The Greenhouse Effect

Because the Earth contains a limited supply of fossil fuels, and we are using them up rapidly, and it will take ~100 million years or more for nature to make a lot of new ones, we must replace fossil fuels with some other energy sources or face really huge problems. But, if we burn most of the fossil fuels that still exist in the Earth and let the carbon dioxide accumulate in the atmosphere before building a sustainable energy system, we will live in a really different climate that will make our lives much harder. The scientific community knows this really well. Several lines of information are involved, starting with the effect of carbon dioxide on the greenhouse effect.

If not for the greenhouse effect, we humans probably would not be here. The Earth’s atmosphere allows the shortwave radiation (what we usually call "sunlight") coming from the sun to pass through to the Earth’s surface, without too much interference. (There is a little interference. Among other things, blue light is scattered off air molecules a bit more than red light is, so the blue light bounces around in the atmosphere and reaches our eyes from all directions in the sky, whereas the red comes more directly from the sun, which is why the sky is blue.) The windows in a greenhouse similarly allow sunlight to enter easily.

But, the sunlight heats the Earth or the inside of a greenhouse, which then radiates longwave radiation (infrared radiation) back upwards. As we saw way back at the Redwoods, the total energy reaching the planet on average equals the total energy leaving, but the arriving energy is mostly shortwave electromagnetic radiation (light that we can see) while the leaving energy is mostly longwave electromagnetic radiation (infrared radiation that we cannot see without special sensors). The windows of a greenhouse do not allow longwave radiation to pass through easily; some gets through, but some is trapped.

When the sun rises after a cold night, energy enters a greenhouse but has trouble leaving, and the extra energy warms the greenhouse. Warming makes the floor of the greenhouse emit more longwave radiation, forcing some through the glass until a new balance is reached between incoming and outgoing radiation, but with the greenhouse at a higher temperature than would occur without the windows of the greenhouse. Some gases in the atmosphere act in the same way as the windows on the greenhouse, intercepting some of the outgoing infrared longwave radiation and keeping that energy in the Earth system. As described in the Enrichment, carbon dioxide really is the most important greenhouse gas, even though water vapor intercepts more of the outgoing radiation from the Earth; methane and some others also matter. Without these greenhouse gases in the atmosphere, the Earth would be mostly frozen.

Human-Caused Greenhouse Warming

Human activities are increasing the concentrations of greenhouse gases in the atmosphere. Carbon dioxide, mostly from the burning of fossil fuels and also from the burning of forests and a few other sources, is by far the most important greenhouse gas, because of its direct effects and because the air warmed by the carbon dioxide picks up more water vapor, also a greenhouse gas. In the USA in the early part of the 21^st century, fossil-fuel burning produced about 15 tons of carbon dioxide per person per year! The greenhouse-gas methane, which is produced in cow guts, landfills, rice paddies, and other places where plant material or other carbon-carrying compounds break down in the absence of abundant oxygen, also has been increasing, and human activities also are increasing a few other less-important greenhouse gases.

The world is undoubtedly warming, as shown by thermometers, including thermometers far from cities and in weather balloons and on satellites, in the ground and in the ocean, analyzed by government and university scientists and other scientists in many different countries, including with industrial funding. The thermometer records are confirmed by changes in temperature-sensitive types of snow and ice, NOT the top of Antarctica at -50^o, which won't melt even with a fairly large warming, but we see reductions in seasonal river and lake ice, seasonally and "permanently" frozen ground, springtime snow, mountain glaciers, the edges of the Greenland and Antarctic ice sheets, and more. The vast majority of significant changes in where plants and animals live, and when during the year they do things, are also in the direction expected from warming.

Furthermore, there is very high confidence that the observed warming is from the observed rise in carbon dioxide. The physics is unavoidable; rising carbon dioxide causes warming. This basic understanding of the greenhouse effect, and the role of carbon dioxide and other gases, was worked out in the 1800s, with the first calculation of the warming effect of fossil-fuel burning in 1896. That was before quantum mechanics was discovered; newer calculations using quantum mechanics are more precise. Much of the important quantum-mechanical work on this subject was done by the U.S. Air Force after World War II. They were not primarily studying global warming, but instead were worrying about such things as the appropriate sensors on heat-seeking missiles to shoot down enemy bombers. (With the wrong sensor, the missile wouldn’t "see" the infrared radiation from the hot exhaust of the enemy engine because carbon dioxide is in the way.) And, carbon dioxide interacts with infrared radiation going from Earth to space in the same way. Satellites confirm this every day. Indeed, before satellites were invented, scientists predicted what satellites would see when they were launched and looked back at the energy leaving the Earth, and those predictions were right.

