Access to groundwater is crucial to life on Earth, but rising temperatures could render it undrinkable for 75 million people within the next 76 years.

In a new study, researchers at the Karlsruhe Institute of Technology (KIT) based their projections on two climate scenarios, SSP 2–4.5 and SSP 5–8.5, which represent different socioeconomic pathways and greenhouse gas concentrations.

SSP 2–4.5 falls in the midrange of future greenhouse gas trends, while SSP 5–8.5 represents the upper extreme.

The study indicates that by 2100, groundwater temperatures will rise by 35.78 degrees Fahrenheit in one scenario and 38.3 degrees Fahrenheit in the other.

"Our results show how important it is to take action to protect groundwater and find lasting solutions to counteract the negative impact of climate change on groundwater," study author Dr. Susanne Benz, from the Institute of Photogrammetry and Remote Sensing at KIT, said in a statement.

"Depending on the scenario, as many as several hundred million people could be affected by 2100. There are already about 30 million people living in regions where the groundwater is warmer than stipulated in the strictest drinking water guidelines," Benz said.

"That means it may not be safe to drink the water there without treatment. It may need to be boiled first, for example. The drinking water also gets warmed up in water pipes by heat in the ground."

The research suggests that by 2100, due to varying climate changes and population trends, anywhere from 77 to 188 million people could be affected by SSP 2–4.5 and 59 to 588 million under SSP 5–8.5.

The temperature of groundwater plays a crucial role in water quality by influencing chemical, biological, and physical processes.

Warmer groundwater also affects groundwater-dependent ecosystems, aquatic biogeochemical processes, geothermal energy potential, and river thermal regimes. Moreover, it poses challenges to biodiversity and risks to carbon and nutrient cycles.

"Under certain conditions, rising groundwater temperatures can lead to increasing concentrations of harmful substances like arsenic or manganese. These higher concentrations can have a negative impact on human health, especially when groundwater is used as drinking water," Benz said.

Earth's climate is warming as greenhouse gases trap heat in the atmosphere, limiting its release. Oceans absorb much of this heat, yet soil and groundwater also serve as significant reservoirs for thermal energy.

However, there is currently limited understanding of how Earth's surface warming affects groundwater—the largest unfrozen freshwater reserve—across different regions and over time.

"To close this gap, we have simulated the projected changes in global groundwater temperatures through 2100. We can provide maps showing global groundwater temperatures at various depths beneath the Earth's surface. The maps show that the world's highest groundwater warming rates can be expected at locations with a shallow groundwater table and/or high atmospheric warming," Benz said.

The researchers discovered that mountainous regions with deep water tables, such as the Andes and the Rocky Mountains, are expected to experience the least amount of warming.

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