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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
within the atmosphere (Ramanathan et al., 2001a). As shown in Chapter 4 (see Box 4-1) the TOA forcing observed during INDOEX over the Indian subcontinent was close to zero, but the surface forcing was −14 W m−2 and tropospheric forcing was about +14 W m−2. As shown by several general circulation model (GCM) sensitivity studies (Ramanathan et al., 2001b; Chung and Ramanathan, 2003; Menon et al., 2002b), in spite of the near-zero TOA forcing, the introduction of absorbing aerosols results in a large surface cooling (−0.5 to −1 K) of the North Indian Ocean and South Asia, a large lower tropospheric warming (0.5 to 1 K), and large changes in regional precipitation. This means that in addition to calculating radiative forcing at the tropopause, one must also quantify radiative forcing at the surface and its atmospheric distribution.
THEORETICAL DEVELOPMENT OF THE RADIATIVE FORCING CONCEPT
The concept of radiative forcing is based on the hypothesis that the change in global annual mean surface temperature is proportional to the imposed global annual mean forcing, independent of the nature of the applied forcing. The fundamental assumption underlying the radiative forcing concept is that the surface and the troposphere are strongly coupled by convective heat transfer processes; that is, the earth-troposphere system is in a state of radiative-convective equilibrium (RCE; see Box 1-3). In the present context, the term “convective heat transfer” refers to heat transport by all types of vertical motions ranging from small (few meters) to planetary scales. The net result of radiative-convective equilibrium is that the vertical temperature profile within the troposphere (the so-called lapse rate) is largely determined by convective heat transport, while the vertically averaged surface-troposphere temperature is regulated by radiative flux equilibrium at the tropopause. RCE models were initially used to determine the vertical temperature profile of stellar (Chandrasekhar, 1947; Ambartsumyan, 1958) and planetary atmospheres (e.g., Chamberlain, 1960; Gierasch and Goody, 1968; Cess, 1972). Its first application to the Earth’s atmosphere with a proper treatment of convective heat transport and the radiative transfer effects of infrared active gases and clouds was published by Manabe and Strickler (1964) and Manabe and Wetherald (1967).
According to the radiative-convective equilibrium concept, the equation for determining global average surface temperature of the planet is