Since the November election, several big-city U.S. mayors have sent President-elect Donald Trump and many congressional Republicans a message on climate change: You won't stop our fight.

Earlier this month, mayors from dozens of U.S. cities joined a global climate change summit in Mexico City, where they vowed to step up their efforts to reduce greenhouse gas emissions. They circulated an open letter to Trump, signed by 48 mayors, challenging the incoming president to support city-led efforts to ward off global warming.

“While we are prepared to forge ahead even in the absence of federal support, we know that if we stand united on this issue, we can make change that will resonate for generations," they wrote. "We have no choice and no room to doubt our resolve. The time for bold leadership and action is now.”

The mayor who circulated that letter, Eric Garcetti of Los Angeles, told Governing that changes in Washington will do little to slow efforts like his, and, in fact, many U.S. cities are committed to moving even more aggressively on greenhouse gas emissions in the coming years.

“There is absolutely a sense of urgency about our cities and our world today," he said. "Cities bear the brunt of climate change, and cities produce the biggest part of the emissions in the world. So this will be up to cities to solve."

That urgency came through in a report called “Deadline 2020” that was issued by the C40 Cities Climate Leadership Group in Mexico City. It warns that the world’s biggest cities have to act quickly in order for last year’s global Paris Agreement to have any chance of reaching its targets for limiting global warming's impact.

“We have four years to change the world,” the C40 group stated. “For wealthier, higher-emitting cities that means an immediate and steep decline [in emissions].”

A skeptic of climate change, Trump has filled his Cabinet with other climate skeptics and deniers. For example, he has nominated Oklahoma Attorney General Scott Pruitt, who led the legal battles against President Obama's plans to fight global warming, to head the Environmental Protection Agency and Rick Perry, the former governor of oil-rich Texas, to lead the Department of Energy.

The American mayors who appeared in Mexico City offered a mix of defiance and reassurance to the global crowd that echoed the messages they’ve been sending to the incoming Trump administration back home.

“It is important that we all recognize that a single election doesn’t change who we are in cities in America,” said Washington, D.C., Mayor Muriel Bowser at the C40 summit.

(Flickr/C40 Cities)

Phoenix Mayor Greg Stanton also remained adamant about the need for further action, even though it would likely set up clashes with his state's Republican-controlled legislature.

“The reality is we are not going backwards on sustainability or fighting climate change," he told the mayors gathered in Mexico. "This fight isn’t about one leader or one person. This is about our kids and their kids."

U.S. mayors have shared concerns about climate change for more than a decade. In fact, more than 1,000 cities joined an effort to limit greenhouse gas emissions following the failure of the United States to join the 1997 Kyoto Protocol.

They launched a wide variety of measures, mostly focusing on reducing the carbon footprint of government activities. But government activity is only a small slice of what produces that pollution.

Cities don’t have the far-reaching regulatory powers of states or the federal government to directly limit greenhouse gas pollution, but they do have many tools at their disposal.

Garcetti, for example, released a 108-page sustainability plan for Los Angeles with action items that run the gamut -- from launching a green technology incubator and building transit to adding charging stations for electric vehicles and reducing carbon dioxide emissions at the port of Los Angeles. He also helped Chinese cities partner with U.S. cities on the issue. Last week, Garcetti signed a measure to make sure existing buildings use water and electricity more efficiently -- a move the city estimates will reduce L.A.’s greenhouse gas emissions 10 percent by 2025.

“There is no legal way for the federal government to stop us from doing things,” said Garcetti. “There’s no way they can pass a proposal that says, ‘No, you can’t buy electric vehicles. No, you have to make buildings less energy efficient. You can’t have your own municipal utility, and you have to go back to generating more power from coal.’”

At the Mexico summit, Portland, Ore., Mayor Charlie Hales said his city has been working for decades to reduce greenhouse gas emissions, but he anticipated that it would go even further. In fact, the Portland City Council just last week passed three measures to further combat climate change: It approved a measure designed to get 50,000 electric vehicles in the city by 2030; backed a proposal to require home energy scores on homes that are up for sale; and banned export facilities for coal, oil and natural gas within the city.

He said that the emphasis on green initiatives helped Portland’s economy. One analysis shows there were 46,900 jobs associated with green initiatives in the city and just 280 jobs linked to transporting fossil fuels on ships and barges.

“This isn’t just some starry-eyed hope. The economy is based now on talent and green innovation. We have the numbers,” said Hales.

For all of the tough talk, though, the mayors also indicated that they were trying to adjust to new federal priorities.

They are, for example, worried about the loss of federal funds -- not just for defying the Trump administration on climate change but on other issues, such as how they respond to any federal crackdowns on illegal immigration.

Seattle Mayor Ed Murray, though, pointed out that the federal government’s financial leverage was limited. In Seattle’s budget of $5 billion, he said, only $85 million came from the federal government.

“That’s no small number, but, in the larger budget, the federal government has been receding in support for cities for decades,” he said.

The Seattle region, like the Los Angeles and Phoenix areas, is also counting on federal money to help build major extensions to its light rail network.

But Garcetti, who said he’s spoken to Trump about transit on the phone since the election, said Trump grew up in New York City and knows the value of transit.

“I’m not worried that this is an area he doesn’t care about," said Garcetti.

In fact, Trump has vowed to invest $1 trillion in infrastructure over the next decade. In the meantime, Bowser, the mayor of Washington, D.C., said government leaders worried about climate change should be developing projects now that would be ready for a major infrastructure initiative.

That way, she said, they can tell the Trump administration, “You want to invest $1 trillion? I have these projects teed up in my city, and I want them done in a sustainable way.”