WASHINGTON — A day after the United Nations issued its most urgent call to arms yet for the world to confront the threat of climate change, President Trump boarded Air Force One for Florida — a state that lies directly in the path of this coming calamity — and said nothing about it.

It was the latest, most vivid example of Mr. Trump’s dissent from an effort that has galvanized much of the world. While the United Nations warned of mass wildfires, food shortages and dying coral reefs as soon as 2040, Mr. Trump discussed his successful Supreme Court battle rather than how rising seawaters are already flooding Miami on sunny days.

The president’s isolation is not just from the world: In California, New York, Massachusetts and other states, governments and companies are pushing ahead with regulations and technological innovations to reduce greenhouse gas emissions.

That bottom-up activism is a source of hope for those who have watched in despair since last year when Mr. Trump declared he would pull the United States out of the Paris climate accord. But experts say it is no substitute for the world’s largest economy, and second-largest emitter of carbon dioxide, turning its back on the fight.