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Why does CO2 affect climate?

Earth is continuously heated by the sun but retains essentially none of that heat. We can tell this because, for example, the temperature drops rapidly at night, and year after year average temperatures are nearly the same. How is this thermal balance maintained? Earth gets heat as energetic “visible” light emitted by the hot sun. Earth, being much cooler, re-emits this heat as less energetic “infrared” light.

Sunlight, or visible light, passes through the atmosphere and reaches the surface. Infrared light, on the other hand, is absorbed by water, CO2, and some other gases in air. The atmosphere re-emits this heat. Some of the re-emitted heat goes out to space, but some goes back down to the Earth’s surface, providing an additional warming. This extra warming associated with re-emission of outgoing Earth heat or light is called the “greenhouse effect”. The greenhouse effect traps some Earth heat on the planet, making it warmer than it would be if there were no water or CO2 in the atmosphere. In a similar way, blankets trap our body heat and make our beds warmer than they would be if our body heat could escape directly to the air in the room.

Industrial activities have increased the CO2 concentration of air from a pre-industrial level of 280 ppm to the present value of about 400 ppm. Accompanying this increase has been a global warming of about 1˚ C over the past 160 years. Climate scientists have worked diligently to understand the response of global temperature to CO2, and how much Earth will warm in the future. The leading tool for making these estimates is computer models of climate. In these models, effects of climate processes are expressed with equations, and the computer solves these equations to determine the global (and regional) temperature associated with a given CO2 level. It turns out that average global temperature should increase by a constant amount for every doubling of the CO2 concentration. Hence the results are expressed as the temperature increase associated with raising the CO2 concentration from the pre-industrial value (280 ppm) to 560 ppm. This term is called, “climate sensitivity.”

The climate system is complex and cannot be perfectly simulated by models. Different models represent the various processes in different ways, and predict very different climate sensitivities. Overall, the range of climate sensitivities predicted by sophisticated climate modes is from about 1.5˚ C to 4.5˚ C, with a mean of 2.5-3˚ C. The warming of about 1˚ C, associated with a CO2 increase of about 40%, is compatible with these predictions.