Quantify and compare climate responses from regional radiative forcings in different climate models and on different timescales (e.g., seasonal, interannual), and report results in climate change assessments. Specific focus should be given to

  • regions in which forcing could interact with modes of climate variability (e.g., El Niño/Southern Oscillation [ENSO], Antarctic Oscillation, Arctic Oscillation) and result in major teleconnections (e.g., forcing over the tropical Pacific from biomass burning affecting ENSO and therefore drought in the United States, Australia, and other distant regions);

  • regions in which there are significant anthropogenic forcings due to anthropogenic emissions or land-use modifications—for example, North America and Europe (industrial emissions, reduction in sulfur dioxide [SO2] emissions), Asia (black carbon emissions, land-use change), and the Amazon (deforestation); and

  • major geopolitical regions with large anticipated socioeconomic changes or vulnerability to climate change and variability.

Determine the Importance of Nonradiative Forcings

Several types of forcings—most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry—impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. These aerosol forcings are sometimes referred to as thermodynamic forcings because they affect spatial patterns of diabatic heating. In some cases, aerosols may be able to modify the hydrological cycle without changing the global average surface temperature. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere; the biogeochemistry of vegetation biomass and soils; or plant species composition. Nonradiative forcings have been shown in a few studies to have first-order effects on regional and global climate, although the globally averaged impacts are not yet sufficiently quantified to allow a careful comparison with forcing from greenhouse gases.

No metrics for quantifying nonradiative forcing have been accepted. Unlike traditional radiative forcing, which can be directly related to surface temperature, nonradiative forcings are not easily linked to a single climate variable. No single metric will be applicable to all nonradiative forcings. Nonradiative forcings generally do have radiative impacts, so one option would be to compare them by quantifying these radiative impacts. Al-



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