Little by little, he explains, over 15,000 acres of salt ponds – largely ecological dead zones that had been transferred from industrial companies to the state – are being brought back to functional ecosystems. They provide important habitat for species like the Western snowy plover and California least tern, add recreational trails for Bay Area residents, and provide flood protection for the San Francisco Bay – a needed adaptation in an era of rising seas. "It's inspiring but challenging," says Mr. Halsing, the executive project manager.

The work to restore the Bay Area's tidal marshes is just one example of a strategy that has been gaining attention in the past few years from climate change experts. Often described as "nature-based climate solutions," this strategy encompasses a wide range of conservation and restoration approaches involving trees, mangroves, soil, and marshlands.

Many current projects – like the South Bay Salt Pond Restoration Project – have locally targeted goals: improved habitat for species or resilience to climate change-related events like hurricanes, floods, or fire. But investing in such approaches at a large scale has another potential benefit, too, say experts: harnessing the natural ability of trees, plants, and soil to store carbon.

"Nature figured out how to solve the toxic carbon dioxide problem 3 billion years ago when it invented photosynthesis, and we're trying to invent similar processes now to solve carbon dioxide in the atmosphere. So why not use nature," explains Peter Ellis, a forest carbon scientist with The Nature Conservancy, who co-authored a landmark study in 2017 showing that natural climate solutions could accomplish about one-third of the mitigation work required in the next decade to keep warming below 2 degrees Celsius.

Could Planting One Trillion Trees Actually Work?

Those promoting natural climate solutions emphasize that it's just one piece of a puzzle that also requires a major shift away from fossil fuels and carbon-based energy. But many experts are seeing these natural solutions as low-hanging fruit that have yet to be tapped at a large scale.

In January, the World Economic Forum launched the ambitious One Trillion Trees initiative, with the goal of planting and conserving 1 trillion trees around the globe in the coming decade. Even President Donald Trump signed on.

The initiative has received some criticism, even among climate activists, who worry it's overly simplistic, takes emphasis off of the energy shift that needs to happen, and will encourage poorly conceived projects that might perpetuate other environmental issues.

And some climate experts have argued that the claims made by natural-solutions proponents in general are lofty and overly optimistic – that they couldn't come close to reducing carbon dioxide at the magnitude some studies have found.

But those debates, ultimately, are unproductive, says James Mulligan, a senior associate in the World Resources Institute's food, forests, and water program. Climate solutions, he notes, aren't a zero-sum game. Nature-based solutions won't ever be enough on their own, says Mr. Mulligan, but they have some big upsides, particularly that most are relatively low cost, some have more bipartisan appeal, and many are "win-win," with none of the "losers" that can be a byproduct of other strategies.

"The question for me is: would this help? And the answer is yes," says Mr. Mulligan. "Do I think we can restore a trillion trees to the planet? Probably not. ... In the U.S., our analysis shows we could restore 60 billion trees to the American landscape." That, he says, would be a "tall order," but would remove about a half a gigaton of CO2 per year.

"That's a meaningful wedge," he says. "And that's just one nature-based solution."

Protection Before Planting?

All trees – and all nature-based solutions – aren't created equal. And many advocates stress that it makes sense to focus on the ecosystems with the most to offer, or the methods that yield the biggest dividends.

"We need to protect first, to hold the line," says Mr. Ellis of The Nature Conservancy, explaining that he views good management of existing ecosystems as being even more important than restoration.

Certain ecosystems, like mangroves and peatlands, are of vital importance to conserve, says Will Turner, senior vice president of global strategies for Conservation International. In those ecosystems, the soil stores so much carbon that losing much more of it in coming years would be devastating, he says.

But to Dr. Turner, conservation and restoration are two sides of a coin, both necessary. Protecting critical ecosystems like tropical forests and mangroves that are being destroyed at a steady rate is crucial in terms of reducing current emissions, he says. But removing carbon dioxide already in the atmosphere is also necessary, if there is any hope of keeping warming below 2 degrees Celsius.

"We have a long way to go before we have any technology that is capable of removing CO2 from the atmosphere at scale except trees," says Dr. Turner. "We'd be foolish not to invest incredibly heavily in regrowing forests."

Despite all the potential of natural climate solutions, most of the examples being tried so far are at a relatively small scale.

WRI's plan for 60 billion trees planted in the U.S. over the next 20 years, Mr. Mulligan notes, would require about $4 billion a year in federal subsidies. But many of these efforts are "happening at the pace and scale of the conservation sector," he says. And that figure, while relatively modest in terms of government spending, is far beyond what the nonprofit community can handle.

Dr. Turner, of Conservation International, agrees. What the conservation community has done well, he says, is shown how these projects can work, how technology can be used to monitor and verify emissions reductions, and how financial mechanisms can allow governments or corporations to invest in these strategies.

Discovering an Ecosystem in Every Backyard

Meanwhile, part of the beauty of nature-based solutions, Dr. Turner says, is that – while some may certainly have more payoff than others in terms of climate mitigation – "there is something that can happen anywhere. Every community has an option to protect a forest or grow a forest or protect a grassland, or to better manage grazing lands so you can get greater carbon stored in the soil."

And many of those solutions – like the marsh restoration taking place in the San Francisco Bay – offer significant local benefits that go far beyond potential emissions reduction: habitat for endangered species, cleaner air and water, recreation opportunities for residents, flood risk mitigation at a time of rising seas.

In the Bay Area, emissions mitigation isn't a real driver of the restoration work, and the carbon market for wetlands isn't as robust as that for forests. But that doesn't mean those benefits don't exist, says Letitia Grenier, co-director of the Resilient Landscapes Program for the San Francisco Estuary Institute.

In her role at the institute, Dr. Grenier looks for creative ways to harness the natural benefits of ecosystems in ways that work for both people and nature – and they are plentiful, she says.

"One of the things climate change has shown us is that we live in ecosystems," says Dr. Grenier. "Not only do we impact ecosystems, but our ecosystem impacts us." In many instances, she says, when she looks at, say, a large watershed, the system is essentially broken. Too many discordant elements have been introduced.

"Suddenly, our system is not working for us," says Dr. Grenier. "Climate change is creating the realization of that, and the opportunity to fix it."