Speaking in Alaska, he said countries including his own were not doing enough to stop global warming.
World powers meet in Paris in December with the aim of agreeing to curb global temperature rises.
Mr Obama said now was the time “to protect the one planet we’ve got while we still can”.
“On this issue – of all issues – there is such a thing as being too late,” he told a meeting of foreign ministers from
countries with Arctic interests soon after arriving in Alaska. “That moment is almost upon us.”