That may be overly optimistic. Sixteen American states and Puerto Rico have pledged to stick to the commitment that the United States made in the Paris agreement to reduce emissions by at least 26 percent by 2025. Those states are on track to keep their promise.

But they represent less than a half of the country’s population, and the United States as a country will most likely fall short of its Paris pledge as Mr. Trump dismantles environmental regulations, according to a 2017 study by the Rhodium Group, a private economic research company. And a group led by Michael R. Bloomberg, the United Nations special envoy on climate change, and Gov. Jerry Brown of California, came to the same conclusion in a report that relied on the same data.

The Paris accord is written in such a way that the United States, in fact, remains in the pact even though it announced its intent to pull out. The actual withdrawal does not happen until 2020.

Mr. Guterres is planning a summit meeting next year to goad world leaders to raise their emissions reductions targets. But few countries are even close to meeting the targets they set under the Paris agreement, which was drafted in November and December in 2015, according to independent analyses.

His warnings came a week after the World Meteorological Organization, a United Nations agency, reported that a barrage of extreme weather events had made 2017 the costliest year on record for such disasters, with an estimated $320 billion in losses.