Confronting the Crisis: The Era of Megadroughts Begins

Chile has experienced a staggering fifteen-year megadrought, the longest in a millennium, which has significantly depleted the country’s water reserves and even impacted essential mining operations. This case highlights how rising temperatures are contributing to lengthy droughts and severe water shortages in vulnerable areas around the world. Droughts are often recognized only when they harm agriculture or visibly damage forests, prompting essential questions: Can we reliably track extreme multi-year droughts and assess their effects on ecosystems? What insights can we gain from the drought trends of the past four decades?

To investigate these questions, the researchers analyzed global meteorological data and modeled drought occurrences from 1980 to 2018. Their findings show a concerning increase in the duration, frequency, and intensity of multi-year droughts affecting a wider area. “Every year since 1980, the drought-affected regions have expanded by about fifty thousand square kilometers on average — roughly equivalent to the size of Slovakia, or the combined area of the US states of Vermont and New Hampshire — resulting in significant harm to ecosystems, agriculture, and energy production,” explains ISTA Professor Francesca Pellicciotti, the main investigator for the WSL-funded EMERGE Project behind the study. The researchers aim to reveal the potential long-term consequences of ongoing droughts globally while supporting policymakers in preparing for more frequent and severe megadroughts in the future.

Revealing unnoticed extreme droughts

The international team utilized CHELSA climate data, prepared by WSL Senior Researcher Dirk Karger and extending back to 1979. They identified anomalies in rainfall and evapotranspiration — the evaporation of water from soil and plants — and assessed their effects on ecosystems globally. This approach allowed them to recognize multi-year drought occurrences in both well-researched and harder-to-access areas of the world, especially in regions like tropical forests and the Andes where observational data are scarce. “Our method not only mapped well-known droughts but also illuminated extreme droughts that had been overlooked, such as the one that impacted the Congo rainforest from 2010 to 2018,” Karger notes. The variation in drought responses among forests across different climates may explain this oversight. “While temperate grasslands have faced the most consequences in the past forty years, boreal and tropical forests seem to withstand drought better and even exhibit unexpected effects during initial drought stages.” However, how long can these forests endure the pressures of climate change?

Diverse effects on ecosystems

The combination of rising temperatures, prolonged droughts, and increased evapotranspiration has led to drier ecosystems, with some areas exhibiting browning patterns despite experiencing heavier rainfall at times. To track drought impacts effectively, scientists have been utilizing satellite imagery to observe changes in vegetation greenness over time. This method proves effective for temperate grasslands, but tracking greenness over dense tropical forest canopies is more challenging, which can lead to underestimating drought impacts in those regions. To ensure more consistent results globally, the team devised a multi-step analysis that better addresses changes in heavily vegetated areas and categorized droughts by severity since 1980. As expected, megadroughts had the most immediate detrimental effects on temperate grasslands. ‘Hotspot’ areas included the western United States, central and eastern Mongolia, and especially southeastern Australia, coinciding with two well-documented ecological droughts. Conversely, the study also explored the paradoxical effects observed in tropical and boreal forests. Tropical forests can mitigate anticipated drought impacts if they possess sufficient water reserves to counterbalance the decrease in rainfall, whereas boreal forests and tundras exhibit different reactions — with the warmer climate allowing for an extended growing season, as plant growth is primarily limited by temperature rather than water availability.

The evolution of droughts over time and space

The results clearly illustrate an ongoing trend of escalating megadroughts: the team created the first comprehensive global representation of megadroughts and their impact on vegetation at high resolution. However, the long-term ramifications for the planet and its ecosystems largely remain uncharted waters. Meanwhile, the data aligns with the widely observed greening in the pan-Arctic region. “However, with persistent extreme water shortages, trees in tropical and boreal areas could die, leading to lasting damage in these ecosystems. Particularly, boreal vegetation may take a long time to recover from such a climatic crisis,” Karger warns. Pellicciotti hopes the findings will reshape perceptions of droughts and enhance preparedness: “Currently, mitigation strategies primarily see droughts as seasonal events, which starkly contrasts with the longer and more severe megadroughts we will encounter in the future,” she states. “We aim for the publicly available drought inventory to aid policymakers in developing practical preparation and prevention strategies.” As a glaciologist, Pellicciotti is also keen to explore the impact of megadroughts on mountainous regions and their glaciers’ potential role in mitigating drought effects. She leads a collaborative initiative titled “MegaWat — Megadroughts in the Water Towers of Europe — From Understanding Processes to Strategies for Management and Adaptation.”

Project and funding information The current study was conducted as part of the EMERGE Project by the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), with Professor Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA) as the Principal Investigator. The research received financial support from the Extreme Program of WSL for the EMERGE project.