Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory, provides this answer.
The earth¿s surface absorbs visible radiation from the sun, which causes heating. At the same time the surface and the atmosphere emit infrared radiation back to space, which produces cooling. Our eyes cannot see infrared radiation but we can feel how our skin absorbs it when we are standing next to a hot object without touching it. Over a long period the earth¿s surface temperature will remain approximately constant because the amount of heat absorbed as visible light is equal to the amount emitted as infrared light.
Nitrogen, oxygen and argon together comprise more than 99 percent of the atmosphere. None of these three gases absorb either visible or infrared light; both types penetrate the entire atmosphere. It is as though, when it comes to the absorption and emission of light, the atmosphere¿s three main components do not exist!
The next most abundant gases--water vapor and carbon dioxide--do absorb a portion of the infrared heat radiated by the earth's surface, thereby preventing it from reaching space. Instead of dissipating into space, the infrared radiation that is absorbed by atmospheric water vapor or carbon dioxide produces heating, which in turn makes the earth¿s surface warmer. This is known as the greenhouse effect and without it our planet¿s surface would likely be frozen, like Mars. The heat absorbed by water vapor and carbon dioxide is shared with all the nitrogen, oxygen and argon, because the latter molecules are always bumping into water vapor and carbon dioxide as they mix in the atmosphere. This effect makes the atmosphere act somewhat like a blanket that becomes thicker when amounts of water vapor, carbon dioxide and other ¿greenhouse gases,¿ such as methane and nitrous oxide, increase. The top of the blanket remains cold and continues to emit about the same amount of infrared to space but below the blanket it gets warmer because it is more difficult for the heat to rise to the top.
The heating effect of extra carbon dioxide, methane, nitrous oxide and many other minor gases can be calculated with confidence based on the absorption properties that have been measured carefully in the laboratory. Currently, the total heating produced by the increases of all 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.
Water vapor is excluded from the above calculation because it is an intimate and highly variable part of the climate system itself in the form of clouds, rain, snow and other weather. The long-lived greenhouse gases, however, can be considered an external forcing clearly influenced by human action. Most climatologists expect that on average the atmosphere¿s water vapor content will increase in response to surface warming caused by the long-lived greenhouse gases, further accelerating the overall warming trend.
It will be difficult to slow or stop this global warming, thanks to the oceans, which are warming as well. Currently, the amount of infrared heat radiated back to space is slightly less than what we absorb from the sun due to the increase in greenhouse gases. This excess energy slowly warms the oceans. Although it takes them a very long time to heat up, once they have they will release more infrared radiation and the Earth will emit as much back to space as it receives from the sun. But the planet¿s surface will be warmer, because a larger fraction of that infrared will be blocked by the blanket of greenhouse gases. Thus, we can expect about another 0.5 degree Celsius of warming even if the amount of carbon dioxide in the atmosphere were to stop increasing today, which is unlikely as we continue to burn coal, oil and natural gas for our increasing energy needs.