resulting from the interactions of water vapor, clouds, and radiation; and the massive heat fluxes associated with the motions of the air and oceans and the exchanges between them, among other phenomena, beyond quantified understanding of anthropogenic forcing itself. To evaluate anthropogenic forcing specifically, greater knowledge is also needed of tropospheric aerosols and the carbon cycle.

The primary characteristics of the climate system must be documented through consistent long-term observations. Finally, the subtlety of slow change over long timescales, in contrast to diurnal, seasonal, and interannual variations, can disguise its potential long-term severity and thus limit society's willingness to address potential problems in advance. The problem is much exacerbated, of course, by the uncertainty in our ability to forecast such change. All these considerations further underscore the importance of achieving better understanding of climate change patterns on decade to century timescales, including their rate and range of variability, likelihood and distribution of occurrence, and the sensitivity of climate to changes in forcing (natural and anthropogenic). With such improved understanding, we ultimately hope to forecast and detect change (distinguishing natural from anthropogenic), providing a foundation on which future policy decisions and infrastructure management can be rationally based.

A number of Research Imperatives must be met to understand climate change on decadal to centennial timescales:

  • Natural climate patterns. Improve knowledge of decadal- to century-scale natural climate patterns, their distributions in time and space, optimal characterization, mechanistic controls, feedbacks, and sensitivities, including their interactions with, and responses to, anthropogenic climate change.

  • Paleorecord. Extend the climate record back through data archeology and paleoclimate records for time series long enough to provide researchers with a better database to analyze decadal- to century-scale patterns. Specifically, achieve a better understanding of the nature and range of natural variability over these timescales.

  • Long-term observational system. Ensure the existence of a long-term observing system for a more definitive observational foundation to evaluate decadal- to century-scale variability and change. Ensure that the system includes observations of key state variables as well as external forcings.

  • Climate system components. Address those issues whose resolution will most efficiently and significantly advance our understanding of decadal- to century-scale climate variability for specific components of the climate system.

  • Anthropogenic perturbations. Improve understanding of the long-term responses of the climate system to the anthropogenic addition of radiatively active constituents to the atmosphere and devise methods of detecting an-



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