Sources, intensities, and implications of subsurface warming in times of climate change

Anthropogenic warming of the atmosphere is one if not the most pressing challenge we face in the 21st century. While our state of knowledge on human drivers of atmospheric warming is advancing rapidly, little so can be said if we turn our view toward the Earth's interior. Intensifying land use and atmospheric climate change condition the changing thermal state of the subsurface at different scales and intensities. Temperature is proven to be a driving factor for the quality of our largest freshwater resource: groundwater. But there is only insufficient knowledge on which sources of heat exist underground, how they relate in their intensity of subsurface warming, and which consequences this warming implies on associated environments, ecosystems and resources. In this review, we propose a differentiated classification based on (1) the geometry of the heat source, (2) the scale at which the subsurface is heated, (3) the process that generates the heat, and (4) the intention of heat release. Furthermore, we discuss the intensities of subsurface warming, the density of induced heat fluxes, as well as their abundance, and draw implications for depending processes and ecosystems in the subsurface and the potential of recycling this waste heat with geothermal installations.