red)
    • 10 μm (micrometers, 10^-5 m) for infrared (IR)
    • 100 μm for thermal IR
    • 1000 μm (10^-3 m) for far IR
    • 1 cm (10^-2 m) for microwaves
    • 10 cm for radar
    • 1 m for FM radio and TV
    • 10 m to 1000 m (10³ m) for AM radio
• Types of Radiation (Center):
  • From top to bottom, the types of electromagnetic radiation are labeled:
    • Gamma rays
    • X-rays
    • Ultraviolet (UV)
    • Visible light (with a color gradient from purple at 400 nm to red at 700 nm)
    • Near IR (infrared)
    • Thermal IR
    • Far IR
    • Microwaves
    • Radar
    • FM radio, TV
    • AM radio
• Temperature Scale (Left Side):
  • The left side shows the temperature of objects whose energy peaks at specific wavelengths, based on blackbody radiation principles.
  • Temperatures are marked with corresponding radiation types:
    • 29,000°K (Kelvin) for objects emitting gamma rays
    • 290°K for objects emitting in the solar spectrum (visible light)
    • 29°K for objects emitting in the Earth's thermal IR spectrum
    • 2.9°K for objects emitting in the microwave spectrum
• Spectral Curves:
  • Two curves are overlaid on the diagram, showing the blackbody radiation spectra:
    • A yellow curve labeled "Solar" peaks around the visible light range (290°K), indicating the Sun's emission spectrum.
    • A blue curve labeled "Earth's" peaks in the thermal IR range (29°K), indicating Earth's emission spectrum.
• Visible Light Section:
  • The visible light portion (400 nm to 700 nm) is highlighted with a color gradient, transitioning from purple (400 nm) to blue, green, yellow, orange, and red (700 nm).

The diagram effectively illustrates the relationship between wavelength, type of electromagnetic radiation, and the temperature of objects emitting that radiation, emphasizing the Sun's peak in visible light and Earth's peak in thermal infrared.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

In the realm of physics, a blackbody is an idealized material that absorbs perfectly all EM radiation that it receives (nothing is reflected), and it also releases or emits EM radiation according to its temperature. Hotter objects emit more EM energy, and the energy is concentrated at shorter wavelengths. The relationship between temperature and the wavelength of the peak of the energy emitted is given by Wien’s Law, which states that the wavelength, lambda, is:

$\lambda =0.0029/T$ (λ is in m, T in kelvins)

But the energy emitted covers a fairly broad range, as described by Planck’s Law, as shown below:

The spectra of energy emitted from idealized blackbodies of different temperatures. Notice that the peaks of the spectra are shifted to longer wavelengths for cooler objects. For reference, the average surface temperature of Earth is 288 °K.

Click for a text description of Blackbody emissions graph.

The image is a graph titled "Blackbody Emission from Objects of Different Temperatures," illustrating the energy emitted by blackbodies at various temperatures as a function of wavelength, based on Planck's law of blackbody radiation. The graph also references Wien's Law to highlight the wavelength at which the energy emission peaks for each temperature.

• Title: "Blackbody Emission from Objects of Different Temperatures" is written at the top.
• Axes:
  • X-Axis (Wavelength): Labeled "Wavelength (μm)" and ranges from 0 to 30 micrometers (μm), with major ticks at intervals of 5 μm (0, 5, 10, 15, 20, 25, 30).
  • Y-Axis (Energy): Labeled "Energy Emitted" with arbitrary units, ranging from 0 to 6, with major ticks at intervals of 1 (0, 1, 2, 3, 4, 5, 6).
• Data Representation:
  • Three curves are plotted, each representing the energy emitted by a blackbody at a specific temperature:
    • 300°K (Kelvin): Plotted in blue.
    • 400°K: Plotted in green.
    • 500°K: Plotted in red.
  • Each curve shows the energy emitted across the wavelength range, with a distinct peak indicating the wavelength at which the maximum energy is emitted.
• Curve Characteristics:
  • 300°K (Blue Curve): Peaks around 10 μm, with a maximum energy of about 1 unit, then gradually decreases toward longer wavelengths.
  • 400°K (Green Curve): Peaks around 7.5 μm, with a maximum energy of about 2 units, showing a higher and sharper peak compared to the 300°K curve.
  • 500°K (Red Curve): Peaks around 5.5 μm, with a maximum energy of about 5 units, exhibiting the highest and sharpest peak among the three curves.
• Annotation:
  • A label near the 500°K curve states: "energy peaks at a wavelength of 0.0029/T (Wien's Law)," explaining that the peak wavelength is inversely proportional to the temperature (T) of the blackbody, as per Wien's Displacement Law. The constant 0.0029 is in meter-Kelvin units (m·K), so when divided by temperature in Kelvin, it gives the peak wavelength in meters (which is then converted to μm in the graph).
• Overall Trend:
  • As the temperature increases from 300°K to 500°K, the peak of the emission curve shifts to shorter wavelengths (from ~10 μm to ~5.5 μm), and the total energy emitted (the area under the curve) increases significantly, consistent with the Stefan-Boltzmann Law and Wien's Law.

The graph visually demonstrates how blackbody radiation varies with temperature, showing that hotter objects emit more energy and at shorter wavelengths, which is a fundamental concept in understanding thermal radiation and its role in climate science (e.g., Earth's and the Sun's emission spectra).

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The total amount of energy radiated from an object is also a function of its temperature, in a relationship known as the Stefan-Boltzmann law, which looks like this:

where σ is the Stefan-Boltzmann constant, which is 5.67e-8 Wm^-2K^-4 (this is another way of writing 5.67 x 10^-8; so 100 is 1e2, 1000 is 1e3, one million is 1e6, etc.), T is temperature of the object in °K, and so F has units of W/m². If you multiply this by the surface area of an object, you get the total rate of energy given off by an object (remember that Watts are a measure of energy, Joules, per second). As you can see, the amount of energy emitted is very sensitive to the temperature, and that can be seen in the figure above if you think about the area beneath the curves of different color. This sensitivity to temperature is very important in establishing the radiative equilibrium or balance of something like our planet — if you add more energy, that warms the planet, and then it emits more energy, which tends to oppose the warming effect of more energy added. Conversely, if you decrease the energy added, the planet cools and emits far less energy, which tends to minimize the cooling. This is a very important example of a negative feedback mechanism, one that works in opposition to some imposed change. The thermostat in your house is another good example of a negative feedback — it works to stabilize the temperature in your house, bringing it into radiative equilibrium.

The version of the Stefan-Boltzmann law described above applies for an ideal blackbody object, but it can easily be adapted to describe all other objects by including something called the emissivity, as follows:

Here, epsilon is the emissivity, which is a unitless value that is a measure of how good an object is at emitting (giving off) energy via electromagnetic radiation. A blackbody has epsilon=1, but most objects have lower emissivities. A very shiny object has an emissivity close to 0, and human skin is between 0.6 to 0.8.

As mentioned earlier, an ideal blackbody will absorb all incident light, but in the real world, things absorb only part of the incident light. The fraction of light that is reflected by an object is called the albedo, which means whiteness in Latin. Black objects have an albedo close to 0, while white objects have an albedo of close to 1.0. The table below lists some representative albedos for Earth surface materials. Most of these albedos are sensitive to the angle at which the sunlight hits the surface; this is especially true for water. When the Sun is at angles of 40° and higher relative to the horizon, the albedo of the water is fairly constant, but as the angle decreases from 40°, the albedo increases dramatically so that it is about 0.5 at a Sun angle of 10° and 1.0 at a sun angle of 0°. You are aware of this in the form of glare coming off the water in the early morning or in the evening before sunset.

Most people have an intuitive sense for the effects of albedo on reflectance and solar energy absorption. This is why people wear white clothes in hot sunny climates and dark clothes in cold sunny climates. What should you wear if it is cloudy and cold?

In the above table, we see that the Earth’s average albedo is 0.31, but there is considerable variation in this value over the surface of the Earth and over time as well — this spatial and temporal variation in albedo of the Earth is shown in the figure below.

The albedo of the Earth during February and July as averaged over the period from 1974 to 1978.

Credit: Graphics created by David Bice © Penn State University is licensed under CC BY-NC-SA 4.0; data from Mitchell and Wallace, 1992, J. Climate, 5, 1140-1156.

We have already mentioned the idea of radiative equilibrium, where the incoming energy and the outgoing energy are in balance, resulting in a steady temperature, but now we are in a position to combine a few other ideas to express this notion in a simple equation that is at the heart of all climate models. Before we begin, we introduce the solar constant, which is the amount of incoming solar electromagnetic radiation per unit area. Just for your information, this amount is measured on a plane perpendicular to the Sun's rays and at the mean distance from the Sun to the Earth.

We begin with the energy (in units of W/m²):

Here, S is the solar constant — 1370 W/m², and a is the albedo, which is about .31 based on satellite measurements. Then we deal with the energy out, using the Stefan-Boltzmann law:

Combining energy in (E_in) and energy out (E_out), we get:

Now, we can solve this to find what the equilibrium temperature of our planet is:

adding numbers, $T=(343(1-.31)5.67e-8)1/4=254°K=-19°C=-2.2°F$

Yikes! This is too cold — we know the mean temperature of the Earth is more like 15°C (288°K or 59°F).  What have we left out? The simple answer is the emissivity, which makes sense since we know the Earth is not an ideal blackbody. (Remember that emissivity is a measure of how good an object is at emitting (giving off) energy via electromagnetic radiation; in the above, we have effectively assumed an emissivity of 1, which is for a perfect black body material). Using the equation above, let’s see what that emissivity number should be:

$$S(1-a)=\epsilon \sigma T4$$$$\epsilon =S(1-a)/(\sigma T4)=343(1-.31)(5.67e-8)(2884)=0.606$$

So, then, even if all of these equations have you seeing stars, what does this basically mean? There is something about the Earth that prevents it from emitting as much energy as it should. What is this something? It is the greenhouse effect — the key that makes our planet a nice place to live.

Insolation — Incoming Solar Radiation

It all starts with the Sun, where the fusion of hydrogen creates an immense amount of energy, heating the surface to around 6000°K; the Sun then radiates energy outwards in the form of ultraviolet and visible light, with a bit in the near-infrared part of the spectrum. By the time this energy gets out to the Earth, its intensity has dropped to a value of about 1370 W/m² —as we just saw this is often called the solar constant (even though it is not truly constant — it changes on several timescales):

Sun's energy shining onto Earth

Click for a text description of Sun's Energy SHining onto Earth.

