What is the mega-drought? How scientists define extreme water shortages

Mega-droughts are exceptionally severe, long-lasting, or widespread compared to normally occurring dry spells

By James Dinen

Southwestern North America has been dry for so long that scientists use the term ‘mega-drought’ to describe drought conditions that have led to extreme wildfires and water shortages for decades. . But what exactly is a mega-drought?

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

For example, depending on the method used to identify them, there have been either 30, 41, or 56 droughts longer than five years in southwestern North America in the past 2,000 years, depending on a new large-scale review of drought records directed by Benjamin Cook at Columbia University in New York.

Fearing that these inconsistent definitions could 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 mega-drought is a exceptionally severe, long-lasting or widespread drought compared to droughts in a given region in the past 2000 years.

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

Cook and his colleagues say their definition is broad enough to encompass severe droughts that have occurred across the world over the past 2,000 years, while recognizing that a mega-drought is more than just a particularly dry spell. severe.

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

When the droughts got mega

The term mega-drought was first introduced into the scientific literature by researchers in Colorado in 1998. From records including historical records, archaeological remains, lake sediment analyzes and patterns in tree rings which reflect wet and dry years, they describes several droughts that occurred in southwestern 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 hit the central United States in waves for about a decade starting in 1930. and created the Dust Bowl.

Over time, other researchers have used the mega-drought to describe exceptionally bad droughts around the world. “It’s such a squishy term,” says Connie Woodhouse at the University of Arizona, co-author of the 1998 paper. The term became much more common when the current drought in southwestern North America began to look like multi-decade mega-droughts of previous centuries, says Woodhouse. Now stretching into its second decade, the mega-drought is the driest 22-year period the region has seen in more than 1,200 years.

With the the risk and severity of mega-drought is expected to increase this century in many parts of the world due to global warming caused by human emissions of greenhouse gases, there is an urgent need to understand past drought patterns, how our actions can worsen these droughts and what we can do to mitigate the worst damage in the most vulnerable regions.

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