It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Friday, January 17, 2025
The megadroughts are upon us
Forty-year study: Extreme droughts will become more frequent, severe, and extensive
Institute of Science and Technology Austria
image:
The Yeso reservoir in central Chile during a megadrought peak in Summer 2020.
Increasingly common since 1980, persistent multi-year droughts will continue to advance with the warming climate, warns a study from the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), with Professor Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA) participating. This publicly available forty-year global quantitative inventory, now published in Science, seeks to inform policy regarding the environmental impact of human-induced climate change. It also detected previously ‘overlooked’ events.
Fifteen years of a persistent, devastating megadrought—the longest lasting in a thousand years—have nearly dried out Chile’s water reserves, even affecting the country’s vital mining output. This is but one blatant example of how the warming climate is causing multi-year droughts and acute water crises in vulnerable regions around the globe. However, droughts tend only to be noticed when they damage agriculture or visibly affect forests. Thus, some pressing questions arise: Can we consistently identify extreme multi-year droughts and examine their impacts on ecosystems? And what can we learn from the drought patterns of the past forty years?
To answer these questions, researchers from the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) and the Institute of Science and Technology Austria (ISTA) have analyzed global meteorological data and modeled droughts between 1980 and 2018. They demonstrated a worrying increase in multi-year droughts that became longer, more frequent, and more extreme, covering more land. “Each year since 1980, drought-stricken areas have spread by an additional fifty thousand square kilometers on average—that’s roughly the area of Slovakia, or the US states of Vermont and New Hampshire put together—, causing enormous damage to ecosystems, agriculture, and energy production,” says ISTA Professor Francesca Pellicciotti, the Principal Investigator of the WSL-funded EMERGE Project, under which the present study was conducted. The team aims to unveil the possible long-lasting effects of persistent droughts around the globe and help inform policy preparing for more frequent and severe future megadroughts.
Unveiling extreme droughts that flew under the radar
The international team used the CHELSA climate data prepared by WSL Senior Researcher and study author Dirk Karger, which goes back to 1979. They calculated anomalies in rainfall and evapotranspiration—water evaporation from soil and plants—and their impact on natural ecosystems worldwide. This allowed them to determine the occurrence of multi-year droughts both in well-studied and less accessible regions of the planet, especially in areas like tropical forests and the Andes, where little observational data is available. “Our method not only mapped well-documented droughts but also shed light on extreme droughts that flew under the radar, such as the one that affected the Congo rainforest from 2010 to 2018,” says Karger. This discrepancy is likely due to how forests in various climate regions respond to drought episodes. “While temperate grasslands have been most affected in the past forty years, boreal and tropical forests appeared to withstand drought more effectively and even displayed paradoxical effects during the onset of drought.” But how long can these forests resist the harsh blow of climate change?
Contrasting impacts on ecosystems
The persistently rising temperatures, extended droughts, and higher evapotranspiration ultimately lead to dryer and browner ecosystems, despite also causing heavier precipitation episodes. Thus, scientists can use satellite images to monitor the effect of drought by tracking changes in vegetation greenness over time. While this analysis works well for temperate grasslands, the changes in greenness cannot be tracked as easily over dense tropical forest canopies, leading to underestimated effects of drought in such areas. Thus, to ensure consistent results worldwide, the team developed a multistep analysis that better resolves the changes in high-leaf regions and ranked the droughts by their severity since 1980. Unsurprisingly, they showed that megadroughts had the highest immediate impact on temperate grasslands. ‘Hotspot’ regions included the western USA, central and eastern Mongolia, and particularly southeastern Australia, where the data overlapped with two well-documented multi-year ecological droughts. On the other hand, the team shed additional light on the paradoxical effects observed in the tropical and boreal forests. While tropical forests can offset the expected effects of drought as long as they have enough water reserves to buffer the decrease in rainfall, boreal forests and tundra react in their distinct way. It turns out that the warming climate extends the boreal growth season since vegetation growth in these regions is limited by lower temperatures rather than water availability.
