Global groundwater warming due to climate change

Climate change is causing a holistic warming of Earth's climate system, primarily driven by increased greenhouse gas concentrations. While oceans absorb most of this additional heat, the terrestrial subsurface and groundwater also act as significant heat sinks. Under stable climatic conditions, seasonal temperature variations penetrate only to depths of 10–20 meters, below which temperatures increase with depth according to the geothermal gradient. However, modern borehole temperature profiles often show temperature inversions (decreasing temperature with depth) extending up to 100 meters due to recent surface warming. These deviations from steady-state temperatures have been used to study terrestrial heat storage and estimate past surface temperatures. While previous studies have examined subsurface warming, the impacts on groundwater resources and associated implications have largely been overlooked. Global-scale groundwater research is advancing with the availability of datasets from sources like GRACE satellites and global hydrological models. However, these studies have primarily focused on groundwater quantity (levels, recharge rates), with global-scale research on groundwater quality, particularly temperature, remaining rare. Water temperature is a critical, yet understudied, groundwater quality parameter in the context of climate change. While global studies of river and lake warming exist, there are no global assessments of climate change impacts on groundwater temperatures (GWTs). This is a significant gap, considering groundwater's role as the largest global reservoir of unfrozen freshwater, supplying water for at least half the world's population and a large portion of global irrigation needs. Groundwater also sustains terrestrial and aquatic ecosystems. Given temperature's importance as a master environmental variable, and observational evidence of groundwater warming globally, the lack of global-scale assessment of this phenomenon presents a critical knowledge gap. Groundwater temperature directly influences biogeochemical processes that affect water quality. Increased temperatures reduce gas solubility, increase organism metabolism, accelerate oxygen consumption, and potentially shift redox conditions. In many aquifers already low in oxygen, small temperature increases could trigger a shift from oxic to hypoxic or anoxic conditions, leading to the mobilization of redox-sensitive constituents like arsenic, manganese, and phosphorus. Elevated levels of these constituents pose significant health risks. Groundwater warming also alters groundwater community composition, potentially impairing nutrient cycling and biodiversity. Shallow soil and groundwater warming can also raise temperatures in water distribution networks above critical thresholds for pathogen growth, impacting human health. The discharge of thermally stable groundwater into surface water bodies influences their thermal regimes. Groundwater inflows can create cold-water plumes offering thermal refuge for aquatic species. Groundwater warming increases ambient water temperatures and the temperatures of these vital refuges. Spring ecosystems, particularly sensitive to temperature changes, will also be affected. While groundwater warming has negative consequences, it can also offer a positive impact as accumulated thermal energy can be harnessed via low-carbon geothermal energy systems. However, increased warming will reduce the efficiency of cooling systems.

The paper reviews existing literature on global climate change and its impact on various environmental systems, highlighting the disproportionate focus on atmospheric and surface water warming compared to groundwater. It points out the scarcity of global-scale studies on groundwater quality, specifically temperature, despite its importance as the largest global reservoir of unfrozen freshwater. Existing research mostly centers on groundwater quantity rather than quality, leaving a critical knowledge gap regarding the impacts of climate change on groundwater temperature and its associated consequences. The authors cite studies illustrating the impacts of temperature on biogeochemical processes, microbial activity, and ecosystem health in various aquatic systems. They also acknowledge the limited understanding of groundwater warming's implications for drinking water resources and geothermal energy potential. This literature review establishes the context and rationale for the study's aim to quantify groundwater temperature changes globally and assess their implications.

The study uses a global-scale heat-transport model to quantify groundwater temperatures and their response to climate change. The model focuses on diffusive heat transport, considering it the dominant mechanism at the spatial scale of the study. The model is 1-dimensional (vertical), simulating heat transfer from the surface to a depth of 100 meters, focusing on temperatures at the water table depth. The exclusion of advection, a simplification, is justified by the study's large spatial scale and the finding that advection has a minor influence on groundwater warming rates under typical conditions. Accurate initial conditions are crucial, so the model starts in 1880, before the significant increase in atmospheric greenhouse gases. The initial temperature profile increases linearly with depth, reflecting the geothermal gradient. Permafrost regions are excluded due to the model's inability to account for latent heat effects during thawing. The model employs an analytical solution to the transient 1D heat diffusion equation, accounting for step changes in surface temperature over time. The model runs within Google Earth Engine (GEE), producing maps and charts of annual mean, maximum, minimum, and seasonal variability of groundwater temperatures at different depths. The model uses gridded ground surface temperature data: ERA-5 data for current (2020) temperatures and CMIP6 data for projected changes (up to 2100) under SSP 2-4.5 and SSP 5-8.5 scenarios. The CMIP6 data are adjusted to match ERA-5 data from 1981-2014. Ground thermal diffusivity is calculated from bulk thermal conductivity and volumetric heat capacity, which vary with water saturation and are based on data from various sources and guidelines (VDI 4640). The geothermal gradient is derived from global heat flow and thermal conductivity data. Water table depth, crucial for GWT analysis, is derived from a previously published global groundwater model, with adjustments to account for model limitations. The model is validated against measured GWT data, showing good accuracy. The analysis of GWT changes is performed between 2000 and 2020 (observed) and projected for 2000-2100 under different scenarios. Accumulated energy is calculated by integrating the difference between simulated and initial temperature profiles. The impact on drinking water quality is assessed by comparing maximum GWTs with drinking water temperature guidelines from the World Health Organization. Population vulnerability is determined using population density data. The study also examines the impact on surface water bodies by comparing simulated GWTs at stream sites with a groundwater-dominated signature. Data are available through the Google Earth Engine app and the Scholars Portal Dataverse.

