altering climate system components, which then almost immediately lead to changes in radiative fluxes; an example is the effect of aerosols on the precipitation efficiency of clouds. Nonradiative forcings create an energy imbalance that does not involve radiation directly; an example is the increasing evapotranspiration flux resulting from agricultural irrigation. This report focuses on the forcing agents and the ways in which they act to create a climate response (i.e., downward arrows in Figure 1-2). Although they are not the primary focus of this report, it is necessary at times to address climate responses because of what they tell us about climate forcings. See Box 1-1 and Appendix C for definitions of important terms.

Some times the climate system is defined more broadly by including the Sun, the lithosphere (the Earth’s crust), or even humans as part of the climate system (e.g., Claussen, 2004; Steffen et al., 2004). For the purposes of this report, however, those elements that impact climate but are not affected by it are considered external to the climate system. Changes in solar output are viewed as a natural external forcing because the Earth does not affect the Sun. Volcanic aerosols are also considered a natural external forcing because Earth’s climate does not impact volcanic activity, except on very long timescales. Increases in CO2 and other greenhouse gases due to human activities are assumed to be an external anthropogenic forcing. Defining the climate system in this way allows separation between external climate forcings and internal climate responses to those forcings. This definition is consistent with that adopted in the recent NRC report on climate change feedbacks (NRC, 2003).

The definition of climate forcing and climate response may vary depending on the timescale under consideration. On the timescale of billions of years, greenhouse gas concentrations may both influence climate, through their radiative properties, and be influenced by climatic variations in weathering rates. On the timescale of millions of years, on the other hand, greenhouse gas concentrations are determined largely by slowly evolving tectonic boundary conditions. In this case, greenhouse gas concentrations can be treated as a forcing, and changes in global mean temperature can be considered a response. Over the past 1000 years, CO2 concentrations appear to have varied in response to surface temperature changes prior to large-scale fossil fuel burning during the nineteenth and twentieth centuries, while during the latter period they can be considered primarily as a forcing of surface temperature changes (Gerber et al., 2003).

Given the conventional focus of climatologists on temperature as well as the clear link between greenhouse gases and surface temperature, studies of long-term changes in climate have emphasized temperature as the primary index for climate change. The concept of “radiative forcing” provides a way to quantify and compare the contributions of different agents that affect surface temperature by modifying the balance between incoming and

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