the potential for major, abrupt climate change as well as for single climate events and clusters and sequences of such events that could be sufficiently disruptive as to raise concerns about U.S. national security. The next two chapters examine how such events might disrupt social, political, and economic systems sufficiently to raise national security concerns.

THE SCIENCE OF CLIMATE PROJECTION

The predictability of weather and climate varies with the time-scale; moreover, predictive efforts face different challenges at different time-scales. Weather forecasting focuses on predictions at time-scales up to about two weeks and is based on the premise that the atmosphere behaves according to a set of deterministic equations such that if the initial state of the atmosphere is known, its evolution can always be determined. Changes or errors in the initial state limit predictability on longer time horizons; two weeks is normally considered the limit of atmospheric predictability.

Climate models are based on the same basic set of equations that predict shorter-term weather variations, but they also include terms that represent a coupling of the atmosphere with the ocean and land surfaces, which inherently have memories of climate longer than the atmosphere does. Climate models have a coarser resolution than weather prediction models, which limits the level of accuracy in their simulations of atmospheric and ocean dynamics and their interactions with climate. Instead of forecasting actual day-to-day changes in weather over a period of a week or more, climate models concentrate on simulating the processes that govern the interannual and longer-term climate variability of the coupled ocean– atmosphere–land system.

To date, climate prediction has focused mostly on two time-scales: seasonal and centennial. Seasonal predictions, like weather forecasts, are dependent on initial values, and thus their ability to make predictions relies on information provided by initial ocean conditions (Latif et al., 2010), particularly sea surface temperatures, which strongly influence atmospheric circulation. The greatest contributor to predictive skill on a seasonal time-scale has been an understanding of the dynamics of the El Niño–Southern Oscillation (ENSO), which influences the yearly variability of rainfall and temperatures over broad sectors of the globe and even global mean temperatures.

On centennial time-scales, the evolution of climate remains chaotic and irregular and depends on external changes in radiative forcing (the influence of a factor, such as solar radiation or anthropogenic changes in atmospheric composition and land cover, on the balance of incoming and outgoing energy in the Earth-atmosphere system). Thus on such time-scales, climate projections are sensitive to assumptions about how future radiative forcing



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