Leaders from 75 countries gathered this month — virtually, of course — to mark the fifth anniversary of the Paris climate accord and to outline more ambitious plans to cut planet-warming emissions in the critical years ahead.

The United Kingdom vowed to stop funding fossil fuel projects abroad. The European Union said it will push to cut emissions 55 percent by 2030. Canada detailed plans to ramp up its tax on carbon to more than $130 per ton over the same period. A handful of smaller countries pledged to reach net-zero emissions by 2035, if not sooner. Three dozen nations have declared a “climate emergency” inside their borders.

“They recognize the urgency and the stakes,” said António Guterres, secretary general of the United Nations. “It’s time for all countries to do the same.”

The one glaring absence at the anniversary gathering: the United States. The nation that has historically spewed more greenhouse gases into the atmosphere than any other, and the only country to withdraw from the global pact, was nowhere in sight.

But just before the Dec. 12 event got underway, President-elect Joe Biden blasted out a statement, vowing to rejoin the Paris agreement “on day one” and to restore the United States as a world leader in climate action.

“I’ll immediately start working with my counterparts around the world to do all that we possibly can, including by convening the leaders of major economies for a climate summit within my first 100 days in office,” he said.

Biden promised to put the nation on a path to achieve net-zero emissions by 2050, to ensure that the shift toward cleaner energy brings new U.S. jobs, and to “listen to and engage closely with the activists, including young people, who have continued to sound the alarm and demand change from those in power.”

Biden’s promises no doubt provided relief to many world leaders, who are eager for the United States to rejoin an effort that remains far from meeting the goals forged five years ago in Paris.

But Biden may not find as warm a reception as he hopes when the United States steps back onto the global stage in 2021. Instead, he is likely to encounter a hopeful but wary world.

“The U.S. has to do a lot to rebuild trust,” said Harjeet Singh, global climate lead for the advocacy group ActionAid. “It really needs to find a way to convince the world that this time, the U.S. is genuinely ready to do its share.”

Many in the international community have harbored resentment about the United States’ seeming inability, or unwillingness, to live up to its lofty promises to the rest of the world when it comes to climate change.

President Trump announced early in his tenure that he would withdraw the nation from the Paris accord — insisting that following through with the nonbinding pledges of the Obama era could economically disadvantage the country.

But the United States has wavered even before Trump, most notably on a key global climate treaty forged in 1997, known as the Kyoto Protocol. The United States signed that global climate treaty in 1998 under the Clinton administration, but did not ratify the deal and backed away from it under President George W. Bush.

Over the years, the United States has failed to hit its own emissions-cutting targets — even as overall emissions have decreased. It also has not adequately contributed to a fund meant to help vulnerable nations that have done little to cause climate change but are most affected by it.

“There’s no papering it over,” John Holdren, an environmental policy professor at Harvard who served as President Barack Obama’s top science adviser, said of the loss of trust the United States has suffered internationally during the Trump era. “It’s going to involve more than just saying we are back. We are going to have to demonstrate we are back, and we are going to have to demonstrate it powerfully.”

Christiana Figueres, a Costa Rican diplomat and key architect of the Paris agreement, told reporters recently that the best thing Biden can do to prove that he is serous about global climate action is to make progress domestically.

“The U.S. will have to do its homework at home first in order to regain credibility,” she said, adding: “Yes, the Biden administration has put out their plans. But we’re going to have to see the plans being enacted. We’re going to have to see the rollbacks of the rollbacks.”

That part won’t come easily.

Biden can make headway using executive orders and shaping policies at the Environmental Protection Agency, the Interior Department and other corners of the government. But any far-reaching national climate blueprint, or any major stimulus spending on green energy, will need the blessing of a deeply divided Congress.

Unlike Obama, Biden also will confront a rockier relationship with China, currently the world’s largest emitter. And he faces intense pressure from activists to move quickly, as well as the growing urgency created by raging wildfires, crippling hurricanes and other climate-fueled catastrophes.

Despite those obstacles, the new president will begin with a measure of goodwill from many countries that have been aching for U.S. leadership on climate once again.

“The U.S. pull-out from the Paris Agreement was certainly a heavy blow, but not the fatal strike everyone feared it would be,” said Janine Felson, the acting chair of the Alliance of Small Island States, a group of 44 islands and low-lying coastal nations that act as a bloc at international climate talks. “While it has certainly emboldened some large carbon emitters to do less, no one else left, avoiding the worst case scenario of a total collapse.”

Countries already getting battered by climate change “do not have the luxury of time to hold grudges and would welcome any meaningful climate action by the U.S. that goes beyond empty rhetoric,” she said in an emailed statement.

Guterres, the U.N. chief, also is eager for the next chapter with Biden at the helm.

“We look forward for a very active U.S. leadership in climate action from now on, as the U.S. leadership is absolutely essential,” he told reporters after the Paris anniversary gathering. “The United States is the largest economy in the world. It is absolutely essential for our goals to be reached.”