The announcement was all the more notable for being made with the governments of Mexico and Canada, which agreed to implement their own version of some of the state-level policies.

“This is a statement of confidence that we can lick this problem,” Jay Inslee, the governor of Washington and a Democrat, told me. “We have to fight the despair that some people feel because of the Trump administration’s climate denial and lack of planning. We have to defeat that.”

“If man is going to destroy our fellow man, it’s either going to be by nuclear proliferation or because we ignore the simple reality that our environment is getting warmer, it’s going to overwhelm us, and we are approaching a tipping point,” said Dannel Malloy, the Democratic governor of Connecticut, at the Thursday event. “We simply need to do everything in our individual power and as states together to hold that back.”

The governors are acting as members of the U.S. Climate Alliance, a coalition of states that have promised to abide by the Paris Agreement on climate change. The alliance is made up of 14 Democratic governors, including Ricardo Rosselló of Puerto Rico, and three popular Republican governors from traditionally blue states: Larry Hogan of Maryland, Charlie Baker of Massachusetts, and Phil Scott of Vermont.

They issued the new policies at the Global Climate Action Summit in San Francisco, a conference of countries, companies, and local governments that have committed to fighting climate change. Part pep rally, part technology exhibition, the summit is meant to inspire flagging environmental hopes while celebrating the legacy of Jerry Brown, California’s outgoing Democratic governor, who has helped turn the state into a climate-policy powerhouse.

The summit is also global recognition that state and city governments are, for now, the sole hope for aggressive American climate policy. The U.S. Climate Alliance is the star of that show: It is by far the largest coalition to emerge since Trump took office. Taken together, the 16 states in the alliance represent 40 percent of the U.S. population and roughly half of its GDP.

But the alliance also represents the pitfalls of a subnational approach. Despite essentially forming the world’s third-largest economy, the 16 Climate Alliance states represent only a quarter of U.S. carbon emissions, according to 2015 data from the Energy Information Administration.

So it remains an open question how successfully states and cities can fight climate change, especially if the federal government pushes the other way. A recent Yale report found that the current slate of state and local policies would only get the United States half of the way to meeting its 2030 carbon-cutting promises under the Paris Agreement.

The Thursday announcement encompassed five policies, several of which have been described before. Some of them are targeted directly at Trump’s actions.