Flooding off SH16 Waimauku after Cyclone Gabrielle hit New Zealand. Photo / Michael Craig
Flooding off SH16 Waimauku after Cyclone Gabrielle hit New Zealand. Photo / Michael Craig
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.
The study provides an emerging picture of distortions in the total amount of water both above ground andalso in aquifers deep beneath 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, or 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,” said Matthew Rodell, deputy director of earth sciences at Nasa Goddard. “GRACE sort of breaks those boundaries and provides information everywhere.”
In a paper published last week in the journal Nature Water, Rodell and Bailing Li, an assistant research scientist at the University of Maryland, analysed the satellite data to measure water-cycle extremes. They uncovered 505 wet episodes 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.
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.
Rodell and 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 realised 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 Nino, 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 such as 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.
Graphic / The New York Times
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 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,” Fasullo said.
