OTTAWA — The timing was coincidental, but the meeting had a new urgency.

One day after President Trump withdrew the United States from the Paris climate accord — saying he was elected to serve Pittsburgh, not Paris — the transport minister in Prime Minister Justin Trudeau’s cabinet visited with the mayor of Pittsburgh to discuss climate change.

The meeting was part of a broad strategy by the Canadian federal government to work directly with American states and cities on global warming, and to become a leader on the issue. Though the effort began several months ago, Mr. Trump’s rejection last week of the Paris agreement has energized it.

“I think it’s disappointing that the United States’ administration has not stepped up,” said Catherine McKenna, Canada’s environment minister, referring to Mr. Trump’s decision on the climate pact. “But the United States is bigger than the administration.”

Mr. Trudeau has made battling climate change a top priority since his election in 2015, and Canadian provinces and American states have been dealing directly with one another on a variety of initiatives. Quebec and California have linked their cap-and-trade programs — an effort that Ontario, the most populous province, is set to join. Ontario and Manitoba participate in a climate coordination group primarily made up of Midwestern states.