Scientists have long cautioned that warming temperatures would lead to wetter and drier global extremes — increasingly severe rainfall, more intense droughts. A new study shows where that may already be happening.

Global intensity of

wet and dry extremes

Source: Rodell and Li, Nature Water (2023), based on analysis of NASA Grace and Grace-FO data. Note: Multi-year events are assigned to the year with peak intensity.

The study provides an emerging picture of distortions in the total amount of water both above ground and also in aquifers deep beneath the Earth’s surface, where most of the freshwater that humans depend upon comes from.

It relies on data from NASA’s Gravity Recovery and Climate Experiment mission, known as Grace, which uses satellites that can detect changes in gravity to measure fluctuations in water where other satellites can’t see. That way, it can provide information about locations where there are otherwise no gauges or wells.

“For most of the world, we just don’t have data on how groundwater storage is changing,” Matthew Rodell, the deputy director of earth sciences at NASA Goddard, said. “Grace sort of breaks those boundaries and provides information everywhere.”

In a paper published Monday in the journal Nature Water, Dr. Rodell and Bailing Li, an assistant research scientist at the University of Maryland, analyzed the satellite data to measure water-cycle extremes. They uncovered 505 wet and 551 dry episodes between 2002 and 2021, then assigned each one an “intensity,” in order to rank them. The intensity rankings took into account the severity of an episode as well as its duration and the amount of land area affected.

Most Intense Dry Events,

2002 to 2021

Aug. 2015–Jan. 2017

Record-breaking heat

amplified by El Niño

fueled megafires

across the country.

2. Southern and Central Africa

Jan. 2005–Nov. 2006

The dry spell caused an

acute food shortage, with

mass livestock die-offs.

3. Southeast Brazil

March 2020–Dec. 2021

Much of the region

remains in drought.

4. Russia and Central Asia

Sept. 2011–Jan. 2014

Russia, a global wheat

exporter, suffered huge

crop losses due to drought.

July 2018–April 2021

Drought across Europe

returned with the La Niña

that lasted through 2022.

6. Northwest Canada and Alaska

Feb. 2017–May 2020

Unusually hot, dry

weather plagued

Alaska, disrupting local

vegetation and wildlife.

One aspect of the Grace data is that it measures changes that persist over longer periods of time. In effect it tracks slower-moving disasters that unfold over months or years, not momentary flash floods during an otherwise normal season.

Most Intense Wet Events,

2002 to 2021

March 2019–Dec. 2021

Lakes overflowed,

flooding homes

and farmland.

Aug 2018–April 2021

Slow-moving but

widespread floods

submerged the

midwest in 2019.

March 2010–Nov. 2012

Rainfall displaced so

much water, it reversed

global sea level rise for

18 months.

Dec. 2008–May 2010

Hundreds of

thousands were

left homeless.

Sept. 2016–June 2019

Multiple flash floods

overwhelmed towns

across Russia over

several months.

6. Southern Africa

Nov. 2010–March 2013

Increased rainfall

linked to La Niña

displaced thousands.

Dr. Rodell and Dr. Li initially set out to rank the worst droughts and periods of increased rainfall over the past 20 years of available satellite observations. When reviewing the results, however, they soon realized both types of events were more common — and were growing more severe — toward the end of the study period.

To see if global warming could be behind the changes, the researchers compared the correlation of monthly wet and dry intensities with global average temperatures and other known climate factors.

They found that global average temperatures had a more significant correlation than the other indicators, including El Niño, the occasional shift in Pacific Ocean water temperatures that can have significant effects on heat and precipitation. The finding strengthens the possibility that, as the world warms, we’ll see more frequent and stronger extremes.

A few regions stood out. The tropics are experiencing more intense wet spells, and continental regions are seeing a trend toward drought.

However, 20 years of observations is short in terms of climate time scales. Simply looking for correlations like this “is going to be limited in its ability to tease these things apart. It’s not trivial to do,” said John Fasullo, a scientist at the National Center for Atmospheric Research, who wasn’t involved in the study.

There are other limitations to the Grace data. Global measurements are monthly and are published with a delay, making it impractical for tracking events as they unfold. The data also has an effective resolution about the size of the state of Illinois, which isn’t ideal for seeing changes on a more local level.

The analysis also excludes regions known to be undergoing long-term human impacts or persistent ice melt that were identified in a previous study led by Dr. Rodell. They include California’s Central Valley, where agricultural use has depleted aquifers, and Greenland, where the ice sheet has melted.

Groundwater changes, particularly at the extremes, remains a topic in need of further study, especially to analyze the influence of global warming. But for precipitation over shorter time periods, the relationship is more clear. “One of the robustly detected aspects of water cycle extremes is the increase of intense precipitation with climate change,” said Dr. Fasullo.