Obama, who did not mention Trump by name, made a quick appearance at the conference hosted by his former chief of staff, Chicago Mayor Rahm Emanuel. He said it was an "unusual time" with the U.S. as the only country to walk away from the Paris agreement, but it was a chance for local leaders to come together and fulfill promises the country has made.
"Ultimately the work is done on the ground," Obama said. "Cities and states and businesses and universities and nonprofits have emerged as the new face of American leadership on climate change."
Chicago officials billed the North American Climate Summit, which began Monday evening, as the first of its kind for the city. Leaders elsewhere have taken similar action, despite Trump's announcement earlier this year that the U.S. would pull out of the 2015 Paris agreement, 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.
The idea is to fill the void left by the actions of the Republican president, who has worked to reverse much of Obama's approach to foreign policy, Chicago officials said. Trump has said the terms of the agreement should be more favorable to businesses and taxpayers.
The Chicago charter calls for mayors to achieve a percent reduction in greenhouse gas emissions that's equal to or more than what is outlined in the Paris agreement. It also calls for them to work with scientific and academic experts to find solutions. Some mayors have specifically agreed to commitments to expand public transportation and invest in natural climate solutions such as tree canopy and vegetation.
Emanuel said the current resident of the White House - not mentioning Trump by name - and his environmental officials are in denial on climate change despite facts.
"Climate change can be solved by human action," he said. "We lead respectively where there is no consensus or directive out of our national governments."
Mayors from 51 cities including Paris, Mexico City, San Francisco and Phoenix attended the summit.
Paris Mayor Anne Hidalgo said city residents will be the victims if action isn't taken.
"We cannot afford to be cautious," she said.
Follow Sophia Tareen on Twitter at https://twitter.com/sophiatareen.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.