Climate Change and Drought: a Precipitation and Evaporation Perspective

Many studies have shown that greenhouse gas (GHG)-induced global warming may lead to increased surface aridity and more droughts in the twenty-first century due to decreased precipitation in the subtropics and increased evaporative demand associated with higher vapor pressure deficit under warmer temperatures. Some recent studies argue that increased water use efficiency by plants under elevatedCO(2) may reduce the evaporative demand and therefore mitigate the drying. Here we first discuss the model-projected changes in precipitation amount and frequency that affect the surface water balance and aridity and then the changes in actual and potential evapotranspiration under GHG-induced warming. The effects of the GHG-induced warming and changes in plants' physiology under elevated CO2 on precipitation, soil moisture, and runoff are quantified and compared by analyzing different model experiments with and without the physiologic response. The surface drying effect of GHG-induced warming is found to dominate over the wetting effect of plants' physiology in response to increasing CO2, leading to similar surface drying patterns in climate model simulations with or without the physiologic response in the twenty-first century. Part of the drying comes from increased dry spells (i.e., more dry days) and a flattening of the histograms of drought indices asGHGs increase, with the latter leading to widespread increases in hydrological drought even over areas with increasing mean runoff. Because of this, the change pattern of the mean cannot be used to represent drought changes. Consistent with the projected drying in the twenty-first century, recent analyses of model experiments suggest wetter land surfaces during the last glacial maximum, which implies that dusty air during cold glacial periods may have resulted from other factors, such as stronger winds and more dust sources, rather than drier land surfaces. Finally, the drying in the subtropics does not appear to be just a transient response to increased GHGs, as the warming and precipitation change patterns do not vary significantly over time in 500-year simulations with increased CO2 contents by a fully coupled climate model.