A new study sheds light on another impact of the climate crisis—a “time bomb” for the world’s groundwater reserves.

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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.