The climate system begins with energy from the Sun. At Earth’s distance from the Sun, the light has an intensity of 1370 W/m² — this value is sometimes called the solar constant, although it does change over time. The Earth is so far away from the Sun that the incoming rays of energy are all essentially parallel (thin black lines within the yellow region). Note that the sunlight strikes the planet perpendicular to the surface near the equator, but it strikes at an oblique angle near the poles, such that L2>L1. This means that the insolation is more concentrated near the equator and weaker near the poles. In other words, the heat flux density (W/m²) is greater at the equator than at the poles.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Since the Earth spins, the insolation is spread out over an area 4 times greater than the disk shown in the figure above, so the solar constant translates into a value of 343 W/m². This is a bit less than six 60 Watt light bulbs shining on every square meter of the surface, which adds up to a lot of light bulbs since the total surface area of Earth is 5.1e14 m². How much energy do we get from the Sun in a year? Take 1370 W/m², multiply by the area of the disk (pi x r² where r=radius of Earth, 6.37e6 m), and this gives us an answer in Watts, which has units of joules per second, so if we then multiply by the number of seconds in a year, then we get the total energy in joules per year. The number is staggering — 5.56e24 Joules of energy — and is 10,000 times greater than all of the energy generated and consumed by humans each year.

Earth's Orbit, Axial Tilt, and Seasons

Click for a text description of Earth's Orbit, Axial Tilt and the Seasons

The Earth orbits around the Sun with its spin axis (the line connecting the North and South Poles) tilted at 23.4° from a line perpendicular to the orbital plane. This tilt, or obliquity, gives rise to the variation in seasons, and the larger the tilt angle, the greater the contrast in seasons (this tilt changes on a timescale of about 40,000 years). If the tilt were 0°, there would be no real difference between winter and summer; the difference in distance between perihelion (the closest point of the orbit) and aphelion (the farthest point) is very small at present, but this, too, changes. The degree of ellipticity is called the orbital eccentricity; it changes on timescales of 95,000, 125,000, and 405,000 years. A very nice animated version of Earth’s orbit can be found here.

• Central Elements:
  • The Sun is depicted in the center as an orange circle.
  • Earth's elliptical orbit around the Sun is shown as a black oval path, with Earth positioned at four points corresponding to the seasons.
• Earth’s Positions and Seasons:
  • Summer (Northern Hemisphere):
    • Earth is shown on the left side of the orbit, tilted with the Northern Hemisphere facing the Sun.
    • Labeled "Summer Northern Hemisphere."
    • An annotation reads: "the South Pole receives no sunlight during this part of the orbit."
    • The position is marked as "Aphelion – Jul. 4, 1.01671 AU," indicating Earth is farthest from the Sun (1.01671 Astronomical Units).
  • Spring (Northern Hemisphere):
    • Earth is shown at the top of the orbit, with its axis tilted at an angle.
    • Labeled "Spring Northern Hemisphere."
  • Winter (Northern Hemisphere):
    • Earth is shown on the right side of the orbit, tilted with the Northern Hemisphere facing away from the Sun.
    • Labeled "Winter Northern Hemisphere."
    • An annotation reads: "the North Pole receives no sunlight during this part of the orbit."
    • The position is marked as "Perihelion – Jan. 4, 0.98329 AU," indicating Earth is closest to the Sun (0.98329 Astronomical Units).
  • Fall (Northern Hemisphere):
    • Earth is shown at the bottom of the orbit, with its axis tilted at an angle.
    • Labeled "Fall Northern Hemisphere."
• Axial Tilt Annotation:
  • A label on the right side of the diagram reads: "Earth’s spin axis is tilted relative to the orbital plane," explaining the cause of seasonal variations.
  • The tilt angle is marked as 23.5° on the Earth diagrams.
• Visual Details:
  • Each Earth is depicted as a globe with visible continents, primarily showing North and South America.
  • The tilt of Earth’s axis is indicated by a dashed line running through the center of each Earth, with the North Pole (marked with a small symbol) tilted toward or away from the Sun depending on the season.
  • Arrows on the orbital path indicate the direction of Earth’s movement around the Sun (counterclockwise).

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The insolation is not constant over the surface of the Earth — it is concentrated near the equator (first figure on the page) because of the curvature of the Earth. But, the situation is complicated by the fact that the Earth’s spin axis is tilted by 23.4° relative to a line perpendicular to the Earth’s orbital plane (see the second figure on the page), so that as Earth orbits around the Sun, the insolation is concentrated in the Northern Hemisphere (the Northern Hemisphere summer) and then the Southern Hemisphere (winter in the Northern Hemisphere). This tilt of the spin axis, also called the obliquity, is the main reason we have seasons.

Insolation with latitude graph

Click for a text description of Insolation vs. Latitude

The image is a graph titled "Insolation v. Latitude," showing the variation in average daily insolation (solar energy received per unit area) across different latitudes on Earth. The graph includes three curves representing insolation at different times of the year, with a note indicating Earth's tilt angle as 23.45°.

• Axes:
  • X-Axis (Latitude): Labeled "Latitude," ranging from -80° (Southern Hemisphere) to 80° (Northern Hemisphere), with major ticks at intervals of 20° (-80, -60, -40, -20, 0, 20, 40, 60, 80). The equator is at 0°.
  • Y-Axis (Insolation): Labeled "Average Daily Insolation W/m²," ranging from 0 to 600 watts per square meter (W/m²), with major ticks at intervals of 100 W/m² (0, 100, 200, 300, 400, 500, 600).
• Data Representation:
  • Three curves are plotted, each representing the average daily insolation at different times:
    • Winter Solstice (Dec. 21): Plotted in blue, labeled "winter Dec. 21 solstice."
    • Summer Solstice (June 21): Plotted in red, labeled "summer solstice."
    • Annual Average: Plotted in green, labeled "Annual avg."
• Curve Characteristics:
  • Winter Solstice (Dec. 21, Blue Curve):
    • Shows high insolation in the Southern Hemisphere, peaking at around 500 W/m² near -20° latitude.
    • Insolation decreases sharply toward the Northern Hemisphere, dropping to nearly 0 W/m² at 80° latitude (North Pole), reflecting minimal sunlight during the Northern Hemisphere's winter.
  • Summer Solstice (June 21, Red Curve):
    • Shows high insolation in the Northern Hemisphere, peaking at around 500 W/m² near 20° latitude.
    • Insolation decreases sharply toward the Southern Hemisphere, dropping to nearly 0 W/m² at -80° latitude (South Pole), reflecting minimal sunlight during the Southern Hemisphere's winter.
  • Annual Average (Green Curve):
    • Shows a more balanced distribution, peaking at around 400 W/m² near the equator (0° latitude).
    • Insolation gradually decreases toward both poles, reaching around 150 W/m² at ±80° latitude, reflecting the yearly average sunlight received.
• Overall Trend:
  • The graph illustrates how Earth's axial tilt causes significant seasonal variations in insolation, with the Northern Hemisphere receiving more sunlight during the summer solstice (June 21) and the Southern Hemisphere receiving more during the winter solstice (Dec. 21).
  • The annual average curve shows that the equator receives the most consistent and highest insolation year-round, while the poles experience the greatest seasonal extremes.

The graph effectively demonstrates the relationship between latitude, Earth's tilt, and the distribution of solar energy, which drives seasonal climate patterns

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The tilt of the spin axis also means that day length changes, and these changes are most dramatic at the poles, which experience 24 hours of daylight during their summers and no daylight during their winters. The varying day length, along with the angle of incidence of the Sun’s rays, combine to control the average daily insolation variation (see figure above). On a yearly average, the equatorial region receives the most insolation, so we expect it to be the warmest, and indeed it is.

Earlier, we mentioned the Solar Constant — a measure of the amount of solar energy reaching Earth. In reality, this value is not a constant because the Sun is a dynamic star with lots of interesting changes occurring. One of the best known of these changes is the solar cycle, related to sunspots. Sunspots are dark regions on the surface of the Sun related to intense magnetic activity, and measurements have shown that the greater the number of sunspots, the greater the energy output of the Sun. Early observations of these sunspots revealed a pronounced cyclical pattern to them, varying on an 11-year cycle, as shown below.

Sunspot History and Solar Constant, comparing the number of sunspots per calendar year.

Click for a text description of the Sunspot History and Solar Constant graph.

The image is a graph showing two overlapping time series, likely representing climate-related data such as temperature anomalies or another paleoclimate proxy, over a period of time. The graph lacks specific labels for the axes and title, but the y-axis appears to represent a measurement scale, and the x-axis likely represents time, with specific years marked on the right side.

• Axes:
  • Y-Axis: The vertical axis ranges from -50 to 200, with major ticks at intervals of 50 (-50, 0, 50, 100, 150, 200). The units are not specified but could represent temperature anomalies (e.g., in °C or a proxy like δ¹⁸O in ‰).
  • X-Axis: The horizontal axis is not explicitly labeled with a time scale, but specific years are marked on the right side, suggesting a time series. The years are not shown on the x-axis itself but are inferred from the right-side labels.
• Data Representation:
  • Two curves are plotted:
    • A blue line representing one dataset.
    • A pink line representing another dataset.
  • Both lines show similar patterns, indicating they might be measuring related variables or the same variable from different sources.
• Curve Characteristics:
  • Both the blue and pink lines exhibit significant variability, with frequent peaks and troughs.
  • The values generally fluctuate between -50 and 150, with occasional peaks reaching close to 200.
  • The two lines closely follow each other, suggesting a strong correlation between the datasets, though there are slight differences in amplitude and timing of peaks.
• Year Markers (Right Side):
  • Specific years are marked on the right side of the graph, corresponding to certain points on the curves:
    • 1967 at the top (around 150 on the y-axis).
    • 1964.5 (around 100 on the y-axis).
    • 1964 (around 50 on the y-axis).
    • 1963.5 (around 0 on the y-axis).
    • 1963 (around -50 on the y-axis).
  • These years suggest the data spans at least from 1963 to 1967, though the full time range of the graph is not clear without x-axis labels.
• Overall Trend:
  • The graph shows cyclical fluctuations with no clear long-term trend over the visible period.
  • The data appears to oscillate around a mean value (possibly around 50 on the y-axis), with periodic increases and decreases.
  • The close alignment of the blue and pink lines indicates consistency between the two datasets, possibly representing different measurements of the same phenomenon or related climate variables.

The graph likely represents a paleoclimate or climate variability record, showing short-term fluctuations over a few years in the 1960s, but without specific labels, the exact nature of the data (e.g., temperature, isotopic ratios, or another proxy) remains unclear.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0;

Here, in blue, we see the annual number of sunspots and in red we see the reconstructed solar intensity or Solar Constant. The reconstruction is made by studying the relationship between sunspot number and solar intensity in the last few decades, where we have good direct measurements of the solar intensity — this provides a relationship that is fairly simple and directly proportional. Higher sunspot numbers correspond to higher solar intensity. Both records are characterized by a strong 11-year cycle, often called the sunspot cycle.

The magnitude of variation in the Solar Constant, however, is quite small, and we shall see in our lab activity for this module that this amounts to a very small change in the temperature of the Earth.

