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 century(1,2). Coastal aquifers are a nexus(3) of the world's oceanic and hydrologic ecosystems and provide a water source for the more than one billion people living in coastal regions(4,5). Saltwater intrusion caused by excessive groundwater extraction is already impacting diverse regions of the globe(5-7). Synthesis studies(8,9) and detailed simulations(10-13) 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.