groundwater in some coastal areas of Nassau County, a major suburb of New York City, is increasingly turning salty. That shift, called saltwater intrusion, is the result of decades of pumping fresh water out of wells for homes and irrigation, creating space for saltwater from the ocean to seep into the underground aquifers once filled with freshwater.

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The growing stress on groundwater can be caused by population growth, as on Long Island. But climate change also plays a role. As temperatures rise, water demand increases. Warmer weather also means more water evaporates before seeping into the ground, reducing the rate at which water drawn from underground aquifers can be replaced.