Finding 4.2: Progress is likely on a number of important problems in climate modeling over the coming decades through a combination of increasing model resolution, advances in observations and process understanding, improved model physical parameterizations and stochastic methods, and more complete representations of the Earth system in climate models.

THE WAY FORWARD

There is generally a tension between different lines of progress in climate modeling. For instance, do we allocate resources to increased resolution or to increased model complexity (i.e., Earth system model development)? There is no one-size-fits-all answer, but instead the approach should be problem driven. Some problems that are of great societal relevance, such as sea-level rise and climate change impacts on water resources, require increased model complexity, and progress is likely through the addition of new model capabilities (ice-sheet dynamics and land-surface hydrology, in these examples). In other cases, such as improved model skill in regional precipitation and extreme weather forecasts, increased resolution and “scalable” physical parameterizations are the highest priorities for extending model capabilities. Other problems, such as water resource management, require both increased resolution and complexity.

The committee finds that an important direction forward is for Earth system models to be developed with realistic representations of ice-sheet dynamics and ice-ocean-atmosphere interactions in order to provide improved projections of sea-level rise. Such models will also improve understanding of glacial-interglacial cycles and millennialscale climate variability during glacial periods. Coupled with sophisticated models of terrestrial and marine carbon cycles, investigations of glacial cycles could shed light on natural carbon sources and sinks and the future evolution of the atmospheric carbon pool.

A number of important scientific and societal questions require detailed and meaningful climate projections at local to regional scales. The committee recommends that the U.S. climate modeling community pursue high-resolution model runs in the coming decades. Specifically, at least one national modeling effort in the next decade should aim to simulate historical and future climate change (i.e., the period 19002100) at a resolution of less than 5 km, to enable eddy- and cyclone-resolving models of ocean dynamics and more realistic representation of land-surface exchanges with the atmosphere. In addition, at least one national modeling effort in the next 20 years should aim for century-scale simulations at resolutions of 1-2 km, to allow cloud-



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