Humans exist on a short leash. A person can only last around three days without drinking water. Put that way, human life is absurdly fragile; plenty of other organisms can go far longer. Just think of your houseplant. To make matters more precarious, that essential substance is growing harder to come by. You’ve heard about this: climate change and population growth is a devastating twofer.

But water scarcity doesn’t just apply to the water you can see. It isn’t just bathtub rings in Lake Mead. Of the seven and a half billion people currently residing on our fair planet, two billion rely on groundwater—water that’s seeped through the layers of soil and rock to become stored in underground deposits called aquifers. Some of this water is exceptionally old. Some of it did all its seeping when dinosaurs lumbered around. Filling some of these aquifers can take an extraordinarily long time, so on a timescale relevant to any human lifetime, much of it is not a renewable resource.

Some aquifers are closer to the surface, and can recharge more quickly. These respond fast to rainfall, for example; a good soak will replenish them. But being closer to the surface isn’t without its drawbacks. They’re more accessible to humans, so they’re already likely to be withdrawn faster than they can be filled up. And their proximity to the surface makes them more vulnerable to drought and contamination—two things on the rise amid climate change and population growth.

In a paper published this week in Nature Climate Change, scientists from the UK, Australia, Canada, Germany, France and the US used modeling and hydrological data sets to find how devastating climate change could be to aquifers. They found that climate-related changes to rainfall in the next century will make it harder for 44% of the world’s aquifers to recharge, particularly the shallower ones we rely on to fill up faster.

That means within the next 100 years, nearly half the world’s groundwater supply will become less reliable; since humanity’s need for water is certainly not diminishing, the newly deprived aquifers will begin to deplete—or deplete faster than they already are.

You can understand why this is a problem. Cities, states, countries, all want to be sure their future water supply will, well, be there. Remember, we only last three days.

In arid places like El Paso, Texas, rewriting water-use laws and a decades-long educational effort has managed to halt a decline in the aquifer it relied on. In many aquifer-reliant communities, engineers have designed ways to deliberately recharge their aquifer by storing water in seepage-friendly ponds. But based on this most recent study and previous research, it’s clear that those efforts will have to ramp up in the coming decades; falling levels in aquifers will be a more common crisis.

The water won’t forget about this

But in the same paper, the researchers surfaced yet another threat to our general safety as a water-drinking species. Some aquifers take such a long time to respond to changes like increased rainfall or more severe drought that they have yet to even register the climate change that’s already wreaking havoc on the above-ground world. To understand our true future water supply, they write, we must consider the “hydraulic memory of groundwater systems.”

This is an unexpected twist. It is also quite poetic. Our water systems will remember present-day climate change far longer than you or I, because they may take far longer to respond to it. Like mountains and canyons, underground water systems are a geologic thing. And geologic time moves differently, on a scale that makes a human lifetime laughable. Or more aptly, negligible.

“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,” Mark Cuthbert, a groundwater specialist at Cardiff University in the UK and lead author on the paper, told AFP. “We know there are these massive lags.”

That means the climate impacts of today will not show up in certain aquifers for a long time; how much water will be available in the future is very much being determined now.

“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,” Cuthbert said.