Mr. Suga, the Japanese prime minister, acknowledged the realities of the global time differences by beginning his remarks to his fellow leaders with a cheery, “Good morning, good afternoon and good evening.”

As the speeches went on, Mr. Kerry joined Mr. Biden at the conference table to welcome other world leaders to the summit — although he said with a laugh to Mr. Biden, “If you can call it a summit.”

Douglas Brinkley, a presidential historian, said the inability of the United States government to solve basic computer glitches made him uneasy about its larger promises.

“It was amazing to see how bad the technology is and it makes you think, how are we going to solve climate change when you can’t even do video linkage for world leaders?” he said.

Despite the technical glitches, leaders expressed open relief at working with a United States administration that embraces science and again acts as part of the global community.

“The Paris Agreement is humanity’s life insurance,” said Ursula von der Leyen, head of the European Commission. “It is so good to have the U.S. back on our side.”

Brad Plumer contributed reporting.

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