The expected absence of Biden served to underscore a warning that came Tuesday in the speech that opened the meeting of the United Nations General Assembly. "The climate crisis is the defining issue of our time. It must be the first priority of every government and multilateral organization," said UN Secretary-General António Guterres. "And yet climate action is being put on the back burner — despite overwhelming public support around the world."

The closed-door meeting slated for Wednesday, to be hosted by Guterres and the president of Egypt, is meant as an informal exchange on climate matters ahead of the UN's November climate summit in the Egyptian resort of Sharm el-Sheikh. Biden is expected to host a Global Fund to Fight AIDS, Tuberculosis and Malaria event at the same time, according to people familiar with the matter. The two events are set to take place across Manhattan, already snarled by traffic from visiting diplomats' motorcades, and the president's schedule has been compressed by his attendance at Queen Elizabeth's funeral.