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CHICAGO — Former President Barack Obama says cities, states and nonprofit groups have emerged as “the new face of leadership” on climate change.

He briefly spoke Tuesday to a summit of mayors from around the world gathered in Chicago to address concerns about climate change since President Donald Trump rejected the Paris climate accord. The mayors signed a charter that echoes portions of the 2015 Paris agreement.

Obama didn’t mention Trump by name, saying only that the U.S. was in an “unusual” position as the sole country to reject the Paris agreement.

Trump announced earlier this year that the U.S. would pull out of the Paris accord, which involves nations setting benchmarks to reduce emissions of heat-trapping gases. The U.S. won’t technically back out until 2020 because of legal technicalities.

Mayors from more than 50 cities attended the summit, which began Monday evening.

Mexico City Mayor Angel Mancera says in a statement that his sprawling capital of about 9 million is committed to combating climate change through programs such as ensuring all residents have access to alternative transportation like walking and cycling, as a matter of land use policy. Mancera says the city is trying to accelerate a transition to soot-free engines and procure zero-emissions buses by 2025 with an overall zero-emissions goal for most of the city by 2030.

San Francisco Mayor Ed Lee says rising water levels and affordable housing are concerns for his city.