When our planet absorbs and emits energy, the temperature changes, and the relationship between energy change and temperature change of a material is wrapped up in the concept of heat capacity, sometimes called specific heat. Simply put, the heat capacity expresses how much energy you need to change the temperature of a given mass. Let’s say we have a chunk of rock that weighs one kilogram, and the rock has a heat capacity of 2000 Joules per kilogram per °C — this means that we would have to add 2000 Joules of energy to increase the temperature of the rock by 1 °C. If our rock had a mass of 10 kg, we’d need 20,000 Joules to get the same temperature increase. In contrast, water has a heat capacity of 4184 Joules per kg per °K, so you’d need twice as much energy to change its temperature by the same amount as the rock.

Cooling history of air and water

Click for a text description of Cooling history of air and water

This image is a line graph showing the cooling of air temperature over time in comparison to a constant water temperature. The graph plots temperature in Kelvin against time in hours, illustrating the cooling process of air.

• Graph Type: Line graph
• Y-Axis: Temperature (K)
  • Range: 17 K to 293 K
• X-Axis: Hours
  • Range: 0 to 200 hours
• Data Representation:
  • Water Temperature (T_water): Red line
    • Constant at 293 K (labeled as 2) throughout the 200 hours
  • Air Temperature (T_air): Blue line
    • Starts at 155 K (labeled as 1) at 0 hours
    • Decreases rapidly within the first 50 hours, approaching 17 K
    • Levels off near 17 K after 100 hours, remaining stable through 200 hours
• Trend:
  • Air temperature cools significantly from 155 K to near 17 K within the first 100 hours
  • Water temperature remains unchanged at 293 K

The graph demonstrates the rapid cooling of air over time while the water temperature remains constant, highlighting the difference in thermal behavior between air and water over a 200-hour period.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The heat capacity of a material, along with its total mass and its temperature, tell us how much thermal energy is stored in a material. For instance, if we have a square tub full of water one meter deep and one meter on the sides, then we have one cubic meter of water. Since the density of water is 1000 kg/m³, this tub has a mass of 1000 kg. If the temperature of the water is 20 °C (293 °K), then we multiply the mass (1000) times the heat capacity (4184) times the temperature (293) in °K to find that our cubic meter of water has 1.22e9 (1.2 billion) Joules of energy. Consider for a moment two side-by-side cubic meters of material — one cube is water, the other air. Air has a heat capacity of about 1000 Joules per kg per °K and a density of just 1.2 kg/m³, so its initial energy would be 1000 x 1 x 1.2 x 293 = 351,600 Joules — a tiny fraction of the thermal energy stored in the water. If the two cubes are at the same temperature, they will radiate the same amount of energy from their surfaces, according to the Stefan-Boltzmann law described above. If the energy lost in an interval of time is the same, the temperature of the cube of air will decrease much more than the water, and so in the next interval of time, the water will radiate more energy than the air, yet the air will have cooled even more, so it will radiate less energy. The result is that the temperature of the water cube is much more stable than the air — the water changes much more slowly; it holds onto its temperature longer. The figure above shows the results of a computer model that tracks the temperature of these two cubes.

One way to summarize this is to say that the higher the heat capacity, the greater the thermal inertia, which means that it is harder to get the temperature to change. This concept is an important one since Earth is composed of materials with very different heat capacities — water, air, and rock; they respond to heating and cooling quite differently.

The heat capacities for some common materials are given in the table below.

Earlier, we noticed that if you do the energy balance calculation to figure out the temperature of our planet, it suggests that Earth should be -19 °C, which is 34 °C colder than the observed average global temperature of 15 °C. Why is Earth warmer than it should be? The answer lies in the greenhouse effect — gases in our atmosphere (including CO₂, CH₄ (methane) and H₂O water vapor) trap much of the emitted heat and then re-radiate it back to Earth’s surface. This means that the energy leaving our planet from the top of the atmosphere is less than one would expect given the known temperature of our planet. As mentioned earlier, this effect can be represented in the simple energy balance equation as a term called the emissivity.

The fact of this greenhouse effect comes out of the very simple calculation we did above, but it can also be observed in great detail from satellite measurements of the infrared energy leaving Earth’s atmosphere.

As we discussed on the topic of black body radiation, the temperature of a body (a planet, for instance) gives us a sense of what the spectrum of energy should look like — that is, a range of wavelengths and intensity of radiation at those wavelengths. For the Earth, this spectrum, as seen from satellites looking down on the surface, is very different from the expected. The figure below shows the difference between the expected and the observed.

CO₂ H₂0 CH4 O₃ absorb energy differences

Click for a text description of the diagram above

The image consists of two related diagrams illustrating the spectrum of energy emitted by Earth and the absorption of this energy by greenhouse gases across different wavelengths. The diagrams highlight the role of greenhouse gases in trapping Earth's emitted energy, contributing to the greenhouse effect. The image is credited to Robert Rhode and includes a source link.

• Top Diagram (Energy Emission Spectrum):
  • Axes:
    • X-Axis (Wavelength): Labeled "Wavelength," ranging from 1 μm (micrometer) to 70 μm, with major ticks at 1, 5, 10, and 70 μm.
    • Y-Axis (Energy Emitted): Labeled "Energy Emitted," with no specific units provided, but the scale is relative, showing the intensity of emitted energy.
  • Data Representation:
    • Yellow Curve: Represents the theoretical blackbody emission spectrum for a planet at Earth's temperature (~288 K) without greenhouse gases, peaking around 10 μm.
    • Red Curve: Represents the actual spectrum of energy emitted by Earth, showing significant reductions at certain wavelengths due to absorption by greenhouse gases.
    • The area between the yellow and red curves is shaded red, indicating the energy absorbed by the atmosphere.
  • Trend: The yellow curve follows a smooth blackbody radiation curve, while the red curve shows dips at specific wavelengths where greenhouse gases absorb energy, reducing the amount of energy escaping to space.
• Bottom Diagram (Absorption by Greenhouse Gases):
  • Title and Label: The bottom diagram shows the "% Absorption From All Greenhouse Gases" and breaks down the absorption contributions from individual gases.
  • Axes:
    • X-Axis (Wavelength): Matches the top diagram, ranging from 1 μm to 70 μm, with major ticks at 1, 5, 10, and 70 μm.
    • Y-Axis (Absorption): For the topmost graph, labeled "% Absorption From All Greenhouse Gases," ranging from 0 to 100%, with major ticks at 0, 50, and 100%.
  • Data Representation:
    • Topmost Graph (Yellow with Blue Shading): Shows the percentage of Earth’s emitted energy absorbed by all greenhouse gases combined (blue shading) at each wavelength, with the red curve from the top diagram overlaid to show the emitted energy that escapes.
    • Individual Gas Absorption Graphs: Below the combined absorption graph, separate graphs show the absorption spectra for specific greenhouse gases:
      • Water Vapor (Blue): Strong absorption around 5–7 μm and beyond 20 μm.
      • Carbon Dioxide (Green): Significant absorption around 4 μm and 15 μm.
      • Oxygen and Ozone (Gray): Absorption primarily around 9–10 μm.
      • Methane (Brown): Absorption around 3.5 μm and 7–8 μm.
      • Nitrous Oxide (Dark Brown): Minor absorption around 4.5 μm and 8 μm.
    • A label on the right side reads: "these absorption spectra add up to the total absorption spectrum for green-house gases," indicating that the combined absorption (topmost graph) is the sum of the individual contributions.
  • Trend: The absorption graphs show that different gases absorb energy at specific wavelengths, with water vapor and carbon dioxide being the most significant contributors to the overall absorption.

The diagrams together illustrate how greenhouse gases absorb Earth’s outgoing infrared radiation, reducing the energy that escapes to space and contributing to the greenhouse effect. The absorption spectra of individual gases highlight their specific roles in this process.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

In fact, the same thing happens to the energy the Earth receives from the Sun — various gases in the atmosphere absorb that energy, so the amount we receive on the surface is less than what arrives at the top of the atmosphere.

Spectrum of Solar Radiation

Click for a text description of Spectrum of Solar Radiation

The image is a graph titled "Spectrum of Solar Radiation," showing the energy intensity of solar radiation across different wavelengths, comparing the radiation above the atmosphere to that at sea level. It highlights the effects of atmospheric absorption by various gases.

• Title: The title at the top reads: "Spectrum of Solar Radiation."
• Axes:
  • X-Axis (Wavelength): Labeled "Wavelength (nm)," ranging from 250 to 2500 nanometers (nm), with major ticks at intervals of 250 nm (250, 500, 750, 1000, 1250, 1500, 1750, 2000, 2250, 2500).
  • Y-Axis (Energy Intensity): Labeled "Energy Intensity (W/m²/nm)," ranging from 0 to 2.5 watts per square meter per nanometer (W/m²/nm), with major ticks at intervals of 0.5 (0, 0.5, 1.0, 1.5, 2.0, 2.5).
• Data Representation:
  • Two curves are plotted:
    • Radiation Above the Atmosphere: Represented by a smooth yellow curve labeled "Radiation Above the Atmosphere," also referred to as "Black Body at 5250°C." This curve approximates the Sun's emission as a blackbody at 5250°C (approximately 5523 K, close to the Sun's surface temperature of ~5773 K).
    • Radiation at Sea Level: Represented by a red curve labeled "Radiation at sea level," showing the solar radiation after passing through the atmosphere.
  • The area between the yellow and red curves is shaded red, indicating the energy absorbed by the atmosphere.
• Spectral Regions:
  • The graph is divided into three regions along the x-axis:
    • UV (Ultraviolet): From 250 nm to ~400 nm.
    • Visible: From ~400 nm to ~700 nm.
    • Infrared: From ~700 nm to 2500 nm.
  • These regions are marked with vertical dashed lines separating UV, visible, and infrared.
• Absorption Bands:
  • Specific absorption bands are labeled along the red curve, indicating where atmospheric gases absorb solar radiation:
    • O₃ (Ozone): Absorbs strongly in the UV range (around 250–350 nm).
    • O₂ (Oxygen): Absorbs around 750 nm.
    • H₂O (Water Vapor): Absorbs at multiple wavelengths, notably around 900 nm, 1100 nm, 1400 nm, and 1900 nm.
    • CO₂ (Carbon Dioxide): Absorbs around 2000 nm.
  • These absorption bands cause dips in the red curve, showing reduced energy intensity at sea level compared to above the atmosphere.
• Curve Characteristics:
  • Yellow Curve (Above Atmosphere): Peaks around 500 nm in the visible range at an intensity of about 2.0 W/m²/nm, then gradually decreases toward longer wavelengths, reaching near 0 W/m²/nm by 2500 nm.
  • Red Curve (At Sea Level): Follows the yellow curve but with significant reductions at specific wavelengths due to absorption. It peaks slightly below 2.0 W/m²/nm around 500 nm and shows pronounced dips corresponding to the absorption bands of O₃, O₂, H₂O, and CO₂.
• Overall Trend:
  • The graph illustrates that while solar radiation above the atmosphere follows a smooth blackbody curve, the radiation reaching sea level is significantly altered by atmospheric absorption.
  • The visible range (400–700 nm) experiences relatively less absorption, allowing most of the sunlight in this range to reach the surface, while UV and infrared regions are more heavily absorbed by atmospheric gases.

