Science: A Drier Future. Global Warming is likely to lead to overall drying of land surfaces.

A Drier Future. Global Warming is likely to lead to overall drying of land surfaces. Sherwood, S & Fu, Q. Science. 14 Feb 2014. Vol 343. pp 737-739

Global temperature increases affect the water cycle over land, but the nature of these changes remains difficult to predict. A key problem is to distinguish between droughts, which are regionally temporary extreme phenomena versus normal dryness. Average dryness depends on precipitation and how fast water evaporates. As the planet warms, global average rainfall increases, but so does evaporation. What is the likely net impact on average aridity?

Most studies of dryness focus on droughts rather than on the usual, background dryness that don’t take into consideration changes in available energy, air humidity, and wind speed that can exaggerate the trend toward more drought in a warming climate, which has undone past claims that drought is on the rise globally, and led to weaker claims about observed drought trends in the most recent Intergovernmental Panel on Climate Change report.

However, that does not mean that conditions will not get drier

The key factor causing drying is that land surfaces (and the air just above them) warm, on average, about 50% more than ocean surfaces. Enhanced warming of land surfaces relative to oceans occurs because continental air masses are drier than maritime ones, which in turn is a consequence of the limited availability of surface water.

The second factor ensuring drying is that water vapor content over land does not increase fast enough relative to the rapid warming there. This increases the aridity.

Positive feedback from soil moisture changes is not needed to explain enhanced land warming, but likely amplifies it in some regions.

The general trend toward a drier land surface appears to rest on relatively firm foundations. The predicted drying would be sufficient to shift large portions of the Earth to new, drier climate categories (although the richer atmospheric CO2 might mitigate the impact on some plants). The background drying is separate from, but may be compounded by, the expected trend toward more intermittent rainfall for a given mean rain rate.

As the above considerations show, focusing on changes in precipitation, as typical in high-profile climate reports, does not tell the whole story—or perhaps even the main story—of hydrological change. In particular, it obscures the fact that in a warmer climate, more rain is needed. Many regions will get more rain, but it appears that few will get enough to keep pace with the growing evaporative demand.

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