How CO2 raises the earth’s temperature is not as simple as people are led to believe. The popular notion is that the sun’s rays go through the atmosphere and are absorbed by land and water, which radiate the energy back up at a longer wave­length. GHGs prevent this radiation from getting back through the atmosphere, thus trapping the energy and heating the earth. This notion is not wrong, but it is over­simplified. Indeed, the gases in the atmosphere are quite transparent to sunlight, which has wavelengths near those we can see. When land and water absorb this light, they radiate part of the energy back to the sky at infrared wavelengths, which we cannot see. The main constituents of the air, N2 (nitrogen) and O2 (oxygen), allow the infrared to get out, but “greenhouse” gases such as CO2, CH4 (methane), and N2O (nitrous oxide) absorb the infrared and are heated up. They then re-radiate the energy both upwards and downwards. Only the downwards part is the energy “trapped” by the greenhouse effect. Actually, the energy radiated to the earth’s surface by the atmosphere is larger than the energy coming directly from the sun [5]. If it were not for GHGs, the average temperature on the earth’s surface would be -19°C (0°F) rather than 16°C (60°F) as it is now. Already we can see what a large effect CO2 has on the earth’s temperature, and why even a small change in its abundance would be worrisome.

The situation is complicated by the fact that water vapor is also a strong GHG, and its amount in the atmosphere changes constantly as water evaporates, forms clouds, and then is removed by rain and snow. But H2O is a short-lived GHG, going in and out of the atmosphere every two weeks or so, while CO2 is a long-lived GHG with an average residence time of four years.3 Furthermore, water forms clouds, which reflect sunlight strongly, and rain and cloud cover vary greatly depending on where you are. It would be impossible to predict the details of changing cloud cover, so the H2O effect has to be treated as an average. This is not as bad as it

sounds because the saturation humidity level, as we all know, increases or decreases with temperature in a predictable way.

Because the water content in the atmosphere changes constantly, climate scien­tists cannot treat H2O as a long-lived GHG like CO2 but only as a modifier of the effects caused by those gases. One can calculate that doubling the CO2 concentra­tion will cause a 1.1°C (2.1°F) rise in temperature, but the presence of H2O will cause a larger change by positive feedback. Positive feedback is a self-enhancing effect like a stock market crash. As stock prices plunge, more people will try to sell their stocks, causing the prices to fall faster. Here, as the temperature rises, more water is evaporated into the atmosphere, where it radiates energy back to earth, further increasing the temperature. It finally settles down at a high value 29°C (85°F). It is the convection of warm air upwards that brings this down to the observed value of 16°C (60°F). It is actually the stoppage of air currents that makes greenhouses work, not the trapping of radiation [5].

Without such mitigating factors, there can be runaway feedback, in which an increase in temperature (caused by CO2) evaporates more water, which “traps” more solar energy, raising the temperature further, until all the water on the planet has been evaporated. This is apparently what happened to Venus, where the sur­face temperature is about 460°C, enough to melt lead. The runaway can also go in the other direction if the planet gets so cold that it snows everywhere, reflect­ing sunlight away so that it gets colder, causing more snow and ice to form. The planet can turn into an ice ball. In geologic times, the earth has had numerous ice ages and interim warm periods but has always escaped from catastrophic run­away feedback. We do not know why, though there are many theories. This is one of the lucky breaks that allowed life, even sentient human life, to arise in an interglacial period.