Biden vows to double aid on climate change, one of the key issues facing leaders.

President Biden said on Tuesday that his administration would seek to double aid aimed at helping developing nations address climate change, raising a pledge he made in April to about $11.4 billion a year by 2024.

The pledge is considered critical to the success of United Nations-led climate talks that are scheduled to take place in November in Glasgow, though whether and when the money will materialize depends on congressional approval.

Climate change is perhaps the most important subject at this year’s General Assembly meeting, with new scientific evidence showing a losing battle in what the United Nations secretary general, António Guterres, has called an existential struggle.

Many developing countries have repeatedly pointed out that rich countries have not delivered the $100 billion a year in aid that they promised under the landmark 2015 Paris climate accord. A tally by the Organization for Economic Cooperation and Development found a nearly $20 billion shortfall.

Earlier in the year, Mr. Biden had pledged $5.7 billion, money that also requires approval from Congress.

Mr. Guterres has warned that a failure to make good on such promises could jeopardize cooperation to rein in global greenhouse emissions and avert the worst effects of warming. “This is a crucial question of trust,” he said at a climate summit organized by the White House last week.

Prime Minister Boris Johnson of Britain, who will host the Glasgow talks, led a preparatory meeting with Mr. Guterres on Monday. Mr. Johnson told reporters afterward that the November gathering would be “a turning point for the world, and it is the moment when we have to grow up and take our responsibilities.”

The scientific consensus is that global temperature rise needs to be limited to 1.5 degrees Celsius. Beyond that threshold, there is a far greater likelihood of devastating consequences, like widespread crop failures and the collapse of the polar ice sheets.

“We are no longer on the wrong path — we are on the edge of the cliff,” Abdulla Shahid, the foreign minister of the Maldives who is serving as president of the General Assembly, told the gathering on Tuesday. The low-lying Maldives is one of several nations at risk of devastating flooding because of rising sea levels.

Altogether, nearly 200 countries have made pledges to reduce or slow down emissions of planet-warming gases under the Paris agreement. But still missing are new pledges from 70 countries, including China, which currently produces the largest share of greenhouse gas emissions, and India and Saudi Arabia, both large economies with a significant climate footprint. Brazil, Mexico and Russia have submitted new pledges that have weaker emissions targets than their previous ones.

Mr. Biden’s revised pledge would make the United States, the largest emitter of planet-warming gases since the start of the industrial era, among the largest global climate donors, though advocacy groups said it still fell short of Washington’s fair share.

“It’s good to see President Biden is upping the amount that the U.S. is contributing,” Mohamed Adow, the director of Power Shift Africa, said in a statement. “However, the U.S. is still woefully short of what it owes.”

Tina Stege, the climate envoy of the Marshall Islands, said: “Watching Biden’s speech today, I thought — this is the announcement we’ve been waiting for. Now we’re looking to Congress to work with Biden to deliver, and to the rest of the G20 to follow suit.”