Aquifers contain the largest store of unfrozen freshwater, making groundwater critical for life on Earth. Surprisingly little is known about how groundwater responds to surface warming across spatial and temporal scales. Focusing on diffusive heat transport, we simulate current and projected groundwater temperatures at the global scale. We show that groundwater at the depth of the water table (excluding permafrost regions) is conservatively projected to warm on average by 2.1 °C between 2000 and 2100 under a medium emissions pathway. However, regional shallow groundwater warming patterns vary substantially due to spatial variability in climate change and water table depth. The lowest rates are projected in mountain regions such as the Andes or the Rocky Mountains. We illustrate that increasing groundwater temperatures influences stream thermal regimes, groundwater-dependent ecosystems, aquatic biogeochemical processes, groundwater quality and the geothermal potential. Results indicate that by 2100 following a medium emissions pathway, between 77 million and 188 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures set by any country.

含水层蕴藏着最大的未冻结淡水，使得地下水对地球上的生命至关重要。令人惊讶的是，人们对地下水如何在空间和时间尺度上对地表变暖做出反应知之甚少。我们专注于扩散热传输，模拟全球范围内当前和预计的地下水温度。我们表明，在中等排放路径下，保守预测地下水位深度（不包括永久冻土地区）的地下水在 2000 年至 2100 年间平均升温 2.1 °C。然而，由于气候变化和地下水位深度的空间变化，区域浅层地下水变暖模式差异很大。预计安第斯山脉或落基山脉等山区的发病率最低。我们说明地下水温度的升高会影响溪流热状况、依赖地下水的生态系统、水生生物地球化学过程、地下水质量和地热潜力。结果表明，按照中等排放路径，到 2100 年，预计将有 7700 万至 1.88 亿人生活在地下水超过任何国家设定的饮用水温度最高阈值的地区。