# Groundwater warming

Maria Klepikova

► To cite this version:

Maria Klepikova. Groundwater warming. Nature Geoscience, 2024, 17, pp.487-488. 10.1038/s41561-024-01461-x. insu-04606925

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# Groundwater warming

Maria Klepikova

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Ongoing climate warming is heating the subsurface. Projections suggest that by the end of the century millions of people will live in areas where groundwater exceeds the highest threshold for drinking water temperatures.

The foundations of modern hydrology have been built within the paradigm of stable thermal regimes within groundwater systems. This presumed thermal stability is particularly important as multiple physical, chemical, and biological processes, that are critical for providing clean drinking water, are sensitive to thermal disturbances¹. Moreover, aquatic organisms rely on groundwater discharge, as they are particularly susceptible to shifts in thermal regimes and groundwater can buffer stream temperatures against long-term air temperature trends and short-term hot and cold extremes². However, recent observations have suggested ongoing atmospheric warming is substantially impacting shallow groundwater temperatures in some regions³⁻⁵. Writing in *Nature Geoscience*, Benz et al.⁶ show that aquifer temperatures across the globe are projected to rise substantially over this century.

Groundwater temperature is influenced by periodic diurnal and seasonal heating and cooling of the land surface. While the amplitude of these short-term temperature fluctuations is strongly attenuated at a depth of a few metres, longer-term changes in air temperature can influence the subsurface down to depths of up to one hundred meters. As a consequence, present-day borehole temperature–depth profiles often show a temperature decrease with depth (that is, the geothermal gradient near the Earth’s surface is reversed for up to 100 m). Groundwater temperature data has revealed local temperature increases in several regions of the globe. Austria, Germany and the Netherlands have exhibited nationwide linear increases in temperature over the past 30 years³,⁴,⁵, and generally elevated groundwater temperature has been observed in central Europe⁷. However, we still have insufficient knowledge of how atmospheric warming influences groundwater temperatures at a global scale. Moreover, the timescale over which groundwater responds to atmospheric conditions is not well understood.

Using a global-scale groundwater temperature model of vertical diffusive heat transport, Benz et al. calculated transient aquifer thermal patterns across the globe. They project that between the years 2000 and 2100, groundwater at the depth of the water table will warm on average by 2.2 °C (according to the most-likely climate change scenario) or by 3.8 °C (according to a high-emission climate change scenario). Their simulation results also reveal that, according to the above scenarios, the depth down to which we can trace the impact of climate warming in the form of reversed temperature–depth profiles will increase from 15 m in 2020 to 40 or 60 m in 2100. The groundwater warming trends exhibit substantial spatial variability, with locations

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**Fig. 1 | Processes related to groundwater warming.** Benz et al. show that ongoing climate warming is modifying the thermal regimes of the shallow subsurface. These changes will impact groundwater chemistry and microbiology as well as the functioning of groundwater-dependent ecosystems. Warmer groundwater could reduce faunal diversity and increase the prevalence of microbes and contaminants.

that have shallow water tables experiencing pronounced seasonal variations, as well as the greatest warming rates. The rate and duration of local climate-induced atmospheric warming also influences the groundwater warming rate.

Direct and indirect consequences of the projected groundwater temperature changes are multiple. The climate warming will benefit from the use of aquifer thermal energy for heating and, in some areas, the accumulated geothermal energy will fulfil heating demands. In contrast, the efficiency of cooling geothermal systems will be reduced. Furthermore, warming generally decreases faunal diversity and accelerates the cycling of organic and inorganic matter¹,² (Fig. 1). The results indicate that, under climate variability and change, by the end of the century between 30 and 500 million people will live in areas where
groundwater temperatures exceed safe drinking water thresholds. In many regions, groundwater warming will also alter the conditions that sensitive groundwater-dependent ecosystems currently rely on.

Benz and colleagues offer a new approach to estimating global groundwater temperature distribution and its time evolution using the assumption of diffusion only conditions (that is, without considering groundwater flow and the associated advection of heat). The simulation results are generally in agreement with available observations at the large scale, but some apparent discrepancies are evident. The authors attribute these to the influence of additional urban heat on measured groundwater temperatures and the impact of heat advection due to groundwater flow. Such factors were not resolved by the global-scale model, but will be important to incorporate into future research. Further advancing our understanding of the physical mechanisms that control the depth and the magnitude of heat advection effects and thermal effects driven by urbanization will require more detailed case studies of this kind. In addition, it seems necessary to examine how recent and future alteration of the state of groundwater systems⁸, either caused by human water use or climate change (changes in recharge,

discharge, or modifications of groundwater flow patterns), may modify subsurface thermal conditions.

Maria Klepikova

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Univ Rennes, CNRS, Géosciences Rennes, Rennes, France.

e-mail: maria.klepikova@univ-rennes1.fr

Published online: 4 June 2024

References

1. Koch, F. et al. *Hydrol. Earth Syst. Sci.* **25**, 3053–3070 (2021).

2. Burns, E. R. et al. *Water Resour. Res.* **53**, 3341–3351 (2017).

3. Bense, V. F., Kurylyk, B. L., de Bruin, J. G. H. & Visser, P. *Water Resour. Res.* **56**, e2019WR026913 (2020).

4. Benz, S. A., Bayer, P., Winkler, G. & Blum, P. *Hydrol. Earth Syst. Sci.* **22**, 3143–3154 (2018).

5. Hemmerle, H. & Bayer, P. *Front. Earth Sci.* **8**, 1–8 (2020).

6. Benz, S. A. et al. *Nat Geosci.* https://doi.org/10.1038/s41561-024-01453-x (2024).

7. Tissen, C., Benz, S. A., Menberg, K., Bayer, P. & Blum, P. *Environ. Res. Lett.* **14**, 104012 (2019).

8. Kuang, X. et al. *Science* **383**, eadf0630 (2024).

Competing interests

The author declares no competing interests.