A great egret sits atop a dead tree in the Everglades, near South Bay, Fla., on Jan. 14, 2005. (J. Pat Carter/AP)

It may not be as obvious a climate symbol as the rapidly warming Arctic. But with President Obama’s climate-focused visit on Earth Day, Everglades National Park could take on new significance as a politically potent case study of how global warming directly impacts people living in the United States.

The chief reason? In the Everglades, the fate of an ecosystem, and the fate of millions of people, are tightly wrapped together — and both are affected by rising seas.

Everglades National Park is an ecological icon because of its liminal nature — its 1.5 million acres lie perched between fresh and saltier water, between marsh and ocean. The unique region was  famously dubbed a “river of grass” and supports vast biological diversity — mangrove forests, sawgrass prairie  and much more.

Moreover, it does this even in the damaged and dramatically shrunken state in which humans have currently left it by diverting much of its historic waters.

[Obama will visit the Florida Everglades on Earth Day — to talk about climate change]

But the Everglades is not just a place to see alligators, crocodiles and manatees. People rely on it, too. The vast water system that feeds the Everglades also helps to fill and refill the Biscayne aquifer, a gigantic underground supply of freshwater upon which southeast Florida’s human residents rely.

“The Everglades as a natural system, and the coastal area as a human system, are really interdependent,” said Len Berry, former head of the Florida Center for Environmental Studies at Florida Atlantic University and now an emeritus professor there. “Folks that live here don’t always realize it, but we are dependent on the Everglades as a retainer of water, and a filter of water.”

“I can be an environmentalist and say we’ve got to save the Everglades, I can also be a practical developer and say we’ve got to save the Everglades, because it’s of practical use to us,” Berry said.

Some people in Florida are already experiencing the spoiling of water supplies with salt water — Hallandale Beach, for instance, had “abandoned six of its eight drinking water wells because saltwater has advanced underground across two-thirds of the city,” the Miami Herald reported in 2011. With further saltwater intrusion, more Floridians could experience the same problem.

The dependence on the Everglades can only increase as Florida’s population booms — it just surpassed New York to become the third-largest state in the country by population, approaching 20 million people, with an additional 5 million or more expected by 2040.

Thus, by visiting the Everglades, Obama may be able to land a shrewd political strike and shift the climate debate more to a clear and present homefront.  That’s particularly pertinent in an ever-more-important swing state that has produced two potential 2016 GOP presidential contenders — Marco Rubio and Jeb Bush — both of whom are going to have to answer a lot of climate change questions in the coming year focused on their own back yard.

[Floridians seem a lot more worried about climate change than Marco Rubio is]

When it comes to climate change and the Everglades, the issue is not so much rising temperatures — although those matter — as it is the way that rising seas will push into the ecosystem, not only imperiling unique wildlife but also potentially spoiling freshwater supplies.

The reason is what Florida International University ecologist Evelyn Gaiser calls “the Swiss Cheese underpinnings of the Everglades, that is easily encroached by the sea.”

The Everglades sit atop a base layer of porous limestone that is easily penetrated by seawater, so that in essence, the flow of freshwater into the Everglades can be thought of as being in a constant competition with seawater.

Freshwater is lighter in weight than salt water, explains the U.S. Geological Survey, meaning that in the area, “a 41-foot column of freshwater is necessary to balance a 40-foot column of saltwater.” We only see what’s at the surface, but these columns mostly abut against one another below it — and humans are, in effect, aiding the saltwater’s progress.

They do so, notes USGS, through changing the landscape — “lowering of freshwater levels by drainage canals or by intensive pumping creates an imbalance that causes the inland movement of saltwater” — but also by driving global warming and rising seas, a kind of third strike after canals and wells.

As a result, saltwater pushes into aquifers. “As the average sea level rises, that allows the denser salt water to move in underneath the freshwater,” explained Florida Atlantic University’s Len Berry. Basically, it’s an “advancing front,” Berry said. “It’s not racing, but it’s creeping.”

Or, as a 2010 report from the Florida Oceans and Coastal Council put it:

Rising sea level will increase the hydraulic backpressure on coastal aquifers, reduce groundwater flow toward the ocean, and cause the saltwater front to move inland, thus threatening to contaminate water-supply wells in coastal areas with seawater.

And we’re just at the beginning of this potential change. Sea level in the Florida area already rose nearly 9 inches during the 20th century, but projections for the 21st century are vastly higher — it could be as much as 78 inches, according to a 2014 study by the National Research Council of the National Academy of Sciences on the progress of Everglades restoration.

Throw in warmer temperatures and a potential decline in rainfall, and you’ve got “insufficient freshwater to sustain the natural and built systems,” the report warns.

Indeed, with a sea level rise of 30 or so inches, “most of Everglades National Park could essentially become an extension of Florida Bay,” warns an Everglades report by the EPA, National Parks Service and Fish and Wildlife Service.

The Everglades do have one card to play against rising seas — a mangrove barrier that can rise with them, at least to an extent, as the plants’ roots filter saltwater and build up a layer of peat beneath them. But scientists don’t know how fast mangroves can rise, and thus, how long the barrier will last. (It is also vulnerable to powerful hurricanes.)

“That’s the general paradigm, mangroves will accrete soils at rates that are commensurate with the rate of sea level rise,” said Florida International University’s Gaiser. But as sea level rise continues and at a faster rate, she continues, “what we’re seeing is that the soils on the other side of the mangroves, on the freshwater marsh, are not similarly accreting, and instead, they’re collapsing. And that’s completely the opposite of the predictions of what a mangrove forest would do.”

Mangroves are one potential last ditch barrier. But the longstanding Comprehensive Everglades Restoration Plan aims to engage the battle against salt water from another strategic position — to “redirect” some freshwater back to the Everglades that had been diverted from it.

Thus, in the Everglades, the fight against climate change — and to push back the sea — is being constantly waged with very high stakes.

This week, we’ll hear a lot of climate news. In particular, John Kerry will become yet another major Obama official (after Interior Secretary Sally Jewell) to visit the Arctic. Kerry is traveling to Iqaluit, Canada, as the U.S. takes over chairmanship of the Arctic Council, an eight-nation body that sets the governance regime for the region.

[The Arctic is ‘unraveling’ due to climate change — and the consequences will be global]

Climate change in the Arctic has vast consequences that stretch far beyond the region itself. But for most of us, the Arctic is very far away. It is  also relatively sparsely populated — which couldn’t be a greater contrast with South Florida, where the consequences of climate change imperil a very large, and very politically diverse, human population.

Which may explain why it’s now becoming central to Obama’s climate agenda. Or, as Joe Pogdor, the former head of Friends of the Everglades  put it many years ago: “The Everglades is a test. If we pass, we may get to keep the planet.”

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