Models that account for the observed increases in the atmospheric concentrations of greenhouse gases and sulfate aerosols are simulating the recent history of observed changes in surface temperature and its vertical distribution with increasing realism. The balance of evidence suggests that there is a discernible human influence on global climate. The reliability of regional-scale predictions is still low, and the degree to which climate variability may change is uncertain. The long atmospheric lifetime of many greenhouse gases, coupled with the thermal inertia of the oceans, means that the warming effect of anthropogenic emissions will be long-lived. Even after a hypothetical stabilization of greenhouse gas concentrations, temperatures would continue to increase for several decades, and sea level would continue to rise for centuries.
Most systems are sensitive to both the magnitude and rate of climate change.
Food Security . On the whole, the effects of climate change over the next century on total global food production may be small to moderate in comparison to the effects of population change and demands for improved nutrition. Water Resources . will be increasingly stressed, leading to substantial economic, social, and environmental costs, especially in regions that are already water-limited and where there is strong competition among users. Human Habitat Loss . Natural Ecosystems . will be degraded because the composition, geographic distribution, and productivity of many ecosystems will shift as individual species respond to changes in climate.