New research from Dartmouth College confirms something climate scientists in the Northeast have speculated about for some time: Rainfall is happening in shorter, more intense bursts with longer dry spells in between storms.
Climate change is a leading driver of this shift, said author Justin Mankin, a professor of geography at Dartmouth. The paper was published recently in the science journal Nature.
“As we concentrate rainfall, we're asking the land surface of Vermont to drink from a fire hose, and it just cannot do that,” Mankin said.
This trend is leading to more flooding and more frequent “flash” droughts between storms, like the one Vermont saw last fall when groundwater levels dropped and many wells in the state ran dry.
"We're asking the land surface of Vermont to drink from a fire hose, and it just cannot do that.”Justin Mankin, professor of geography at Dartmouth College
Until now, Mankin said, much of the established science assumed that how much precipitation a place receives is the biggest driver of how dry the lakes, streams, soil and groundwater there will be. This study upends that.
Among the key findings, Mankin said, is that when and how frequently rain falls is as big a cause of drought as how much precipitation a place sees overall.
Mankin and his coauthor Corey Lesk, a professor of atmospheric science at the University of Quebec in Montreal who was a postdoctoral candidate at Dartmouth at the time of the study, looked at precipitation records over the last 40 years from around the world.
Using a combination of statistical analysis and satellite imagery, they mapped how projected changes in rainfall patterns would affect how water accumulates on the landscape, in river and reservoir levels, soil moisture and groundwater.
They found that if the earth warms by 2 degrees Celsius, roughly a third of the world’s population will be forced to contend with abnormally dry conditions because of rain consolidating, regardless of whether rainfall increases overall.
“What we found is something pretty counterintuitive, which is that the more precipitation gets concentrated into stronger events, separated by longer dry spells — which is kind of what we expect with global warming — the less of that rain tends to stick around on the land,” Lesk said.
This, Lesk said, makes that water less available to ecosystems that have evolved to take it up slowly. It leads to flooding, erosion and water pooling on the land’s surface, where it’s in turn more likely to evaporate at a faster rate because of warmer temperatures driven by climate change.
"That’s why you can have a flood, owing to these very, very heavy rain days, even if it sets up a drought later in the summertime.”Justin Mankin, professor of geography at Dartmouth College
All of this makes the earth’s surface less resilient to shorter dry spells, and creates uncertainty about whether increased rainfall will lead to wetter conditions.
The northeastern United States and southern Quebec are expected to see more rain due to climate change, but more of that rain will likely fall in large storms like the ones that have caused flooding in recent years.
However, the fraction of that water that the landscape can hold onto is going down.
“New England is quite fortunate that the projections suggest additional precipitation, even while the character of that precipitation is going to be delivered in heavier downpours,” Mankin said. “You can have these two things conspiring, and that’s why you can have a flood, owing to these very, very heavy rain days, even if it sets up a drought later in the summertime.”
The researchers say regardless, this marks a stark shift away from how rain has historically fallen in New England and southern Quebec: in small, frequent increments throughout the year.
Mankin said this could mean it takes longer for groundwater to recharge after a dry spell, necessitating deeper wells and reservoirs. It also means Vermont will need to continue to adapt to bigger and more frequent floods.
He and Lesk say more research is needed to understand the impact of more precipitation falling as rain rather than snow.
The researchers are looking next into whether this consolidating of rain is leading to more frequent flash droughts.
