The ultimate magnitude of global warming is likely to be limited mainly by the rate of absorption of CO2 by the oceans. Model calculations of this limit need to take account of the effect of the temperature gradient at the ocean surface on the effective saturation pressure of CO2 above that surface. We present a description of this and other unfamiliar phenomena in terms of Onsager's irreversible thermodynamics. (C) 2010 Elsevier B.V. All rights reserved.