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Abstract
Water vapour 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 (RF) of emitted water vapour have not been formally quantified in the literature. Here these quantities are estimated for surface emission using idealised experiments conducted with the CAM5 global atmospheric model at fixed ocean temperatures. Water is introduced in vapour 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 -10-3 to 5 × 10-4 is found, and an effective radiative forcing of -0.1 to 0.05 W m-2 for the given emissions. Increases in water vapour greenhouse effect are small because additional vapour cannot reach the upper troposphere, and greenhouse-gas warming is outweighed by increases in reflectance from humidity-induced low cloud cover, leading to a near-zero or small cooling effect. Near-surface temperature decreases over land are implied even without evaporative cooling at the surface, due to cooling by low clouds and vapour-induced changes to the moist lapse rate. These results indicate that even large increases in anthropogenic water vapour emissions would have negligible warming effects on climate, but that possible negative RF may deserve more attention.