The study's global-scale model projects a conservative average warming of groundwater at the water table depth of 2.1 °C (0.8, 3.0) between 2000 and 2100 under the SSP 2-4.5 median emission scenario. The warming is even more pronounced under the SSP 5-8.5 high emissions scenario, projecting a 3.5 °C increase. Regional variations are substantial, reflecting spatial variability in climate change and water table depth. Mountain regions like the Andes and Rockies are projected to experience lower warming rates. The model accurately reflects observed groundwater temperature changes, with a root mean square error of 1.4 °C and a coefficient of determination of 0.75. Between 2000 and 2020, GWTs increased by an average of 0.3 °C globally, but regional variations were significant. The depth to which temperature inversion is observed (where temperature decreases with depth instead of increasing due to the geothermal gradient) provides a clear indication of climate change. This depth is projected to increase significantly by 2100, reaching 45 meters under SSP 2-4.5 and 60 meters under SSP 5-8.5. By 2020, the terrestrial subsurface had absorbed a significant amount of heat (14 × 10²¹ J) since the industrial revolution. The study projects a substantial increase in accumulated subsurface energy by 2100 (41 × 10²¹ J under SSP 2-4.5 and 62 × 10²¹ J under SSP 5-8.5). The accumulated heat could be harnessed for geothermal energy, though efficiency will decline for cooling systems. Under the SSP 2-4.5 scenario, 77 to 188 million people are projected to live in areas with maximum groundwater temperatures exceeding the highest drinking water temperature thresholds set by any country by 2100, while the SSP 5-8.5 projects 59 to 588 million. The study also reveals that warming will affect groundwater-dependent ecosystems, specifically impacting river temperatures and thermal refuges for aquatic species. Comparisons with data from the United States shows groundwater warming exacerbates warming in surface water systems.

The findings highlight the significant and largely underestimated impact of climate change on global groundwater temperatures. The projected warming has far-reaching implications for water quality, human health, and ecosystem integrity. The regional variations emphasize the need for localized assessments to guide effective adaptation strategies. The study's conservative estimates underscore the urgency of mitigating greenhouse gas emissions. The potential for harnessing the increased subsurface heat for geothermal energy presents an opportunity to mitigate some negative impacts while also facilitating sustainable heating solutions. However, the risks to water quality and ecosystem health outweigh this benefit, emphasizing the necessity for mitigation efforts. The model's limitations regarding advection and heterogeneity of thermal properties are acknowledged, but the large-scale trends revealed remain highly significant. The integration of these findings with other global models, particularly those modeling river temperatures, is essential for a more comprehensive understanding of the coupled impacts of groundwater warming.

This study provides the first global-scale assessment of groundwater warming due to climate change, projecting significant temperature increases by 2100 with substantial regional variations. The findings underscore the interconnectedness of climate change with water resource management, human health, and ecosystem sustainability. The research highlights the need for urgent mitigation of greenhouse gas emissions and the development of region-specific adaptation strategies. Future research should focus on improving the model's resolution, incorporating advection and other factors to refine projections, and exploring the complex feedback mechanisms between groundwater warming and other environmental processes.

The model simplifies some aspects of heat transport, particularly excluding advection, which may underestimate warming in specific areas with significant groundwater flow. The use of bulk thermal properties for the subsurface might lead to oversimplification of the spatial variability in temperature responses. The projection of future water table elevations is limited to a static assumption, and future changes could affect the accuracy of the temperature projections at the water table depth. The model does not consider feedback mechanisms within the climate system, which could affect both surface temperature and groundwater warming rates. Data limitations affect the accuracy of the model in some regions. The analysis of the effects on surface water bodies is limited to the United States.