How Can We Measure Droughts and Deluges? Weigh the Planet.
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.
km3/month
Global intensity of
wet and dry extremes
30,000
20,000
Wet extremes
10,000
-10,000
Dry extremes
-20,000
2004
2006
2008
2010
2012
2014
2016
2018
2020
km3/month
30,000
Global intensity of
wet and dry extremes
20,000
Wet extremes
10,000
-10,000
Dry extremes
-20,000
2004
2006
2008
2010
2012
2014
2016
2018
2020
km3/month
Global intensity of
wet and dry extremes
30,000
20,000
Wet extremes
10,000
-10,000
Dry extremes
-20,000
2004
2008
2012
2016
2020
km3/
month
Global intensity of
wet and dry extremes
30,000
20,000
Wet extremes
10,000
-10,000
Dry extremes
-20,000
2004
2008
2012
2016
2020
km3/
month
Global intensity of
wet and dry extremes
30,000
20,000
Wet extremes
10,000
-10,000
Dry extremes
-20,000
2004
2008
2012
2016
2020
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
1. Brazil
2. Southern and Central Africa
3. Southeast Brazil
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
March 2020–Dec. 2021
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
Record-breaking heat
amplified by El Niño
fueled megafires
across the country.
Much of the region
remains in drought.
4. Russia and Central Asia
5. Europe
6. Northwest Canada and Alaska
Sept. 2011–Jan. 2014
July 2018–April 2021
Feb. 2017–May 2020
Unusually hot, dry
weather plagued
Alaska, disrupting local
vegetation and wildlife.
Russia, a global wheat
exporter, suffered huge crop
losses due to drought.
Drought across Europe
returned with the La Niña
that lasted through 2022.
Most intense dry events, 2002-2021
1. Brazil
2. Southern and Central Africa
3. Southeast Brazil
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
March 2020–Dec. 2021
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
Record-breaking heat
amplified by El Niño
fueled megafires
across the country.
Much of the region
remains in drought.
4. Russia and Central Asia
5. Europe
6. Northwest Canada and Alaska
Sept. 2011–Jan. 2014
July 2018–April 2021
Feb. 2017–May 2020
Unusually hot, dry
weather plagued
Alaska, disrupting local
vegetation and wildlife.
Russia, a global wheat
exporter, suffered huge
crop losses due to drought.
Drought across Europe
returned with the La Niña
that lasted through 2022.
Most Intense Dry Events, 2002 to 2021
1. Brazil
2. Southern and Central Africa
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
Record-breaking heat
amplified by El Niño
fueled megafires
across the country.
3. Southeast Brazil
4. Russia and Central Asia
March 2020–Dec. 2021
Sept. 2011–Jan. 2014
Russia, a global wheat
exporter, suffered huge crop
losses due to drought.
Much of the region
remains in drought.
5. Europe
6. Northwest Canada and Alaska
July 2018–April 2021
Feb. 2017–May 2020
Unusually hot, dry
weather plagued
Alaska, disrupting local
vegetation and wildlife.
Drought across Europe
returned with the La Niña
that lasted through 2022.
Most Intense Dry Events, 2002 to 2021
1. Brazil
2. Southern and Central Africa
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
Record-breaking heat
amplified by El Niño
fueled megafires
across the country.
3. Southeast Brazil
4. Russia and Central Asia
March 2020–Dec. 2021
Sept. 2011–Jan. 2014
Russia, a global wheat
exporter, suffered huge crop
losses due to drought.
Much of the region
remains in drought.
5. Europe
6. Northwest Canada and Alaska
July 2018–April 2021
Feb. 2017–May 2020
Unusually hot, dry
weather plagued
Alaska, disrupting local
vegetation and wildlife.
Drought across Europe
returned with the La Niña
that lasted through 2022.
Most Intense Dry Events, 2002 to 2021
1. Brazil
2. Southern and Central Africa
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
Record-breaking heat
amplified by El Niño
fueled megafires
across the country.
3. Southeast Brazil
4. Russia and Central Asia
March 2020–Dec. 2021
Sept. 2011–Jan. 2014
Russia, a global wheat
exporter, suffered huge crop
losses due to drought.
Much of the region
remains in drought.
5. Europe
6. Northwest Canada and Alaska
July 2018–April 2021
Feb. 2017–May 2020
Unusually hot, dry weather
plagued Alaska, disrupting
local vegetation and wildlife.
Drought across Europe
returned with the La Niña
that lasted through 2022.
Most Intense Dry Events, 2002 to 2021
1. Brazil
2. Southern and Central Africa
Aug. 2015–Jan. 2017
Jan. 2005–Nov. 2006
Record-breaking heat
amplified by El Niño fueled
megafires across the country.
