The "Greenhouse Effect," and Global Warming

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I hope that over the last few sections you've gotten the idea (which maybe surprised you initially) that the atmosphere itself is an important contributor to Earth's energy budget. That's right, even invisible atmospheric gases (and clouds) emit some radiation toward the earth's surface! The key to understanding this observation lies in our laws of radiation. Recall that Planck's Law tells us that all objects emit radiation at all wavelengths at all times.

As you've learned, the earth's peak emission occurs at infrared wavelengths (from Wien's Law), so what happens to that radiation after it's emitted upward from the surface? Some is absorbed by air molecules, in particular, so-called "greenhouse gases," such as water vapor, carbon dioxide, methane, and nitrous oxide. Of the greenhouse gases, water vapor is the most abundant in the atmosphere, followed by carbon dioxide (although recall that in the overall scheme of the atmosphere, these are trace gases). It turns out that some of the wavelengths that carbon dioxide and water vapor absorb readily (particularly those around 15 microns and a little larger) coincide with the wavelengths of earth's peak emission.

Text description of the Absorption Spectra graph.

The image is a stacked spectral diagram showing how different gases in Earth’s atmosphere absorb electromagnetic radiation across a range of wavelengths. The horizontal axis represents wavelength in microns, extending from about 0.1 to 30 microns, while the vertical axis shows absorptivity, indicating how strongly each gas absorbs radiation at each wavelength. Each horizontal panel isolates a specific gas or group of gases, making it easier to compare their absorption behavior.

From top to bottom, the panels show absorption bands for methane (CH₄), nitrous oxide (N₂O), oxygen and ozone (O₂ and O₃), carbon dioxide (CO₂), and water vapor (H₂O). Each gas exhibits distinct peaks where absorption is strong and flat or empty areas where absorption is weak or absent. Water vapor shows the broadest and most continuous absorption across many wavelengths, while gases such as methane and nitrous oxide absorb mainly in narrower, specific infrared regions.

The bottom panel labeled “Atmosphere” combines these effects, illustrating where the atmosphere as a whole absorbs radiation and where it allows radiation to pass through. Large white regions indicate atmospheric windows, where radiation can travel relatively freely, while dark regions indicate strong absorption by one or more gases. Together, the figure highlights the key role of greenhouse gases in controlling how energy moves through the atmosphere and how different gases contribute at different wavelengths.

Credit:David Babb @ Penn State is licensed under CC BY-NC 4.0(opens in a new window)

Kirchoff's Law tells us that if an object is an efficient absorber of radiation at a particular wavelength, then it's also an efficient emitter of radiation at that wavelength. A consequence of Kirchoff's Law then is that greenhouse gases like water vapor and carbon dioxide also emit IR radiation efficiently at those wavelengths, some of which is emitted toward Earth's surface. The emissions that reach the surface are a major contributor to the "downwelling infrared" traces on the graphs we were using for our energy budgets(opens in a new window).

The fact that greenhouse gases absorb and emit infrared radiation so readily works out very well for humans. Without emissions of downwelling IR from greenhouse gases, the average temperature of Earth's surface would be about 0 degrees Fahrenheit (-18 degrees Celsius). That's a pretty harsh environment for life on earth, and certainly life as we know it could not exist. However, observations show that the average temperature of Earth's surface is about 59 degrees Fahrenheit (15 degrees Celsius), and it's downwelling IR from greenhouse gases that are responsible. Without greenhouse gases, the temperature of Earth's surface would be nearly 60 degrees lower -- much, much colder!

The contributions of downwelling IR from greenhouse gases to warming the planet are called the greenhouse effect. To be honest, the names "greenhouse effect" and "greenhouse gases" are pretty unfortunate, because the processes at work to create the planetary warming are not the same as those in a greenhouse, but I'll touch on that shortly. The bottom line is that the warming from the greenhouse effect is essential to sustaining life as we know it on Earth.

Global Warming

The existence of earth's greenhouse effect is perhaps as important as its distance from the sun in determining the average global surface temperature, so there's no doubt that some greenhouse effect is desirable. But, can we have too much of a good thing? Might the magnitude of the greenhouse effect change if we change the concentration of greenhouse gases in the atmosphere?

Before the Industrial Revolution in the late 1700s, the atmospheric concentration of carbon dioxide was around 280 parts per million, but through the burning of fossil fuels like coal, oil, and natural gas, humans have added carbon dioxide to the atmosphere. The concentration of carbon dioxide in the atmosphere now exceeds 400 parts per million, and you can see the upward trend in atmospheric carbon-dioxide concentration since the late 1950s in the data from the Mauna Loa Observatory(opens in a new window) in Hawaii below.

Text description of the Mauna Loa Atmospheric Carbon Dioxide graph.

The image is a well‑known time series graph showing atmospheric carbon dioxide (CO₂) concentrations measured at the Mauna Loa Observatory in Hawaii. The horizontal axis displays year, ranging from the late 1950s to about 2020, while the vertical axis shows CO₂ concentration in parts per million (ppm). The title and labels indicate that the data are produced by the Scripps Institution of Oceanography and the NOAA Earth System Research Laboratory.

Two lines are visible on the graph. The red jagged line represents monthly average CO₂ measurements, showing a repeating up‑and‑down seasonal pattern. This seasonal cycle reflects natural processes, such as plant growth absorbing CO₂ during parts of the year and releasing it during others. Overlaid on this is a smoother black line, which removes seasonal variability and highlights the long‑term trend.

The most striking feature of the graph is the clear upward trend in atmospheric CO₂ over time. Concentrations rise from about 315 ppm in 1960 to over 410 ppm by 2020, demonstrating a steady and accelerating increase. This visualization, often called the Keeling Curve, provides strong evidence of long‑term growth in atmospheric CO₂ levels and is a cornerstone dataset in climate science.

Credit: Earth System Research Laboratory

Remember that carbon dioxide is the second most important greenhouse gas (behind water vapor) so increasing its concentration gradually results in a stronger greenhouse effect, which means more downwelling IR being emitted toward Earth, causing the planet to warm additionally (a "global warming"). So, if you've read an article or watched a news story about global warming, the strengthening of the greenhouse effect from an increased concentration of greenhouse gases is the basic science behind it. That's far from the whole picture, though, and we'll explore other issues related to global warming and climate change in a later lesson.

Finally, I mentioned earlier that the phrases "greenhouse effect" and "greenhouse gases" are rather unfortunate because the processes involving emission of radiation from gases is a different process than what keeps a greenhouse warm. The name "greenhouse effect" was dubbed in the early 1800s when it was thought that greenhouses stayed warmer because the panes of glass allowed solar radiation to enter, but prevented radiation emitted from plants and other objects inside the greenhouse from escaping. It turns out that a big reason why greenhouses stay warmer inside has to do with the fact that the air inside cannot mix with cooler air outside the greenhouse. The warm air in a greenhouse essentially gets trapped inside the panes of glass, but there is no "trapping" with respect to the atmospheric "greenhouse effect" (even though you may still see it described in terms of "trapping heat"). The atmospheric "greenhouse effect" is all about the absorption and emission of infrared radiation by some atmospheric gases. But, alas, the name "greenhouse effect" stuck, and the rest is history.

The idea of air not being able to mix with cooler air outside a greenhouse leads us to our next topics. Indeed, we've talked a lot about how energy is transferred via radiation, but it's time to look at how energy is transferred through the earth-atmosphere system by contact between objects and by the movement of air (the very movement of air prevented by the panes of glass on a greenhouse). Read on.