Friday, October 22, 2010

Drought under global warming


A recent article from Aiguo Dai, «Drought under Global Warming: A Review», published in Wiley Interdisciplinary Reviews: Climate Change on 19 October 2010, reviews recent literature on drought of the last millennium, and provides an update on global aridity changes from 1950 to 2008. Projected future aridity is also presented based on recent studies and our analysis of model simulations.

The article provides a typology of droughts: meteorological drought; agricultural drought; and hydrological drought, which might be the more difficult to address because it occurs when river stream flow and water storages in aquifers, lakes, or reservoirs fall below long-term mean levels, thus probably requiring systemic adaptation in water allocation patterns and total consumption.

Interesting points from the article are as follows:

- «Successive "megadroughts", unprecedented in persistence (20–40 year) yet similar in severity and spatial distribution to the major droughts experienced in modern day’s North America, occurred during a 400-year-long period in the early to middle part of the second millennium AD over western North America. Compared with these multi-decadal droughts, the modern-day droughts in the 1930s and 1950s had similar intensity but shorter durations.» (p.4, references omitted)

- «One should not use total precipitation alone to measure changes in aridity or drought, as done in many studies. Increased heavy precipitation and reduced light to moderate rain can increase the runoff to precipitation ratio, and increases in surface air temperature and radiative heating can lead to higher atmospheric demand for moisture. These processes can result in drier soils even if the precipitation amount increases.» (p.13, references omitted)

- «Although natural variations (...) have played a large role in the recent drying, the rapid warming since the late 1970s has increased atmospheric demand for moisture and likely altered atmospheric circulation patterns (e.g., over Africa and East Asia), both contributing to the recent drying over land. Since a large part of the recent warming is attributed to human-induced GHG increases, it can be concluded that human activities have contributed significantly to the recent drying trend.» (p.15, reference omitted)

- «Coupled climate models used in the IPCC AR4 project increased aridity in the 21st century, with a striking pattern that suggests continued drying over most of Africa, southern Europe and the Middle East, most of Americas (except Alaska, northern Canada, Uruguay, and northeastern Argentina), Australia, and Southeast Asia.» (p.15)

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