More evidence shows multiyear events are increasing. One scientist said no one had characterized these protracted global droughts and their impacts — until now.
By Kasha Patel, The Washington Post, Jan. 16, 2025
More than 13,000 droughts spanning years have overwhelmed our planet across nearly four decades. These severe droughts are becoming hotter, longer and more devastating as our planet’s global temperature increases, new research has found.
Some of the worst drought effects appear in grassland areas — such as in eastern Australia as well as in the western United States, where Los Angeles is battling a spate of deadly and destructive wildfires. Additionally, researchers recently identified thousands of other multiyear droughts that may be overlooked on a global scale.
“We need awareness of these ‘new’ type of droughts, which last for more than one season or year,” said Francesca Pellicciotti, a hydrologist at the Institute of Science and Technology Austria and co-author of the study published Thursday. “Their effects are amplified — up to the point that some of these systems will not recover anymore.”
While there is more evidence that multiyear events are increasing, Pellicciotti said no one had characterized these long-term droughts across the globe and their impacts — until now.
The team analyzed 13,176 multiyear droughts from 1980 to 2018 through two perspectives: a meteorological standpoint, which largely looked at rainfall as well as temperature, and the ecological impact on vegetation, which helped the researchers gauge the severity.
They found that precipitation deficits in the top 100 multiyear droughts increased over the past four decades. The droughts were also getting hotter, in tandem with global average air temperatures also increasing in recent decades. The years with the largest droughts also followed El Niño events.
“Drought can propagate and reinforce themselves significantly,” said Liangzhi Chen, lead author of the study. “If a drought persists, it will persist longer.”
Still, assessing multiyear droughts and changes to them “nevertheless remains a challenge,” in part because of uncertainties in data and relatively limited information about soil moisture, said climate researcher Jason Smerdon, a professor at Columbia University who was not involved in the study.
Plus, not all droughts affect ecosystems the same way, the study found. Droughts in certain biomes — or geographic areas of certain climates, vegetation and animals — had bigger transformative effects on the land. Severe droughts in tropical rainforests were largely not as severe on vegetation as the ones that occurred in grasslands.
For example, one of the longest-lasting meteorological droughts in the study — persisting from 2010 to 2018 — covered nearly 1.5 million square kilometers in the eastern Congo basin. Another drought in the southwestern Amazon lasted nine years and covered nearly the size of the United Kingdom (213,000 square kilometers) at its peak.
Yet these droughts had relatively tame effects on their surrounding ecosystems when compared to others.
Measuring the health of vegetation using satellite data, the researchers found that droughts with the most impactful effects occurred in temperate grasslands — with hot spots in Mongolia, the western United States and eastern Australia.
In forests, Chen said, even a small change in vegetation can have significant effects. But the researchers found that the effects are greatly multiplied in grassland areas for longer-lasting dry spells. In fact, the productivity of grassland vegetation can decrease by three times as much in the second year of a drought compared with the first year.
The drought consequences are easy to spot in the western United States, which experienced some of the most severe drought by meteorological standards and effects on the surrounding ecosystem. As of Thursday, the Eaton and Palisades fires had destroyed thousands of structures and consumed more than 40,000 acres in nine days.
“When the drought gets longer, the vegetation will be drier and they are more flammable,” said Chen, a researcher at the Swiss Federal Research Institute WSL. “The whole system will be drier and soil will be drier. … It can somehow accelerate the fire to develop spatially.”
The study’s results “add to the growing consensus that we can expect larger, more frequent and more intense droughts as the planet warms,” said Matt Rodell, a hydrologist at NASA who was not involved in the study.
In a separate study, Rodell and his colleague showed a notable increase in the frequency of both major droughts and extreme wet events after 2015 — all of which have been among Earth’s top 10 warmest years on record.
Chen’s study stopped analyzing droughts after 2018, but Chen expects that the researchers would find additional intense droughts had occurred in recent years if they were to expand their dataset.
“This is an important piece of detective work that highlights how the character of droughts globally, such as their length and severity, is important to consider beyond just assessments of their occurrence,” Smerdon said.
https://www.washingtonpost.com/climate-environment/2025/01/16/multiyear-droughts-severe-western-grasslands-fires/