For their study, the researchers were able to fall back on uninterrupted 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 forty years. This is unique and a rare commodity for the researchers. “For us, the data was a godsend,” stresses Peter Bayer, a senior assistant at ETH Zurich’s Geological Institute. Even with some intensive research, they would not have been able to find a comparable series of measurements. Evidently, it is less interesting or too costly for waterworks to measure groundwater temperatures systematically for a lengthy period of time. “Or the data isn’t digitalised and only archived on paper,” suspects the hydrogeologist.
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,” says Bayer, summarising the main results that the project has yielded. The researchers published their study in the journal Hydrology and Earth System Sciences.
The data also reveals 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 forty years. This water heating follows the warming pattern of the local and regional climate, which in turn mirrors that of global warming.
The groundwater reveals how the atmosphere has made several temperature leaps at irregular intervals. These “regime shifts” can also be observed in the global climate, as the researchers write in their study. Bayer was surprised at how quickly the groundwater responded to climate change....