Droughts evolve in time and space
The results show that the trend of intensifying megadroughts is clear: The team generated the first global—and globally consistent—picture of megadroughts and their impact on vegetation at high resolution. However, the long-term effects on the planet and its ecosystems remain largely unknown. Meanwhile, the data already agrees with the observed widely greening pan-Arctic. “But in the event of long-term extreme water shortages, trees in tropical and boreal regions can die, leading to long-lasting damage to these ecosystems. Especially, the boreal vegetation will likely take the longest to recover from such a climate disaster,” says Karger. Pellicciotti hopes the team’s result will help change our perception of droughts and how to prepare for them: “Currently, mitigation strategies largely consider droughts as yearly or seasonal events, which stands in stark contrast to the longer and more severe megadroughts we will face in the future,” she says. “We hope that the publicly available inventory of droughts we are putting out will help orient policymakers toward more realistic preparation and prevention measures.” As a glaciologist, Pellicciotti also seeks to examine the effects of megadroughts in the mountains and how glaciers can buffer them. She leads a collaborative project titled “MegaWat—Megadroughts in the Water Towers of Europe—From Process Understanding to Strategies for Management and Adaptation.”
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Project and funding information The present study was conducted within the scope of the EMERGE Project of the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) with Professor Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA) serving as its Principal Investigator. The research was supported by funding from the Extreme Program of the WSL for the EMERGE project.
The Yeso reservoir in central Chile during a megadrought peak in Summer 2020.
Drone Video from Chile, 2017.The Upper Rio Yeso catchment: a tributary to the Maipo River which serves the Chilean capital, Santiago.
The Yeso region in Central Chile, visibly dry during a peak of the megadrought in the Summer of 2020Facebook
Dead vines in the region around Los Andes in the western catchment area of Aconcagua, a region that has been particularly hard hit by the ongoing drought in Chile.
Global increase in the occurrence and impact of multiyear droughts
Article Publication Date
17-Jan-2025
Multiyear “megadroughts” becoming longer and more severe under climate change
Summary author: Walter Beckwith
American Association for the Advancement of Science (AAAS)
Severe droughts are becoming hotter, longer, and increasingly devastating to ecosystems as climate change accelerates, according to a new study, which reports that temperate grasslands, including in parts of the United States, are facing the worst effects. The findings provide a global quantitative understanding of multiyear droughts (MYDs) – prolonged events lasting years or decades – and offer a benchmark for understanding their global trends and impacts. As droughts become more frequent worldwide, the likelihood of MYDs rises, posing severe threats to both natural ecosystems and human systems. These events deplete soil moisture and reduce streamflow, potentially causing irreversible damage, with consequences that include widespread crop failures, increased tree mortality, diminished ecosystem productivity, and reduced water supplies. Recent MYDs, like the Western U.S. drought (2000–2018) and Chile's ongoing drought (since 2010), have drawn significant concern due to their devastating impacts. However, less prominent MYDs, with fewer perceived consequences, may have gone unnoticed, highlighting the need for comprehensive knowledge of their occurrence and effects. To address this need, Liangzhi Chen and colleagues mapped the global distribution of 13,176 MYDs from 1980 to 2018 using the standardized precipitation evapotranspiration index (SPEI), which measures drought severity based on the balance between precipitation and potential evaporative demand. Chen et al. also assessed these events’ impacts on vegetation through remotely sensed greenness data. The findings show that over the last 40 years, MYDs have occurred on nearly every continent and have become increasingly widespread, hotter, and drier, with global land affected by these events increasing by 49,279 ±14,771 square kilometers annually. What’s more, the authors report that the ecosystem impacts of MYDs varied significantly, with temperate grasslands exhibiting greater declines in greenness compared to subtropical and tropical forests. While grasslands demonstrated low resistance to drought, they often displayed rapid recovery following drought events, highlighting their unique resilience dynamics. In a related Perspective, David Hoover and William Smith discuss the study and its findings in greater detail.
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