As the Earth gets warmer, the risk of concurrent droughts and heat waves, such as those seen in California during 2014, could rise substantially. Precipitation is generally the only variable that scientists use to estimate how often droughts of a certain severity might occur. However, because this method ignores temperature, which can influence drought severity, it could inaccurately calculate the actual risk. AghaKouchak et al. analyzed how unique the 2014 California drought actually was, on the basis of not just precipitation or temperature alone but also those variables combined.
The authors obtained monthly precipitation and temperature data for California between 1896 and 2014 from the National Oceanic and Atmospheric Administration’s U.S. Climate Divisional Database. With these data, the authors could model the California drought’s return period—the number of years, on average, for a drought as severe as the 2014 drought to recur. The authors calculated this quantity, first using a statistical technique based only on precipitation or temperature alone, then using another approach that combined both variables.
The 2014 California drought has a 24-year return period based on precipitation data alone and a 120-year return period based on temperature data alone, the researchers found. However, when the researchers considered the effects of precipitation and temperature together, the number rose to a 200-year event.
The findings suggest that modeling droughts’ return periods based on precipitation alone could inaccurately convey their risks. The authors argue for further research into this multivariate approach, which they suggest could help policy makers and forecasters better prepare the public for future droughts. The method, the authors add, could be adapted for assessing the risks of different combinations of extremes. (Geophysical Research Letters, doi:10.1002/2014GL062308, 2014)
—Puneet Kollipara, Freelance Writer
Citation: Kollipara, P. (2015), When predicting drought risk, do not overlook temperature, Eos, 96, doi:10.1029/2015EO027973. Published on 15 April 2015.