How can global warming be traced to CO2? —J. POPE, WINSTON-SALEM, N.C.

Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Administration Earth System Research Laboratory, explains:

Although carbon dioxide is just a minor constituent of the atmosphere, it is one of the few atmospheric gases capable of trapping the heat radiating from the earth. At the earth's surface, visible radiation from the sun is absorbed, which causes heating. At the same time, the surface emits infrared radiation back to space, which produces cooling. (We cannot see infrared radiation, but we feel our skin absorbing it when we stand next to a hot stove.) The more sunlight is absorbed by the surface, the more radiation is sent out to space, until the heat loss to space equals the heat absorbed from the sun.

The gases that make up more than 99 percent of the atmosphere—nitrogen, oxygen and argon—do not absorb visible or infrared light and thus let both forms of radiation pass through untouched. The next most abundant gases, water vapor and CO2, do absorb a portion of the infrared heat radiated by the earth, thereby preventing it from reaching space. This is known as the greenhouse effect, and without it our planet would very likely have a frozen surface, akin to that of Mars.

Even though carbon dioxide and water vapor make up a small amount of the atmosphere, those molecules share their absorbed heat with all the nitrogen, oxygen and argon molecules they bump into as they mix together. The atmosphere thus acts somewhat like a blanket that becomes more insulating when water vapor, CO2 and other greenhouse gases increase.

The heating effect of extra carbon dioxide, methane, nitrous oxide and many other minor gases can be calculated with confidence based on properties that have been measured carefully in the lab. Currently the total heating produced by the increases of all such long-lived greenhouse gases (excluding water vapor) since preindustrial times is equal to about 1 percent of all solar radiation absorbed at the surface. The effect would be somewhat similar if the sun had started to shine 1 percent more brightly during the 20th century.

That may sound trivial, but small changes in the earth's heat balance can lead to large climatic changes—the ice ages and the warmer periods in between during the past several million years appear to have been separated by global average temperature differences of only about five degrees Celsius in the tropics and eight degrees C in polar regions.

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