Gov. Jerry Brown of California announced the Climate Action Summit, planned for San Francisco in September 2018, in a video.

Gov. Jerry Brown of California on Thursday reinforced his reputation as America’s de facto leader on climate change, announcing to cheering crowds in Hamburg, Germany that his state would gather leaders from around the world for a global warming summit next year.

Speaking by videoconference to the Global Citizens Festival in Hamburg, where President Donald Trump is joining other world leaders for the Group of 20 economic summit, Governor Brown said the president “doesn’t speak for the rest of America” in pulling out of the Paris agreement on climate change.

“Look, it’s up to you and it’s up to me and tens of millions of other people to get it together to roll back the forces of carbonization and join together to combat the existential threat of climate change. That is why we’re having the Climate Action Summit in San Francisco, September 2018,” Governor Brown told the crowd.

“Yes, I know President Trump is trying to get out of the Paris agreement, but he doesn’t speak for the rest of America,” he said. “We in California and in states all across America believe it’s time to act.” Mr. Trump declared in June that the United States would withdraw from the 2015 pact, in which nearly 200 nations pledged to curb greenhouse gas emissions and support poor countries’ plans to develop clean energy and protections against climate-related disasters.

The California summit meeting will bring together the leaders of states, cities, businesses and others who made pledges to curb heat-trapping greenhouse gas emissions as part of the Paris accord and the thousands of others who were galvanized by Mr. Trump’s decision. Days after his announcement, the former New York City Mayor Michael Bloomberg published a letter to the United Nations signed by more than 1,200 mayors, business leaders, university presidents and others declaring “we are still in” the climate deal.

According to Governor Brown’s office, the summit meeting will be the first time an American state has hosted an international climate change conference with the direct goal of supporting the Paris Agreement.

“With the Trump administration’s rather comprehensive moves to reverse the Obama administration’s suite of climate policies, the potential importance and the prominence of all these subnational actions and actors is greater now than ever before,” said Robert Stavins, an energy economist at Harvard University.

The former United Nations climate chief, Christiana Figueres, wearing a T-shirt declaring herself “Stubborn Climate Optimist,” introduced Governor Brown as a “stubborn optimist from a surprising country, the United States.”

Bruce E. Cain, a professor of political science at Stanford University, noted that Governor Brown was no stranger to environmental crusading. “This is a perfect role for him,” Mr. Cain said. “The only problem is he’s got less than two years to play that role, so for him the challenge is how to segue out of office and still be relevant to the debate.”

Governor Brown has spent the past several months countering the president’s message on climate change. He invited Prime Minister Frank Bainimarama of Fiji, who is hosting this year’s United Nations climate change conference, to the governor’s mansion in Sacramento. He also traveled to China, where he encouraged nations to move toward renewable energy and act faster to reduce carbon emissions.

Governor Brown maintained that his goal was not to antagonize the president, but to urge everyone to do more. “It isn’t being cooked up because of Trump,” Governor Brown said in an interview Wednesday. “No nation or state is doing what they should be doing. This is damn serious, and most people are taking it far too lightly than the reality of the threat. You can’t do too much to sound the alarm because so far the response is not adequate to the challenge.”

He predicted the opposition to climate change policies by Mr. Trump and many Republicans would shift as evidence of the consequences of climate change mounted. “If the whole world except the U.S. is sufficiently galvanized, it will only be a short period before the U.S. falls in line,” he said. “I think President Trump unwittingly is serving to stimulate the movement toward decarbonization by his very public, idiosyncratic resistance to both the science, the diplomacy and the politics.”
Correction: July 6, 2017
An earlier version of this article misstated the timing of President Trump’s announcement that the United States would withdraw from the Paris agreement. It was in June, not May.