This seems weird, doesn't it?

Not because of the sentiment. Climate change presents a real danger to humanity, and it’ll hit the humans who live near oceans first. Los Angeles, where Eric Garcetti is mayor, has a population of over 10 million people, a quarter of California’s humans, and the busiest port in the United States. Sea level rise and pollution matter there.

And it’s not weird because Garcetti was wrong about the politics. President Trump has, as you’ve no doubt read, expressed doubt in the reality of climate change (he’s wrong about that) and threatened to withdraw the United States from the Paris Agreement (he could, but it wouldn’t be simple) signed by 200 countries in 2015.

No, it was a strange thing to say because … cities don’t make international treaties. Countries do. But the slightly strange terms of the Paris Agreement—every signatory agreed to voluntary cuts in emissions called Nationally Determined Contributions, and the US Senate didn’t ratify it before it became international law—well, maybe a mayor could sign? Somehow? And that would be awesome?

“We do see ourselves as signatories,” says George Kivork, Mayor Garcetti’s press secretary. “It’s essentially saying, we’re going to continue the steps we said we were going to take when our country was committed to the Paris Agreement. We’re going to continue taking those actions.”

Like a lot of cities around the world, Los Angeles is trying to clean up its act, climate-wise. That means adding transit, tightening pollution controls, even changing zoning laws. In a March statement Garcetti said the city would “reduce greenhouse gas emissions 80 percent by 2050, move toward zero emissions transportation, and pursue our vision of a 100 percent clean energy future.” Los Angeles participates in a bunch of coalitions of mayors aiming to do similar work—Climate Mayors, the Compact of Mayors, the global coalition C40. Paris and London are pushing to reduce the number of cars in their city centers, and 30 US cities have asked automakers to figure out how to deliver over 100,000 electric cars for fleets and other uses—forcing down their costs by buying in bulk, basically.

Which is all great. But that’s different than saying “we’re signing onto Paris.” That tweet wasn’t the first time Garcetti has expressed that commitment. He said it in April, in his State of the City Address after winning re-election (in a walk). Still, cities signing the Paris Agreement? Not a thing. “It’s really of political significance, not of legal significance,” says Dan Bodansky, an international environmental lawyer at Sandra Day O’Connor College of Law at ASU.

That doesn't make Garcetti's point meaningless, though. Cities working in concert—not just Los Angeles alone, but LA and Chicago and Boston and New York and and and—could actually help fight climate change as much or more than a national or international law. “It’s similar to what cities did when the Kyoto summit agreement was not ratified by the United States, back when I was a chief of staff to a mayor,” says Sam Adams, director of WRI US at the World Resources Institute and former mayor of Portland, Oregon. “Hundreds of cities signed up with a pledge to adhere their cities to the agreement.”

They might be able to live up to those pledges, too—because in some respects climate change is their fault. Seventy percent of energy-related carbon dioxide emissions come from cities, and three quarters of that from buildings. From a regulatory perspective, that’s low-hanging fruit. “A national government cannot determine or institute or mandate whether there is a fleet in a city or not, or a subway. It can’t mandate if that bus fleet should be hybrid, compressed natural gas, or electric,” says Seth Schultz, director of science and innovation for C40, a global climate and cities consortium. “They cannot mandate sidewalk setback or a bicycle sharing system. But there are ways cities can control huge swaths of emissions from the transport sector.”

If you add up all the action that’s already taking place by cities and states and counties in the US, we are probably well on our way. Seth Schultz, Director of Science and Innovation, C40

The peculiar history of the Paris Agreement—as distinct from, say, the Kyoto Protocol—makes cities even more able to participate, at least politically. Early on in the Paris process, a faction lobbied to allow “non-state actors” like cities, states, and even businesses to be official signatories to the agreement. “But there was really zero interest by actual states in allowing that, so the references to non-state actors in the Paris Agreement are quite modest,” Bodansky says. Still, the non-state actors had influence. California’s governor, Jerry Brown, was a breakout hit, and Paris mayor led a sidebar, all-mayoral-action conference.

So the non-state actors are kind of doing it on their own. Lobbyists from entrenched petrochemical countries don’t drown cities in money they way they do national-level politicians, and cities don’t get as much money directly from the federal government as they used to. On a district-by-district voting map, cities are typically blue islands amid seas of exurban and rural red. And cities, especially coastal cities, are already seeing the consequences of shifting weather patterns and rising seas. All that gives mayors political coverage to take action. “If you start looking at the non-state actor space in general, there are some people saying we might meet our Nationally Determined Contribution of 26 to 28 percent reduction by 2025,” Schultz says. “If you add up all the action that’s already taking place by cities and states and counties in the US, we are probably well on our way.”

That kind of aggregation of non-state action is called a “synthetic NDC.” It’s not nothing. He says that the 91 member cities in C40 generate 25 percent of global GDP. And unlike, say, transnational corporations, those cities aren’t competing with each other to come up with and protect their best ideas—they share them, freely. “We get our cities to do something, it makes a massive dent.” Schultz says.

That means Garcetti’s talking point isn’t so weird after all. Cities like Los Angeles may have gotten locked out of signing the Paris Agreement, but that doesn’t mean they might not make it work. Presidential administrations are temporary; cities, one hopes, are less so.