China’s top climate change envoy, Xie Zhenhua, apologized as he entered late for a weekend meeting at the United Nations climate summit.

“We have quite busy schedules,” Mr. Xie said, according to two people who were present for the exchange. “Especially for me, I have to meet with John Kerry almost every day.”

In an unexpected development, the United States and China on Wednesday announced in a joint statement that they will both do more to cut fossil fuel pollution this decade. The terms of the deal weren’t groundbreaking — but the fact that agreement occurred at all is notable, considering the badly strained ties between Washington and Beijing over trade, human rights, Taiwan and other serious differences.

Despite that, according to American and Chinese officials, the agreement was the product of months of meetings between Mr. Xie and Mr. Kerry, President Biden’s global climate envoy, before they arrived in Glasgow for the conference, known as COP26. The two also held near daily discussions at the summit, these officials said.

Even as Mr. Biden earlier in the summit publicly scolded President Xi Jinping for not attending in person, setting off a round of sniping from Beijing in return, Mr. Xie and Mr. Kerry continued to quietly meet to discuss whether China could increase its ambition on climate.

Over the past 10 days the two envoys, as well as their negotiating teams, met frequently in one or the other’s delegation office, windowless makeshift white rooms of thin partitions held together by metal hinges, set up in a cavernous exhibition center where the two weeks of talks are being held in Scotland.

Mr. Kerry, 77, and Mr. Xie, 72, have known one another for more than 20 years and both came out of retirement to take on their country’s top climate positions. Speaking through masked interpreters, they talked about their grandchildren, Mr. Kerry’s vacation home and Mr. Xie’s garden before launching into more intense negotiations about coal, methane and greenhouse gas emissions, according to a senior U.S. official who was part of the talks.

On Wednesday the world’s two largest greenhouse gas emitters sounded more like allies in the fight against climate change than fierce rivals, with both Mr. Xie and Mr. Kerry saying both nations were responsible for keeping rising global temperatures from reaching dangerous levels.

“We both see the challenge of climate change is existential and a severe one,” Mr. Xie said. “As two major powers in the world, China and the United States, we need to take our due responsibility and work together and work with others in the spirit of cooperation to address climate change.”

Tensions between the United States and China are at their highest point in years. Mr. Kerry said that he has been “honest” about concerns regarding China’s treatment of Uyghurs and other Turkic Muslims in China’s semiautonomous Xinjiang province, but said his focus has been to be “the climate guy.” He also said the two countries must work together to pivot away from fossil fuels no matter what other issues arise.

António Guterres, the United Nations Secretary General, called it “an important step in the right direction.” Laurence Tubiana, France’s former climate change ambassador, said it shows “that the two countries can cooperate to address the climate crisis.”

But experts agreed that the terms of the deal fall far short of an agreement that Mr. Kerry and Mr. Xie helped broker in 2014 in which both the United States and China jointly announced new targets to cut emissions.

That moment was groundbreaking because China, as a developing country despite its skyrocketing emissions, was exempt under the rules of the U.N. climate body from taking mandatory action to cut emissions. The 2014 deal helped spur the Paris climate agreement one year later, in which nearly 200 nations of all levels of wealth and responsibility for causing climate change agreed to act.