Any scientist who could find a true alternate explanation of the warming would help humanity, and would immediately be famous and receive awards and speaking invitations and other good things; thus, you can be sure that many scientists have tried very hard to find some other explanation for some or all of the observed warming. As noted next, there are some other small influences on temperature, but a lot of effort has totally failed to change our understanding that the carbon dioxide we are releasing is the primary driver of climate change now.

Nature did contribute a little to the warming in the early 1900s, when a few decades passed without major sun-blocking volcanic eruptions and when, coincidentally, the sun brightened a little. But, warming continued after that even though the sun dimmed a little and a few volcanic eruptions threw up particles that cooled the climate by slightly blocking the sun, and as humans put up a lot of particles from smokestacks that also blocked the sun. Humans also caused a little cooling by cutting down a lot of dark sun-absorbing forests and replacing them with more-reflective surfaces, but the warming continued anyway.

We know from geologists and other scientists studying climate history recorded in ice cores and other archives that climate is not significantly influenced by natural changes in cosmic rays, space dust, Earth’s magnetic field, or anything else weird; and, we know that none of these are changing much now anyway. Changes in features of Earth’s orbit are important in changing climate, as we learned back in Module 7 with Milankovitch cycles and ice ages, but over “short” times of centuries or less the orbital changes are too small to affect climate significantly, and in addition, we are at a point in the cycles when the changes are naturally small. Extra heat is not coming out of the ocean or land or ice to warm the air; instead, heat is moving out of the air to warm the ocean and land and melt ice, yet the temperature of the air continues to rise. The Pacific Ocean flips back and forth between El Niño and La Niña, shifting the climate a little warmer and then cooler over a few years, but this cannot create trends in climate, only fluctuations around the trend from other causes.

Furthermore, the "fingerprint" of the warming in space and time is just what is expected from the effects of rising carbon dioxide, and completely unlike the pattern expected from changes in the sun, volcanoes, El Niño, or other natural fluctuations. For example, adding carbon dioxide to the air warms near the Earth’s surface but cools high in the stratosphere where the radiation emitted by the extra carbon dioxide can escape to space and thus cool off other gases in the air, but turning up the sun would warm the stratosphere as well as the surface; the data clearly show the carbon-dioxide pattern of warming down here but cooling up there. Computer models of the climate system, when forced only with the known natural causes of climate change such as changes in the sun and volcanoes, do a good job of simulating the climate changes that happened before greenhouse gases had risen much but do a lousy job of simulating more-recent changes after human emissions of carbon dioxide became large; adding those human-caused effects allows the models to simulate what happened quite accurately up to the present, with rising carbon dioxide most important.

The climate models have repeatedly proven to be accurate, predicting future changes accurately and “retrodicting” past changes without cheating, so those models give us clear guidance for the future. If we continue to burn fossil fuels and release the carbon dioxide, and we don’t do extensive geoengineering by taking immense amounts of carbon dioxide out of the air or else doing the huge task of imitating the volcanoes and blocking the sun ourselves, we will continue to see warming and other changes.

When carbon dioxide rises, direct warming is unavoidable from the basic physics. And, the warming causes other changes, which in turn affect the climate in what we call feedbacks. For example, snow and ice reflect a lot of the sun’s energy, and so help keep the Earth cooler than we otherwise would be. Warming from rising carbon dioxide tends to melt the reflective snow and ice, exposing darker dirt or plants that absorb more sunshine, which causes more warming. The most important feedback is from water vapor—putting warmer air over the vast ocean causes the air to pick up more vapor, which is a greenhouse gas itself and causes more warming.

In total, if we put up enough carbon dioxide to warm the climate by one degree, the actual warming will be roughly three degrees, with uncertainties that include a little less, a little more, or a good bit more, but not a good bit less. For more on feedbacks and uncertainties, and for the stabilizing feedbacks that will eventually remove the carbon dioxide we release over ½ million years or so, go to the Enrichment.

So What?

Few people really care about the average temperature of the Earth, but many people care about how changes in that temperature will affect humans and other living things. Impacts are harder to estimate than temperature change, both because of the additional uncertainties involved in estimating how the temperature change will affect the economy and ecology, and because of the involvement of human values. For example, if warming causes species to become extinct, but those species were not economically important, how bad is that? You will find really strong differences of opinion across the political spectrum in the U.S. and the world, with some people really unhappy that we are driving species to extinction and other people apparently not very concerned. (The value of species will be addressed later in this Module.)

