
With climate change, extreme weather events are occurring more and more frequently. This includes not only heavy rain, but also droughts, which now often last for several years or even decades. In the future, these megadroughts could be even longer, larger, hotter and more destructive if trends over the past 40 years continue, a new study shows. The most noticeable damage is to the grasslands in prairies and steppes, which dry out particularly quickly, the team writes in “Science”. However, how megadroughts affect forests is often harder to see.
Weather data shows that droughts are becoming more and more common due to global warming, including multi-year megadroughts such as those that occurred in the USA between 2000 and 2018. In Chile, such a dry phase has been going on since 2010 and has almost completely depleted the country’s water reserves. Such extreme weather events threaten both ecosystems and people. Because soils dry out and rivers carry less water, crop failures occur, drinking water becomes scarce, trees die, forest fires occur and habitats become impoverished. But how often do megadroughts actually occur and how big are the water crises for nature?

Megadroughts are becoming more frequent and more intense
A team led by Liangzhi Chen from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) in Switzerland has now investigated this in more detail. To do this, the researchers used data on precipitation and evaporation to determine where in the world droughts occurred between 1980 and 2018 and how severe they were. They mapped 13,176 megadroughts worldwide, including many previously overlooked droughts in less accessible regions of the world. “Our method not only mapped well-documented droughts, but also uncovered extreme droughts that have gone almost undetected, such as the 2010-2018 drought in the Congo rainforest,” says senior author Dirk Karger from WSL. Using satellite images, the team also analyzed how green the vegetation was – a sign of living, healthy plants.
The analysis showed that megadroughts have occurred on all continents in recent decades, except Antarctica. Over time they became more and more frequent, larger, hotter and drier. “Since 1980, drought areas have expanded by an average of 50,000 square kilometers every year – roughly the area of Slovakia – causing enormous damage to ecosystems, agriculture and energy production,” says co-author Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA). Larger areas of land around the world were affected by megadroughts, particularly after years of the El Niño climate phenomenon.
However, the consequences for the ecosystems were very different because they are located in different climatic regions and different plants occur there. The grasses and shrubs of prairies and steppes, particularly in southeastern Australia, the western United States and Mongolia, suffered most visibly from the drought. Unsurprisingly, the grasslands lost more green color than tundra, subtropical or tropical forests. However, grass areas often recovered quickly after droughts. The steppe plants are therefore the least heat-resistant, but are quite resilient in the long term, according to Chen and his colleagues. In contrast, tropical forests have apparently been able to compensate for drought phases well through their water reserves. Boreal forests also seem to have had little impact on the drought so far. Thanks to global warming, they are currently growing more. But just because the forests are (still) green doesn’t mean they aren’t suffering, as the team emphasizes. The damage is just harder to see.

Better preparation against droughts needed
The long-term consequences of megadroughts on various ecosystems are still largely unknown, but the future is rather bleak. “The violence of multi-year droughts will become increasingly severe with climate change,” says co-author Philipp Brun from WSL. The researchers assume that the effects could be even more devastating in the future. “During extreme water shortages, trees in tropical and boreal regions can die, causing long-term damage to these ecosystems. Boreal vegetation in particular will probably take the longest to recover from such a climate catastrophe,” says Karger.
The findings should help to take more effective and long-term preparation measures against future megadroughts, instead of just preparing for shorter, seasonal droughts as is currently the case.
Source: Liangzhi Chen (Swiss Federal Institute for Forest, Snow and Landscape Research) et al.; Science, doi: 10.1126/science.ado4245