The graph effectively demonstrates the impact of Earth's atmosphere on incoming solar radiation, highlighting the role of specific gases in absorbing energy at different wavelengths.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

How do gases absorb this energy? It is basically a matter of vibrations of gas molecules being in sync with some of the frequencies of energy associated with insolation or infrared energy given off by Earth. You can think of the bonds between atoms in an H₂O molecule like springs that stretch, twist, and bend at specific frequencies (nice animation of H₂O movement), and if energy hits those molecules at just the right frequency, the bonds of the molecule absorb that energy and oscillate and stretch and twist more strongly.

There are numerous ways to demonstrate this heat-trapping ability of some gases — here is a nice laboratory demonstration of heat-trapping — but you can also think of the difference between the cold nighttime temperatures when the air is dry (little water vapor) compared to the warmer nighttime temperatures when the air is humid. The fact of the greenhouse effect is one of the most important things to understand about our climate system. This greenhouse effect, which is probably better described as warming produced by heat-trapping gases, is incredibly powerful — it returns more energy to the surface than we absorb from the Sun, and its strength is closely tied to the global carbon cycle, and thus the oceans, and all the biota on Earth.

Let’s try to put a lot of this together now and have a glance at the energy budget for Earth’s climate. The figure below attempts to illustrate where all the energy goes in the climate system. We start with 100 units of energy, which represents the total amount of energy Earth receives from the Sun in a year.

Energy Flows in the Climate System

Click here for a text description of Energy Flows in the Climate System

The image is a diagram titled "Energy Flows in the Climate System," illustrating the flow of solar energy through Earth's climate system, including the atmosphere and surface reservoirs. It quantifies the energy in units (relative to 100 units of incoming solar radiation) and highlights processes like reflection, absorption, heat transfer, and the greenhouse effect. The diagram is attributed to Kiehl and Trenberth (1997).

• Overall Structure:
  • The diagram is divided into three main sections: incoming solar radiation (left), the atmosphere (center), and the surface reservoir (bottom). Arrows and numerical values indicate the flow and distribution of energy.
• Incoming Solar Radiation (Left Side):
  • 100 units: Represented as a yellow arrow labeled "Incoming Solar Radiation," entering from the top left.
  • 9 units: Reflected off the land surface, labeled "Insolation Reflected off Land Surface" (yellow arrow pointing back upward).
  • 23 units: Reflected by clouds and aerosols, labeled "Insolation Reflected by Clouds & Aerosols" (yellow arrow pointing upward).
  • Total Reflected: 9 + 23 = 32 units of the incoming 100 units are reflected back to space.
• Atmosphere (Center Section):
  • 19 units: Absorbed by the atmosphere, labeled "Absorbed by Atmosphere" (yellow arrow curving into the atmosphere).
  • 16.5 units: Labeled "Atmosphere Reservoir," indicating the energy stored in the atmosphere.
  • 11 units: Lost to space as heat, labeled "Heat Lost to Space" (purple arrow pointing upward).
  • 133 units: Transferred from the surface to the atmosphere, labeled "Heat Transfer to Atmosphere" (red arrow pointing upward).
  • 57 units: Radiated into space from the top of the atmosphere, labeled "Heat Radiated into Space from top Atmosphere" (purple arrow pointing upward).
  • 95 units: Returned to the surface via the greenhouse effect, labeled "Heat Returned to Surface (Greenhouse Effect)" (green arrow pointing downward).
  • A section labeled "Energy Recycles" indicates the cycling of energy between the surface and atmosphere.
• Surface Reservoir (Bottom Section):
  • 49 units: Absorbed by the surface, labeled "Insolation Absorbed by Surface" (yellow arrow pointing downward).
  • 271.2 units: Labeled "Surface Reservoir (30% land; 70% water)," indicating the total energy at the surface.
  • A note within the surface reservoir states: "(boxed values indicate thermal energy stored in a reservoir)," referring to the 271.2 units.
  • Another note states: "(circled values indicate energy flows)," referring to the other numerical values in the diagram.
• Energy Balance Note (Bottom Left):
  • A note at the bottom left reads: "Here, 100 energy units = ~5.56E24 Joules/yr; the total annual solar energy received (averages ~342 W/m² over the surface of the Earth)," providing the scale for the energy units used in the diagram.
• Visual Elements:
  • The diagram uses color-coded arrows to represent different energy flows:
    • Yellow for incoming and reflected solar radiation.
    • Red for heat transfer from the surface to the atmosphere.
    • Purple for heat lost to space.
    • Green for heat returned to the surface via the greenhouse effect.
  • Clouds are depicted in the atmosphere, and the surface is divided into land (30%) and water (70%).

The diagram effectively illustrates the energy balance of Earth's climate system, showing how incoming solar radiation is distributed, reflected, absorbed, and re-radiated, with a significant portion recycled through the greenhouse effect, as quantified by Kiehl and Trenberth in 1997.

Credit: © Kiehl and Trenberth, 1997 Used with permission

When the insolation strikes the atmosphere, 23 units are reflected back to space from clouds and aerosols, which are tiny particles suspended in the atmosphere. Another 19 units are absorbed by the atmosphere, as described in the figure above, thus adding thermal energy to the atmosphere. The remaining 58 units of energy reach the Earth’s surface, where 9 units are reflected back into space, and the remaining 49 units are absorbed by the surface, warming the planet. The Earth’s surface is mostly water, and by virtue of its temperature and heat capacity, it has a lot more thermal energy than the atmosphere (271.2 vs 16.5). Energy flows up from the surface to the atmosphere in a variety of ways — mainly by emission of infrared radiation, heat transfer by evaporation, and then condensation of water. When water evaporates, it “steals” energy from the surface; this energy is needed to make the phase change from liquid to vapor, and the same energy is then released when water vapor condenses to form liquid water droplets. As you can see from the diagram, the combined flow of energy from the surface is greater than the amount we get from the sun! Of this energy given off by the surface, a little bit (7 units) escapes the atmosphere because there are no gases that absorb infrared energy at wavelengths between 10 to 15 microns; the rest is absorbed by the atmosphere, which then emits infrared energy from its top to outer space and from its bottom back to the surface; this atmospheric absorption of infrared energy and its return to the surface is called the greenhouse effect. Since the bottom of the atmosphere is much warmer than the top, much more energy is returned to the Earth’s surface than is emitted to outer space.

The remarkable thing to observe and remember here is that the surface receives almost twice as much energy from the greenhouse effect than it does directly from the Sun! But, if you look at the diagram a bit, you can see that the energy sent to the surface from the atmosphere is essentially recycled energy, whose origin is the Sun.

Energy Flows in the Climate System

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The image is a diagram titled "Energy Flows in the Climate System," illustrating the flow of solar energy through Earth's climate system, including the atmosphere and surface reservoirs. It quantifies the energy in units (relative to 100 units of incoming solar radiation) and highlights processes like reflection, absorption, heat transfer, and the greenhouse effect. The diagram is attributed to Kiehl and Trenberth (1997).

• Overall Structure:
  • The diagram is divided into three main sections: incoming solar radiation (left), the atmosphere (center), and the surface reservoir (bottom). Arrows and numerical values indicate the flow and distribution of energy.
• Incoming Solar Radiation (Left Side):
  • 100 units: Represented as a yellow arrow labeled "Incoming Solar Radiation," entering from the top left.
  • 9 units: Reflected off the land surface, labeled "Insolation Reflected off Land Surface" (yellow arrow pointing back upward).
  • 23 units: Reflected by clouds and aerosols, labeled "Insolation Reflected by Clouds & Aerosols" (yellow arrow pointing upward).
  • Total Reflected: 9 + 23 = 32 units of the incoming 100 units are reflected back to space.
• Atmosphere (Center Section):
  • 19 units: Absorbed by the atmosphere, labeled "Absorbed by Atmosphere" (yellow arrow curving into the atmosphere).
  • 16.5 units: Labeled "Atmosphere Reservoir," indicating the energy stored in the atmosphere.
  • 11 units: Lost to space as heat, labeled "Heat Lost to Space" (purple arrow pointing upward).
  • 133 units: Transferred from the surface to the atmosphere, labeled "Heat Transfer to Atmosphere" (red arrow pointing upward).
  • 57 units: Radiated into space from the top of the atmosphere, labeled "Heat Radiated into Space from top Atmosphere" (purple arrow pointing upward).
  • 95 units: Returned to the surface via the greenhouse effect, labeled "Heat Returned to Surface (Greenhouse Effect)" (green arrow pointing downward).
  • A section labeled "Energy Recycles" indicates the cycling of energy between the surface and atmosphere.
• Surface Reservoir (Bottom Section):
  • 49 units: Absorbed by the surface, labeled "Insolation Absorbed by Surface" (yellow arrow pointing downward).
  • 271.2 units: Labeled "Surface Reservoir (30% land; 70% water)," indicating the total energy at the surface.
  • A note within the surface reservoir states: "(boxed values indicate thermal energy stored in a reservoir)," referring to the 271.2 units.
  • Another note states: "(circled values indicate energy flows)," referring to the other numerical values in the diagram.
• Energy Balance Note (Bottom Left):
  • A note at the bottom left reads: "Here, 100 energy units = ~5.56E24 Joules/yr; the total annual solar energy received (averages ~342 W/m² over the surface of the Earth)," providing the scale for the energy units used in the diagram.
• Visual Elements:
  • The diagram uses color-coded arrows to represent different energy flows:
    • Yellow for incoming and reflected solar radiation.
    • Red for heat transfer from the surface to the atmosphere.
    • Purple for heat lost to space.
    • Green for heat returned to the surface via the greenhouse effect.
  • Clouds are depicted in the atmosphere, and the surface is divided into land (30%) and water (70%).

The diagram effectively illustrates the energy balance of Earth's climate system, showing how incoming solar radiation is distributed, reflected, absorbed, and re-radiated, with a significant portion recycled through the greenhouse effect, as quantified by Kiehl and Trenberth in 1997.

Credit: © Kiehl and Trenberth, 1997 Used with permission

The view of the climate system depicted in the adjacent figure is one of stability — energy flows in and out, in perfect balance, so the temperature of the earth should stay the same. But if we can learn anything from studying Earth’s history, we learn that change is the rule and stability the exception. When change occurs, it almost always brings feedback mechanisms into play — they can accentuate and dampen change, and they are incredibly important to our climate system. There are many good examples of feedback mechanisms, but here are a few to illustrate the idea.

