Former President Obama is in Chicago addressing mayors on climate change, after President Trump made a decision to withdraw the US from the agreement.
Other mayors expected to attend include Ed Lee of San Francisco, Valerie Plante of Montreal and Miguel Angel Mancera of Mexico City.
In June, Trump announced the U.S' withdrawal from the historic global climate agreement reached in 2015, joining only two countries, Syria and Nicaragua, in rejecting the deal.
He explained mayors are stepping up because there's no consensus from national governments.
"All the USA cities signing the charter are making commitments in support of America's Pledge, which sends a strong signal to the world that we will keep moving forward toward our Paris goal, with or without Washington".
Mayors from more than 50 cities attended the summit, which began Monday evening.
Earlier, Obama also discussed climate issues with Paris Mayor Anne Hidalgo.
"We can not afford to be cautious", she said. Some mayors have specifically agreed to commitments to expand public transportation and invest in natural climate solutions such as tree canopy and vegetation. The Republican has said the terms of the agreement should be more favorable to businesses and taxpayers.
"They're entitled to their own opinion, but not their own facts", he said.