Heat below the city

It is hot in the city. All of us living in a big city like Vienna, have experienced extreme summer heat waves in the recent past. Apart from exceptional weather extremes, cities in general have their own local microclimate with a higher mean annual air temperature differing from rural areas. What is observed at the surface is also true for the underground. Groundwater temperatures below cities are 2-3 K above natural background values and, similar to the atmosphere, show an increasing trend with time due to climate change. In consequence, the accumulation of heat in the urban subsurface including urban heat islands demands a fundamental understanding of its effects to groundwater flow, ecology, water quality, and the application of shallow geothermal energy technology. Tipping points that kick off negative cascading effects for both the subsurface energy resources and the valuable ecosystems need to be identified as basis of future management strategies.