A Dartmouth study shows that annual rainfall in much of the world has consolidated over the past four decades into heavier storms with longer dry periods in between.
The findings are the first to show that a year's worth of rainfall packed into bigger and wetter storms means less water for aquifers and ecosystems, even if total precipitation increases. Because soil can absorb only so much water at once, what is not soaked up collects on the surface where it's more readily evaporated.
"It doesn't matter where you are, more consolidated rainfall means less water is available for the land. We show that this phenomenon is consistent worldwide, what physically accounts for it, and what we should expect going forward," says Justin Mankin , the study's senior author and an associate professor of geography at Dartmouth.
"Rainfall concentration is almost as important to land wetness as how much rainfall you get in a year," says first author Corey Lesk , who led the study as a Neukom Postdoctoral Fellow at Dartmouth in Mankin's Climate Modeling and Impacts Group .
"There are only so many days of the year when rain can fall, and if more of it is going back into the atmosphere, there's not much we can do to recapture it," Lesk says.
Lesk and Mankin analyzed global precipitation records from 1980 to 2022 and found that annual rainfall has become more concentrated regardless of whether the local climate is wet or dry, they report in Nature .
The study projects that rainfall will grow more consolidated as global temperatures rise due to climate change. An increase of 2 degrees Celsius (3.6 degrees Fahrenheit) could lead to abnormally dry land conditions for 27% of the world's population, offsetting any rise in total rainfall, Lesk and Mankin report.
"This is not a good effect we've uncovered," says Lesk, who is now a professor of Earth and atmospheric sciences at the University of Quebec in Montreal. "It really exposes the mechanics of how climate change will affect water resources for everyone."
Lesk and Mankin employed an economic tool called a Gini coefficient that is typically used to measure wealth inequality to capture how evenly precipitation fell for a region during a given year. The scale ranged from zero, or equal daily precipitation, to one, meaning all annual precipitation fell in a single day. The higher a region's Gini coefficient, the less evenly distributed its precipitation is.
The United States west of the Mississippi River experienced some of the world's highest levels of rain consolidation, with yearly rainfall for the Rocky Mountains becoming 20% more compacted into heavier downpours, the study finds. Rainfall in South America's Amazon River basin grew 30% more concentrated into heavy storms and longer dry spells, the largest change worldwide since 1980.
The Arctic, Northern Europe, and Canada exhibited as much as a 20% decrease in rain consolidation, meaning that precipitation became more evenly distributed between 1980 and 2022. While that would be a good thing for most of the world, that change likely reflects an increase in snow and rain year-round as these regions become warmer due to climate change.
Southeast Asia, which derives its rain from seasonal monsoons, also saw rainfall spread out more throughout the year, though the researchers are uncertain as to why, Lesk says.
But Southeast Asia and the northern latitudes could see a reversal back to more sporadic rain and longer periods of dryness, the study finds. Lesk and Mankin's climate models project that these regions will see the highest increases in rain consolidation with each degree of global warming.
"There are many reasons physically and socioeconomically to expect that a world with global warming is going to be a much more unequal world," Mankin says. "Precipitation, like wealth, exhibits a highly unequal distribution in the present day, and the expectation is that with global warming, inequality in both the economy and precipitation will increase."
The study presents a new way of thinking about water resources by showing that how and when rain falls during the year is as important as how much falls all year, Mankin says. Climate scientists project that a warmer climate will result in more rain, but it's been less certain if that means more water for the land, he says.
"From a hydrology perspective, we've long believed that what matters is how much precipitation a place gets, less its demand by ecosystems and the atmosphere," Mankin says.
"We discovered that it's not just supply that counts, but also how it's delivered. Rainfall concentration is essentially asking the land to drink from a firehose," he says. "When rainfall is intense, you get more consecutive dry days, but more important is that heavier rains lead to surface ponding that is more easily evaporated by the atmosphere."
An erratic boom-bust cycle of heavy rainfall and long droughts will complicate the management of public water supplies, Mankin says, especially in arid regions where water storage is critical.
California has faced this conundrum in recent years during long-term droughts when atmospheric rivers have drenched the state, he says. Water managers must decide whether to release precious reservoirs of water to collect freshly fallen rainwater with no certainty of how long the new supply will last.
That same calculation could take hold in regions such as the northeastern United States that have relied on a relatively equitable distribution of year-round precipitation. The Nature study shows that expectations for future water supply are increasingly patchier, Mankin says.
"The acceleration of rainfall consolidation raises the imperative to conceive of ways to deal with the simultaneous flood and long-term drought risks. Places we don't typically think of as needing reservoir storage may need it in the future," Mankin says.
"Consolidation of rainfall under global warming will lead to a drier land surface," he says. "What is unresolved is whether future total precipitation changes can keep apace."
