Figure 1.

The thermodynamic context of environmental and ecological systems. The cycling of matter at the planetary scale—as manifested by a strong atmospheric circulation and the global cycles of water and carbon—require engines to continuously operate to keep these cycles running. These engines ultimately run on gradients of radiative fluxes, which result from the vastly different radiative temperatures of the Sun (with T_sun ≈5760 K) and Earth (with T_earth ≈ 255 K). Gradients in radiative fluxes can result in gradients of heating, which drive heat engines such as the atmospheric circulation, or photochemical engines that make use of low entropy solar radiation directly.