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.