Ice — Albedo Feedback

Ice reflects sunlight better than almost any other material on Earth, and in reflecting sunlight, it lowers the amount of insolation absorbed by Earth, which makes it colder. If the Earth becomes colder, more ice may grow, covering more area and thus reflecting even more insolation, which in turn cools the Earth further. Thus cooling instigates ice expansion, which promotes additional cooling, and so on — this is clearly a cycle that feeds back on itself to encourage the initial change. Since this chain of events furthers the initial change that triggered the whole thing, it is called a positive feedback (but note that the change may not be good from our perspective). Positive feedback mechanisms tend to lead to runaway change — some small initial change is thus accentuated into a major change.

Weathering Feedback

Rocks exposed at the surface interact with water and the atmosphere and undergo a set of chemical and physical changes we call weathering. The chemical part of weathering often involves the consumption of carbonic acid (formed from water and carbon dioxide) in dissolving minerals in rocks. This process of weathering is thus a sink for atmospheric carbon dioxide, which is an important greenhouse gas. If you remove carbon dioxide from the atmosphere, you weaken the greenhouse effect and this leads to cooling of the Earth. Like many chemical reactions, this chemical weathering occurs more rapidly in hotter climates, which are associated with higher levels of carbon dioxide. So consider a scenario in which some warming occurs; this will encourage faster weathering, which will consume carbon dioxide, which will lead to cooling. In this case, the initial change triggered a set of processes that countered the initial change — this is called a negative feedback (even though it may have beneficial results) because it works in opposition to the change that triggered it.

Cloud Feedback

Another important negative feedback mechanism involves the formation of clouds. On the whole, clouds in today's climate have a slight net cooling effect — this is the balance of the increased albedo due to low clouds and the increased greenhouse effect caused by high cirrus clouds. As a general rule, as the atmosphere gets warmer, it can hold more water vapor, and with more water vapor, we expect more clouds, and the increased clouds will then tend to limit the warming that initiated the increased clouds — thus we have another negative feedback mechanism.

Positive and Negative Feedbacks — Yin and Yang

In Asian philosophy, yin and yang can be thought of as interacting, interconnected forces that are essential components of a dynamic system. In the Earth system, positive and negative feedbacks are a bit like yin and yang — they are essential components of the whole system that ultimately play an important role in maintaining a more or less stable state. Positive feedback mechanisms enhance or amplify some initial change, while negative feedback mechanisms stabilize a system and prevent it from getting into extreme states. In many respects, the history of Earth’s climate system can be seen as a bit of a battle between these two types of feedback, but in the end, the negative feedbacks win out and our climate is generally stable with a limited range of change (excepting, of course, a few extremes such as the Snowball Earth events back around 750 Myr ago).

Positive and Negative Feedback Mechanisms

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The image consists of two sets of diagrams illustrating feedback mechanisms in the climate system, specifically focusing on positive and negative feedback loops. The diagrams use arrows and labels to show the relationships between different climate variables.

• Top Section: Positive Feedback Mechanism
  • Left Diagram (Cooling Cycle):
    • Components and Flow:
      • "Cooling" (in blue) leads to "Ice Growth" (with a "+" sign indicating a positive relationship).
      • "Ice Growth" leads to "Increase Albedo" (more ice reflects more sunlight).
      • "Increase Albedo" leads to "Less Insolation Absorbed" (less solar energy absorbed due to higher reflectivity).
      • "Less Insolation Absorbed" loops back to "Cooling," completing the cycle.
    • Description: This cycle shows that cooling promotes ice growth, which increases albedo (reflectivity), reducing the absorption of solar energy and further enhancing cooling—a self-reinforcing loop.
  • Right Diagram (Warming Cycle):
    • Components and Flow:
      • "Warming" (in red) leads to "Ice Melting" (with a "+" sign indicating a positive relationship).
      • "Ice Melting" leads to "Decrease Albedo" (less ice means less reflectivity).
      • "Decrease Albedo" leads to "More Insolation Absorbed" (more solar energy absorbed due to lower reflectivity).
      • "More Insolation Absorbed" loops back to "Warming," completing the cycle.
    • Description: This cycle shows that warming causes ice to melt, decreasing albedo, which increases the absorption of solar energy and further enhances warming—another self-reinforcing loop.
• Bottom Section: Negative Feedback Mechanism
  • Title: "Negative Feedback Mechanism" is written below the positive feedback section.
  • Left Diagram (Warming to Cooling):
    • Components and Flow:
      • "Warming" (in red) leads to "Increased Weathering" (with a "−" sign indicating a negative relationship).
      • "Increased Weathering" leads to "Weaker Greenhouse" (weathering removes CO₂ from the atmosphere, reducing the greenhouse effect).
      • "Weaker Greenhouse" leads to "Cooling" (in blue).
      • "Cooling" loops back to "Warming," completing the cycle.
    • Description: This cycle shows that warming increases weathering, which weakens the greenhouse effect by removing CO₂, leading to cooling—a self-regulating loop that counteracts the initial warming.
  • Right Diagram (Cooling to Warming):
    • Components and Flow:
      • "Cooling" (in blue) leads to "Decreased Weathering" (with a "−" sign indicating a negative relationship).
      • "Decreased Weathering" leads to "Stronger Greenhouse" (less CO₂ removal allows the greenhouse effect to strengthen).
      • "Stronger Greenhouse" leads to "Warming" (in red).
      • "Warming" loops back to "Cooling," completing the cycle.
    • Description: This cycle shows that cooling reduces weathering, allowing CO₂ to accumulate and strengthen the greenhouse effect, leading to warming—a self-regulating loop that counteracts the initial cooling.
• Visual Elements:
  • Arrows indicate the direction of influence between variables.
  • "+" signs in the positive feedback diagrams indicate that the variables reinforce each other.
  • "−" signs in the negative feedback diagrams indicate that the variables counteract each other.
  • "Cooling" is written in blue, and "Warming" is written in red to differentiate the temperature changes.

The diagrams effectively illustrate how positive feedback mechanisms (like the ice-albedo feedback) amplify climate changes, while negative feedback mechanisms (like the weathering-greenhouse feedback) act to stabilize the climate system by counteracting changes.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Earth’s climate systems are characterized by thresholds, levels that once crossed herald a new climate state. For example, we often talk about the 1.5 or 2^oC thresholds across which the impacts of climate change become dangerous, for example including long heatwaves, devastating droughts and more common extreme weather events. Thresholds can become tipping points if, once crossed, there is no going back from the new climate state, at least temporarily. One of the best examples of a tipping point is the cessation in Atlantic meridional overturning circulation (AMOC) which we will learn about in Module 6 and which is a key driver of heat transport around the globe. This circulation drives the formation of ocean deep waters which in return feeds the Gulf Stream which warms northern latitudes including Western Europe, making them more habitable. It also fuels monsoon rains in places like India. In the past, AMOC has turned off, driving Northern Europe into an ice age. The system is highly complex and difficult to predict, but there have been warning signs that it has been edging closer to that potentially devastating tipping point in recent decades. The system becomes more variable as a tipping point is reached, and that variability is currently showing signs of increasing. Tipping points may involve positive feedbacks. For example, melting and disintegration of the West Antarctic Ice sheet will lower planetary albedo resulting in further melting, and at some stage the feedback will make the system irreversible, at least temporarily. Another example is the melting of permafrost in the Arctic region. Tipping points can lead to cascading changes if they impact one another, for example, significant melting of Antarctic ice can cause enough warming to exacerbate permafrost melting. More examples of tipping points are showing in the figure below.

Possible tipping elements in the climate system

Click for a text description of the Tipping Elements map.

The image is a world map highlighting various regions with potential climate tipping points, where small changes in climate conditions could lead to significant, often irreversible shifts in the Earth system. The map uses color-coded regions and labels to indicate specific tipping elements and their potential impacts.

• Map Overview:
  • The map is a global projection centered on the Atlantic Ocean, showing continents in gray with specific regions highlighted in colors (red, green, blue, orange) to indicate climate tipping points.
  • Labels are placed over the highlighted regions to describe the tipping elements and their potential changes.
• Highlighted Regions and Tipping Points:
  • Arctic and Northern Regions:
    • Melt of Greenland Ice-Sheet: Highlighted in orange over Greenland, indicating potential ice loss due to warming.
    • Climatic Change-Induced Ozone Hole?: Highlighted in orange over the Arctic, suggesting possible changes in atmospheric chemistry.
    • Boreal Forest Dieback: Highlighted in green over northern North America (Canada and Alaska) and northern Eurasia (Siberia), indicating potential forest loss due to climate stress.
    • Permafrost and Tundra Loss?: Highlighted in blue over northern Siberia, suggesting potential thawing of permafrost and loss of tundra ecosystems.
  • Atlantic Ocean:
    • Atlantic Deep Water Formation: Highlighted in orange in the North Atlantic, indicating potential disruption of ocean circulation patterns like the Atlantic Meridional Overturning Circulation (AMOC).
  • South America:
    • Dieback of Amazonas Rainforest: Highlighted in green over the Amazon Basin, indicating potential forest loss due to drying and deforestation.
  • Africa:
    • Sahel Greening: Highlighted in green over the Sahel region (south of the Sahara Desert), suggesting potential vegetation growth due to changing rainfall patterns.
    • West African Monsoon Shift: Highlighted in green over West Africa, indicating potential changes in monsoon patterns.
  • Indian Subcontinent:
    • Indian Monsoon Chaotic Multistability: Highlighted in red over India, suggesting potential unpredictable shifts in monsoon behavior.
  • Antarctica:
    • Instability of West Antarctic Ice Sheet: Highlighted in blue over West Antarctica, indicating potential ice sheet collapse and sea level rise.
    • Changes in Antarctic Bottom Water Formation?: Highlighted in blue around Antarctica, suggesting potential disruption of deep ocean water formation.
  • Global Ocean:
    • Change in ENSO Amplitude or Frequency: Highlighted in red over the equatorial Pacific Ocean, indicating potential changes in the El Niño-Southern Oscillation (ENSO) patterns.
• Color Coding:
  • Red: Used for regions with potential changes in ENSO and the Indian Monsoon, indicating high-impact, dynamic shifts.
  • Green: Used for regions like the Amazon, Sahel, West Africa, and boreal forests, indicating potential ecosystem changes (dieback or greening).
  • Blue: Used for polar regions (Siberia, Antarctica), indicating ice and permafrost-related tipping points.
  • Orange: Used for the Arctic, Greenland, and North Atlantic, indicating ice melt and ocean circulation changes.
• Visual Elements:
  • The map uses a gray background for continents not highlighted, with colored overlays for the tipping point regions.
  • Labels are written in black, with question marks in some cases (e.g., "Climatic Change-Induced Ozone Hole?" and "Permafrost and Tundra Loss?") indicating uncertainty or ongoing research.

