*The increase in heat-trapping greenhouse gases due to human activities are projected to be amplified by feedback effects, such as changes in water vapor, snow cover, and sea ice. As atmospheric concentrations of carbon dioxide and other greenhouse gases increase, the resulting increase in surface temperature leads to less sea ice and snow cover helping to raise temperatures even further. As snow and sea ice decrease, more of the Sun�s energy is absorbed by the planet instead of being reflected back to space by the underlying snow and sea ice cover. Present evidence also suggests that as greenhouse gases increase, evaporation increases leading to more atmospheric water vapor. Additional water vapor acts as a very important feedback to further increase temperature. Our present understanding suggests that these feedback effects account for about 60% of the warming. The magnitude of these feedback effects and others, such as changes in clouds, remain a significant source of uncertainty related to our understanding of the impact of increasing greenhouse gases. Increases in evaporation and water vapor affect global climate in other ways besides increasing temperature such as increasing rainfall and snowfall rates.
The increase in greenhouse gas concentrations in the atmosphere implies a positive radiative forcing, i.e., a tendency to warm the climate system.
THOMAS R. KARL, DIRECTOR
NATIONAL CLIMATIC DATA CENTER
NATIONAL ENVIRONMENTAL SATELLITE DATA AND INFORMATION SERVICES
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
COMMITTEE ON GOVERNMENTAL AFFAIRS
UNITED STATES SENATE
July 18, 2001
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