of study in understanding the forcing of change and variability in the atmosphere, and they are of particular relevance to understanding anthropogenic change.

To characterize the climate system sufficiently, we need to take it apart, understand how its pieces work, and then “reassemble” the system. We also need to understand how the system responds to perturbations. Two Research Imperatives can guide this process. After presenting these imperatives, we turn to specific issues and key Scientific Questions for each major type of climate system component.

• Climate system components: Address those issues in terms of individual climate components whose resolution will most efficiently and significantly advance our understanding of dec-cen climate variability.

• Anthropogenic perturbations: Improve understanding of the climate system's long-term response to anthropogenic additions of radiatively active constituents to the atmosphere and devise methods of detecting the anthropogenic signal over the background noise of dec-cen climate variability.

Atmospheric water content (in all three phases), distribution of radiatively active gases, and aerosol concentrations all directly force the climate system and its principal operating agent, the atmospheric circulation. Atmospheric circulation plays a key role in redistributing physical and chemical properties, such as heat, moisture, and aerosols between source and sink regions, thus determining the regional variations of climate. In doing so, atmospheric circulation also directly controls the distribution in space and time of temperature and fresh water and thus the nature and distribution of ecosystems, surface radiation (via distribution of aerosols, which influence cloud distribution and formation physics), and sea level change (via distribution of heat and moisture and ice melt and decay). Atmospheric circulation, induced by fluctuating local and remote boundary conditions, also communicates changes from one location to another. This action often manifests itself in the form of storms. In addition, atmospheric circulation influences the location and disposition of large-scale climate patterns.

Our current understanding of atmospheric processes and large-scale circulation suggests that one critical area of research is the feedback and interactions