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In each of the six sections of this chapter, the discussion is partitioned into subsections dealing with the influence of the particular climate-system component on climate attributes, the evidence of variability and change of that component on dec-cen time scales, and the mechanisms through which that component operates within the climate system. At the end of each section there is a discussion of the principal outstanding issues associated with that climate-system component, as well as an overview of some of the key observational and modeling priorities that will help resolve the outstanding issues. The discussion of the requisite observational and modeling strategies is not intended to be comprehensive; rather, it provides a broad perspective on the types of research initiatives that are most likely to be productive.

Finally, we wish to emphasize that this chapter deals with all the components of the climate system that influence dec-cen variability, whether that variability be natural, anthropogenically induced, or anthropogenically modified natural.

Atmospheric Composition and Radiative Forcing

Changes in solar output—either in terms of total radiative flux (the solar constant), or in terms of the spectral distribution of this radiation—will directly influence the radiative environment and energy budget at the Earth's surface, the response of the climate system, and the response of many life forms. Moreover, changes in the atmospheric concentration of a number of trace constituents directly influence the transfer of radiative energy throughout the atmospheric column, and therefore the energy balance in the atmosphere, including the temperature at the Earth's surface. Such direct climate influences are modified by myriad feedbacks that indirectly affect surface temperatures and radiative fluxes, the hydrologic cycle, storm frequency and intensity, sea level, and ecosystem structure and functioning. Increasing the skill with which such feedbacks can be quantified is a principal challenge for earth system science over the next decade.

The primary reason for the current widespread concern about global climate change is that human activities are increasing the greenhouse effect of the atmosphere and the tropospheric aerosol burden, and weakening the stratospheric ozone shield against ultraviolet radiation. Greenhouse gases (e.g., H2O, CO2, CH4, N2O, chlorofluorocarbons, and O3 in the troposphere) warm the Earth' s surface by trapping a portion of the outgoing longwave-radiation flux. Atmospheric aerosols tend to cause surface cooling by scattering solar radiation back into space (although they can produce the opposite effect if they consist of very dark material or if they are over a bright surface such as snow or ice), and they exert indirect effects by providing nucleation sites for the formation of cloud droplets. The net influence of the myriad feedbacks responding to changes in atmospheric gas and aerosol content has yet to be determined. Better understanding of these climatic influences will be fundamental to our ability to predict the nature and magnitude of the climate' s response to anthropogenic change in any of the forcing factors.

Radiative forcing is affected not only by anthropogenic changes, but also by natural variations in the sun's output and by the input and distribution of volcanic aerosols. Largely unpredictable, these elements exert measurable influence over the Earth's radiative budget and atmospheric chemical interactions, and account for some of the natural dec-cen variability in the Earth's climate. Solar output, volcanic aerosol contributions, and atmospheric gases and aerosols thus represent the main forcings, natural and anthropogenic, of the climate system. In this respect, they are distinct from the components of the climate system discussed in the other sections of this chapter, and changes in them will drive responses in those other components. Ultimately we need to be able to differentiate climate variations driven by changes in the forcings (internal or external) from variations that are the expression of internal or coupled modes of variability, which will occur even when forcing is steady. Our efforts to understand the behavior of climate variations may be furthered by the fact that the forcings and responses may vary with latitude or regional characteristics, possibly relating specific forcings to specific responses or climatic fingerprints. For example, the stratospheric warming by volcanic aerosols in the Northern Hemisphere winter is greater in low latitudes than in high latitudes (Labitzke and Naujokat, 1983; Labitzke and McCormick, 1992). The differential heating produces a larger pole-to-equator temperature gradient, which in turn increases the zonal winds and enhances the stratospheric polar vortex. The stronger polar vortex may affect the vertically propagating tropospheric planetary waves, and so modify the tropospheric circulation and alter surface air temperature (Mao and Robock, 1998). Thus, radiative influences associated with aerosols may differ from those driven by other types of radiative forcing in the high latitudes.

Influence on Attributes

The solar radiation striking the Earth, however it may be modified by the atmosphere's components, fundamentally mediates the Earth's energy budget and climate through a complex array of feedbacks. In the process, it influences all of the climate attributes discussed in Chapter 2. These feedbacks include changing the atmospheric concentration of water vapor, itself the major greenhouse gas; changing cloudiness; changing the surface albedo due to changes in snow, ice, and vegetative cover; changing source and sink rates for carbon dioxide, methane, and nitrous oxide; changing the formation rates for tropospheric ozone and aerosols; and changing the transport and storage of heat in the oceans. Each of these feedbacks further influences the surface temperature and radiative fields, which in turn alter the evaporation of water from, and precipitation onto, land and water

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