Vulnerability of coastal aquifers to groundwater use and climate change

Climate change and human population growth are expected to have substantial impacts on global water resources throughout the twenty-first century1, 2. Coastal aquifers are a nexus3 of the world\textquoterights oceanic and hydrologic ecosystems and provide a water source for the more than one billion people living in coastal regions4, 5. Saltwater intrusion caused by excessive groundwater extraction is already impacting diverse regions of the globe5, 6, 7. Synthesis studies8, 9 and detailed simulations10, 11, 12, 13 have predicted that rising sea levels could negatively impact coastal aquifers through saltwater intrusion and/or inundation of coastal regions. However, the relative vulnerability of coastal aquifers to groundwater extraction and sea-level rise has not been systematically examined. Here we show that coastal aquifers are more vulnerable to groundwater extraction than to predicted sea-level rise under a wide range of hydrogeologic conditions and population densities. Only aquifers with very low hydraulic gradients are more vulnerable to sea-level rise and these regions will be impacted by saltwater inundation before saltwater intrusion. Human water use is a key driver in the hydrology of coastal aquifers, and efforts to adapt to sea-level rise at the expense of better water management are misguided.