Background Information Greenhouse gases are any gases that absorb infra-red radiation . By trapping infra-red radiation they contribute to global warming through a process known as the greenhouse effect. To learn more about the effects of global warming visit the Effects of Global Warming unit.
Water vapor is the largest contributor to the greenhouse effect.
The amount of water vapor in the atmosphere is determined primarily by the water cycle rather than by human activity. Water evaporates from the surface. Eventually this water condenses and returns to the surface as precipitation. Once global warming begins to occur the amount of water vapor becomes subject to a positive feedback effect. The warmer the air gets the more water vapor it can hold and the more evaporation will occur.