Abstract
It has been pointed out that climatological-mean precipitation-evaporation difference (P–E) should increase under global warming mainly through the increasing saturation level of moisture. This study focuses on evaporation changes under global warming and their dependency on the direct warming effect, on the basis of future projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Over most of the tropical, subtropical and midlatitude regions, the direct contribution from surface temperature increase is found to dominate the projected increase in evaporation. This contribution is nevertheless offset partially, especially over the oceans, by contributions from weakening surface winds and increasing near-surface relative humidity. Greater warming of surface air than of the sea surface also acts to reduce surface evaporation, by reducing both the exchange coefficient and humidity contrast at the surface. Though generally of secondary importance, this contribution is the dominant factor over the subpolar oceans. Over the polar oceans, the effect of sea-ice retreat dominantly contributes to the evaporation increase in winter, whereas the reduced exchange coefficient and surface humidity contrast coupled with the sea-ice retreat account for most of the response during summertime. Over the continents, changes in the surface exchange coefficient, reflecting changes in soil moisture and vegetation among other factors, are important to modulate the direct effects of the warming and the generally reduced surface air relative humidity.
Highlights
As the global climate is warming due to increasing concentration of greenhouse gases (GHGs) in the atmosphere, an expected consequence is an overall intensification of the global hydrological cycle (Huntington 2006; Oki and Kanae 2006), manifested as an enhancement of atmospheric moisture transport, i.e., precipitation-evaporation difference (P–E), with P and E representing the climatological-mean precipitation and evaporation rates at a given location
This study focuses on evaporation changes under global warming and their dependency on the direct warming effect, on the basis of future projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5)
As the global climate is warming due to increasing concentration of greenhouse gases (GHGs) in the atmosphere, an expected consequence is an overall intensification of the global hydrological cycle (Huntington 2006; Oki and Kanae 2006), manifested as an enhancement of atmospheric moisture transport, i.e., P–E, with P and E representing the climatological-mean precipitation and evaporation rates at a given location
Summary
As the global climate is warming due to increasing concentration of greenhouse gases (GHGs) in the atmosphere, an expected consequence is an overall intensification of the global hydrological cycle (Huntington 2006; Oki and Kanae 2006), manifested as an enhancement of atmospheric moisture transport, i.e., P–E, with P and E representing the climatological-mean precipitation and evaporation rates at a given location. Held and Soden (2006) have shown how this effect is relevant for describing the first-order response of the hydrological cycle simulated under the global warming condition in the models from the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4) Another important parameter for the hydrological cycle is evapotranspiration, through which moisture is supplied to the atmosphere. We limit our attention to simple and direct physically-based questions concerning evapotranspiration changes projected in global climate models under the increased GHGs. We hereafter use the general term ‘‘evaporation’’ to refer to the total water flux from the surface to the atmosphere, including sublimation, evaporation of water intercepted by the canopy and transpiration.
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