2 With a global average irradiance of 342 W/m , the Sun is by far the largest source of energy for planet Earth. In comparison, the internal energy produced by Earth 2 itself is only about 0. 087 W/m (Pollack et al. , 1993), which in turn is 3. 5 times 2 larger than the 0. 025 W/m of heat produced by the burning of fossil fuels. About 31% (31 units) of the solar energy which arrives at the top of the - mosphere is re?ected back to space by scattering from clouds, aerosols, and the Earth´s surface. Almost 20 units of solar radiation are absorbed in the atmosphere. The remaining 49 units are absorbed at the surface. Evaporation of water at the Earth´s surface consumes 23 units, and 7 units are transferred to the atmosphere by heat conduction. On balance 19 units are lost from the Earth´s surface as infrared radiation, however consisting of 114 going upward and 95 returning from the - mosphere to the Earth´s surface (see also Rosenfeld, 2006). There is thus a sixfold recycling of energy. This is the greenhouse effect, established by the presence of watervaporandofothergreenhousegases,CO ,CH ,N O,andCFCs,intheatmo- 2 4 2 sphere. Due to human activities the latter have been increasing in the atmosphere, 2 causing climate warming through an energy imbalance of 2. 5-3 W/m , more than 100 times larger than the heat released into the atmosphere by the burning of fossil fuels.
