Global warming is even reflected in the groundwater, a study shows: In the last few decades, the higher surface temperatures have led to a significant warming of the groundwater in Bavaria, data from 35 measuring stations show. At a depth of 20 meters, the water is now on average almost one degree warmer than it was 30 years ago. A similar trend can be assumed in other regions of Germany. The increased temperatures underground could have problematic effects on underground ecosystems, say the researchers. However, part of the heat could also be used for sustainable energy generation with the help of geothermal systems.
As is well known, when the air warms up, this also affects the substances with which it comes into contact. This so-called thermal coupling also shapes the geology: if the average temperatures in the atmosphere rise, the soil and the fluids in the water cycle warm up too. At some point a warming trend will therefore also be reflected in the groundwater. “In contrast to the atmosphere, however, the subsurface reacts very slowly,” says Peter Bayer from the Martin Luther University in Halle-Wittenberg. Since the subsoil does not react to short-term temperature fluctuations or the seasons and thus rather shows long-term trends, it is a particularly good indicator of climate change, explains the geologist.
“Fever measurements” in the groundwater
“Warming effects underground have already been shown, but overall there is still little data,” emphasizes Bayer. That is why he and his colleague Hannes Hemmerle have now devoted an investigation to the subject – in Bavaria. The two geologists measured the temperatures of the groundwater there at 35 stations at different depths. They were able to compare this data with measurement results that had already been recorded in the 1990s. During this time, the average air temperature in Bavaria rose by 1.05 degrees Celsius, say the researchers. The measuring stations are distributed throughout the state – thus the comparative data could provide a wide-ranging insight into the development of groundwater temperatures in the entire region, explain Bayer and Hemmerle.
The comparisons showed: Apart from a few exceptions, which have to do with regional peculiarities of the groundwater supply, there was a clear warming trend in the deep water reservoirs. “From around 15 meters, climate change has a very clear impact, short-term local or seasonal fluctuations are then no longer recorded,” explains Hemmerle. As the geologists report, the groundwater at a depth of 20 meters is now on average almost 0.9 degrees Celsius warmer than it was in the 1990s. At a depth of 60 meters, the temperature was around 0.3 degrees.
Geothermal use possible
The results cannot be transferred one-to-one to Germany as a whole. “However, it can be assumed that the trend is the same,” says Hemmerle. “The groundwater will probably continue to warm even if it reacts too late to the air temperatures and will continue to react to rising atmospheric temperatures in the future,” says the geologist. The scientists say it is still difficult to assess what consequences this warming could have for life underground. However, it is known that higher temperatures influence the growth of microbes and thus also the metabolism in the subsurface. But it is also possible that subterranean ecosystems that are adapted to very constant temperatures could come under pressure, according to Bayer and Hemmerle.
The scientists emphasize that the consequences of climate change could also be seen as something positive. Because the warmth of the groundwater can be used as a sustainable energy source through geothermal systems. In these systems, the comparatively warm water at a depth of over 15 meters is used to generate heating water in winter. For this purpose, the low heat is, as it were, stepped up by the principle of the heat pump. A higher starting temperature of the groundwater naturally has a positive effect. “This means that at least some of the heat could be reused with the help of geothermal energy,” Bayer concludes.
Source: Martin Luther University Halle-Wittenberg, specialist article: Frontiers in Earth Science, doi: 10.3389 / feart.2020.575894