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.