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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
FIGURE 1-3 Energy budget for the atmospheric components of the climate system. SOURCE: Kiehl and Trenberth (1997).
RADIATIVE FORCING: PERTURBATION TO EARTH’S ENERGY BUDGET
The various physical processes that contribute to Earth’s global annual mean energy budget are shown in Figure 1-3. The Earth receives a continuous influx of energy from the Sun. About 69 percent of this energy is absorbed at the Earth’s surface or by the atmosphere, while the rest is reflected back to space. At the same time, the Earth and its atmosphere emit energy to space, resulting in an approximate balance between energy received and energy lost. The so-called greenhouse gases, such as water vapor, CO2, methane (CH4), nitrous oxide (N2O), halocarbons, and ozone (O3), modify this balance by absorbing and then re-emitting some of the outgoing radiation.1 Small particles in the atmosphere (aerosols) also ab-
Each object in the Earth system (e.g., a gas molecule in the atmosphere, the surface of the Earth itself) absorbs and emits energy at a rate that depends on the object’s temperature. For this reason, greenhouse gases are most effective when located in parts of the atmosphere that are much colder than the Earth’s surface, such as the upper troposphere. Greenhouse gases present in this region absorb energy emitted by the surface, but emit less energy to space because they are much colder than the surface.