A new study sheds light on another impact of the climate crisis—a "time bomb" for the world's groundwater reserves.
It's a key issue, as roughly two billion people worldwide rely on groundwater as their main source of freshwater, and many of these reserves are already being overdrawn.
In contrast to surface water, groundwater is stored beneath the ground's surface, held in porous rock, sand, and soil. That water seeps out, or "discharges," into waterways. The groundwater is also replenished in what is called "recharge" when precipitation falls. As such, a balance is created. But events like drought or extreme downpours—features of a warming planet—have an impact on restoring that balance.
Assessing groundwater model results along with hydrologic data sets, the team of researchers behind the new study found very differing timescales for how groundwater is going to respond to climate change, with groundwater in wetter areas expected to experience a far shorter response time than reserves in more arid areas.
"Our research shows that groundwater systems take a lot longer to respond to climate change than surface water, with only half of the world's groundwater flows responding fully within 'human' timescales of 100 years," said lead author Mark Cuthbert of Cardiff University's School of Earth and Ocean Sciences and Water Research Institute in a statement.
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While some regions could bounce back in less than 10 years, including parts of central Africa and the U.S. Midwest, other areas—including Australia, the Sahara region, and parts of the central U.S.—were shown to have response times of thousands of years.
"This means that in many parts of the world," Cuthbert continued, "changes in groundwater flows due to climate change could have a very long legacy. 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."
Speaking to Agence France-Presse, Cuthbert added, "Groundwater is out of sight and out of mind, this massive hidden resource that people don't think about much yet it underpins global food production."
The authors conclude, in part, that it "is critical therefore that climate change adaptation strategies that shift reliance to groundwater in preference to surface water should also take account of lags in groundwater hydrology and include appropriately long timescale planning horizons for water resource decision making."
The study was published Monday in the journal Nature Climate Change.