“The world has those that follow and those that lead. And those that lead, some lead in the wrong direction and some lead in the right direction,” Mr. Guterres said at a news conference Monday at United Nations headquarters. “You are of those that lead and have always led in the right direction,” he told Mr. Bloomberg, “and it is an enormous privilege for me to be able to work so closely with you.”

In a statement, Mr. Guterres said he had asked Mr. Bloomberg to help him encourage governments and private businesses to rapidly reduce the emissions of greenhouse gases to meet the targets of the Paris climate accord.

“The Secretary General and Michael Bloomberg share the perspective that the emissions gap needs to be closed soon to limit global temperature increase to below 2 degrees Celsius,” Mr. Guterres’s office said in a statement.

President Trump has said the United States will pull out of the Paris climate agreement. He has rolled back a host of regulations to protect the environment, appointed climate science deniers to key posts in the administration and championed the use of fossil fuels, defying the scientific consensus that emissions from those fuels are a major cause of global warming.

On the campaign trail, Mr. Trump described climate change as a “hoax.”

Mr. Bloomberg, who was mayor of New York City from 2002 to 2013, has teamed up with one of Mr. Trump’s most outspoken critics, Gov. Jerry Brown of California, to press cities and states to reduce their own greenhouse gas emissions to help the United States meet its Paris agreement targets.