GLASGOW, Scotland — Speaking at a high-profile climate summit that attracted more than 100 world leaders, President Biden apologized Monday for the Trump administration’s inaction on climate, lending a personal note to his administration’s efforts to sharply reverse the U.S. position on what he views as an existential issue.

“I guess I shouldn’t apologize — but I do apologize for the fact that the United States and the last administration pulled out of the Paris accords and put us sort of behind the eight ball a little bit,” said Biden, speaking briefly at a breakout meeting of world leaders at the COP26 climate summit.

Presidents rarely apologize for broad U.S. policies, even when they are pushing to change them, and the message carried additional resonance for being delivered overseas. Biden’s comments, which came during a smaller meeting after his address to the opening session of the climate conference, highlighted the policy U-turn that he is seeking and underscored how Biden is using this foreign trip to move America away from his predecessor’s policies.

During the Trump administration, the United States, the world’s largest economy, walked away from the Paris climate accord and was virtually invisible at international climate talks. The Biden administration in contrast has shown up in force in Glasgow, with a delegation featuring not only the president, but also a big majority of his Cabinet and a sizable group of career officials. American luminaries such as former president Barack Obama and former vice president Al Gore are also taking part.

“This is the decisive decade,” Biden said in remarks to fellow world leaders. “To state the obvious, we meet with the eyes of history upon us.”

President Biden called for "a decade of ambition" on climate change on Nov. 1 in his opening remarks at the COP26 summit in Glasgow, Scotland. (The Washington Post)

Biden plans to spend the next two days in Scotland working to reestablish the United States as a leader on climate issues. Shortly before he spoke Monday, the White House released a strategy to reach net zero emissions by 2050 and announced that more than 70 countries have joined an initiative led by the United States and the European Union to reduce methane emissions by 30 percent by the end of the decade.

The president will also work with Congress to set aside $3 billion a year to help developing countries adapt to climate change. That sum, which will go toward protecting the world’s poorer citizens from rising seas and temperatures, is part of the $11.4 billion Biden already announced at the U.N. General Assembly in September.

Biden said the United States has a special responsibility to lead on climate because it is a major greenhouse gas contributor. “I want to emphasize again, those of us who have deforested a long time ago, those of us who have taken actions a long time ago to cause the problems we have, we have to be ready to step up,” he said.

The president sought to put the issue in historical terms, signaling that he was thinking of his legacy and that of his generation of leaders. Speaking at the smaller forum, Biden mused about what “future historians” would write about how heads of state acted in this decade, whether they would say “we let this final chance to stem the crisis slip through our fingers” or “in the 2020s, we stepped up.”

For all the dramatic talk and sweeping promises, the limits of the conference were evident, too.

The leaders of two major greenhouse-gas-producing countries, China’s Xi Jinping and Russia’s Vladimir Putin, declined to attend the conference. Given the growing confrontation between those two powers and Western nations, their decision to snub the summit could forecast difficulty in persuading them to join global climate change efforts.

Some climate activists dismissed the goals set by Biden and other world leaders in Glasgow as woefully inadequate to address the looming disaster. Others noted that Biden so far has been unable to pass the climate program embodied in two major bills before Congress, his infrastructure plan and his social spending package.

“Biden is at Glasgow empty-handed, with nothing but words on paper,” said Varshini Prakash, executive director of the Sunrise Movement. “It is humiliating and fails to meet the moment that we’re in.”

Biden signaled a recognition of his struggles at home, acknowledging that he has a sales job to do among some voters about the importance of reducing emissions. “That’s the next big case I have to make,” he said.

Still, the president’s pledge to pour billions into developing countries as they try to fend off the floods, droughts and famines resulting from climate change came as a surprise to those watching closely. It was read by some as a way to counter China, which has been pursuing a program to construct major projects in developing countries.

“Biden’s unexpectedly strong emphasis on financing for developing countries can be seen as a direct attempt to counter China’s influence through its Belt and Road global infrastructure efforts,” said Paul Bledsoe, a former Clinton White House climate adviser who is now with the Progressive Policy Institute.

Biden’s aid for developing countries needs congressional approval, like the bulk of his promises at the summit, many of which are tied up in his multitrillion-dollar legislative agenda awaiting action by Congress. Biden touted his “Build Back Better” agenda in Glasgow, saying it would enable the United States to meet his goal of cutting emissions by 50 percent or more by the end of the decade. The Biden White House has also set a goal of making the power sector 100 percent carbon-free by 2035.

“My Build Back Better framework will make historic investments in clean energy, the most significant investment to deal with the climate crisis that any advanced nation has made, ever,” Biden told leaders during his remarks Monday morning.

But as Biden was touting his plan in Scotland, a new hurdle for the legislation emerged at home. Sen. Joe Manchin III (D-W.Va.), a key moderate vote in the Senate, told reporters Monday that he is not ready to support the social spending legislation, asserting that its revenue-raising portions are “shell games” and that the bill is a “recipe for economic crisis.”

That unexpected rebuke left administration officials highlighting other ways Biden has begun to make progress on his pledge to cut emissions, for example, though administrative action and by scrubbing the U.S. code to find ways that various agencies can incorporate climate goals as they make policy.

The president’s top domestic climate adviser, Gina McCarthy, told reporters in a call Sunday afternoon that the administration would show up to Glasgow on “strong footing” because of the president’s efforts to move the gears of the federal government.

“We have and will continue to use every agency and every tool at our disposal to marshal a climate response,” McCarthy said, noting that the administration had embraced higher fuel-efficiency standards for cars, advanced wind and solar projects, and begun working to reduce emissions of potent greenhouse gases such as methane and hydrofluorocarbons, or HFCs.

U.S. officials also downplayed concerns about whether the Biden administration’s actions will pass constitutional muster. The Supreme Court on Friday granted a request from 19 Republican-led states and the coal industry to consider their challenge to the Environmental Protection Agency’s authority to curb greenhouse gases from power plants, potentially threatening another plank of Biden’s climate agenda.

The GOP states are trying to prevent the EPA from issuing the type of sweeping emissions controls that were proposed by the Obama administration. The Supreme Court put Obama’s Clean Power Plan on hold in 2016, and it was never implemented.

Despite Biden’s domestic challenges, the U.S. presence in Glasgow was unmistakable as the administration sought to send the message that America was reasserting its leadership role. A large venue called the U.S. Center staged dozens of panel discussions.

Among its first events Monday was a session titled “America All In.” Others dealt with “Equitable Deep Decarbonization” and “Tech For Net Zero.” The U.S. Center is run by the State Department and emblazoned with an epigraph from Biden that reads: “We’ve already waited too long to deal with this climate crisis and we can’t wait any longer. We see it with our own eyes, we feel it, we know it in our bones and it’s time to act.”

John F. Kerry, Biden’s special envoy for climate, swung by the U.S. Center to mingle with a crowd of diplomats, journalists and others. Kerry noted that such an American presence had been missing under President Donald Trump.

Under Trump, the United States sent only scaled-down delegations to U.N. climate talks. Trump officials in Katowice, Poland, in 2018 drew the scorn of protesters when a top White House adviser on energy and climate sat on a panel and talked up the benefits of fossil fuels.

For the most part, U.S. officials kept a low profile both that year and in Madrid in 2019, the last time a U.N. climate summit took place. Unlike other nations, the United States had no pavilion to highlight its accomplishments and policies and no high-level officials working to steer the outcome.

William Booth and Steve Mufson contributed to this report.