Barring a stupendous reversal in greenhouse gas emissions, the rising Atlantic will cover much of Miami by the end of this century. The economic effects will be devastating: Zillow Inc. estimates that six feet of sea-level rise would put a quarter of Miami’s homes underwater, rendering $200 billion of real estate worthless. But global warming poses a more immediate danger: The permeability that makes the aquifer so easily accessible also makes it vulnerable. “It’s very easy to contaminate our aquifer,” says Rachel Silverstein, executive director of Miami Waterkeeper, a local environmental protection group. And the consequences could be sweeping. “Drinking water supply is always an existential question.”

County officials agree with her. “The minute the world thinks your water supply is in danger, you’ve got a problem,” says James Murley, chief resilience officer for Miami-Dade, although he adds that the county’s water system remains “one of the best” in the U.S. The questions hanging over Miami and the rest of Southeast Florida are how long it can keep its water safe, and at what cost. As the region struggles with more visible climate problems, including increasingly frequent flooding and this summer’s toxic algae blooms, the risks to the aquifer grow, and they’re all the more insidious for being out of sight. If Miami-Dade can’t protect its water supply, whether it can handle the other manifestations of climate change won’t matter.

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