In a one-two punch, the US plans to pledge at least $2.5bn and up to a maximum of $3bn over the next four years to help poor countries invest in clean energy and cope with rising seas and extreme weather, according to those briefed by administration officials.

The White House told campaign groups and thinktanks that the figure was conditional on other countries making ambitious funding commitments. The American contribution would be capped at 30% of the fund, and the US would stump up the full $3bn only if the fund met its initial $10bn target, the official said.

The financial commitment will be unveiled as world leaders gather for the G20 summit in Brisbane, sending a powerful signal of Obama’s determination to act on climate change despite the Republican takeover of Congress in mid-term elections.

The pledge to the Green Climate Fund was seen as critical to UN negotiations for a global climate deal. Developing countries have said they cannot sign on to emissions cuts at climate talks in Lima later this month without the funds.

Analysts said the $2.5bn figure under discussion before the Brisbane summit was just about enough to demonstrate that the US was willing to put up the cash.

“I think it’s a good signal for unlocking the negotiations for Paris in 2015,” said Alex Doukas, an international climate policy analyst at the World Resources Institute. Congress will still have to authorise the funds. But some analysts argue that it will be difficult for Republicans to cut out climate finance entirely.

The pledge from Obama could also help spur Britain and other countries to pay into a fund that so far has raised just under $3bn, well short of its initial $10bn target.

Jake Schmidt, who follows international climate negotiations for the Natural Resources Defence Council, said: “He is trying to use the G20 as a way to put pressure bilaterally and otherwise for countries to put their targets and their financing on the table.”

There were early signs the strategy was paying off. The Japanese prime minister, Shinzo Abe, was expected to announce a pledge of up to $1.5bn to the fund at the Brisbane summit, press reports said.

The financial commitments from the US and Japan are in strong contrast with Canada’s and Australia’s positions, which have said they will not contribute to the climate fund.

Indeed the announcement could again embarrass the G20 host country, Australia, which has been fiercely resisting climate change discussions distracting from its desired focus on “economic growth and jobs”.

The Australian government was caught off guard when Obama and his Chinese counterpart Xi Jinping unveiled climate pledges on the eve of the summit.

As revealed by Guardian Australia, Australia has been arguing against behind-the-scenes diplomatic efforts for G20 leaders to promise to make contributions to the fund.

The prime minister, Tony Abbott, had previously insisted Australia would not make any contributions to it, although it is understood the Department of Foreign Affairs and Trade, which leads Australia’s negotiating position in international climate talks, has been considering whether Canberra should make a pledge. Sources said no final decision had been made.

Asked about the fund before last year’s UN climate meeting in Warsaw, the prime minister said: “We’re not going to be making any contributions to that.” It was reported that at one of its first cabinet meetings the Abbott government decided it would make no contributions to the fund that was described as “socialism masquerading as environmentalism”.

Abbott disparaged the fund at the time, comparing it to a domestic fund championed by the former Greens leader Bob Brown, which he wants to abolish.

He told the Australian newspaper: “One thing the current government will never do is say one thing at home and a different thing abroad. We are committed to dismantling the Bob Brown bank [the Clean Energy Finance Corporation] at home so it would be impossible for us to support a Bob Brown bank on an international scale.”

The government also pointedly dissented from support for the fund in a communique from last November’s Commonwealth Heads of Government meeting – a stance backed by Canada.

The Green Climate Fund aims to help poorer countries cut their emissions and prepare for the impact of climate change, and is seen as critical to securing developing-nation support for a successful deal on reducing emissions at the UN meeting in Paris next year.

More than $2.8bn had been pledged before the US commitment – including $1bn by France and almost $1bn by Germany. More pledges are expected at a special “pledging” conference in Berlin on 20 November. Britain has said it will make a “strong” contribution at that meeting.

Federal ministers raised strong objections to Australia’s commitment to the Green Climate Fund during the cabinet discussion before Warsaw, a meeting to which Australia controversially declined to send the environment minister, Greg Hunt, or any ministerial representative. (Foreign minister Julie Bishop will be attending this year’s meeting in Lima.)

In opposition Bishop raised strong concerns that money from the foreign aid budget was being directed towards the climate change fund. “Climate change funding should not be disguised as foreign aid funding,” she said, accusing the former government of introducing the now-repealed carbon tax to pay for contributions to the fund.

“This is a tax to gather revenue to redistribute it around the economy and to buy themselves some brownie points at the United Nations,” she said in 2011.

A showdown over the Green Climate Fund had been looming for next week, when a pledging conference was scheduled in Berlin.

However, it appears that Obama wanted to get out ahead of other countries – and focus the attention of G20 countries more firmly on climate change.

“He is seizing the opportunities that come his way to demonstrate to the world that the US is not going to backtrack on the progress he has made for the last five years, and that he is firmly committed to getting a strong deal in Paris,” said Pete Ogden, a former White House adviser who is now the international climate and energy director at the Centre for American Progress.

“I think this is certainly about him showing that he is making no apologies for helping to build up an effective domestic climate policy and he is making no apologies for wanting to help lead global efforts to combat climate change. People around the world look at us and see what happened in the mid-terms. If they had any reason for concern that he would be diminished, I think the evidence of the last couple of days is going to put that to rest.”

Heather Coleman, a climate analyst at Oxfam, said the US-China deal earlier in the week had helped lay the groundwork for Obama to offer a pledge on climate finance.

“Now that we have demonstrated that China is willing to move forward it does make it more palatable for the US to put more money on the table for international climate finance which everyone knows is the essential key to unlocking negotiations. Without finance you just don’t get a global climate deal,” she said.

The ballpark figure of $2.5bn to $3bn is not that much higher than the $2bn pledged to climate finance by George Bush in 2008.

“Ultimately this is money that will be appropriate by Congress, but the fact is for decades Congress has been investing in multilateral funds that support the efforts of countries to cut emissions and build cleaner economies,” Ogden said.

Suzanne Goldenberg, the guardian 

(Additional reporting by Lenore Taylor, Guardian Australia political editor)

This article first appeared in the guardian 

edie is part of the Guardian Environment Network

Action inspires action. Stay ahead of the curve with sustainability and energy newsletters from edie