The Earth's environment is in a state of continuous change. The climate system, for example, is highly variable, with conditions changing significantly over the span of seasons, from year to year, and over longer timescales. Fluctuations in the amount of energy emitted by the Sun, slight deviations in the Earth's orbit, volcanic injections of gases and particles into the atmosphere, and natural variations in ocean temperatures and currents, all cause variability and changes in climate conditions. Many scientific observations indicate that the Earth may be undergoing a period of relatively rapid change on timescales of decades to centuries, when compared to historical rates of change on similar timescales. Much scientific evidence indicates that these changes are likely the result of a complex interplay of several natural and human-related forces.  

Although humans are relative newcomers in the vast scale of the Earth's geological history, we have become agents of environmental change, at least on timescales of decades to centuries. Atmospheric emissions of greenhouse gases and pollutants, and extensive changes in the land surface, have potential consequences for global and regional climate, weather, and air quality, the Earth's protective shield of stratospheric ozone, the distribution and abundance of many plant and animal species, and the health of ecosystems and their ability to provide life-supporting goods and services.

The complexity of the Earth system and the interconnections among its components make it a complex scientific challenge to document change, diagnose its causes, and develop useful projections of how natural variability and human actions may affect the  global environment in the future. Because of these complexities and the potentially profound consequences of climate change and variability, climate has become a capstone scientific and societal issue for this generation and the next, and perhaps even beyond.

The challenge for the U.S. Global Change Research Program (USGCRP) is to provide the best possible scientific basis for documenting, understanding, and projecting changes in the Earth's life-support systems, and the role for the Climate Change Research Initiative (CCRI) is to reduce the significant remaining uncertainties associated with human-induced climate change and facilitate full use of scientific information in policy and decisionmaking on possible response strategies for adaptation and mitigation.