The map effectively illustrates the global distribution of climate tipping points, highlighting regions where the climate system may undergo significant and potentially irreversible changes due to global warming and other climate stressors.

CodeOne (blank map), DeWikiMan (additional elements), CC BY-SA 4.0, via Wikimedia Commons

Tipping points represent one of the most dire threats of climate change due to their irreversible nature. Because of this, they are very much a key driver of the warming thresholds and emissions targets.

Energy Flows in the Climate System

Click here for a text description of Energy Flows in the Climate System

The image is a diagram titled "Energy Flows in the Climate System," illustrating the flow of solar energy through Earth's climate system, including the atmosphere and surface reservoirs. It quantifies the energy in units (relative to 100 units of incoming solar radiation) and highlights processes like reflection, absorption, heat transfer, and the greenhouse effect. The diagram is attributed to Kiehl and Trenberth (1997).

• Overall Structure:
  • The diagram is divided into three main sections: incoming solar radiation (left), the atmosphere (center), and the surface reservoir (bottom). Arrows and numerical values indicate the flow and distribution of energy.
• Incoming Solar Radiation (Left Side):
  • 100 units: Represented as a yellow arrow labeled "Incoming Solar Radiation," entering from the top left.
  • 9 units: Reflected off the land surface, labeled "Insolation Reflected off Land Surface" (yellow arrow pointing back upward).
  • 23 units: Reflected by clouds and aerosols, labeled "Insolation Reflected by Clouds & Aerosols" (yellow arrow pointing upward).
  • Total Reflected: 9 + 23 = 32 units of the incoming 100 units are reflected back to space.
• Atmosphere (Center Section):
  • 19 units: Absorbed by the atmosphere, labeled "Absorbed by Atmosphere" (yellow arrow curving into the atmosphere).
  • 16.5 units: Labeled "Atmosphere Reservoir," indicating the energy stored in the atmosphere.
  • 11 units: Lost to space as heat, labeled "Heat Lost to Space" (purple arrow pointing upward).
  • 133 units: Transferred from the surface to the atmosphere, labeled "Heat Transfer to Atmosphere" (red arrow pointing upward).
  • 57 units: Radiated into space from the top of the atmosphere, labeled "Heat Radiated into Space from top Atmosphere" (purple arrow pointing upward).
  • 95 units: Returned to the surface via the greenhouse effect, labeled "Heat Returned to Surface (Greenhouse Effect)" (green arrow pointing downward).
  • A section labeled "Energy Recycles" indicates the cycling of energy between the surface and atmosphere.
• Surface Reservoir (Bottom Section):
  • 49 units: Absorbed by the surface, labeled "Insolation Absorbed by Surface" (yellow arrow pointing downward).
  • 271.2 units: Labeled "Surface Reservoir (30% land; 70% water)," indicating the total energy at the surface.
  • A note within the surface reservoir states: "(boxed values indicate thermal energy stored in a reservoir)," referring to the 271.2 units.
  • Another note states: "(circled values indicate energy flows)," referring to the other numerical values in the diagram.
• Energy Balance Note (Bottom Left):
  • A note at the bottom left reads: "Here, 100 energy units = ~5.56E24 Joules/yr; the total annual solar energy received (averages ~342 W/m² over the surface of the Earth)," providing the scale for the energy units used in the diagram.
• Visual Elements:
  • The diagram uses color-coded arrows to represent different energy flows:
    • Yellow for incoming and reflected solar radiation.
    • Red for heat transfer from the surface to the atmosphere.
    • Purple for heat lost to space.
    • Green for heat returned to the surface via the greenhouse effect.
  • Clouds are depicted in the atmosphere, and the surface is divided into land (30%) and water (70%).

The diagram effectively illustrates the energy balance of Earth's climate system, showing how incoming solar radiation is distributed, reflected, absorbed, and re-radiated, with a significant portion recycled through the greenhouse effect, as quantified by Kiehl and Trenberth in 1997.

Here 100 energy units = 5.56e24J/year, the total annual solar energy received averages 342 W/m^@ over the surface of the Earth

Incoming solar radiation: 100

Insolation Reflected by Clouds and Aerosols: 23

Insolation Reflected off Land Surface: 9

Insolation Absorbed by Surface: 49

Atmosphere Reservoir: 16.5

Surface Reservoir (30% Land, 70% Water): 271.2

Heat Transfer to Atmosphere: 133

Heat Lost to Space: 11

Heat returned to Surface (Greenhouse Effect): 95

Heat Radiated into Space from top of Atmosphere: 57

Credit: © Kiehl and Trenberth, 1997 Used with permission

The diagram we have just been considering (repeated above), presents a good overview of how energy flows through the Earth’s climate system, but it does not give us a sense of how that energy is distributed across the surface of the globe and there are some important things to be learned from looking at this spatial pattern. For many years now, satellites have been monitoring these energy flows using spectrometers that measure the intensity of energy at different wavelengths flowing to the Earth and from the Earth. So, let’s see what can be learned from a quick study of these satellite views. First, we consider the insolation at the top of the atmosphere averaged for the month of March.

The insolation averaged over March 2003 from NASA’s CERES satellite. In March, the insolation is at its greatest right at the equator and drops off to nearly zero at the poles. Note that this is the insolation before it interacts with the atmosphere, so it is a fairly simple pattern.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Of course, not all of this insolation strikes the surface — remember that just 49% of it reaches the ground. If we then look at the insolation reaching the ground, we see the following:

The total radiation reaching the surface of the Earth for March 1985-1989, from NASA’s ERBE experiment. The red and orange areas receive higher levels of radiation, while the blue areas are receive lower levels of radiation.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Notice that the highest flux is about 190 W/m²; far less than the maximum of almost 440 W/m² that reaches the top of the atmosphere. The difference is due to reflection from clouds, reflection from the surface, and absorption by atmospheric gases.

Now, let’s look at what comes back from the Earth, in the form of long wavelength energy, for the same time period.

The emitted long wavelength energy, averaged over March 1985-1989, from NASA’s ERBE experiment. This is the emitted infrared energy after it interacts with the atmosphere (the satellite is way above the atmosphere, looking down on Earth). The complex pattern reflects variability in surface temperature and concentrations of greenhouse gases.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

At the simplest level, we see that the tropics emit much more energy than the poles. This makes sense since we know they are warmer, and the Stefan-Boltzmann law tells us that the amount of energy emitted varies as the fourth power of temperature, and the tropics are warmer because they receive much more insolation (see figures above). Looking closely at the nearest above image, we see some interesting variations near the tropics — look at South America, Central Africa, and Indonesia, where the emitted energy is far less than we see elsewhere at these same latitudes. Why is this? Is it colder there? No, it is not colder there, which leads to another question — is the atmosphere above these regions absorbing more of the infrared energy emitted by the surface? Recall that one of the main heat-absorbing gases is water, and where you have a lot of water, you have a lot of clouds. So, let’s have a look at the typical average cloud cover for this time of year.

The cloud fraction, averaged over March 2005, from NASA’s CERES satellite. White is 100% cloud cover, while dark blue is 0% cloud cover. Note that the white areas of high cloud cover correspond to the regions where there is less long wavelength energy emitted.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

So, it is indeed the case that the amount of energy leaving the Earth varies according not only to the temperature but also to the concentration of heat-absorbing gases such as water.

Recall that we are focused on the energy budget here and whenever you do a budget, at the end, you look at the balance between what is coming in and what is going out. So, let’s do that now with the energy as measured by the satellites.

The difference between energy coming in and energy leaving the Earth for March 1985-1989, from NASA’s ERBE experiment. The red areas are places where more energy is coming in than is going out, while the blue areas are places where more energy is leaving than is coming in. So the red areas have a surplus of insolation, while the blue areas are running a deficit relative to insolation.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

As can be seen in the figure above, the tropics receive more energy than they emit, while the poles emit more than they receive. This picture can also be seen in a somewhat simpler diagram in which we average the net energy flow at each latitude.

The insolation reaching the surface averaged over March 1960, from NASA’s ERBE experiment.

Click for a text description of the Net Energy: Insolation - LW Emission graph.

The image is a graph titled "Net Energy: Insolation - LW Emission, March, 1960," showing the net energy balance (incoming solar radiation minus outgoing longwave radiation) across different latitudes on Earth for the month of March 1960. The graph highlights regions of energy surplus and deficit.

• Axes:
  • X-Axis (Latitude): Labeled "Latitude," ranging from -90° (South Pole) to 90° (North Pole), with major ticks at intervals of 30° (-90, -60, -30, 0, 30, 60, 90). The equator is at 0°.
  • Y-Axis (Net Radiation): Labeled "Net Radiation (W/m²)," ranging from -160.0 to 160.0 watts per square meter (W/m²), with major ticks at intervals of 32.0 W/m² (-160.0, -96.0, -32.0, 32.0, 96.0, 160.0). The zero line represents a balance between incoming and outgoing radiation.
• Data Representation:
  • The graph is a single curve plotted in red, representing the net energy (insolation minus longwave emission) at each latitude.
  • The area above the zero line (positive values) is shaded gray and labeled "Energy Surplus," indicating where incoming solar radiation exceeds outgoing longwave radiation.
  • The areas below the zero line (negative values) are shaded with a crosshatch pattern and labeled "Energy Deficit," indicating where outgoing longwave radiation exceeds incoming solar radiation.
• Curve Characteristics:
  • Energy Surplus: The curve peaks around the equator (0° latitude), reaching a maximum of approximately 96 W/m², indicating a significant energy surplus in the tropics. The surplus extends from about -30° to 30° latitude.
  • Energy Deficit: The curve dips below the zero line at higher latitudes, showing an energy deficit. It reaches a minimum of around -160 W/m² at the poles (-90° and 90° latitude), indicating a significant energy deficit in polar regions.
  • The curve crosses the zero line (net energy balance) at approximately -30° and 30° latitude, marking the transition between energy surplus and deficit zones.
• Overall Trend:
  • The graph shows a clear latitudinal pattern: the tropics (near the equator) experience an energy surplus due to high insolation and relatively lower longwave emission, while the polar regions experience an energy deficit due to low insolation (especially in March, during the polar night in the Southern Hemisphere) and high longwave emission.
  • The energy surplus in the tropics drives global atmospheric and oceanic circulation, as excess energy is transported toward the poles to balance the energy deficit.

The graph effectively illustrates the latitudinal variation in Earth's energy balance for March 1960, highlighting the role of solar insolation and longwave emission in creating energy surpluses and deficits that influence global climate patterns.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The atmosphere and oceans are constantly flowing, and this motion is critical to the climate system. What makes them flow? In general, the movement is due to pressure differences — things flow from regions of high pressure to low pressure and the resulting surface winds distribute heat at the Earth's surface.

