1.2.5. Horizontal Energy Transfers
Figure 1.3 illustrates how the energy transfers of the Earth-atmosphere system are in equilibrium. On a global scale and over a time period of several days and more this assumption is valid, and is adequate for an understanding of the causes of climate change covered in chapter 2. However, the real world is more complex than this.
If energy fluxes are calculated for different areas around the globe, one finds that between about 40�N and 35�S the incoming solar radiation is greater than the outgoing terrestrial radiation. Elsewhere (i.e. nearer the poles), there is a net radiation deficit, that is, more radiation is lost than received (Trewartha & Horn, 1980; Figure 1.4).
To restore equilibrium to this balance a meridional interchange of heat exists from the tropics to the poles (Figure 1.5). If this energy transfer did not occur, the equator would be 14�C warmer on average than now, whilst the North Pole would be 25�C colder (Barry & Chorley, 1992). This latitudinal transfer of energy occurs in several ways, involving the movement of sensible heat (convection processes caused by heating, rising and dispersion of surface air), latent heat (evapotranspiration processes involving evaporation of water vapour from the oceans and transpiration from land plants) and ocean currents (section 1.3).
As well as this movement of heat energy, there are other transfers which occur and must be balanced according to thermodynamic and physical principles. These include the transfer and balance of mass, momentum and moisture. The movement of heat involves the movement of air (sensible heat), and moisture or water vapour (evapotranspiration). If a packet of air moves from the equator to the poles, this air must be replaced by colder air returning from the poles, having released its heat. In other words, the fluxes of air masses at specific locations around the Earth must be in equilibrium. By similar reasoning, both moisture and momentum fluxes must balance (Cubasch & Cess, 1990).
This picture is further complicated by the rotation of the Earth, which introduces a Coriolis Force on the moving atmosphere, and the axial tilt of the Earth, which affects the seasonal and latitudinal distribution of solar radiation. However, these phenomena are more usually covered in the study of meteorology, which is beyond the scope of this guide. Barry & Chorley (1992) provide a more detailed discussion for those interested. Figure 1.6 schematises a rudimentary Earth atmospheric circulation.