Overview

The Earth's global climate system consists of the atmosphere, oceans, land, and continental glaciers as illustrated in Fig. 8.1. In this framework, variables such as salinity, soil moisture, and flora are integral to the functioning of this dynamic system. All of these variables are climate variables. Several of these climate forcings were discussed in a regional context in Chapters 5 through 7. Here we present a global perspective. This definition of climate is broader than the definition of climate as long-term weather statistics (Pielke, 1998).

A climate forcing is defined as “an energy imbalance imposed on the climate system either externally or by human activities” (National Research Council, 2005). Climate forcing can be separated into radiative and non-radiative forcing following the definitions provided in National Research Council (2005). A radiative forcing is reported in the climate change scientific literature as a change in energy flux at the tropopause, calculated in units of watts per square meter. A non-radiative forcing is a climate forcing that creates an energy imbalance that does not immediately involve radiation. An example is the increasing latent heat flux resulting from agricultural irrigation. A direct forcing is a climate forcing that directly affects the radiative budget of the Earth's climate system. For example, this perturbation may be due to a change in concentration of the radiatively active gases, a change in solar radiation reaching the Earth, or changes in surface albedo.