What is megadrought? How scientists define extreme water shortages

Megadroughts are exceptionally severe, long-lasting or widespread relative to normally occurring dry stretches

By James Dinneen

South-western North America has been in drought for so long that scientists use the term “megadrought” to describe the parched conditions that have led to extreme wildfires and water shortages for decades. But what exactly is a megadrought?

Currently, the term is loosely used to refer to any particularly bad drought, which itself is simply defined as a period of below-normal water availability. But how long or severe a drought has to be to constitute a megadrought is inconsistent in the scientific literature, says Gerald Meehl at the National Center for Atmospheric Research in Colorado.

For instance, depending on the method used to identify them, there were either 30, 41 or 56 droughts lasting more than five years in south-western North America in the past 2000 years, according to a new large-scale review of drought records led by Benjamin Cook at Columbia University in New York.

Concerned that these inconsistent definitions can muddy the waters when it comes to understanding past and current trends – and their future implications – Cook and a team of international climate researchers have proposed a new definition: A megadrought is a drought that is exceptionally severe, long-lasting or widespread relative to droughts in a given region over the past 2000 years.

This takes into account the fact that droughts are relative to the typical water use and climate of a particular area. A megadrought in the Amazon would be considered an extremely wet period in Arizona, for instance.

Cook and his colleagues say their definition is broad enough to encompass the severe droughts that have occurred across the globe in the past 2000 years, while recognising that a megadrought is more than just a particularly bad dry spell.

“We should really reserve that term for droughts that would be unprecedented in the palaeoclimate record,” says Park Williams at the University of California, Los Angeles, and part of the team that proposed the new definition.

When droughts went mega

The term megadrought was first introduced into the scientific literature by researchers in Colorado in 1998. From records including historical documents, archaeological remains, analyses of lake sediment and patterns in tree rings that reflect wet and dry years, they described several droughts that occurred in south-western North America during the 1500s and 1600s. They called these droughts “mega” because they were more severe and lasted longer than the worst known droughts of the 20th century, such as the one that came in waves across the central US for about a decade starting in 1930 and created the Dust Bowl.

Over time, other researchers used megadrought to describe exceptionally bad dry spells around the world. “It’s such a squishy term,” says Connie Woodhouse at the University of Arizona, who co-authored the 1998 paper. The term became much more common when the current ongoing drought in south-western North America started to resemble the multi-decade megadroughts of previous centuries, says Woodhouse. Now stretching into its second decade, the megadrought is the driest 22-year stretch the region has seen in more than 1200 years.

With the risk and severity of megadrought expected to increase this century in many regions around the world due to global warming caused by human greenhouse gas emissions, there is an urgent need to understand past drought patterns, how our actions may be making such droughts more severe and what we can do to mitigate against the worst harms in the most vulnerable regions.

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