Humans need water. We need it for farming, bathing, washing clothes and, of course, drinking. We're not tardigrades after all. (They can without water for 10 years; we can only go for three days.)

Climate change is reshaping our world, and its effect on water are disastrous, including longer droughts, increased rainfall and making access to water more difficult. About 2 billion people get their water from the ground, but how climate change affects that water source hasn't been studied as much.

That access may be threatened, however, according to a study published in the journal Nature Climate Change, which found that more than half the world's groundwater systems could take 100 years to respond to changes in the environment. This could have drastic repercussions for how we live, from difficulty locating water to drink to curtailing the global food supply.

A vital resource

An aquifer at the edge of a field Some aquifers are close to the surface, like this one. (Photo: sheris9/Shutterstock)

Groundwater is, as its name implies, fresh water that's stored underground in aquifers. It arrived at these underground storage containers after trickling through soil and rock over the course of thousands of years. Rainfall and melting snow contribute to the recharge, or the replenishment, of groundwater, but some of this water goes into lakes, rivers and oceans before we pump it up to the surface. This helps to maintain the balance of the aquifers and the water system overall.

Some of these aquifers take an incredibly long time to recharge. Technically, groundwater is a renewable resource, but we shouldn't treat it as one, according to a 2015 study from Nature Geoscience, because only 6 percent of groundwater around the world is replenished over the course of a human lifetime.

Groundwater collects in a container on a farm Groundwater is an important facet of the global food supply. (Photo: PK photograph/Shutterstock)

Billions of people rely on groundwater. We bring it to the surface using pumps or we collect it from wells. We drink it, water crops with it and so much more. Water that we pull from closer to the surface is fresher than water from deeper in the ground, but closer-to-the-surface water is more prone to contamination and more vulnerable to drought. These are two risk factors that have increased with climate change.

And as our population increases, so does the demand on the food chain, which also relies on groundwater. Groundwater supplies are already being stressed. The 2015 study found that some communities in Egypt and in the U.S. Midwest are already tapping into those deeper aquifers to the get the water they need.

"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," Mark Cuthbert from Cardiff University's School of Earth and Ocean Sciences told Agence France-Presse. Cuthbert is one of the authors of the Nature Climate Change study.

Aquifers take a long time to adjust

Cuthbert and his fellow researchers used groundwater model results and hydrologic datasets to figure out how groundwater supplies were responding to changes in the climate.

What they found was that 44 percent of aquifers would struggle to recharge over the next 100 years due to climate change-influenced rainfall. Their models demonstrated that the shallower aquifers, the one that we rely on the most, would be hit particularly hard by these changes. In general, groundwater in wetter, more humid locations reacts to changes on a shorter timescale than more arid regions, like deserts. In wetter areas, the response time is much longer, at least from a human perspective.

This may seem weird, but things like droughts and floods can have more immediate effects on the wetter areas because those aquifers are closer to the ground than those in arid areas. These areas suffer the slings and arrows of climate change much more quickly and much more noticeably. Aquifers in some deserts, however, are still feeling the effects of changes in the climate from tens of thousands of years ago.

A well in the Sahara Desert in Morocco Aquifers in the Sahara don't respond quickly to change. (Photo: astudio/Shutterstock)

"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 to AFP. "We know there are these massive lags."

This lag means communities in arid areas won't experience the effects of contemporary climate change on their aquifers until generations from 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.

The researchers conclude that regions have to make plans for groundwater that take into account both the present and the future — change the plan makers won't be alive to see.

"There may also be initially 'hidden' impacts on the future of environmental flows required to sustain streams and wetlands in these regions," they wrote. "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."

Groundwater is an 'environmental time bomb'
Scientists report that it could take 100 years for the world's groundwater supplies to respond to climate change, potentially leaving populations without water.