Are megadroughts upon us?

International researchers who are monitoring climate change say persistent multiyear drought events – sometimes called megadroughts – pose a growing threat to nature and humans.

A team of researchers from the Institute of Science and Technology Austria produced a 40-year global quantitative inventory to inform policy about the environmental impact of human-induced climate change. It also detected previously “overlooked” events.

The team, writing in an article in the January edition of Science, pointed to its research showing that multiyear droughts (MYDs): “…have become drier, hotter, and led to increasingly diminished vegetation greenness.

“The global terrestrial land affected by MYDs has increased at a rate of 14,771 square kilometres per year from 1980 to 2018.

“Temperate grasslands have exhibited the greatest declines in vegetation greenness during MYDs, whereas boreal and tropical forests have had comparably minor responses.”

The team provided an inventory of 13,176 multiyear droughts and listed the top ten saying: “Recent MYDs in central Chile, the western United States, and Australia have alarmed the scientific community, public media, stakeholders, and decision-makers because of their devastating impacts.

“The longest-lasting MYD covered the eastern Congo basin in Africa and persisted for almost a decade, from 2010 until 2018, with its largest extent in 2014 covering an area of 1,494,226km². 

“The MYD that occurred in the southwestern Amazon lasted 9 years, and the annual coverage gradually increased from 112,540 km² in 2010 to 212,925 km² in 2014, which is nearly equivalent to the size of the United Kingdom.

“Severe societal and ecological disruptions were seen in several prominent multiyear drought events, such as the US Dust Bowl in the 1930s (1931 to 1939), the Australian millennium drought (2001 to 2009), a California drought (2012 to 2016), and the central European drought (2018 to 2022).

“With MYDs becoming more common, this global quantitative inventory of the occurrence, severity, trend, and impact of MYDs provides an important benchmark for facilitating more effective and collaborative preparedness toward mitigation of and adaptation to such extreme events.”

The team has concerns that the standardised precipitation index (SPI), which is broadly used to monitor and characterise meteorological drought conditions, does not go far enough to describe the impact of drought.

“In a warming world, excess bottom-up evaporation can substantially amplify ecosystem water deficits during droughts, challenging the effectiveness of a drought index that is solely derived from precipitation in measuring drought stress.

“Prolonged drought can also deplete deep water storage, as seen during the fourth year of a multiyear drought event in California, which led to a substantial increase in tree mortality.”

Australian drought researcher Albert van Dijk led a team which this month published the annual report on trends in floods and droughts, which include multiyear droughts.

Because the Science journal article was accompanied by a news release headed: “The megadroughts are here,” Cosmos asked him if Australia had been experiencing megadroughts?

“I think the article takes care to call them multiyear droughts but the journal editor took some liberty putting the header “megadroughts” over it,” van Dijk says.

“I suppose there probably isn’t a formal definition of ‘megadrought’ but it’s a term that comes from the paleoclimatology literature where they see extreme events in tree rings etc and those can easily be of 1 in 500-year events or more rare again, and usually last more than a decade. 

“The recent Western US drought can still be considered a megadrought as it’s been about 2 decades by now, and our Millennium Drought arguably might be called one.”

Two regions in Australia are experiencing 4 years of very low rainfall, (see BoM image above) but van Dijk doesn’t class them as megadroughts either.

“Unlikely. You could call 4 years of below rainfall a multiyear drought if it hasn’t previously happened more often than every 20 or 25 years on average – a common definition of a drought in Australia and elsewhere.

“For much of Australia low rainfall would need to occur for multiple years to be called a drought anyway, but not always: the 2019/20 black summer was the culmination of a relatively short but very intense drought, and in the Australian context could be called a so-called ‘flash drought’.

“Flash droughts are increasingly common globally but usually found in wetter areas (such as the forests that burnt that summer).  In drier regions such a flash drought wouldn’t be much noticed as a prolonged lack of rainfall would be quite normal.”

So when does a multiyear drought or megadrought simply morph into permanent climate change?

“That’s a really good question,” van Dijk says. “It depends on your expectations, I guess.

“For example, Southwest Western Australia and more recently much of Victoria have shifted to different climate and now systematically get less winter rainfall.

“You could probably call that a megadrought if you expected it to change back any moment, but the majority of climate scientists see it as a global warming driven shift that’s unlike to change back until greenhouse gas concentrations fall again, which at best would be several decades hence.”

Megadroughts: a prediction a decade ago