WASHINGTON – President Biden on Thursday put four years of official climate rejection behind the United States, saying America would cut its global warming emissions at least in half by the end of the decade.

Address by 40 heads of state at the start of a two-day virtual summit to demonstrate United States commitment to the Paris Agreement, the Donald J. Trump After giving up, Mr Biden tried electroplating other countries to take more aggressive steps. He saw the challenge of avoiding catastrophic warming as an economic opportunity for America and the world.

“This is a moral imperative, an economic imperative,” said Biden. “A moment of danger, but also a moment of extraordinary opportunity.”

In quick succession, Japan, Canada, Great Britain and the European Union committed themselves to steeper cuts. But China, India and Russia have not made any new emissions pledges, and even Mr Biden’s commitment to cut US greenhouse gases by 50 to 52 percent below 2005 levels by the end of the decade will be extraordinarily difficult to deliver, economically and politically be.

Energy experts said it would require a dramatic overhaul of American society, including the virtual elimination of coal for electricity and the replacement of millions of gasoline-powered cars with electric vehicles.

And the ambitions of the Biden government have summed up their toughest diplomatic challenge: dealing with China. While the United States is the largest emitter in history, China’s emissions are currently the largest, which only adds to the problems that are simmering both Republicans and Democrats in Beijing.

Republicans immediately asked why Americans should sacrifice if Chinese coal pollution is likely to inundate profits from US emissions cuts, at least in the short term.

Senator Mitch McConnell, the Republican leader, said Thursday that China had “shamelessly” radiated more and more. “Their share of greenhouse gas emissions is now almost twice that of the United States,” he said.

The stakes are enormous, for Mr. Biden and for the planet. Unless nations prevent global temperatures from rising more than 1.5 degrees Celsius above pre-industrial levels, the global economy will suffer $ 23 trillion in losses from natural disasters and the spread of disease by mid-century, so a report from Swiss Re, one of the world’s largest insurance providers for other insurance companies.

American credibility has been marred by years of joining and subsequent abandonment of efforts to combat climate change. If it does not achieve its new goals or if it reverses course again with a new administration, confidence in the United States would decline even further.

The latest pledge puts the United States almost on par with Europe, but still behind Britain. On climate finance, the Biden government pledged to double its contribution to helping developing countries fight climate change to an estimated $ 5.7 billion by 2024. However, like many promises made by Mr Biden, this would require the approval of Congress. And even that level would only match what many other rich countries did years ago. Experts said the climate finance announcement was far from ambitious.

That underscores a central fact: Mr Biden’s promise is little more than that – a promise. Contrary to what the European Union and the UK made this week, the US target is not anchored in law. Mr Biden’s $ 2.3 trillion infrastructure plan, which includes the money and measures to reduce carbon pollution, has not yet been put into law, let alone supported by a divided Congress.

Mr Biden is urging the heads of his cabinet agencies to enact climate change policies across the federal government in the executive branch, from new standards for vehicle fuel consumption to regulations limiting fossil fuel extraction on public land to new financial regulations to contain Wall Street investing in heavily polluting industries. However, it is unlikely that these rules alone will result in the large emissions reductions required to meet Mr Biden’s ambitious new target. And as the Trump administration has shown, they could be undone by a future White House without much trouble.

John Kerry, Mr. Biden’s global climate envoy, said the changes in the marketplace have happened so quickly that he believes the United States will not only meet its new goal but will exceed it.

“It’s not easy,” he admitted. “Is it feasible? Yes. Are we likely to surpass it? I expect yes. “

Republicans denounced the new emissions target as illegal and unrealistic. Patrick Morrisey, the West Virginia attorney general, called it a “radical” plan and a “domestic and foreign policy flaw of almost unfathomable proportions”.

Updated

April 22, 2021, 4:48 p.m. ET

To overcome this domestic opposition, Mr. Biden must bring the world, especially China. Several major industrialized nations announced aggressive new targets at the summit.

Japanese Prime Minister Yoshihide Suga raised his country’s emissions reduction target by the end of the decade to 46 percent from 2013, down from 26 percent, and at the last minute the country said it would “keep trying to get an even higher cut of 50 percent”. The Biden government had put heavy pressure on Japan to announce a 50 percent target.

