The radiative forcing and Global Warming Potential of near-surface water vapor emissions

Water vapor is the most abundant and powerful greenhouse gas in Earth's atmosphere, and is emitted by human activities. Yet the global warming potential (GWP) and radiative forcing of emitted water vapor have not been formally quantified. The fact that H₂O is understood primarily as a feedback constituent does not mean these forcings cannot be quantified, and the relatively new concept of "effective radiative forcing" allows for this to be done. Here they are estimated for near-surface emission using idealized experiments with the CAM5 global atmospheric model at fixed ocean temperatures. Water is introduced in vapor form at rates matching total anthropogenic emissions (mainly from irrigation) but omitting the local evaporative cooling seen in irrigation simulations. A 100-year GWP for H₂O of less than 5 × 10^-4 is found, and an effective radiative forcing of less than 0.05 W m^-2 (likely near zero) for present emissions. Increases in water-vapor greenhouse effect are small because added vapor cannot reach the upper troposphere, and because any small warming is offset by increases in reflectance from low-level clouds due to higher humidity at low levels. Interestingly, vapor emissions can reduce the average near-surface temperature over land even without the evaporative cooling to which such temperature reduction is usually attributed in irrigation simulations. We conclude that, in contrast to current thinking, the radiative effects of anthropogenic water vapor emissions can be quantitatively compared to those of other greenhouse gases. This comparison confirms reassuringly that even large increases in near-surface emissions would have negligible direct effects on the planetary radiative balance.