Short version: The Earth adjusts its temperature to send back to space as much energy as is received from the Sun. But, the Sun’s shortwave energy passes easily through the air while some of the Earth’s longwave energy is intercepted by carbon dioxide and other “greenhouse” gases, making the Earth warmer than it otherwise would be, with more warming when more greenhouse gas is added to the air. This warmer air picks up water vapor and melts reflective snow and ice, making the total warming even larger.
Friendlier, but longer version: Think about a factory making cars. Many small parts go in, and a few big cars come out. But, the total amount of stuff going in is very nearly the same as the total amount coming out. If they were very different for very long, the factory would either fill up with parts or run out of them. The factory may need to adjust its rate of making cars to match the rate at which parts arrive, speeding up by hiring more workers when the parts arrive rapidly, and slowing down by sending workers home or out for coffee when parts arrive slowly. Keep reading for the longer version!
You can think of energy in the Earth’s climate in a way that is similar to the materials entering and leaving the car factory. Almost all the energy for the Earth system comes from the Sun. About 30% of this is reflected from clouds and the land surface and the other 70% is absorbed and heats the Earth. (The reflected fraction is called the “albedo”, so we say that the Earth’s albedo is about 30%. We don’t worry much about the heat coming up from the deep Earth because it is almost 4000 times smaller than the absorbed heat from the sun.)energy
You know that when the sun rises in the morning, the temperature in the air can go up a lot, quickly. If all of the Sun’s energy stayed on Earth, everyone would be dead from overheating in much less than a year.
But, warmer things lose energy to colder things. Suppose you turn on an electric stove. As the temperature of the heating element (the “burner”) rises, it begins to heat the pot of water on top to boil the water for your spaghetti. If there isn’t a pot of water on top, you can see the burner begin to glow, radiating energy.
The burner is “glowing” even before you can see the glow, as you could prove to yourself if you watched it while wearing special glasses that can “see” in the infrared, which is a longer wavelength of energy than visible light. As the burner gets hotter, it radiates more energy. And, while it continues to radiate long wavelengths such as the infrared you can’t see, a hotter burner shifts more of its energy to shorter wavelengths you can see, going to red and then orange and yellow as it warms up.
If you keep giving the burner the same amount of energy, its temperature will increase until the outgoing and incoming energy are equal, and then the temperature will stabilize. If you then supply energy more rapidly, the burner will warm to a new level that radiates the extra energy. Always, the burner tends to that temperature at which incoming and outgoing energy are equal, a balance like the stuff going into and out of the factory. But, electricity comes in and electromagnetic radiation goes out, much the way car parts go in and cars come out of the factory.
For the Earth, energy from the very hot Sun comes in, mostly in the short wavelengths of light we can see, and we send back infrared radiation at longer wavelengths. But on average, the total amount of energy going out is just about the same as the total coming in. At the present time, this incoming and outgoing energy is not exactly equal — a bit less is leaving, which means that the Earth is warming.
When a car drives out of the factory, there may be tiny cracks in the pavement that the tires roll over easily. And, the road may go down into a small valley and up the other side, again causing no trouble for the car tires. But, if there is a pothole of the wrong size in the way, the tire may drop in, bending the rim, blowing the tire and getting the car stuck. Going really slowly might allow the car to ease through the pothole without damage, and going really fast might jump the pothole, but a car at the wrong speed in the wrong place can fall into the pothole and get into trouble.
We are all familiar with such situations, in which interactions happen when the size or energy is “right”, but otherwise there is almost no interaction. This is very common in the air. The shortwave radiation from the sun does interact with clouds, and the very shortwave (ultraviolet) interacts with ozone (which helps protect us from skin cancer caused by the high-energy radiation), but otherwise most of the light from the sun passes easily through the air. However, the infrared radiation going back up from the Earth does interact with certain gases in the air, which are often called greenhouse gases. Radiation tends to be absorbed if it is at or near those wavelengths with the right energy to make a particular molecule wiggle or spin in a particular way.
A molecule that is wiggling or spinning because it absorbed radiation has extra energy—it is hotter than it was. It usually will quit wiggling or spinning by colliding with a neighboring molecule and passing the extra energy along; occasionally, the extra energy will be sent out as radiation instead. Most of the energy absorbed by molecules in the air was going up from the surface, and if they “re-radiate” the energy it goes in a random direction, which has the effect of reducing the radiation going to space and sending some back to Earth. In the more common case of collisions, even a rare greenhouse gas can heat the atmosphere by repeatedly absorbing energy and then colliding with non-greenhouse molecules.
Without greenhouse gases, the Earth's average surface temperature would be well below freezing —about 18°C or -0.4°F.
Want to learn more?
Read over Enrichment titled The Simplest Climate Model to learn more about this.