Canadian Prime Minister Justin Trudeau raised his country’s target from 30 percent to 40 percent to 45 percent compared to 2005. President Moon Jae-in of South Korea announced an end to public finances for coal-fired power plants overseas. Even President Jair Bolsonaro of Brazil, an ally of Mr Trump who has historically denied the science of climate change, pledged to end illegal deforestation by 2030 despite seeing the sharpest surge in Amazon destruction in 12 years.

President Xi Jinping promised that China would “strictly limit” increasing coal consumption over the next five years and phase it out in the next five years. This could prove significant as China is by far the world’s largest consumer of coal and continues to expand its fleet of coal-fired power plants. Coal is the dirtiest fossil fuel.

Mr. Xi also reiterated his commitment from last year to reduce CO2 emissions to zero by 2060. In a clear reminder to his host, he said that developed countries have a responsibility to act faster to reduce emissions.

But the United States cannot tame climate change on its own, Biden said. America accounts for about 15 percent of global emissions, a point the President, Mr. Kerry, and Secretary of State Antony J. Blinken have repeatedly raised.

“All of us, and especially those of us who represent the world’s largest economies, need to be strengthened,” said Biden.

His new target and summit received the expected approval of environmental groups, who described the number as an aggressive but achievable goal that would help America achieve its long-term goal of carbon neutrality by 2050.

Former Vice President Al Gore called the target “a groundbreaking step for our country” and the summit as evidence of America’s ability to galvanize other countries even after four years of former President Donald J. Trump’s isolated agenda.

“We are still in a period in history when the United States remains the only nation that can provide effective leadership to the world community,” Gore said in an interview.

“Other countries may feel that the period of history is over, but there is no substitute for US leadership,” added Gore. “Although our nation’s reputation in the world has been damaged over the past four years, perhaps particularly on the climate issue, most nations will welcome the restoration of the US’s traditional role.”

In particular, major fossil fuel industry associations such as the American Petroleum Institute and the Chamber of Commerce praised Mr. Biden for his international commitment to climate change and did not directly criticize or question the ability to achieve it.

Some climate activists, particularly from poorer countries that are the least polluted but suffer the worst of climate change, said the United States has a duty to do far more.

“This summit is a major turning point that is now drawing attention to the stragglers and concrete short-term action,” said Mohamed Adow, director of Power Shift Africa, a research organization based in Nairobi, Kenya. Rich, polluting countries, he said, “need to come back with much stronger pledges, including climate finance for poorer nations.”

Thursday’s summit made history as one of the first all-digital gatherings of world leaders, and the White House showed that even it wasn’t immune to the zoom-induced disruption that frustrated distant workers during the pandemic.

The opening speeches by Mr. Biden and Vice President Kamala Harris were plagued by painful echoes, apparently the result of overlapping microphone or loudspeaker devices. The screen on whitehouse.gov then moved to show Mr Biden sitting alone at a small conference table against a brightly lit blue background while other world leaders beamed in.

Mr Blinken introduced President Vladimir V. Putin of Russia, but obviously signals were crossed when the screen switched between Mr Putin and President Emanuel Macron of France and Mr Putin sat in stony silence.

Mr. Suga, the Japanese Prime Minister, acknowledged the reality of global time differences by opening his remarks to his counterparts with a cheerful “Good morning, good afternoon and good evening”.

As the speeches continued, Mr Kerry sat down with Mr Biden at the conference table to welcome other world leaders to the summit – although he said to Mr Biden with a laugh, “If you can call it a summit.”

Douglas Brinkley, a historian for the president, said the U.S. government’s inability to solve basic computing problems worried him about his bigger promises.

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

Despite the technical problems, the heads of state and government were openly relieved to work with a US government that is dedicated to science and is once again part of the global community.

“The Paris Agreement is the life insurance of mankind,” said Ursula von der Leyen, Head of the European Commission. “It’s so good to have the US on our side again.”

Brad Plumer contributed to the coverage.