2.1.4 The heat balance at the top of the atmosphere: geographical distribution

The geographical distribution of the net incoming solar radiation at the top of the atmosphere (i.e., the incoming minus the reflected solar radiation) that is absorbed by the Earth is a function of the insolation distribution as well as of the regional variations of the planetary albedo (Fig. 2.12). The latter is influenced by several factors, including the albedo of the surface (see section 1.5) and the presence of clouds which reflect a significant fraction of the incoming solar radiation back to space. The influence of clouds is particularly evident in the Tropical Regions, where it explains, for instance, why the absorbed solar radiation is larger in the relatively cloud free eastern Equatorial Pacific than in the cloudier western Pacific. At high latitudes, the surface albedo is high because of the high zenith distance (Sun low above the horizon) and the high reflectance of snow and ice (see section 1.4.2). This high surface albedo at high latitudes amplifies the latitudinal variations in solar radiation associated with the Earth's geometry (Fig. 2.11), resulting in a difference of nearly a factor of five in annual mean absorbed solar radiation at the poles, compared to the equator.

The Stefan-Boltzmann law says that the longwave radiation emitted is a function of the temperature of the emitting surface. A difference of about 50°C between the equator and the poles roughly corresponds to a variation in the emitted thermal radiation of about 50 W m^-2, which is in reasonable agreement with the estimated values (Fig. 2.13). The presence of clouds and water vapour also has a large influence. Indeed, water vapour is a strong greenhouse gas. It absorbs part of the infra-red radiation emitted by the surface before re-emitting radiation, generally at a lower temperature as clouds are located higher in the atmosphere (see section 2.1.2). This results in less outgoing longwave radiation. As a consequence, the maximum outgoing longwave radiation is found above warm dry areas such as the subtropical deserts. More generally, wet equatorial areas generally emit less radiation than dry tropical areas (Fig. 2.13).

When averaged over longitude, the outgoing longwave radiation clearly shows less latitudinal variation than the net incoming solar radiation absorbed by the Earth. As a consequence, the absorbed solar radiation outbalances the outgoing radiation in regions located between roughly 40°S and 40°N, while a net deficit in the net radiative flux at the top of the atmosphere (RF_TOA) is observed poleward of 40°N and 40°S (Fig. 2.14). RF_TOA also displays some longitudinal variations, the most spectacular being probably the net negative flux over the Sahara because of the dry conditions there and of the high albedo of its sand.