Some disagreement probably should be expected about some aspects of climate change. The climate in many places is already too hot for humans and many of our animals and crops to live comfortably; making these places hotter will not be good for us. Other parts of the world, generally with many fewer people, are too cold for comfortable human habitation today, so warming them could be good for the humans living there, although there will be much bad for them as the melting ice floods their coasts, and melting ice in permafrost causes their roads and buildings to settle and break, and warming makes it harder for native peoples there to preserve their traditional lifestyles, and…. Note that most of the use of fossil fuels that drives warming is done by relatively wealthy people living in colder places, while the greatest harm will be done to poorer people in hotter places, raising large ethical issues.

A few of the big impacts of more warming include:

• Too much heat. If we continue to rely on fossil fuels, later this century some parts of the Earth could have heat waves that will be fatal to all unprotected people—a healthy person sitting naked in the shade in the wind drinking water will die from overheating. Long before then, heat stress to people, their animals and crops may drive climate refugees unless major adaptations are made such as exceptionally widespread installation of air conditioning. We already are seeing an increasing number of deaths from overheating, despite the fact that nowhere has yet become hot enough to be truly unsurvivable for a careful, healthy person.
• More floods and more droughts. Warmer air carries more water vapor, picked up from the vast ocean as well as from the land surface, and thus warmer air can generate more, faster rainfall when conditions are right, causing flooding. Clothes dryers and hair dryers have heating elements to dry faster, and in the same way, warmer air can bring on more droughts faster after the rain stops.
• Sea-level rise. Warming ocean water causes it to expand, and warming melts mountain glaciers and the edges of the great ice sheets, releasing their stored water that eventually reaches the ocean.
• Strong storms becoming stronger, because more energy is available to them.
• Ecological displacements. This is expected to cause extinction of many species, and may spread diseases to new places (malaria is limited by cold, for example, and so its spread will be favored by warming).
• More forest-fire weather, with the potential for western-type fires spreading into places including the Appalachians where such fires have not been observed or prepared for.
• And much more.

Note that costs go up much faster than the temperature. With a little bit of sea-level rise, for example, we deal with “nuisance flooding” by staying away from certain flooded roads at certain times, but otherwise we go on with our lives. A little more sea-level rise, though, will require highly expensive engineering in some places to keep salt water out of our drinking water, or will require abandonment of some roads and neighborhoods and even whole cities or else expensive engineering to protect them. Thus, the warming so far has not been hugely expensive because we started with some capacity to adapt to it, but we are using up that adaptive capacity, and the full scholarship shows clearly that costs will rise rapidly with further warming.

Uncertainties

There are of course uncertainties in all of this, but these uncertainties really provide a strong argument for working harder to reduce future warming, not a reason to wait until we’re more certain. And, they provide a strong argument for more research to reduce the uncertainties.

Here’s a short video explaining the uncertainties related to climate change, and a few thoughts on what this might mean to us.

Video: Uncertainties (3:44 minutes)

First, consider an analogy to a familiar issue—estimating how long it will take to drive somewhere, and how much the trip will cost. Suppose your phone tells you that a trip will take an hour with current traffic. If all goes well, and the traffic clears, you might save five minutes, and maybe you’re a really good driver and can save five more minutes. But, if things go wrong, you might be run over by a drunk driver and never make it at all, or arrive three weeks later after you get out of the hospital. The uncertainties include outcomes that are a little better, a little worse, or a lot worse than your most-likely estimate, but not a lot better. Your response to these uncertainties includes buying a car with crumple zones and antilock brakes and air bags, and putting on your seat belt to protect you, and if you’re transporting a baby putting them in an approved safety car seat, and paying police to arrest drunk drivers, and perhaps you donate to Mothers Against Drunk Drivers—you spend a lot of money on something you do not expect to happen, because it could happen and would be devastating if it did happen.

Uncertainties with climate change are often similar. Drs. Anandakrishnan and Alley study the possibility of ice-sheet collapse and sea-level rise. With warming, the expansion of ocean water and melting of mountain glaciers is virtually guaranteed to raise sea level, giving roughly ½ m (1.5 feet) of rise by the year 2100 under small warming, 1 m (3 feet) of rise by 2100 under large warming, and uncertainties of about ⅓ the rise…if the ice sheets behave. But, if certain possible events actually happen, the rise could be double or triple that, or even more, with no really good possibility to offset the really bad one. With costs are rising faster than sea level, and the uncertainties including the possibility that too much warming will cause very large, rapid sea-level rise, the costs could go up immensely. We will return to economics next, but the uncertainties do motivate more effort now to slow down the warming, and more research now to reduce the uncertainties.