The greenhouse earth: A view from space

The natural greenhouse effect of the Earth is strongly influenced by the radiative effects of water vapour and clouds in the atmosphere, which control the energy absorbed from the sun, and that lost through thermal emission to space. Any perturbations to the climate balance, for example through so-called 'radiative forcing' due to increasing CO2 amounts, variations in solar constant, or other causes, can be amplified by the feedback processes that involve water in its various phases. The radiative cooling of the Earth in the absence of clouds has recently been shown to be dominated by emission from upper-tropospheric water vapour, in the far infrared portion of the spectrum, and this is illustrated: observations of this radiative flux, and of the distribution of water vapour in the upper troposphere, are urgently needed. The role of clouds is discussed, and it is noted that their response to global warming is not presently unambiguously determined with available models, due to the complexity of competing processes: again, as in the cloud-free case, more accurate global observations are needed. The paper is illustrated by data from satellite experiments, most notably the Earth Radiation Budget Experiment sponsored by NASA.