Researchers used long-term temperature measurements of groundwater flows around the cities of Cologne and Karlsruhe, where the operators of the local waterworks have been measuring the temperature of the groundwater, which is largely uninfluenced by humans, for 40 years.

"For us, the data was a godsend," said Peter Bayer, a senior assistant at ETH Zurich's Geological Institute in Switzerland.

Evidently, it is less interesting or too costly for waterworks to measure groundwater temperatures systematically for a lengthy period of time.

Based on the readings, the researchers were able to demonstrate that the groundwater is not just warming up; the warming stages observed in the atmosphere are also echoed.

"Global warming is reflected directly in the groundwater, albeit damped and with a certain time lag," said Bayer.

The data also shows that the groundwater close to the surface down to a depth of around sixty metres has warmed up statistically 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 that of global warming.

The groundwater shows how the atmosphere has made several temperature leaps at irregular intervals. These "regime shifts" can also be observed in the global climate.

The Earth's atmosphere has warmed up by an average of 0.13 degrees per decade in the last 50 years.

And this warming doesn't stop at the subsoil, either, as other climate scientists have demonstrated in the last two decades with drillings all over the world.

However, the researchers only tended to consider soils that did not contain any water or where there were no groundwater flows.

The research was published in the journal Hydrology and Earth System Sciences.