resolving physics. There is ample evidence that resolving these highly interactive, nonlinear, and thermodynamically irreversible processes provides for qualitatively better simulations of Earth’s climate. Such a resolution will permit improved representation of many features of the climate, such as explicit resolution of mesoscale ocean eddies and the spatial scales of land-surface and hydrologic variability.

The committee recognizes that these suggested efforts are not trivial and will require a substantial investment in manpower, computing power, and financial capital. It is also not certain that increases in resolution will reduce uncertainty. However, improvements in model capability and resolution can be expected to advance understanding of the high-priority climate science questions discussed in this chapter. The “grand challenges” outlined here all refer to societally relevant questions where progress can be anticipated in the next 10-20 years, with highest priority given to the questions of climate sensitivity, regional climate change, climate extremes, and sea-level rise. Each of these is central to provision of critical information for climate policy decisions and climate change adaptation.

Recommendation 4.1: As a general guideline, priority should be given to climate modeling activities that have a strong focus on problems that intersect the space where (i) addressing societal needs requires guidance from climate models and (ii) progress is likely, given adequate resources. This does not preclude climate modeling activity focused on basic research questions or “hard problems,” where progress may be difficult (e.g., decadal forecasts), but is intended to allocate efforts strategically.

Recommendation 4.2: Within the realm where progress is likely, the climate modeling community should continue to work intensively on a broad spectrum of climate problems, in particular on longstanding challenges such as climate sensitivity and cloud feedbacks that affect most aspects of climate change (regional hydrologic changes, extremes, sea-level rise, etc.) and require continued or intensified support. Progress can be expected as resolution, physical parameterizations, observational constraints, and modeling strategies improve.

Recommendation 4.3: More effort should be put toward coordinated global and regional climate modeling activities to allow good representation of landsurface hydrology and terrestrial vegetation dynamics and to enable improved modeling of the hydrologic cycle and regional water resources, agriculture, and drought forecasts. This will require better integration of the various national climate modeling activities, including groups that focus on models of surface hydrology and vegetation dynamics. The annual climate modeling forum discussed in Chapter 13 might provide a good vehicle for a working group with this focus.

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