RENEE MONTAGNE, HOST:
A warming world is causing the oceans to rise. We know that. Some places, though, like south Florida are already feeling the effects. Streets are flooding more often, for example. As NPR's Christopher Joyce reports, there's a much more insidious threat from rising sea levels in Florida. And it's one that people can't see. The ocean is threatening the region's supply of fresh water.
CHRISTOPHER JOYCE, BYLINE: The city of Coral Gables has seen a lot of nuisance flooding lately. But the mayor, James Cason, says few people are paying much attention to the cause.
JAMES CASON: I've never received an email in five years on sea level rise, which, for me, was just - I was flabbergasted. I asked other mayors in some series of meetings we've had. And they said, well, that's the same with us.
JOYCE: Cason and other mayors are worried. Some of them are talking about building walls and sand dunes to hold back the ocean. The problem is - the ocean is actually penetrating underneath the land in south Florida. Walls and dunes can't stop that.
And that invasion could drastically change life for people here and millions of others in south Florida. To understand why, I drove to the Everglades, a swampy wilderness the size of Delaware.
A few miles in, I park on the shoulder of the road. There's an experiment going on here to understand what could happen as the ocean invades.
It's tall sea grass to the horizon, a few twisted cypress trees and the pale blue sky. A guy with a mosquito net over his face is stirring water in a vat the size of a Jacuzzi.
JOE STACHELEK: We're making, basically, artificial seawater here.
JOYCE: Joe Stachelek and Tiffany Troxler from Florida International University are making salt water. They're pumping it out into the wetland. Troxler's experiment involves dosing the plants and soil with it - a preview of what the ocean could do.
TIFFANY TROXLER: As sea level rises, the saltwater wedge moves inland.
JOYCE: The wedge of seawater will move through the bedrock - yes, through rock.
TROXLER: Our underlying rock is limestone. That limestone is very porous. It's almost like Swiss cheese in some areas.
JOYCE: A rising sea increases the pressure of the ocean on the coastal bedrock. Eventually, the ocean wins and moves in. We walk out into the test site through the sawgrass and the underlying peat, which is a fancy name for muck.
So I think the secret is to step on where the grass is, rather than where the muck is.
That muck is rich stuff, though, full of nutrients and microorganisms that feed the river of grass. The team has laid a metal boardwalk over the peat so you can walk around without sinking up to your waist. Out here, the grass is patchier. In some places, the peat is slumping - collapsing downwards.
TROXLER: When we start to lose the structure of the plants, essentially, this peat that's otherwise held together by roots becomes a soupy pond.
JOYCE: What we were walking through.
JOYCE: As salt water is hosed into the ground, the sawgrass plants actually pull up some of their roots. The plants look like teeth protruding from receding gums. This, says Troxler, could be the future of the Everglades. And here's the thing - the Everglades are like a sponge - a sponge that feeds the Biscayne Aquifer, a giant cell of fresh water that lies underneath the land.
TROXLER: We get over 90 percent of our fresh water from the Biscayne Aquifer.
JOYCE: Millions of thirsty people in south Florida draw their water from that aquifer. So if seawater seeps out from underneath, it's infiltrating and contaminating the freshwater aquifer as well as the Everglades above it. That's starting to worry some people like Julie Hill-Gabriel, who's the expert on the Everglades at the Florida Audubon Society.
JULIE HILL-GABRIEL: What we do in the Everglades is a hundred percent going to affect you and your neighborhood. You know, when you turn on the tap - whether you have enough fresh, clean water.
JOYCE: For millennia, fresh water flowed south to the Everglades, the largest flooded grassland in America. But that fresh water was diverted to irrigate agricultural fields and to keep it from flooding homes. Environmental groups like Audubon have been trying to restore the natural flow of fresh water to the Everglades, mostly to preserve wildlife there.
But now, Hill-Gabriel says there's a new reason to repel the invading sea. Putting more fresh water into the Everglades sponge could push back the invading ocean.
HILL-GABRIEL: It just really compounds the urgency to move that fresh water south because it's the No. 1 thing, really, we can do to help push off the salt water that's now moving inland.
JOYCE: At least, that's the theory. It's one that Mayor Cason in Coral Gables wants his constituents to think about when they think about the Everglades.
CASON: It's not just so they can go see the alligators. It's because they want to see - make sure the drinking water on which we all depend is not contaminated.
JOYCE: Christopher Joyce, NPR News.
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