Friday, December 27, 2013

Global Warming Will Intensify Drought, Says New Study

Terry Hash pauses after searching in the cracked soil for cotton seeds in his 175-acre cotton field in Garfield on Thursday, August 18, 2011. Hash planted 800 acres of cotton, corn, wheat and sorghum, and almost all of it was destroyed by the drought. Despite having insurance, Hash said he worries about how he is going to pay his farm loans and borrow more money for next season's crops. 'Lots of sleepless nights,' Hash said. 'You lay in bed wondering what the hell you're going to do.' (Credit: Jay Janner) Click to enlarge.A very recent study by Trenberth et al., "Global warming and changes in drought" published in Natural Climate Change has investigated the way droughts are measured. They discuss various drought metrics such as the Standardized Precipitation Index which is based entirely on precipitation, the Standardized Precipitation and Evapotranspiration Index which includes ET effects, and the Palmer Drought Severity Index (PDSI) which balances precipitation, evaporation, runoff, and includes local soil moisture and vegetation.

This most sophisticated metric has led different teams of researchers to very different conclusions about drought trends.  One study reports "little change in drought over the past 60 years" while another, "increasing drought under global warming in observations and models". How could researchers come to such different conclusions?  It turns out that various versions of the PDSI have differing algorithms for calculating ET which partially explains the differences.

First, quantifying evapotranspiration is very challenging; the observational spread is large as are regional uncertainties.  Second, the choice of baseline period is crucial.  The first study mentioned above used a 1950-2008 baseline which includes human impacts while the second study baseline is limited to 1950-1979 which, while having less human influence turned out to be a wetter than normal period.

Global Warming Will Intensify Drought, Says New Study

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