What Causes Global Warming?

What is the basis for the predictions concerning global warming? There are several gases in the air, collectively called greenhouse gases, that trap the infrared radiation emitted from the earth. Although these gases make up a small percentage of our atmosphere, they significantly affect the amount of infrared light energy leaving the earth. Therefore, they disrupt the heat budget and are a cause of global warming.

Since the start of the industrial revolution, human activities have caused a steady increase in the concentration of some greenhouse gases to unprecedented levels in our earth's recent history. The most abundant of these is carbon dioxide. It accounts for about 64 percent of the total absorption of infrared energy by greenhouse gases. The level of atmospheric carbon dioxide increased by about 11 percent between 1958 and 1988. Currently, the level is rising at about 0.4 percent annually because we are adding more carbon to the atmosphere than is being removed, by about 5 gigatonnes annually (1 gigatonne = 1 x 1012 kilograms). Almost all of this is the result of burning of fossil fuels and deforestation, which are both manmade sources. The other 36 percent of the greenhouse effect is due primarily to: methane gas, nitrogen oxides, ozone, and chlorofluorocarbons (CFCs).

Water vapor (or a cloud, in other words) is a greenhouse gas that has not been included in the above percentages, because its total effect on global climate change is not fully understood. For example, when water vapor's physical state changes (water, snow, ice), its effect on global warming changes. However, we do know that water vapor's total effect on global warming is greater than carbon dioxide's.

The Atmospheric Radiation Measurement (ARM) Program is designed to better understand how water vapor affects climate change. The data gathered from our three field research sites are providing us with some important information about this greenhouse gas and its impact on global warming.

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