Movement of air masses from high to low-pressure regions.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The pressure changes are themselves due to density and height differences — higher density in the air or the oceans leads to higher pressures. The density differences are due to changes in composition and temperature; this works slightly differently for air and water. In air, the important compositional variable is water vapor content — more water means lower density air. When we say more water, we mean that for a given number of molecules in a volume of air, a greater percentage of them are water, and water, as shown below, is lighter than a nitrogen molecule, which is the most abundant molecule in our atmosphere.

Density of Air

Inverse relationship with Temperature:

Higher temp = lower density → rising air

Weight of H₂O= 18

Lower temp = higher density → sinking air

Weight of N₂ = 28

Inverse relationship with water content:

More water = lower density → rising air

Less water = higher density → sinking air

As indicated above, density differences can cause either rising or sinking of air masses. Because Earth’s gravity decreases as you move away from the surface, there is a kind of equilibrium profile of density with height above the surface, as shown by the green curve below:

Lowering density of an air mass causes it to elevate

Click for a text description of the density of air masses.

The image is a simple graph labeled "Equilibrium profile of density," depicting the relationship between density and an unspecified variable, likely altitude or depth, in a fluid medium such as the atmosphere or ocean. The graph consists of a single curve on a black background with minimal labeling.

• Curve:
  • The curve is plotted in cyan and shows a smooth, exponential-like decay.
  • It starts at a higher value on the left side and decreases as it moves to the right, approaching a lower value asymptotically.
  • The curve suggests that density decreases with increasing altitude (if the x-axis represents height in the atmosphere) or increases with depth (if the x-axis represents depth in the ocean).
• Axes:
  • The graph lacks explicit axes labels or numerical scales.
  • The x-axis likely represents the variable with which density changes (e.g., altitude, depth, or another parameter), increasing from left to right.
  • The y-axis likely represents density, decreasing from left to right as the curve slopes downward.
• Trend:
  • The curve indicates an equilibrium state where density decreases exponentially with increasing altitude (or increases with depth), which is typical in atmospheric or oceanic profiles due to gravitational effects and pressure gradients.

The graph visually represents a fundamental concept in atmospheric or oceanic science, showing how density varies in a stable, equilibrium state, likely influenced by factors such as gravity, temperature, and pressure. The lack of specific axes labels makes the exact context (e.g., atmosphere vs. ocean) ambiguous, but the shape of the curve is characteristic of such profiles.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

If we lower the density of air at the surface from A to B, then the air rises from B to C. Then, if we increase the density of air at point C, moving it to D, it will sink back down to point A near the surface.

We start with the movement of the atmosphere, which we will try to make as simple as possible by first concentrating on the flow as seen in a vertical slice from pole to pole. The story begins at the equator, where air is warmed and lots of evaporation adds water to the air, giving it a low density:

Air masses spread out laterally when they reach the tropopause.

Click for a text description of the air masses spreading out diagram.

The image is a simple graph depicting a temperature profile in the atmosphere, likely illustrating the concept of the lapse rate or temperature variation with altitude. The graph consists of a single curve on a black background with minimal labeling.

• Curve:
  • The curve is plotted in blue with a dashed line style.
  • It forms a smooth, parabolic shape, starting at a higher value on the left, reaching a peak at the center, and then decreasing symmetrically to a lower value on the right.
  • The curve likely represents temperature as a function of altitude, with the peak indicating a temperature maximum (possibly at the tropopause or another atmospheric layer) and the decrease on either side representing cooling with increasing altitude.
• Axes:
  • The graph lacks explicit axes labels or numerical scales.
  • The x-axis likely represents altitude, increasing from left to right.
  • The y-axis likely represents temperature, with the peak at the center indicating the highest temperature in the profile.
• Additional Element:
  • At the bottom center of the graph, there is a small rectangular box with the symbol "L" inside it, possibly indicating the lapse rate (the rate of temperature decrease with altitude) or another related parameter.
• Trend:
  • The curve suggests a temperature profile where temperature increases with altitude up to a certain point (the peak) and then decreases, which is characteristic of the troposphere-stratosphere transition in the atmosphere.

The graph visually represents a fundamental concept in atmospheric science, showing how temperature varies with altitude in a simplified manner, likely focusing on the lapse rate or a specific atmospheric layer.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

This air rises until it gets to the top of the tropopause, which is a bit like a lid on the lower atmosphere. It then diverges, with some of the air flowing north and some flowing south. As it rises and moves away from the equator, the air gets colder, water vapor condenses and rains out and the air grows drier — the cooling and drying both make the air grow denser and by the time it reaches about 30°N and 30°S latitude, it begins to sink down to the surface.

Sinking of air masses at sub-tropical latitudes.

Click for a text description of the sinking of air masses diagram.

• Diagram Overview:
  • A simple diagram illustrating a low-pressure system in the atmosphere.
  • Features a blue dashed parabolic curve on a black background.
• Curve Details:
  • The blue dashed curve dips to a minimum at the center.
  • Represents a pressure profile with altitude or horizontal distance.
  • Indicates the lowest pressure at the center of the curve.
• Labeling:
  • A light blue box labeled "Low pressure" is placed at the center below the curve.
  • Marks the point of lowest pressure in the system.
• Surrounding Elements:
  • Two red upward-curving shapes on either side of the "Low pressure" label.
  • Indicate higher pressure regions surrounding the low-pressure center.
• Interpretation:
  • Suggests the structure of a low-pressure system where air rises at the center.
  • Often associated with weather phenomena like storms due to the pressure gradient.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The sinking air is dense and dry, creating zones of high pressure in each hemisphere that are associated with very few clouds and rainfall — these are the desert latitudes. The sinking air hits the ground and then diverges. Some flows south and some flows north; the parts of this divergent flow that return towards the equator complete a loop or a convection cell, a Hadley Cell, named after Hadley, a famous meteorologist. Now, let’s turn our attention to the air that flows away from the equator. Moving along the surface, it warms and picks up water vapor, and so, its density decreases, and it eventually rises up when it gets to somewhere between 45° and 60° latitude in each hemisphere.

Rising of air masses between 45 and 60 degrees.

Click for a text description of the Rising of air masses diagram.

• Diagram Overview:
  • A simple diagram illustrating a high-pressure system in the atmosphere.
  • Features a blue dashed parabolic curve on a black background.
• Curve Details:
  • The blue dashed curve peaks at the center, forming a convex shape.
  • Represents a pressure profile with altitude or horizontal distance.
  • Indicates the highest pressure at the center of the curve.
• Pressure Elements:
  • Two red upward-curving shapes on either side of the center, indicating low-pressure regions.
  • Three light blue boxes with a "J" symbol, placed at the far left, center, and far right below the curve, representing high-pressure points.
• Interpretation:
  • Suggests the structure of a high-pressure system where air sinks at the center.
  • Often associated with clear, stable weather due to the sinking air in the high-pressure region.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0.

Once again, the rising air runs into the tropopause (which is lower at these higher latitudes) and diverges, with some of it returning toward the equator, thus completing another convection cell called the Ferrel Cell. The air that flows pole-ward sinks down at the poles, creating yet another convection cell known as the Polar Hadley Cell. These convection cells create bands of low and high pressure that roughly follow lines of latitude that exert a big influence on the climate at different latitudes. The air flowing within these convection cells does not simply move north and south as depicted above — the Coriolis effect alters the flow directions, giving us a surface pattern that is dominated by winds flowing east and west.

Simplified global atmospheric circulation.

Click for a text description of the Simplified global atmospheric circulation image.

• Diagram Overview:
  • A diagram titled "Simplified View of Global Circulation in the Atmosphere."
  • Depicts a cross-section of Earth's atmosphere, showing circulation patterns from the equator to the poles.
• Atmospheric Layers and Latitudes:
  • The troposphere is shown at the top, with the tropopause as a wavy line.
  • Latitudes are marked: 0° (equator), 30°, 60°, and 90° (poles) on both hemispheres.
• Circulation Cells:
  • Hadley Cells: Near the equator (0° to 30°), with rising air at the equator (low pressure) and sinking air at 30° (high pressure).
  • Ferrel Cells: Between 30° and 60°, with rising air at 60° (low pressure) and sinking air at 30° (high pressure).
  • Polar Hadley Cells: Between 60° and 90°, with rising air at 60° (low pressure) and sinking air at the poles (low pressure).
• Pressure and Flow Patterns:
  • Red arrows indicate rising air (low pressure, labeled "LOW P") at 0° and 60°, and sinking air (high pressure) at 30° and the poles.
  • Blue arrows show the direction of air movement within the cells, looping between high and low pressure zones.
  • Polar fronts are marked at 60° latitude in both hemispheres, where air masses converge.
• Weather Patterns:
  • Green labels indicate dominant weather patterns:
    • "Precipitation Dominates" at 0° and 60° (rising air leads to rain).
    • "Evaporation Dominates" at 30° (sinking air leads to dry conditions).
• Interpretation:
  • Illustrates the three-cell model of atmospheric circulation (Hadley, Ferrel, Polar Hadley).
  • Shows how pressure differences drive global wind patterns and weather, with precipitation and evaporation varying by latitude.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

Note that the boundary between the Polar Hadley Cell and the Ferrel Cell (often called the Polar Front, and associated with the mid-latitude jet stream) is highly variable, with big loops in it. These loops, or waves, change over time to a much greater extent than the boundaries between the other convection cells.

The Coriolis Effect arises because our planet is spinning, which means that objects near the equator are moving at much faster velocities than objects at higher latitudes. If you were standing on the equator, you would be traveling at about 1600 km/hr; if you were standing at the North Pole, you would be traveling at 0 km/hr. This means that a parcel of air moving across the surface moves into regions where the whole planet is traveling either slower or faster. The physics of this phenomenon are well-understood, and without getting into the mathematics behind it, we can summarize it with 4 simple statements:

1. objects moving in the Northern Hemisphere get deflected to the right as you look in the direction of motion;
2. objects moving in the Southern Hemisphere get deflected to the left as you look in the direction of motion;
3. the strength of this effect, this deflection, is greater as you approach the poles; and
4. the strength of the effect is more important at higher velocities (e.g., a glacier does not respond to Coriolis).

Let’s think for a minute about what this general circulation of the atmosphere does. Among other things, it mixes the atmosphere quite thoroughly, and this means that the concentrations of things like greenhouse gases get homogenized. It also means that heat gets transferred. Polar air finds its way toward lower latitudes, where it cools the surface and in so doing warms itself, and warmer air finds its way to higher latitudes, where it gives up its heat to the surroundings and thus cools.

But this general circulation does more — it drives the circulation of the surface waters in the ocean.

