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 Nino-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-

TABLE 4.1 Examples of Research Needs Related to Improving Fundamental Understanding of Climate Forcings, Feedbacks, Responses, and Thresholds in the Earth System

• Extend understanding of natural climate variability on a wide range of space and time scales, including events in the distant past.

• Improve understanding of transient climate change and its dependence on ocean circulation, heat transport, mixing processes, and other factors, especially in the context of decadal-scale climate change.

• Improve estimates of climate sensitivity, including theoretical, modeling, and observationally based approaches.

• Expand observations and understanding of aerosols, especially their radiative forcing effects and implications for strategies that might be taken to limit the magnitude of future climate change;

• Improve understanding of cloud processes, and cloud-aerosol interactions, especially in the context of radiative forcing, climate feedbacks, and precipitation processes.

• Improve understanding of ice sheets, including the mechanisms, causes, dynamics, and relative likelihood of ice sheet collapse versus ice sheet melting.

• Advance understanding of thresholds and abrupt changes in the Earth system.

• Expand understanding of carbon cycle processes in the context of climate change and develop Earth system models that include improved representations of carbon cycle processes and feedbacks.

• Improve understanding of ocean dynamics and regional rates of sea level rise.

• Improve understanding of the hydrologic cycle, especially changes in the frequency and intensity of precipitation and feedbacks of human water use on climate.

• Improve understanding and models of how agricultural crops, fisheries, and natural and managed ecosystems respond to changes in temperature, precipitation, CO2 levels and other environmental and management changes.

• Improve understanding of ocean acidification and its effects on marine ecosystems and fisheries.

SOURCE: These research needs (and those included in each of the other six themes in this chapter) are compiled from the detailed lists of key research needs identified in the technical chapters of Part II of this report.

induced climate change. Continued research on the mechanisms and manifestations of natural climate variability in the atmosphere and oceans on a wide range of space and time scales, including events in the distant past, can be expected to yield additional progress.

Some of the largest risks associated with climate change are associated with the potential for abrupt changes or other climate "surprises" (see Chapters 3 and 6). The paleoclimate record indicates that such abrupt changes have occurred in the past, but our ability to predict future abrupt changes is constrained by our limited understand ing of thresholds and other nonlinear processes in the Earth system. An improved understanding of the likelihood and potential consequences of these changes will be important for setting GHG emissions-reduction targets and for developing adaptation strategies that are robust in the face of uncertainty. Sustained observations will be critical for identifying abrupt changes and other climate surprises if and when they occur, and for supporting the development of improved abrupt change simulations in climate models. Finally, since some abrupt changes or other climate surprises may result from complex interactions within or among different components of coupled human-environment systems, improved understanding is needed on multiple stresses and their potential role in future climate shifts (NRC, 2002a).

Improved understanding of forcings, feedbacks, and natural variability on regional scales is also needed. Many decisions related to climate change impacts, vulnerability, and adaptation could benefit from improvements in regional-scale information, especially over the next several decades. As discussed in Theme 7, these improvements require advances in understanding regional climate dynamics, including atmospheric circulation in complex terrain as well as modes of natural variability on all time scales. It is especially important to understand how regional variability patterns may change under different scenarios of global climate change and the feedbacks that regional changes may in turn have on continental- and global-scale processes. Regional climate models, which are discussed later in this chapter, are a key tool in this area of research.

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