sol yields from biogenic VOCs may also depend on the preexisting supply of anthropogenic aerosols, further complicating the feedback loops.

Atmospheric chemistry plays a critical role in aerosol formation and contributes to other climate-chemistry feedbacks driven by changes in land cover. Deposition of reactive nitrogen (nitrate, ammonium) may significantly affect carbon uptake by ecosystems, and climate change in turn will affect the terrestrial emission and atmospheric chemistry of nitrogen oxides and ammonia. Biogenic changes in nitrogen oxide and VOC emissions will affect the concentration of the hydroxyl radical (OH), the main sink for methane, and will also affect ozone. Like the land-cover impacts and feedbacks that are involved in the carbon cycle, understanding of these effects requires coupling of sophisticated, dynamic ecosystem and land-surface models.

The advance of coupled land-surface, vegetation, boundary-layer, and aerosol chemistry models promises to be an exciting frontier that may transform aspects of climate modeling, and climate model utility in, for example, air quality and land-use simulations. It may pave the way for unification of current efforts in air pollution modeling and in human-climate interactions, discussed further below. In the context of decadal to centennial climate change, these short-term processes influence climate system sensitivity through cumulative effects on radiative transfer and cloud properties. Aerosol chemistry, through direct and indirect effects on atmospheric absorption and scattering, are one of the greatest sources of intermodel climate variability.

How Will Climate Change on Regional Scales? How Will This Affect the Water Cycle, Water Availability, and Food Security?

Climate change impacts and adaptation activities are most strongly manifest on regional scales, where ecological and human systems are adapted to a specific set of historical climate “normals.” Agriculture, water resource management, transportation, energy systems, recreational activities, wildfire hazards, and biological systems are all vulnerable to shifts from these historical normals, creating a demand for climate models that can provide accurate and detailed regional information. This demand is a challenge for the current generation of models, particularly with respect to simulation of regional precipitation; climate models need improved skill on regional scales to address this need. Issues concerning rainfall and the hydrologic cycle are of foremost concern. Simulation of ecosystems, ice-ocean interactions, and severe weather, among other climate processes of interest, also require model skill at regional scales.

Accurate simulation of regional precipitation patterns and trends is difficult. Current models are generally limited in their ability to simulate regional precipitation pat-

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