Even parts of the world mercifully spared flash flooding are getting used to seeing reports, but the concept of a “flash drought” is much less familiar. Nevertheless, climatologists warn this is a real phenomenon becoming more dangerous as global temperatures rise.
Flash droughts are not as fast as flash floods – they involve soil drying out in days or weeks, rather than a torrent of water arriving after hours of driving rain. Nevertheless, researchers warn in Nature Communications that they share the key feature of taking people by surprise, with all the consequences that implies.
The paper charts flash droughts over the period 2000-2020 to see if global heating has increased the flash drought phenomenon. Overall they found that the number of droughts that come on within a month has been stable over that time. There's no counterpart of parts of Australia having a “one in 100 year” flood a month after a “one in 500 years” flood.
However, the fastest category of flash droughts, those that occur within just five days, have increased by between 3 and 19 percent. That's important, because, as with floods, the fastest onset droughts are the ones hardest to prepare for and five-day onset droughts make up around 40 percent of the total. Certain parts of the world, already prone to flash droughts, have seen rises of over 20 percent during this period.
These regions include South and Southeast Asia, between them home to almost a third of the world's people, as well as central North America. Although the authors cannot definitively link the increase to anthropogenic global warming, they doubt it's a coincidence.
"Every year, we are seeing record-breaking warming episodes, and that is a good precursor to these flash droughts," Professor Zong-Liang Yang of the University of Texas at Austin said in a statement. "The hope and purpose [of this research] is to minimize the detrimental effects."
Although our ancestors have been surviving – or not – flash droughts since before they were humans, the phenomenon was not understood until remote sensing technology allowed us to measure soil moisture over a wide area. The term was only coined 20 years ago. The combination of high temperatures, lack of rain, and falling atmospheric humidity can see soil dry up unexpectedly quickly.
Droughts can kill millions when food supplies dwindle in the second or third year of low rainfall. By comparison, flash droughts may seem a minor problem, with normality often restored within weeks or months.
However, the authors of the paper stress that timing is crucial. In 2012, a flash drought in the central United States caused $35.7 billion dollars of damage to the corn crop by hitting just at the time when water was most needed. Global markets had little time to prepare for shortages and spiking prices.
The places most prey to flash droughts are those with the largest changes in humidity between seasons. “We should pay close attention to the vulnerable regions with a high probability of concurrent soil drought and atmospheric aridity,” said co-author Professor Shuo Wang of the Hong Kong Polytechnic University.