What is the Greenhouse Effect?
The "greenhouse effect" is a naturally occurring phenomenon that results from the ability of certain gases, such as water vapor and carbon dioxide, to change the radiant energy balance of the Earth (see figure). These greenhouse gases keep the planet habitable. They absorb the infrared wavelengths of radient energy more efficiently than they absorb the radient energy at solar wavelengths. Thus, greenhouse gases allow solar radient energy to pass through the atmosphere to be absorbed at the Earth's surface, but they trap most of the radient heat emitted from the Earth's surface in the lower atmosphere, not allowing it to escape to space
Greenhouse gases influence the Earth's temperature by allowing solar radiant energy to pass through the atmosphere, where it is absorbed at the surface, and subsequently trapping most of the radiant heat that is re-emitted into the atmosphere. If it were not for this natural greenhouse effect, the Earth's surface would be some 33°C (60°F) colder than it is today. The greenhouse analogy arises from the fact that greenhouse gases act like the glass panels of a greenhouse, allowing short wave radiant energy from the Sun to pass through the atmosphere, but preventing some of the longer planetary infrared wavelength radiation from escaping back through the atmosphere.
The current scientific debate associated with "global warming" focuses on the sensitivity of the Earth's climate to an enhanced greenhouse effect and resulting feedbacks caused by increased atmospheric concentrations of greenhouse gases that result from human activities. For example, a warmer Earth would cause increasing evaporation and hence atmospheric water vapor. Because atmospheric water vapor is a strong greenhouse gas, this could lead to further warming. Increased water vapor and changes in atmospheric circulation could also create changes in cloud amounts; however, more clouds would, on average, have a cooling effect. Similar feedbacks occur as a result of ocean, land and sea-ice processes, many of which are highly temperature dependent. A better understanding of the extent to which these feedbacks affect the means and extremes of the Earth's climate will help scientists to better estimate the environmental, economic, and human health risks from an enhanced greenhouse effect.