The greenhouse gases now in the air do keep the Earth’s surface warmer than it otherwise would be, and adding more greenhouse gases will cause more warming. There is nothing new, surprising, or honestly controversial in any of this. With a calculation something like the one in The Simplest Climate Model (read more about it in the enrichments), the French scientist Jean Fourier discovered in 1824 that something was keeping the Earth’s surface anomalously warm, and among the hypotheses he considered was that the atmosphere is acting something like glass holding heat in a container (perhaps the origin of the comparison to a greenhouse; see The Discovery of Global Warming [1]). The British physicist John Tyndall showed in 1859 that gases in the air, including water vapor and carbon dioxide, were contributing to the greenhouse effect. And, in 1896, the Swedish physical chemist and Nobel Prize winner Svante Arrhenius did a fairly good job of calculating the global warming from the carbon dioxide released by the human burning of fossil fuels. (Through history, scientists have actually been better at calculating the effects of greenhouse gases than at realizing just how incredibly skillful fossil-fuel companies would become at supplying large quantities.)
The science of the greenhouse effect thus is not some new discovery but has a long history compared to such “recent” science as relativity (Albert Einstein, 1905) or quantum mechanics (Max Planck, 1900). The pioneers who explored radiation in climate science were giants of physics, chemistry, and mathematics, who saw the strong interactions between laboratory studies and application to the atmosphere.
Much of the work on the details of the interaction between radiation and gases in the air was done by the US Air Force just after World War II and applied to topics such as sensors on heat-seeking missiles, as told in the introduction to this chapter. A missile uses a sensor to “see” the infrared radiation from a hot engine, but greenhouse gases such as carbon dioxide and water vapor block the view in some wavelengths by absorbing that radiation. Because the gases interact with radiation traveling in any direction, and there is much more energy in those wavelengths going up from the sun-warmed Earth than coming down from military bombers, the warming influence of the greenhouse gases is unavoidable.
Earth: The Operators' Manual
This 9-minute clip will appear three times within modules 4 and 5 this week. To see a short clip on the Air Force's role in understanding the physics of the atmosphere and the warming effect of CO2, watch the first 1 minute and 20 seconds. The material that follows this 1 minute and 20 seconds will be covered later in this module as well as in Module 5.
Adding more greenhouse gases does increase the temperature more. Put on more blankets on a cold night, and heat leaves you more slowly, making you feel warmer. But, if you put a really good stopper in the drain of your sink to keep the water in, adding more plugs doesn’t slow down the drainage still more. We thus know situations in which the job is only partly done so that adding more workers or blankets or plugs will do more, but we know other situations in which the job is completely or almost completely done and adding more help doesn’t make a difference.
For carbon dioxide and other greenhouse gases, the job is not done, and adding more does turn up the temperature. This is mostly because the greenhouse gases are very good at absorbing energy of certain wavelengths, but only somewhat good at absorbing slightly different wavelengths. So, while the outgoing radiation in the lower part of the atmosphere is completely blocked for the just-right wavelengths, that outgoing radiation is only partially blocked for the almost-right wavelengths; adding more greenhouse gas increases blockage of the almost-right radiation.
Furthermore, if you go up in the atmosphere, the air gets thinner, and at some height there is so little greenhouse gas that the just-right wavelengths are only partially blocked. Adding more of greenhouse gases such as carbon dioxide increases this height. The temperature at this height adjusts to radiate to space as much energy as is received from the Sun, and, the physics of the atmosphere cause the temperature to increase downward (squeezing air under higher pressure does work on the air that increases its temperature), so raising the height from which radiation escapes warms the surface.
A molecule of a greenhouse gas has more of a warming influence when the gas is rarer; very roughly, each doubling of atmospheric carbon dioxide has the same effect on surface temperature. Going from the level of carbon dioxide in the air before the industrial revolution, 280 parts per million by volume (280 ppm) to twice that, 560 ppm, and letting the climate come into balance will warm the surface by about 3 C. How much more carbon dioxide must be added to the atmosphere to warm the surface by another 3 C?
Click for answer.
Because warmer things begin to radiate more energy very quickly, the Earth’s climate is very strongly stabilized, as noted in The Simplest Climate Model. Other processes may stabilize the Earth system by reducing changes or destabilize by amplifying changes.
Some stabilizers can be very important but tend to be very slow. We saw in the last chapter that warming reduces oxygen in the ocean, which makes the burial of organic matter easier. And, because the organic matter grew from carbon dioxide in the air, burying rather than burning the dead bugs lowers atmospheric carbon dioxide. Thus, if something such as a brighter Sun causes warming, fossil-fuel formation reduces the size of the warming. However, we also saw that fossil-fuel formation is a slow process because most plants are still “burned” by bacteria or living things; fossil-fuel formation can be very important over a few hundred thousand years or longer, but not over a few thousand years.
Some carbon dioxide is also picked up from the air by rain, forming a weak acid that breaks down rocks in a process called “weathering”, because the weather is involved. The chemicals released from the rocks are used to make shells, some of which contain carbon dioxide. (A coral reef or a clamshell is calcium carbonate, usually written as CaCO3, but sometimes written as CaO•CO2, showing more clearly that it contains carbon dioxide.) Chemists use Bunsen burners in their labs for good reasons; warming almost always makes chemical reactions go faster. So, if the temperature goes up, chemistry removes carbon dioxide from the atmosphere more rapidly. If something such as a brighter sun raises the Earth’s temperature, this “rock weathering feedback” can remove enough carbon dioxide to cool the climate back close to the starting temperature in approximately ½ million years.
