Barack Obama is poised to arrive in Copenhagen tomorrow with additional pledges of cash for poor countries which will suffer the most from global warming, a day after America's promise to support a $100bn a year climate fund.

Obama's arrival has been the most anticipated event of the 10-day summit, which has lurched between optimism and rank despair. He will seek to make a decisive impact, building on the announcement today by Hillary Clinton, the secretary of state, who said for the first time that America would support a $100bn global climate change fund from 2020. But she will be a tough act to follow, as the statement was seen by delegates as a gamechanger.

Obama is expected to add an extra boost of momentum by beefing up America's share in a $10bn a year fast-track aid package. That aims to cushion poor countries from the impact of climate change and promote rainforest preservation starting next year. He is also expected to outline little-known provisions in the climate bill passed by the House of Representatives that would direct some $4bn a year from the auction of emission allowances to a fund to help developing countries adapt to climate change and deploy clean technology.

He is also expected to call more forcefully on the Senate to pass climate change law, critical to the eventual success of Copenhagen. "I've got a sense that she set the table, and he is going to deliver the knock-out punch," said Earl Blumenauer, part of the delegation of Democratic congressmen to the talks.

Clinton gave no specifics on how America would raise its share of the $100bn fund, and she made her offer contingent on overcoming an atmosphere of mistrust to reach a deal at Copenhagen. "It is no secret that we have lost precious time in these past days," she said. "In the time we have left here, it can no longer be about us versus them — this group of nations pitted against that group. We all face the same challenge together."

She also said the deal must include an international regime to monitor and verify pledges by developing countries to curb their emissions. Clinton said there could be no deal without such checks in place. "If there is not even a commitment to pursue transparency, that's kind of a dealbreaker for us," she said.

Clinton's appearance here — only hours after the summit's Danish hosts had given up hopes of reaching a deal — was widely credited with pulling the negotiations back from the brink.

Environmental organisations said America still needed to provide details about the sources of funding, and how it would be distributed. Clinton would only say there would be public and private investment, and that America was exploring several different potential soures of funding.

But British officials said last night that the US move puts the onus on the European Union to decide whether to make good on its promise to raise its emissions reductions target to 30% in the event of strong action at the summit. EU officials were meeting to discuss the next move tonight . .

Alden Meyer, director of strategy for the Union of Concerned Scientists, said America's support for the $100bn climate fund gave China the cover it needed to back down gracefully from the showdown over accountability. "Now China can be magnanimous and say it is acting in solidarity with its brothers and sisters in Africa [who will benefit from the money], and that it is not going to stand in the way of a deal," he said.

Although industrialised countries had cobbled together a package of short-term aid for African countries and low-lying islands that will suffer the worst ravages of global warming, there had been little movement on mobilising the billions that will be required over the long haul.

Clinton's intervention helps supply that crucial missing link. The $100bn figure was formally put on the table at the conference by the Ethiopian prime minister, Meles Zenawi, who is head of the African group of nations. It is much lower than many developing nations say is necessary to help them adapt to climate change and develop green technologies, with esitmates ranging up to $600bn a year.

The American offer to puts its share into the $100bn climate fund is unlikely to win over all objections to the deal from African countries and small island states. "If nobody is going to be alive to get a dime of it, how far does this take us?" asked Hama Arba Diallo, a member of parliament from Burkina Faso.

Amisa Elamia, the prime minister of the Pacific island nation of Tuvalu, said he would be unable to sign on to an agreement unless it sought more stringent emissions cuts that would limit global warming to 1.5C. Negotiators are currently discussing limiting warming to 2C.

"Over the last few days we have faced considerable pressure to accept a deal based around 2C. We have not yielded to this pressure because our future is not negotiatiable," he said. "I will not sign anything that is not 1.5C."