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as well as interactions among these components. Numerous decisions about climate change, including setting emissions targets and developing and implementing adaptation plans, rest on understanding how the Earth system will respond to greenhouse gas (GHG) emissions and other climate forcings. While this understanding has improved markedly over the past several decades, a number of key uncertainties remain. These include the strength of certain forcings and feedbacks, the possibility of abrupt changes, and the details of how climate change will play out at local and regional scales over decadal and centennial time scales. While research on these topics cannot be expected to eliminate all of the uncertainties associated with Earth system processes (and uncertainties in future human actions will always remain), efforts to improve projections of climate and other Earth system changes can be expected to yield more robust and more relevant information for decision making, as well as a better characterization of remaining uncertainties.

Research on forcing, feedbacks, thresholds, and other aspects of the Earth system has been ongoing for many years under the auspices of the U.S. Global Change Research Program (USGCRP) and its predecessors (see Appendix E). Our analysis—the details of which can be found in Part II of the report—indicates that additional research, supported by expanded observational and modeling capacity, is needed to better understand climate forcings, feedbacks, responses, and thresholds in the Earth system. A list of some of the specific research needs within this crosscutting theme is included in Table 4.1, and the subsections below and the chapters of Part II include additional discussion of these topics. Many of these needs have also been articulated, often in greater detail, in a range of recent reports by the USGCRP, the National Research Council, federal agencies, and other groups.

Climate Variability and Abrupt Climate Change

Great strides have been made in improving our understanding of the natural variability in the climate system (see, e.g., Chapter 6 of this report and USGCRP, 2009b). These improvements have translated directly into advances in detecting and attributing human-induced climate change, simulating past and future climate in models, and understanding the links between the climate system and other environmental and human systems. For example, the ability to realistically simulate natural climate variations, such as the El Niño-Southern Oscillation, has been a critical driver for, and test of, the development of climate models (see Theme 7). Improved understanding of natural variability modes is also critical for improving regional climate projections, especially on decadal time scales. Research on the impacts of natural climate variations can also provide insight into the possible impacts of human-

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