absence, the earth’s temperature would be too low to support life as we know it. Among the GHGs, water vapor is by far the most dominant, but other gases augment its effect through greater trapping of heat in certain portions of the electromagnetic (light) spectrum.

In addition to the natural greenhouse effect outlined above, a change is under way in the greenhouse radiation balance. Some GHGs are proliferating in the atmosphere because of human activities and increasingly trapping more heat. Direct atmospheric measurements made over the past 50 years have documented steady growth in the atmospheric abundance of carbon dioxide (CO2). In addition to these direct, real-time measurements, ice cores have revealed the atmospheric CO2 concentrations of the distant past. Measurements using air bubbles trapped within layers of accumulating snow show that atmospheric CO2 has increased by nearly 35 percent over the Industrial Era (since 1750), compared with its relatively constant abundance over at least the preceding 10,000 years (see Figure 2-1). The predominant causes of this increase in CO2 are the combustion of fossil fuels and deforestation. Further, the abundance of methane has doubled over the Industrial Era, although its increase has slowed during the past decade for reasons not clearly understood. Other heat-trapping gases are also increasing as a result of human activities. Scientists are unable to state with certainty the rate at which these GHGs will continue to increase because of uncertainties in future emissions, as well as in how these emissions will be taken up by the atmosphere, land, and oceans. They are certain, however, that once in the atmosphere, these gases have a relatively long residence time, on the order of a century (IPCC 2001). This means they become well mixed across the globe.

There is no doubt that the composition of the atmosphere is affected by human activities. Today GHGs are the largest human influence on atmospheric composition. The increase in GHG concentrations in the atmosphere implies a positive radiative forcing (i.e., a tendency to warm the climate system).

Increases in heat-trapping GHGs are projected to be amplified by feedback effects, such as changes in water vapor, snow cover, and sea ice. As atmospheric concentrations of CO2 and other GHGs increase, the resulting rise in surface temperature leads to less sea ice and snow cover, causing the planet to absorb more of the sun’s energy rather than reflecting it back to space, thereby raising temperatures even further. Present evidence also suggests that as GHGs lead to rising temperatures, evaporation

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