Study: floods, droughts more intense in past 20 years due to higher global temperatures

Sunday, March 19, 2023

Wildfire during Brazil's dry season on September 20, 2015.
Image: Agência Brasil.

On Monday, Nature Water published a study by two NASA Goddard Space Flight Center scientists concluding the "strong" link between the global mean surface temperature and water-related extreme weather led the increase in the severity of events like floods and droughts in the past two decades.

Authors Matthew Rodell and Bailing Li wrote results suggest "rising temperatures may be driving an increase in the total intensity and related metrics of hydroclimatic extreme events that cannot be attributed to the sporadic occurrence of" regular climactic variations, such as the El Niño–Southern Oscillation.

They identified 1,066 regions that experienced periods of too much, or not enough rainfall from 2002 to 2021 with a new algorithm using observations from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellites.

This classified 505 'wet' clusters and 561 'dry' clusters, with an increasing severity of events in the seven years since 2015, which were recognized as among the warmest in history. From 2015 to 2021, the number of 'most extreme' events, among the 30 wettest or driest, increased to four from three per year.

Monday's study noted an increase in the wet-to-dry event ratio from 15° S to 20° N, and a decrease in three areas between 15° N and 50° N, supporting the hypothesis that wet and dry areas are inversely affected by extreme events.

The most intense dry event observed was a record-breaking South American drought from 2015-2016, while the greatest pluvial had been ongoing since 2002 through 2021, affecting most of sub-Saharan Africa above 10° S and leading to flooding and Lake Victoria to rise to its highest levels ever in 2020, inundating homes and infrastructure.

Rodell, the lead author, told the Associated Press (AP): "I was surprised to see how well correlated the global intensity was with global mean temperatures."

Speaking to CNN, he explained: "We thought, well, maybe this is related to global warming, because we do know the past seven or so years have been the hottest on record [...] Sure enough, there was a significant correlation between this total worldwide intensity of these events and the temperature record."

About the study's reliability, he said: "What I feel more confident about is that as the world warms, we’re going to see the greater global intensity of all the wet and dry events increase, meaning they’re going to be more frequent, larger and more severe in total [...] What happens in a regional sense is a little bit more difficult to say with certainty.

"The study is another way for people to recognize that climate change affects everyone [...] It’s not just about the temperature rising on average around the world, it’s the actual weather events that have severe impacts on people that may be increasing in intensity and frequency."

Richard Seager, a climate specialist of Columbia University's Lamont–Doherty Earth Observatory, told the AP: "Looking forward into the future, in terms of managing water resources and flood control, we should be anticipating that the wetter extremes will be wetter and the dry extremes will get drier [...] everything’s going to get amplified on both ends of the dry-wet spectrum."

On Rodell and Li's use of metrics from GRACE, he said: "This study uses novel [satellite] data to confirm that human-driven warming is driving the climate system to more extremes of extended both wet and dry episodes."

Park Williams, a University of California, Los Angeles bioclimatologist, told the AP: "It’s incredible that we can now monitor the pulse of continental water from outer space [...] I have a feeling when future generations look back and try to determine when humanity really began understanding the planet as a whole, this will be one of the studies highlighted."

Flooded streets in Accra, Ghana on May 14.
Image: Fquasie.

Meanwhile, CNN cited climatologist and Brown University Institute for Environment and Society Director Kim Cobb, who called the study "a new lens on our rapidly changing water cycle, linking many of the headlines about droughts, floods and wildfires to this global analysis.

"This finding really reinforces the trends that we see from analysis of rainfall data and climate model output, and in that sense adds significant evidence to inform emergency planning and response, infrastructure planning, agricultural practices, and water resource management under continued warming."

Rodell has been the Acting Deputy Director of Earth Sciences for Hydrosphere, Biosphere, and Geophysics at Goddard since 2021.

Li has been an assistant research scientist at the Hydrological Sciences Laboratory at Goddard, and at the University of Maryland, College Park since 2017.

In its 2021 report, the United Nations' Intergovernmental Panel on Climate Change reported a 70% increase in the frequency of droughts that would typically occur every ten years and a 30% increase in the frequency of comparable flooding.

Moreover, scientists have said global warming could amplify the occurrence of 'weather whiplash', rapid swings between extreme conditions like in the US state of California, which recently experienced heavy rain and snowstorms after a megadrought.

The National Integrated Drought Information System estimates 20% of annual extreme weather-induced losses in the US economy stem from droughts and floods.