ANCHORAGE — President Obama on Monday issued a global call for urgent action to address climate change, declaring that the United States was partly to blame for what he called the defining challenge of the century and would rally the world to counter it.

“Climate change is no longer some far-off problem; it is happening here, it is happening now,” Mr. Obama said here at an international conference on the Arctic. “We’re not acting fast enough. I have come here today, as the leader of the world’s largest economy and its second-largest emitter, to say that the United States recognizes our role in creating the problem, and we embrace our responsibility to help solve it.”

In remarks that bordered on the apocalyptic, Mr. Obama warned that the effects of global warming that have hit the Arctic the hardest would soon engulf the world — submerging entire countries, annihilating cities and leaving fields barren — unless more was done to reduce emissions. Four times in a 24-minute speech, he repeated his assertion that “we’re not acting fast enough.”

The president spoke at the beginning of a three-day Alaska trip choreographed to lend vivid visual justification — in the form of receding glaciers, eroded shorelines and rising seas — to his drive for an international accord to reduce heat-trapping emissions leading up to a United Nations summit meeting in Paris in December.