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Climate change rule of thumb gets rained on

When the Earth warms, the evaporation of water speeds up.  Land areas are more quickly sapped of their moisture while excess water gets sucked into the atmosphere for downpours.  Due to this dynamic, the prevailing wisdom has long been global warming should cause dry areas to get drier and wet areas wetter – a so-called DDWW theory.

Yet new research suggests the rain rich are not getting richer nor the poor poorer over most of the Earth’s land areas. At least so far.

In a new study published in Nature Geosciences, a group of scientists from Switzerland, Germany and the U.S. find no meaningful trends in combined indicators of moisture – precipitation, evapotranspiration, and potential evaporation (the examination of combined trends make the study “unique” says lead author Peter Greve) – over 75 percent of the Earth’s land surface from 1948 to 2005.

Over the remaining 25 percent of the Earth where moisture changes have been observed, the scientists say the DDWW theory only checks out half the time.  In half of the region in which there is a meaningful trend in precipitation, the opposite of what the theory DDWW predicts occurs: dry areas get wetter, and wet areas drier.

The study’s news release summarizes:

Some regions which should have become wetter according to the simple DDWW formula have actually become drier in the past – this includes parts of the Amazon, Central America, tropical Africa and Asia. On the other hand, there are dry areas that have become wetter: parts of Patagonia, central Australia and the Midwestern United States.
Nevertheless, the ‘wet gets wetter’ rule is largely confirmed for the Eastern United States, Northern Australia and northern Eurasia. ‘Dry gets drier‘ also corresponds to indications in the Sahel region, the Arabian Peninsula and parts of Central Asia and Australia.

While the DDWW theory hasn’t played out as expected over land, the study notes it has had better success over the oceans.

The study is an important reminder that due to the complexity of atmospheric circulation patterns, the behavior of precipitation is incredibly difficult to predict.

“We conclude that aridity changes over land, where the potential for direct socio-economic consequences is highest, have not followed a simple intensification of existing patterns,” the study’s abstract states.

Link: Study: Global warming to heighten risk of Southwestern megadroughts

Although the DDWW theory may represent an oversimplication (to date, over land areas), it doesn’t change the overall expectation for greater precipitation extremes as the Earth warms.

A January 2014 study from the Journal Nature explains the rationale for projections for increased drought:

Climate change is adding heat to the climate system and on land much of that heat goes into drying. A natural drought should therefore set in quicker, become more intense, and may last longer. Droughts may be more extensive as a result. Indeed, human-induced warming effects accumulate on land during periods of drought because the ‘air conditioning effects’ of water are absent. Climate change may not manufacture droughts, but it could exacerbate them and it will probably expand their domain in the subtropical dry zone.

Whether or not drought has already increased is controversial, at least on the global scale.

While a global drought signal isn’t entirely clear, heavy precipitation events have shown an increase. Per the 2014 U.S. National Climate Assessment:

Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming… This extra moisture is available to storm systems, resulting in heavier rainfalls.

A 2013 study in the Journal of Climate states: “[A] picture of a world with intensifying heavy precipitation events over the majority of land seems confirmed by CMIP5 model projections for the end of the twenty-first century.”

Perhaps a take home message here is that we should expect more precipitation extremes, but exactly where (and when) and in what direction (drier or wetter) remains unresolved in many areas.

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