Rock-weathering Thermostat-When the air is cold, CO2 builds up in the air, warming; when the air is warm, CO2 is removed from the air, cooling. Volcanoes supply CO2 to air—rate is (nearly) independent of climate, and solid CaSiO3. Rock weathering remove CO2 from air— CaSiO3 + 3H2O+ 2 CO2 becomes Ca2+ +H4SiO4 +2HCO3- . This process works faster when it’s warmer. The products from the previous reaction fall into a body of water where shell growth occurs. The equation for this is Ca2+ +H4SiO4 + 2HCO3- becomes CaCO3 + SiO2 + 3H2O + CO2 . CaCO3 and SiO2 undergo shell subduction, which feeds back into the volcano. Other products are released into the ground
The Earth's climate is a complex system, and like most other complex systems, it is, partially controlled by many feedbacks. Feedbacks can affect many things. If we think about temperature, if a warming or cooling affects other processes that in turn change the temperature, those other processes are called feedbacks. A feedback that works against the initial temperature change to reduce its size is said to be stabilizing or negative; a feedback that increases the size of the initial change is amplifying or positive. The most important stabilizing feedbacks for Earth’s temperature are the almost instantaneous increase in radiation leaving the planet when the temperature rises, and the faster removal of carbon dioxide from warmer air to form shells and fossil fuels over hundreds of thousands of years.
At the in-between times, however, the most important feedbacks are positive. As a result, climate changes over years to millennia can be almost as large as changes over much longer times.
The most important of these positive feedbacks is warmer air picking up more water vapor from the ocean and plants, and carrying that vapor along, thus strengthening the greenhouse effect (or, colder air picking up less water vapor…).
Want to learn more?
Read the Enrichment titled Carbon Dioxide is more Important than Water Vapor as a Greenhouse Gas.
The air doesn’t know why it is warm, so anything that warms the air—brighter sun, or more greenhouse gas, or alien ray guns—will increase evaporation from the ocean, amplifying the warming.
Note that this does NOT mean that the warming “runs away” and the Earth burns up, but just that the total warming is made larger by the feedback. Suppose the sun becomes enough brighter to warm the planet by 1 degree, based on the simplest climate model in the Enrichment, which doesn’t include the water-vapor feedback. Including the effects of the extra water vapor would increase warming to almost 2 degrees.
Another important feedback is linked to snow and ice. Most surfaces (forests, grasslands, cities, oceans, even deserts) absorb most of the sunshine that reaches them, but snow and ice reflect most of the sunshine reaching them. Warming melts snow and ice, causing the Earth to absorb more sunshine, which causes more warming. This ice-albedo feedback is not nearly as strong as the water vapor feedback under modern Earth conditions, because most of the snow and ice occur in places and at times without a lot of sunshine (mostly in the winter, near the poles, and often under clouds that already are reflecting the sunshine; note that this feedback would be much more important if the temperature were cold enough for the ice to extend near the equator).
But, the water-vapor and ice-albedo feedbacks interact with each other. If the sun becomes brighter or carbon dioxide is increased by fossil-fuel burning, the resulting warming melts snow and ice and picks up more water vapor. Each of these causes more warming. But, the warming from the extra water vapor also melts some snow and ice, and the warming from loss of snow causes more water vapor to be picked up. Under modern Earth conditions, this still doesn’t “run away”, but it amplifies the warming still more. (The warming did “run away” on Venus, evaporating the oceans and causing the surface today to be hot enough to melt the metal lead; and such a fate awaits Earth most of a billion years in the future as the sun slowly brightens, although if we hang around and keep learning, we could “geoengineer” our way out of the problem, perhaps using techniques that will be discussed later in the course.)
The best current estimate is that, including changes in vegetation and clouds as well as snow and water vapor, doubling the concentration of carbon dioxide in the air and letting the climate come into balance will cause a warming of roughly 3°C, with fairly high confidence that the number is not less than 1.5°C or more than 4.5°C (or, a most-likely warming of 5.4°F, with the range of possibilities primarily between 2.7°F to 8.1°F). This number is usually called climate sensitivity and is widely discussed in climate science. Of the roughly 3°C warming from doubled CO2, the direct effect of the carbon dioxide on Earth’s radiation is just over 1°C (roughly 2°F), with the rest coming from the positive feedbacks. The stabilizing effect of warmer bodies radiating more energy is included here. Some additional amplifiers are omitted (melting of seasonal snow and sea ice are included, but not melting of the Greenland and Antarctic ice sheets, for example), so over many centuries or millennia, the warming may be somewhat larger than 3°C. The very slow stabilizers are also omitted, but they do not become important until even further into the future. Despite hopes that the climate sensitivity might be low, the most recent studies have made it less and less likely that sensitivity is as low as 1.5-2oC (2.7-3.6oF), with a value close to 3oC (5.4oF) looking fairly likely.