The dry spell caused an
acute food shortage, with
mass livestock die-offs.
3. Southeast Brazil
4. Russia and Central Asia
March 2020–Dec. 2021
Sept. 2011–Jan. 2014
Russia, a global wheat
exporter, suffered huge
crop losses due to drought.
Much of the region
remains in drought.
5. Europe
6. Northwest Canada and Alaska
July 2018–April 2021
Feb. 2017–May 2020
Drought across Europe
returned with the La Niña
that lasted through 2022.
Unusually hot, dry weather
plagued Alaska, disrupting
local vegetation and wildlife.
Most Intense Dry Events,
2002 to 2021
1. Brazil
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.
5. Europe
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
1. Central Africa
2. North America
3. Australia
March 2019–Dec. 2021
Aug 2018–April 2021
March 2010–Nov. 2012
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Lakes overflowed,
flooding homes
and farmland.
4. South America
5. Eastern Russia
6. Southern Africa
Dec. 2008–May 2010
Sept. 2016–June 2019
Nov. 2010–March 2013
Increased rainfall
linked to La Niña
displaced thousands.
Multiple flash floods
overwhelmed towns
across Russia over
several months.
Hundreds of
thousands were
left homeless.
Most Intense Wet Events, 2002 to 2021
1. Central Africa
2. North America
3. Australia
March 2019–Dec. 2021
Aug 2018–April 2021
March 2010–Nov. 2012
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Lakes overflowed,
flooding homes
and farmland.
4. South America
5. Eastern Russia
6. Southern Africa
Dec. 2008–May 2010
Sept. 2016–June 2019
Nov. 2010–March 2013
Increased rainfall linked to La Niña displaced thousands.
Multiple flash floods overwhelmed towns across Russia over several months.
Hundreds of thousands were left homeless.
Most Intense Wet Events, 2002 to 2021
1. Central Africa
2. North America
March 2019–Dec. 2021
Aug 2018–April 2021
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Lakes overflowed,
flooding homes and
farmland.
4. South America
3. Australia
Dec. 2008–May 2010
March 2010–Nov. 2012
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Hundreds of
thousands were
left homeless.
5. Eastern Russia
6. Southern Africa
Sept. 2016–June 2019
Nov. 2010–March 2013
Increased rainfall
linked to La Niña
displaced thousands.
Multiple flash floods
overwhelmed towns
across Russia over
several months.
Most Intense Wet Events, 2002 to 2021
1. Central Africa
2. North America
March 2019–Dec. 2021
Aug 2018–April 2021
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Lakes overflowed,
flooding homes
and farmland.
4. South America
3. Australia
Dec. 2008–May 2010
March 2010–Nov. 2012
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Hundreds of
thousands were
left homeless.
5. Eastern Russia
6. Southern Africa
Sept. 2016–June 2019
Nov. 2010–March 2013
Increased rainfall
linked to La Niña
displaced thousands.
Multiple flash floods
overwhelmed towns
across Russia over
several months.
Most Intense Wet Events, 2002 to 2021
1. Central Africa
2. North America
March 2019–Dec. 2021
Aug 2018–April 2021
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Lakes overflowed,
flooding homes
and farmland.
4. South America
3. Australia
Dec. 2008–May 2010
March 2010–Nov. 2012
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Hundreds of
thousands were
left homeless.
5. Eastern Russia
6. Southern Africa
Sept. 2016–June 2019
Nov. 2010–March 2013
Increased rainfall
linked to La Niña
displaced thousands.
Multiple flash floods
overwhelmed towns
across Russia over
several months.
Most Intense Wet Events, 2002 to 2021
1. Central Africa
2. North America
March 2019–Dec. 2021
Aug 2018–April 2021
Slow-moving but
widespread floods
submerged the
midwest in 2019.
Lakes overflowed,
flooding homes
and farmland.
4. South America
3. Australia
Dec. 2008–May 2010
March 2010–Nov. 2012
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
Hundreds of
thousands were
left homeless.
5. Eastern Russia
6. Southern Africa
Sept. 2016–June 2019
Nov. 2010–March 2013
Multiple flash floods
overwhelmed towns
across Russia over
several months.
Increased rainfall
linked to La Niña
displaced thousands.
Most Intense Wet Events,
2002 to 2021
1. Central Africa
March 2019–Dec. 2021
Lakes overflowed,
flooding homes
and farmland.
2. North America
Aug 2018–April 2021
Slow-moving but
widespread floods
submerged the
midwest in 2019.
3. Australia
March 2010–Nov. 2012
Rainfall displaced so
much water, it reversed
global sea level rise for
18 months.
4. South America
Dec. 2008–May 2010
Hundreds of
thousands were
left homeless.
5. Eastern Russia
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.