Urban environments are often hotter than rural areas due to the way city structures trap and generate excess heat. This phenomenon, called urban heat islands, was first observed in the 1800s, but a new way of gauging it could help us make the most of this untapped source of geothermal energy.

Scientists in Germany and Switzerland have developed a means of estimating groundwater temperature hidden under the surface of our cities, based on surface temperatures and the density of buildings as measured by satellites.

A range of factors, including population density, vegetation levels, surface sealing, industrial structures, and transport all contribute to why cities are often hotter than the country.

But while scientists have long used satellites to measure heat on the surface, the relationship between ground temperatures and underground temperatures has not been fully explored.

To better understand how thermal conduction operates between the surface and underground heat islands (aka subsurface urban heat islands, or SUHI), the researchers compared above-ground and underground temperatures in four German cities: Berlin, Munich, Cologne, and Karlsruhe.

They found that the older a city is, the more pronounced its underground heat. For example, in an older city like Cologne, the researchers found a greater correlation between its surface and underground heat islands than the relatively young Karlsruhe.

Approximately 95 percent of the areas studied were found to have a higher groundwater temperature than surface temperature, which the researchers attribute to a greater abundance of underground anthropogenic heat sources, such as cellars of buildings and sewers.

It's this higher temperature in groundwater that could be used as a source of sustainable energy, for heating in winter and cooling in summer, with the help of geothermal or groundwater heat pumps.

All up, the cities showed an 80 percent spatial correlation between heat islands on the surface and those underground, suggesting satellite measurements of surface temperature alone are not sufficient to reliably gauge the temperature of groundwater.

However, once population density and cellar temperatures were also taken into account, the researchers were able to estimate groundwater temperatures with a mean absolute error of just 0.9 Kelvin.

Given they have so far only analysed four cities, the researchers acknowledge that further research will need to be conducted in order to refine the system.

But nonetheless, the study, which is published in Environmental Science & Technology, could help us to better establish cost- and time-effective ways to remotely assess the geothermal value of heat stored underneath our cities.

"This method can be applied for a first assessment of underground heat islands and, hence, of ecological conditions in the groundwater and of the geothermal potential," said Philipp Blum, a geoscientist with the Karlsruhe Institute of Technology. "No complex groundwater temperature measurements and interpolations are required."