Umberto Gimenez loves alligators. He gives them nicknames such as "Smile" and "Momma Gator" and laughs when he thinks of their antics.

Gimenez, an airboat captain, has found his paradise in Florida's Everglades National Park, a natural gem in the southeastern US state at risk from climate change.

"It's an amazing place and there's only one in the world," he says.

The largest wetland in the United States is under threat, and has become a battleground for one of the most sweeping ecological conservation efforts on Earth.

Gimenez hopes the efforts will help preserve the park.

But time is running short, and global warming is sabotaging a subtropical wilderness that is home to more than 2,000 species of animals and plants.

The primary threat comes from the sea.

The Everglades, like all of south Florida, is almost flat, which makes the ecosystem extremely vulnerable to rising sea levels, one of the biggest consequences of temperature increases.

The passage of salt water into the freshwater wetlands can have disastrous effects.

The region stores and filters the water that nine million of Florida's population of nearly 21 million depends on.

Once salt penetrates subterranean aquifers, they can be ruined.

In addition, salt water risks destroying the habitat for much of the rare fauna and flora in the area.

Intensifying droughts and reduced rainfall, other consequences of climate change, are also causes for concern.

"As a massive peatland that builds up organic soils over time, this ecosystem has sequestered huge amounts of carbon that are locked in the soils that contribute to the formation of habitats," explains Steve Davis, chief science officer at the Everglades Foundation, a non-governmental organization.

A lack of fresh water not only ends carbon sequestration, it also causes the release into the air of what was stored in the soil.

A double climate disaster.

Multi-billion-dollar project

Gimenez puts on sunglasses, ties a bandanna around his head, and jumps barefoot into his airboat along with Davis.

The boat starts up and speeds through a carpet of green with the water hidden below the vegetation.

It feels like floating on grass.

For thousands of years, water accumulated north of the Everglades in the rainy season, shaping the landscape by moving very slowly as it followed the slight slope of the terrain.

In the last century, however, the natural flow was diverted to allow for urban and agricultural growth in south Florida.

In doing so, it altered the ecosystem of the 1.5-million-acre (607,000-hectare) wetlands, weakening it in the face of climate change.

In 2000, Congress approved a project, funded equally by Florida and the federal government, to protect the area, which whs declared a biosphere reserve by UNESCO in 1976.

Its initial cost was $7.8 billion.

The goal was "to store water, to clean it and to flow that water in the most natural way back to the national park," according to Davis.

To achieve this, scientists devised a complex system of canals, dikes, dams, and pumps.

They also designed artificial marshes to filter the water and rid it of nutrients that damage the wetland.

At the same time, sections of road that blocked water flow to the park were raised.

"Everglades restoration is the model for other ecosystem restoration efforts whether it's wetlands like the Pantanal (in South America) or estuaries like the Chesapeake Bay," Davis says.

"We have the same kind of issues here," he adds. "It's about ensuring the proper quantity of clean water moving through the ecosystem."

Delays

The effects of rehabilitation are already noticeable. Davis gets off the boat, dips his hands into the clear water and scoops up a dark glob from the bottom.

It is periphyton, a mixture of algae, bacteria and microbes, the presence of which indicates healthy water quality.

Despite making some progress, only one of the 68 major projects in the original 2000 plan has been fully completed.

The delays are mainly due to a lack of federal funding.

According to the Everglades Foundation, between $4 billion and $5 billion have been spent so far on the restoration project, with Florida contributing 70 percent and Washington just 30 percent.

The urgency caused by climate change could, however, give a boost to the conservation plan.

President Joe Biden included $350 million for the Everglades in his fiscal 2022 budget, $100 million more than in 2021.

In April, Florida Governor Ron DeSantis signed an agreement with the US Army Corps of Engineers for the construction of a reservoir west of Palm Beach which will cost $3.4 billion.

The size of the island of Manhattan, it "will store a lot of water that will go south, rehydrate these wetlands, recharge the aquifer and push back against sea level rise," Davis says.