changes have been especially common when the climate system itself was being altered.

The current understanding of abrupt climate change is discussed in a recent National Research Council report Abrupt Climate Change: Inevitable Surprises (NRC, 2002). That report provides more detailed recommendations for research needed to improve our understanding of and ability to predict possible abrupt changes in the future, including enhanced research on possible causes of abrupt change. Here, a few recommendations that pertain specifically to radiative forcing are identified. The committee notes that the recommended research will require long-term efforts.

RECOMMENDATIONS:

• Investigate the magnitude, spatial patterns, and temporal variation of radiative forcing that may cause the climate system to cross a threshold (e.g., shutdown of the thermohaline circulation).

• Conduct societal impact studies to investigate the magnitude of future forcing that would cause a crossing of a threshold in societal vulnerability.

• Determine the probability that future radiative and nonradiative forcings (e.g., reductions in aerosol emissions, continued tropical deforesta tion) could induce an abrupt climate change.

The most important step for improving understanding of forcings is to obtain a robust record of radiative forcing variables, both in the past and into the future. A robust observational record is essential for improved understanding of the past and future evolution of climate forcings and responses. Existing observational evidence from surface-based networks, other in situ data (e.g., aircraft campaigns, ocean buoys), remote sensing platforms, and a range of proxy data sources (e.g., tree rings, ice cores) has enabled substantial progress in understanding, but important shortcomings remain. The observational evidence needs to be more complete both in terms of the spatial and spectral coverage and in terms of the quantities measured. Long-term monitoring of forcing and climate variables at much improved accuracy is necessary to detect and understand future changes.

Carefully attributing past climate changes to known natural and anthropogenic forcings provides information on how such forcings may impact large-scale climate in the future. Instrumental records of past climate