Groundwater is projected to warm by 2-3.5 degrees Celsius before the turn of this century, potentially risking water quality and safety, apart from threatening ecosystems depending on the resource, a new research has found.

The "world's first global groundwater temperature model" predicted the highest warming rates in Central Russia, Northern China and parts of North America, and the Amazon rainforest in South America.

A team of researchers, led by those from the Karlsruhe Institute of Technology, Germany, said while a lot of focus on climate change concerns weather events and water availability, we also need to think about how it impacts groundwater, critical to life on the planet.

Warming of groundwater can adversely impact ecosystems relying on them, they said.

"Rivers rely on groundwater to keep flowing during dry times. Warm waters hold less dissolved oxygen," explained study co-author Gabriel Rau from the University of Newcastle, UK.

The model also estimated that by 2100, 60-600 million people globally could be living in areas where groundwater exceeds the highest threshold for drinking water temperature guidelines set by any country.

According to the World Health Organization, currently only 18 out of 125 countries have temperature guidelines for drinking water.

Warmer groundwater raises the risk of disease-causing microbial growth, thereby, affecting drinking water quality and potentially the lives of people, said Rau.

"This is especially concerning in areas where access to clean drinking water is already limited, and in areas where groundwater is consumed without treatment," he added.

Based on how heat spreads in water, the researchers modelled current groundwater temperatures and also projected changes between 2000-2100 around the world. Groundwater is present beneath the Earth's surface in pore spaces in rocks and soils.

"We show that groundwater at the depth of the water table (excluding permafrost regions) is conservatively projected to warm on average by 2.1 degrees Celsius between 2000 and 2100 under a medium emissions (scenario)," the authors wrote in the study published in the journal Nature Geoscience. Under this scenario, emissions trends do not markedly change from historical patterns.

The model, however, showed that under a high-emissions scenario, or fossil fuel-driven development, the groundwater temperature could rise by 3.5 degrees Celsius.

The warming of groundwater can be a cause for concern, although it depends on whether the climate change issue can be mitigated by cutting down greenhouse gas emissions, the researchers said.

Warmer groundwater can also impact water quality by affecting its chemistry and microbiology, they said.

"If temperatures increase, we may see significant impacts to our local aquatic animals, including their spawning processes which will impact industries and communities that are reliant on these ecosystems," said co-author Dylan Irvine, Charles Darwin University, Australia.

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