"Global warming is reflected directly in the groundwater, albeit damped and with a certain time lag," said Peter Bayer, senior assistant at ETH Zurich's Geological Institute in Switzerland.
The data reveals that the groundwater close to the surface, down to a depth of around 60 metres has warmed up significantly in the course of global warming over the last 40 years.
This water heating follows the warming pattern of the local and regional climate which, in turn, mirrors the pattern of global warming.
The groundwater reveals how the atmosphere has made several temperature leaps at irregular intervals.
"We were quite surprised at how quickly the groundwater responded to climate change," Bayer added.
For their study, researchers were able to fall back on uninterrupted long-term temperature measurements of groundwater flows around the cities of Cologne and Karlsruhe in Switzerland, where local waterworks have been measuring the temperature of the groundwater, which has been largely uninfluenced by humans for 40 years.
The earth's atmosphere has warmed up by an average of 0.13 degrees Celsius per decade in the last 50 years.
According to Bayer, it is plausible that the natural groundwater flow is also warming up in the course of climate change.
The study appeared in the journal Hydrology and Earth System Sciences.