The oceans swirl and twirl under the influence of the winds, Coriolis, salinity differences, the edges of the continents, and the shape of the deep ocean floor. We will discuss ocean circulation in detail in Module 6, but since ocean currents are critical agents of heat transport, we must include them here as well. In general, the surface currents of the oceans are driven by winds, Coriolis, and the edges of continents, and the deep currents that mix the oceans are driven by density changes related to temperature and salinity as well as the shape of the deep ocean floor.

The pattern of circulation is shown in the figure below, which represents the average paths of flow; on a shorter term, the flow is dominated by eddies that spin around.

Surface ocean current patterns.

Click for a text description of the Ocean surface currents image.

• Map Overview:
  • A world map titled "Ocean Surface Currents," showing major ocean currents and gyres.
  • Continents are in beige, oceans in white, with currents depicted as arrows.
• Ocean Gyres:
  • North Pacific Gyre: Labeled in green, with currents like the Kuroshio and California currents (blue arrows).
  • North Atlantic Gyre: Labeled in green, includes the Gulf Stream and North Atlantic Drift (red arrows).
  • South Pacific Gyre: Labeled in green, with the Peru Current (blue arrows).
  • South Atlantic Gyre: Labeled in green, includes the Brazil Current (red arrows).
  • South Indian Gyre: Labeled in green, with the Agulhas Current (red arrows).
• Equatorial Currents:
  • North Equatorial Currents: Labeled in both Pacific and Atlantic (blue arrows).
  • South Equatorial Currents: Labeled in Pacific, Atlantic, and Indian Oceans (blue arrows).
  • Equatorial Counter Currents: Labeled between North and South Equatorial Currents (red arrows).
• Circum-Antarctic Currents:
  • Labeled in green around Antarctica, with blue arrows indicating the Antarctic Circumpolar Current.
  • Antarctic Subpolar: Labeled in green, with blue arrows showing subpolar flow.
• Other Currents:
  • Oyashio Current: In the North Pacific (blue arrows).
  • West Australian Current: In the Indian Ocean (blue arrows).
  • Benguela Current: In the South Atlantic (blue arrows).
• Flow Patterns:
  • Red arrows indicate warm currents (e.g., Gulf Stream, Brazil Current).
  • Blue arrows indicate cold currents (e.g., Peru Current, Oyashio Current).
  • Arrows show the direction of flow, forming large gyres in each ocean basin.
• Interpretation:
  • Illustrates the global pattern of ocean surface currents driven by wind and the Coriolis effect.
  • Highlights the role of gyres in redistributing heat and influencing climate.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

In this map, the different colors correspond to the warm currents (red), cold currents (blue), and currents that move mostly along lines of latitude and thus do not transport waters across a temperature gradient (black). These latter currents may involve warm or cold water, but they do not move that water to warmer or colder places. As mentioned earlier, these arrows depict average flow paths, but on a shorter timescale, the water is involved in eddies that move along the directions indicated by these arrows. These ubiquitous eddies are important since they mix up the surface of the oceans, just as swirling a spoon in a coffee cup mixes the coffee. There are several ways of forming eddies, including intermittent winds combining with the Coriolis effect, opposing currents interacting with each other, and currents interacting with coastlines. As this pattern of currents indicates, surface ocean circulation moves a lot of warm water to colder portions of the Earth; it also moves cold water back down to warmer regions — the net effect is to exchange heat and bring the tropics and the poles a little closer to each other in terms of temperature. Or, in other words, this (along with the winds) moves surplus energy from the tropics to the regions of energy deficit near the poles.

It is important to realize that these currents, by themselves, would eventually homogenize the temperature on the surface, were it not for the huge difference in solar energy between the tropics and the poles. In addition, the strength of these air and ocean currents is sensitive to the temperature difference between the poles and the equator — the greater the temperature difference, the stronger the currents.

The surface currents described above are generally confined to the upper hundred meters or so of the oceans, and considering that the average depth of the oceans is about 4000 meters, the surface currents represent a very small part of the ocean system. The rest of the oceans are also in motion, moving much more slowly under the influence of density differences caused by temperature and salinity changes. Cold, salty water is dense, while warm, fresh water is light, and the resulting density differences drive a system of flows sometimes referred to as the thermohaline circulation. In today’s world, there are two principal places where deep waters form — the North Atlantic and Antarctica, as shown below:

The Global Conveyor Belt system of surface and deep currents.

Click for a text description of the Global Conveyor Belt system image.

• Map Overview:
  • A world map titled "Thermohaline Circulation," showing global ocean circulation patterns.
  • Continents are in beige, oceans in white, with currents depicted as colored arrows.
• Circulation Patterns:
  • Surface Flow (Red Arrows): Warm surface currents flow from the equator toward the poles, e.g., in the Atlantic from the Gulf of Mexico to the North Atlantic.
  • Deep Flow (Blue Arrows): Cold deep currents flow from the poles toward the equator, e.g., from the North Atlantic southward.
  • Deepest Flow (Purple Arrows): The deepest currents, primarily in the Southern Ocean, connecting the Atlantic, Indian, and Pacific Oceans.
• Key Features:
  • Yellow dots mark areas of deep water formation, mainly in the North Atlantic (near Greenland) and the Southern Ocean (near Antarctica).
  • Arrows show a continuous loop, indicating the global conveyor belt of ocean circulation.
• Annotations:
  • A note in the bottom right reads: "about 1 kyr to make a loop," indicating the circulation takes about 1,000 years to complete.
  • The map is credited to "Rahmstorf, 2002" in the bottom left.
• Color Coding:
  • Red arrows for surface flow (warm water).
  • Blue arrows for deep flow (cold water).
  • Purple arrows for the deepest flow.
• Interpretation:
  • Illustrates the thermohaline circulation, driven by temperature and salinity differences.
  • Shows how surface and deep currents connect globally, redistributing heat and influencing climate.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0, modified from Rahmstorf, 2002

In the North Atlantic, warm, salty water from the Gulf Stream comes into contact with cold Arctic air, and as the water cools it becomes very dense and sinks to the bottom of the ocean — this is called the North Atlantic Deep Water (NADW). When NADW forms, a tremendous amount of heat is transferred from the water to the air; this heat is equivalent to about 30% of the thermal energy received by the whole polar region, so it can influence the Arctic climate in a major way. In the Antarctic, as sea ice forms at the edge of the ice sheet, pure water is removed from seawater, thus increasing the salinity of the remaining water; the resulting density increase makes this the densest water in oceans, and it sinks to the bottom — this water mass is called the Antarctic Bottom Water (ABW). Of these two deep water flows, the NADW is much greater, and it flows in a complex path, hugging the bottom of the ocean as it moves through the Atlantic and into the Indian and Pacific Oceans, by which point it has warmed and mixed with the surrounding water to rise back up into the surface, where it starts its return path back into the North Atlantic, completing the loop in something like a thousand years. This flow is sometimes called the Global Conveyor Belt (we will talk a lot more about this in Module 6), and it represents an important means of mixing the global oceans.

These deep currents are very important to the global climate system in a couple of ways. One of these ways, described above, is the way that NADW formation influences the Arctic climate; this, in turn, can influence the formation or melting of ice in the polar region, which can trigger the ice-albedo feedback mechanism (see below). Another way these deep currents influence the global climate is by transporting CO₂ to the deep waters of the oceans. The CO₂ is dissolved into the seawater at the surface, so when deep waters form, they bring that CO₂ with them, thus removing it from the atmosphere. What this does is to effectively increase the volume of ocean water that can hold CO₂, which increases the total mass of carbon the oceans can hold. Indeed, these deep currents are already transporting anthropogenic CO₂ and other gases such as CFCs into the deep ocean (we will talk a lot more about this in Modules 5 and 7).

In this activity, we’ll explore some relatively simple aspects of Earth’s climate system, through the use of several STELLA models. STELLA models are simple computer models that are perfect for learning about the dynamics of systems — how systems change over time. The question of how Earth’s climate system changes over time is of huge importance to all of us, and we’ll make progress towards understanding the dynamics of this system through experimentation with these models. In a sense, you could say that we are playing with these models, and watching how they react to changes; these observations will form the basis of a growing understanding of system dynamics that will then help us understand the dynamics of Earth’s real climate system.

End of Module Recap: Please go down this list carefully and make sure you understand all of the points below.

• Earth's climate system is essentially an energy balance system, where the energy in (from sunlight or insolation) is balanced by the energy emitted (energy out) (via heat or infrared radiation).
• The energy in varies with latitude and season (and also orbital cycles like precession, axial tilt, and eccentricity mentioned in Module 1 regarding the Ice Ages). The energy in also varies as a function of the albedo (fraction of sunlight reflected); ice and snow have high albedo, water has low albedo, and the land surface is in between, varying according to the type and density of vegetation.
• The energy out depends on temperature and the greenhouse effect, or emissivity, as described by the Stefan-Boltzmann Law.
• The rate at which different parts of the Earth warm and cool is a function of the heat capacity; a larger heat capacity means that things warm and cool more slowly, and they also store much more heat.
• The greenhouse effect is not a theory — it is directly measured by satellites and represents a kind of energy recycling mechanism wherein particular gases in the atmosphere absorb heat emitted from the surface and then re-radiate some of that heat back to the surface.
• Without the greenhouse effect, our planet would be about 33 °C colder!
• Water, carbon dioxide, and methane are the main greenhouse gases. Water contributes the greatest amount of warming, but the atmosphere is saturated with water, and it cycles through the atmosphere very fast, so it cannot drive climate change (even though it is a very important part of the climate system). Carbon dioxide, on the other hand, is not close to saturation, and it cycles through the atmosphere more slowly, so it can drive climate change. Methane is far less abundant in the atmosphere and is quickly converted to carbon dioxide, so it is less important as a greenhouse gas.
• Our climate system is filled with feedback mechanisms. Positive feedback mechanisms like the ice-albedo feedback are triggered by a small climate change and then enhance the strength of that climate change. Negative feedbacks like the weathering feedback are triggered by a small climate change and then act to counter that change — these tend to stabilize the climate.
• If we look at the Earth as a whole, we see that the tropics get more heat than they emit back to space, and the polar regions emit more heat back to space than they get — the balance on a global scale comes about from the transport of heat through the winds and ocean currents.
• Winds and ocean currents are initiated by density differences that create pressure differences (or gradients). Pressure gradients (change in pressure over a certain distance) drive these flows, but the flows are modified by the Coriolis effect caused by the spinning of the Earth.
• Finally, we looked at the effects of historical variations in the four major drivers of the Earth’s climate system — the amount of sunlight, volcanoes, greenhouse gases, and pollutants (aerosols) — on a simple climate model. We saw that combined, they make our climate model warm and cool in a pattern that is pretty close to the reconstructed temperature history, and that among these, the greenhouse gas effect is by far the most important factor in the climate change of the last 100 years or so.

Assignments

You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply, and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.

Lab

• Lab 3: Climate Modeling.