When President Trump announced plans to withdraw the United States from the Paris climate agreement, a wave of blue states vowed to fill the void left by federal leadership. Those pledges have taken on new significance in the wake of a report from the Intergovernmental Panel on Climate Change that underscores the importance of limiting warming to 1.5 degrees C. While the actions of individual states and cities may not be enough on their own, these regional efforts can provide testing ground for strategies that might work at a larger scale. Of the governments committing to climate action, California is the main driver, both in its size and its ambition. Its efforts, energy economist Danny Cullenward says, could ripple far beyond the state. “The process of planning and thinking through really radical transformations in the energy sector is a process everyone is going to have to go through,” says Mr. Cullenward. The work that happens at the regional level, he adds, could inform broader national efforts.

Last month, a major new climate commitment came out of the United States: a pledge to achieve carbon neutrality by 2045.

It’s the sort of commitment that scientists say is increasingly necessary if the world wants to avoid the most severe repercussions of global warming.

And it involved a major economy – the world’s fifth largest – but not the federal government.

Instead, it was California Gov. Jerry Brown who signed the executive order, in advance of the Global Climate Action Summit in San Francisco in mid-September.

As a major new climate report from the Intergovernmental Panel on Climate Change underscores the need for transformational action, many experts have criticized the United States for its lack of action and for its withdrawal from the Paris Agreement commitments.

But below the federal level, states, cities, and companies are stepping into that leadership void in groundbreaking ways. Seventeen governors of US states and Puerto Rico have pledged to reduce their climate emissions in keeping with the Paris agreement guidelines. More than 3,500 entities – states, cities, businesses, and other institutions – have joined the “We Are Still In” coalition, also pledging to support climate action to meet Paris commitments. It’s a group that represents almost 60 percent of US gross domestic product. If it were a country, it would be the world’s third-biggest economy.

The action, say experts, is encouraging. But it may not be enough.

Since President Trump announced he planned to withdraw from Paris more than a year ago, Nathan Hultman says he’s been “both surprised and heartened by the amount of engagement that we have seen in this country by an extraordinarily diverse set of actors at different kinds of organizational levels.”

“There has been a kind of grassroots momentum and an embracing of the opportunities for fast action on clean energy that do deliver change that we want,” says Dr. Hultman, director of the Center for Global Sustainability at the University of Maryland, and a key author of a recent report put out by Bloomberg Philanthropies that looks at climate action by states, cities, and businesses. At the same time, he cautions, “It can’t be just a half or two-thirds of the states in the US carrying all the burden for the next 20 years. We can’t get there from here if we have partial engagement.”

The Bloomberg report, “Fulfilling America’s Pledge,” looked at the actions taken by the entities in the “We Are Still In” network, as well as actions that could feasibly be taken if states, cities, and companies were to increase their climate engagement. Currently, the authors determined, the US is about halfway to its Paris Agreement target of a 26 to 28 percent emissions reduction by 2025, compared with 2005 levels. A continuation of the current commitments would get the US to about a 17 percent reduction by 2025: roughly two-thirds of the way there. A “plausible best-case scenario,” with more states diving in and adopting more of the best climate strategies, could get the US to a 24 percent reduction by 2025, says Hultman.

Can California lead the charge?

Of the governments committing to climate action, California is the main driver, both in its size and its ambition.

It’s the largest economy to have made a commitment to carbon neutrality, which the latest IPCC report has said the whole world needs to achieve by 2050 if it wants to avoid warming of more than 1.5 degrees C above preindustrial levels.

“California is in the interesting situation of exploring the path to deep decarbonization,” says Danny Cullenward, an energy economist and policy director of Near Zero, a nonprofit that analyzes ways to quickly cut greenhouse gas emissions with a focus on California.

Its efforts, he says, could ripple far beyond the state.

“The process of planning and thinking through really radical transformations in the energy sector is a process everyone is going to have to go through,” says Mr. Cullenward. The work that happens at the regional level, he adds, could inform broader national efforts.

For instance, California’s efforts to achieve 100 percent clean electricity by 2045, as pledged in a new law signed by Governor Brown, is “going to set up a real planning process to start talking about how we transition the grid,” Cullenward says.

California’s effort to regulate vehicle emissions could become template for other states if they survive challenges legal challenges posed by the Trump administration.

Not everyone is happy that California is so aggressive in pursuing its own climate agenda. Carol Seperas, a retired insurance agent in Sacramento, Calif., says she worries Brown’s carbon-neutral pledge could hurt the state’s economy.

“It’s overregulation, and I’m concerned that as time goes by, we’ll fall behind because other states aren’t doing the same thing,” says Ms. Separas, who tends to vote Republican. At the same time, she says, “people who don’t live here don’t understand what California has to deal with in terms of its population, how fast we’re growing, the number of cars on the road. So we have to make sure we do what’s right for the environment – within reason.”

California, though, is not alone in taking action. The states that make up the US Climate Alliance include Colorado, Virginia, and Minnesota, notes Dan Lashof, US director for the World Resources Institute (WRI). “It’s not just the coastal dark blue states,” he says.

Will state-led action be enough?

Many experts say that, while the actions of individual states and cities may not be enough on their own, especially in light of the new IPCC report emphasizing the need to keep warming below 1.5 degrees C, they’re often the testing ground for strategies that can work at a larger scale.

“The early movers create momentum that’s far beyond their own direct impact,” says Derek Walker, vice president of US climate for the Environmental Defense Fund. Cities, for instance are “at the roll-up-your-sleeves and get-things-done-on-the-ground level,” says Mr. Walker. “City actions can be hugely transformational.”

And as more states and cities and companies spearhead climate solutions, often at the behest of voters or customers, the success they find can be catalytic, he says.

“There’s not much ground to stand on anymore to say that there aren’t solutions. When even big companies are saying this is going to be good for our bottom line, there’s less and less solid ground to stand on to be against progress,” says Walker. “It doesn’t even need to be about the science for people out in real America to get behind it, because people are seeing [the effects of climate change] with their own eyes and feeling it with their hands, whether that’s a farmer, whether it’s someone that owns property that’s been damaged by a storm.”

Some individuals, too, say they’re happy to be seeing action at any level, even if they wish more was happening at the federal level. “I’m very proud of my state for being part of it,” says Elizabeth Derderian, a practice support specialist at the Boston Medical Center, of Massachusetts’s pledge to meet Paris targets.

“It really should be the federal government doing it, but it’s not happening, so someone has to do it,” she says.”Hey, if you’re going to drown in a small room or in an enclosed space, which is basically what’s happening right now, even tossing a salad bowl of water out is going to help you more than not doing anything.”

Experts agree. The contributions of states and cities is meaningful, “but it’s nowhere near enough,” says Lashof of WRI. “And in fact, what the latest IPCC report shows is that we should be accelerating US emission reductions, going well beyond the existing pledge, not struggling to get most of the way there. Having the federal government not only not helping, but pushing in the wrong direction, is a huge problem and it gets bigger over time.”

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What those smaller actions do is provide hope, say Lashof and others, that the answers exist and huge technological progress is being made. Where solar cells once cost about $300 a watt, they now cost $0.37 a watt, notes Lashof. “There are examples of most of the solutions we need that are being implemented in various locations. The problem is that they’re not being implemented universally, at the scale that’s really needed given the urgency underlined by the IPCC.”

Elena Weissmann and Martin Kuz contributed to this report from Boston and Sacramento, Calif.