California Gov. Jerry Brown (D) will host world leaders for a summit on climate change in San Francisco in September, in a direct challenge to President Trump’s move to pull out of the Paris climate accord.

In remarks to air at the Global Citizen Festival in Hamburg, Germany, on Thursday, Brown will announce the Global Climate Action Summit that will be held ahead of a United Nations meeting on climate change in December.

“Yes, I know President Trump is trying to get out of the Paris Agreement, but he doesn’t speak for the rest of America,” Brown says in a video message. “We in California and in states all across America believe it’s time to act, it’s time to join together, and that’s why at this Climate Action Summit we’re going to get it done.”

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The Trump administration said in June it would pull out of the Paris agreement, signed by President Obama in 2015. Trump said American obligations under the deal were too steep, and that he would try to renegotiate its terms.

“The United States will withdraw from the Paris climate accord … but begin negotiations to reenter either the Paris accord or a really entirely new transaction on terms that are fair to the United States, its businesses, its workers, its people, its taxpayers,” Trump said in a Rose Garden ceremony on June 1.

Since Trump’s election, and his early moves to roll back his predecessor’s efforts to combat climate change, Brown has emerged as a vocal proponent of the Under2 Coalition, a group of national, state and local governments across the world that aims to keep global temperature rise below 2 degrees Celsius.

Brown traveled to China in June, where he held discussions with Chinese President Xi Jinping and China’s special envoy on climate change. Brown signed agreements with the Chinese government and with provincial governments to reduce carbon emissions.

California, Washington and New York last month created the U.S. Climate Alliance, a group that has now ballooned to 13 states that pledge to adhere to the Paris accord’s goals, even if the federal government reneges on pledges made by the Obama administration.

Brown, who is two years away from retirement, has taken up the mantle of opposition leader on climate change issues in the early months of Trump’s administration. In his State of the State address in January, Brown castigated the “climate deniers” who argue against scientific consensus.

“The science is clear. The danger is real,” Brown said at the time.

His announcement in Hamburg comes one day before the Group of 20 summit begins there.

—Correction: An earlier version of this story misstated where some world leaders would be speaking. They have not confirmed they will speak at the event in San Francisco.