Intense drought and heavy rainfall events occurred more often in the last eight years — the hottest years on record — than in the previous decade, according to a new study released in Nature Water on Monday. Warmer global temperatures are increasing the extent, duration, and severity of these extremes, the authors found, and are having more of an effect than natural climate patterns.
“As the world warms, we’re having more intense and more frequent wet and dry events around the world, which gives us a little insight into what’s going to happen in the future,” said Matthew Rodell, a hydrologist at NASA and co-author of the study. “This is an observation. It’s actual data.”
Rodell said researchers have expected to see more droughts and floods in a warmer world based on climate model predictions, but “it’s been really hard to prove.” This new analysis, which uses direct NASA satellite observations, provides “indisputable” evidence that warmer global temperatures are increasing such extreme events, Rodell said.
The team analyzed 1,056 extreme events from 2002 to 2021, using observations from NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellites. The satellites detect subtle variations in Earth’s gravity field, which are used to measure water storage — including groundwater, soil moisture, snow, ice and surface waters — on land. Comparing current data to a longer-term average, the researchers can map anomalies and determine where water storage on land has increased or decreased. In this study, Rodell and NASA hydrologist Bailing Li used an algorithm that identified areas above or below average for a period of time by at least 16 percent.
While natural and recurring climate patterns, such as El Niño or La Niña, may have exacerbated some of these events, the team found that warmer global temperatures had a greater influence than other factors. Rodell said a correlation analysis of the extreme events found that no other factors were as “high as the correlation with the global mean temperature.”
The team found extreme dry and wet events have been increasing since 2002, but the most intense events have been occurring more frequently since 2015 — when Earth began its run of record-breaking warm years. An average of four extreme events occurred each year since 2015, compared to only three annual events over the previous 13 years.
The study adds to existing research, using rain gauge data, climate models and tree rings, on how a warmer atmosphere is affecting extreme wet and dry events, said climate scientist Daniel Swain, who was not involved in the study. Given the accumulation of evidence, he said it’s “probably not coincidental that the most extreme hydrologic events that you could observe in this record occurred during the warmest years of the record.”
“I think if this were just coming out of the blue and this is the only evidence we had that hydroclimate extremes were becoming greater in a warming climate, it wouldn’t be super strong evidence unto itself,” said Swain, a researcher at the University of California at Los Angeles. “But because it exists in the context of other research, I actually think it’s more interesting and more important.”
On average, the team found a wet or dry extreme event lasted about five to six months. About 70 percent lasted six months or less, and another 10 percent lasted a year or more. Globally, there were 10 percent more dry events than wet events. Wetter events tended to occur near the equator, while drier conditions occurred at the mid-latitudes.
Swain said the analysis provides almost direct measures of the amount of water on Earth’s surface at a given time and location, which is helpful because there is a “really incomplete rain observation network globally,” especially outside of wealthy nations.
The most intense event overall over the past two decades — dry or wet — was a wet event that engulfed all of central Africa, which began in 2019. Water levels at Lake Victoria rose by one meter and caused flooding in the surrounding region. It was three times as big as any other wet or dry event in their observations, Rodell said. “It’s just amazing how huge it was in terms of intensity,” he said.
The second most intense wet event occurred from 2018 to 2021 over much of central and eastern North America. During this time period, Rodell said parts of the United States were receiving a lot more rain and snow than normal. Aquifers, lakes and rivers were full and soil was generally moist. Some flooding occurred in the Midwest.
The most intense dry event observed was a short-lived but record-breaking drought in northeastern South America from 2015 to 2016, the study said. Three of the other most intense droughts have occurred in recent years as well, including drought in Brazil and the southwestern United States and recent drought across Europe.
Many of these extreme dry and wet events, including the flood in central Africa, drought in Brazil and the southwestern United States, are still ongoing in 2023.
The team did exclude some areas, such as central California and northern India, where people have pumped groundwater and depleted aquifers. Rodell said these depletions were caused more from direct human water management rather than meteorological drought.
“These findings not only verify model predictions, but also the ‘dry gets drier, wet gets wetter,’ hypothesis,” groundwater scientist Melissa Rohde wrote in a separate review article that appeared in Nature on Monday.
Seeing an increase in both dry and wet events may sound counterintuitive, but the physics are two sides of the same coin. During a dry event, the air is warmer and can drive more evaporation from the surface. During a wet event, a warmer atmosphere can hold more moisture (about 4 percent more for every degree Fahrenheit the atmosphere warms) and transport more moisture into an area.
Swain said that such projected increases in intense dry and wet events was largely a prediction but had not been confirmed in observations. Today, the evidence is strong.
“As the world warms, it’s fair to say that we may expect to see more frequent, more intense droughts and wet events,” said Rodell. “Here, we have the evidence that’s already happening.”