land-use patterns, or perhaps CO2 and nitrogen fertilization or climate change, may have been an important sink over the past decades.

  • What are the expected future emissions of CH4, N2O, and volatile organic carbon compounds by soils and vegetation? CH4 production in soils depends strongly on moisture conditions. N2O production is a product of denitrification processes in soils. The emissions of volatile organic carbon compounds (ozone precursors) are strongly species dependent. Changes in these emissions depend on a combination of ecosystem and climate changes.

  • How do dec-cen changes in land use and land cover affect land surface energy balance on dec-cen timescales? The nature of land cover determines its reflectivity and is expected to change with changing climate and human activities. For example, a warmer high-latitude climate will favor the expansion of boreal forest into tundra-dominated regions, with a concomitant lowering of albedo. Desertification, which may have both human and natural components, leads to an increase in surface albedo. The thermal structure, moisture content, and dynamics of the atmosphere will be influenced by the partitioning between sensible and latent heat transferred from the surface.

  • How does vegetation influence the transfer of freshwater through the land surface on dec-cen timescales? Stomatal opening governs the rate of evapotranspiration from the land surface. Increasing CO2 will tend to reduce stomatal conductance, increasing plant water-use efficiency. More evapotranspiration results in a higher water vapor content of the atmosphere over land, more precipitation, and less runoff via rivers and groundwater flow.

  • How do changes in vegetation cover influence the loading and composition of atmospheric aerosols on dec-cen timescales? Vegetation naturally emits aerosol precursors. The nature and amount of these emitted compounds depend on the species. Thus, the distribution of aerosol precursors will change as ecosystems and species respond to climate change and human perturbations. Biomass burning generates aerosols (particularly soot) that influence the regional radiation balance. Desertification produces mineral dust that is transported in the troposphere and exerts a regional radiative forcing. The distribution of these aerosols can be expected to vary on dec-cen timescales in ways related to climatic and human influences.

Hydrological Cycle

The hydrological cycle refers to the origin and fate of water through its many phases in the atmosphere, ocean, land, and biosphere. Water evaporates as vapor from the surfaces of both land and ocean, condenses in the atmosphere, precipitates

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