PARIS--U.S. President-elect Joe Biden pledged Saturday to rejoin the Paris climate accord on the first day of his presidency, as world leaders staged a virtual gathering to celebrate the fifth anniversary of the international pact aimed at curbing global warming.

Heads of state and government from over 70 countries took part in the event--hosted by Britain, France, Italy, Chile and the United Nations--to announce greater efforts in cutting the greenhouse gas emissions that fuel global warming.

The outgoing administration of President Donald Trump, who pulled Washington out of the Paris accord, wasn't represented at the online gathering. But in a written statement sent shortly before it began, Biden made clear the U.S. was waiting on the sidelines to join again and noted that Washington was key to negotiating the 2015 agreement, which has since been ratified by almost all countries around the world.

“The United States will rejoin the Paris Agreement on day one of my presidency,” he said. “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.”

Biden reiterated his campaign pledge that his administration will set a target of cutting U.S. emissions to net zero “no later than 2050.”

Experts say commitments put forward by the international community in the past five years have already improved the long-term outlook on climate change, making the worst-case scenarios less likely by the end of the century. But wildfires in the Amazon, Australia and America, floods in Bangladesh and East Africa, and record temperatures in the Arctic have highlighted the impact an increase of 1.2 degrees Celsius since pre-industrial times is already having on the planet.

“If we don’t change course, we may be headed for a catastrophic temperature rise of more than 3 degrees this century,” U.N. Secretary-General Antonio Guterres said, urging world leaders to declare a “climate emergency.”

The Paris agreement aims to cap global warming at well under 2 C, ideally no more than 1.5 C, by the end of the century. Meeting the temperature target will require a phasing-out of fossil fuels and better protection for the world's carbon-soaking forests, wetlands and oceans.

The U.N. chief called the announced U.S. return to the Paris accord “a very important signal."

“We look forward for a very active U.S. leadership in climate action from now on,” Guterres said. "The United States is the largest economy in the world, it’s absolutely essential for our goals to be reached.”

Biden insisted that the dramatic economic shifts needed would be positive for American workers.

“We have before us an enormous economic opportunity to create jobs and prosperity at home and export clean American-made products around the world, harnessing our climate ambition in a way that is good for American workers and the U.S. economy,” he said.

American representatives at the virtual meeting included Michigan Gov. Gretchen Whitmer, Gov. Charlie Baker of Massachusetts and U.S. business leaders, such as Apple chief executive Tim Cook.

Also absent from the event were major economies such as Australia, Brazil, Indonesia, Mexico, Russia and Saudi Arabia. Most have offered no significant improvements on their existing emissions targets lately.

Environmental campaigners singled out Brazil's recent announcement that it will stick to its target of cutting emissions by 43% over the next decade compared with 2005 levels and aim for net zero by 2060--later than most other countries.

By contrast, an agreement Friday by European Union members to beef up the continent’s 2030 targets from 40% to at least 55% compared with 1990 levels was broadly welcomed, though activists said it could have aimed even higher.

China, the world's biggest emitter, also surprised the world in September by announcing a net zero target of 2060, with emissions peaking by 2030. In his speech Saturday, Chinese President Xi Jinping provided further details on his country's medium-term goal for improving energy efficiency and ramping up electricity generated from renewable sources of power such as wind and solar.

But Xi also cautioned that “unilateralism will lead us nowhere”--a veiled reference to discussions in the EU to impose tariffs on goods imported from countries that have less stringent emissions standards than the 27-nation bloc. The issue is likely to dominate discussion between China, the EU and the U.S. in coming years.

The Maldives, an Indian Ocean nation made up of low-lying islands that are particularly vulnerable to sea level rise, announced Saturday that it will now aim to achieve net zero by 2030, one of the most ambitious goals worldwide. Bhutan and Suriname claim to have already achieved that goal.

The 189 countries that are party to the Paris agreement are required to submit their updated targets to the United Nations by the end of the year. This would normally have occurred at the annual U.N. climate summit, but the event was postponed for a year because of the pandemic.

The gathering, now scheduled to take place in Glasgow, Scotland, in November 2021, will see haggling over financial support for poor countries to cope with climate change, and fine-tuning the rules for international markets in emissions trading. Britain, next year's host, announced this month that it's aiming to cut emissions by 68% over the next decade and end state support for fossil fuel industry exports.

Former U.N. climate chief Christiana Figueres, who was a key player at the Paris negotiations, said leaders had a duty to be optimistic about their ability to curb global warming.

“Because if we don’t, the alternative is unthinkable,” she said. “None of us adults alive today want to have on our shoulders the responsibility of turning over a world that is a world of misery for generations to come.”