NASA - National Aeronautics and Space Administration

Growing evidence indicates that human activity is altering the climate in significant and potentially hazardous ways. The most recent assessment from the Intergovernmental Panel on Climate Change asserts that global temperature may rise by 2–5 °C (4–9 °F) during the next 100 years in response to rising greenhouse gas concentrations.1 Current predictions also suggest that regional climates may experience significant changes in the frequency and intensity of precipitation, shifts in surface vegetation and soil fertility, and rises in global sea level, to give some examples. Indeed, some changes are already evident, including the dramatic reduction in size of many glaciers, the rapid shrinking of the summertime Arctic ice cap, and a 20-cm rise in sea level since preindustrial times. Predictions of future climate, however, are predicated on model simulations. Of necessity, such models approximate climate scientists’ often incomplete knowledge of the fundamental physical processes that govern the evolution of the climate system. Consequently, significant uncertainties remain in current climate-change projections, particularly at the regional level.2