1994 was the third or fourth warmest year on record, suggesting the atmosphere has rebounded from the transient cooling of 0.5 degrees C. caused by Mt. Pinatubo and simulated by the models. Several ancillary pieces of evidence consistent with warming, such as a decrease in Northern Hemisphere snow cover, a simultaneous decrease in Arctic sea ice, continued melting of alpine glaciers, and a rise of sea level, have also been corroborated. The frequency of extreme rainfall events has increased throughout much of the country, suggestive of an intensification of the hydrologic cycle. The longer emissions continue to increase, the greater reductions would eventually have to be to stabilize concentrations at a given level. Accurate predictions are limited by our knowledge of the future emissions and atmospheric concentrations of carbon dioxide, aerosols and other greenhouse gases, the role of clouds and water vapor, and the role of the oceans.
Climate change poses threats to resources both domestically and internationally.
These threats could have significant, but uncertain, socioeconomic consequences. Changes in temperature, precipitation and sea level driven by climate change can add to existing stresses on resources caused by other influences such as population growth, land-use changes, and pollution. For these systems, technological and management options exist to some extent today, although they may be costly to implement. Temperature changes of the magnitude expected from the enhanced greenhouse effect have occurred in the past, but the previous changes took place over centuries or millennia instead of decades. Rates of natural migration and adaptation of species and communities appear to be much slower than the predicted rate of climate change.