Many harsh realities of climate change are kicking in around the world, including extreme weather and sea level rise. But scientists now warn that there’s an overlooked “time bomb” on the horizon as temperatures warm—the global groundwater supply.

Groundwater is moisture in soil and rock, fed by precipitation and stored in aquifers. It is the largest source of freshwater on Earth, and supplies two billion people with water for drinking and crop production.

But over the next 100 years, climate-related rainfall changes could disrupt the process of “recharge,” the term for groundwater replenishment, in an estimated 44 percent of aquifers on the planet, according to a study published Monday in Nature Climate Change.

That means nearly half of the planet’s aquifers are projected to be depleted to varying degrees within a century, which could reduce water access for millions of people. Climate change will also interfere with the remaining aquifers on timescales longer than a century, said the authors, led by Mark Cuthbert, a groundwater expert at Cardiff University.

"The effect we are having now is going to have this really long lag-time in terms of climate change,” Cuthbert told AFP.

"This could be described as an environmental time bomb because any climate change impacts on recharge occurring now, will only fully impact the baseflow to rivers and wetlands a long time later,” he said.

Cuthbert and his colleagues mapped out the global groundwater supply and modeled regional responses to climate change over various timescales. The team called the variations in response time “hydraulic memory.”

They found that humid areas, like the Amazon Basin or the Florida Everglades, are more likely to be sensitive to recharge problems in the short term. Aquifers in arid areas, like the Sahara Desert, have a long hydraulic memory and may take millennia to respond to current climate shifts.

It may seem counterintuitive that groundwater aquifers in arid regions, which are by definition parched, are less sensitive to climate change. But according to Cuthbert’s team, extreme flooding and drought in humid areas has a more immediate effect on aquifers because the water table in those regions is close to the surface.

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Desert aquifers are normally deeper underground, allowing them to remain more independent from changes on the surface (except in areas with heavy irrigation). The downside is that current climate fluctuations could start a slow chain reaction that throws off groundwater recharge patterns in arid regions millennia from now.

"Parts of the groundwater that's underneath the Sahara currently is still responding to climate change from 10,000 years ago when it was much wetter there," Cuthbert said. "We know there are these massive lags."

The climate crisis understandably sparks concern about the world our children and grandchildren will inherit. These new insights about the hydraulic memory of groundwater are a reminder that our actions will also have consequences far beyond these familiar generations.