Developing nations are prepared to make concessions on climate change targets if the G8 fulfils its side of the bargain in the run-up to the climate change talks in Copenhagen in December, a key negotiator told the Guardian today.

The developing countries want the G8 nations to sign up to a 40% cut by 2020, but that figure is off the radar of the EU and, given the unwieldy legislation laboriously passing through the senate, not a possibility for the US.

In important forward steps this week, the G8 agreed to cut its emissions by 80% by 2050 and said worldwide emissions should fall 50% by the same date.

However, the value of this pledge has been reduced by the lack of an agreed start date from which the emission cuts should be measured, making it a distant promise.

Luis Alfonso de Alba, the lead co-ordinator on climate change for the developing countries at the G8, told the Guardian that their call for a 25-40%cut in developed nations’ emissions by 2020 was based on what UN climate change scientists had recommended.

The Mexican diplomat gave some ground, saying: “It does not have to be a specific target of 40%.

“That is what we hope to achieve, but this is a process of negotiation.”

He said a G8 commitment to a 2020 target was “fundamental”, adding: “It is logical that developing countries will commit once they have certainty that developed countries are commiting themselves.

“We need to see the mid-term targets go much higher, and we want to see all the developed countries, including the US, move at the same pace.

“We still need to see numbers. We respect the internal debate in the US, but it is important for the US to understand that this is a global issue and a multilateral negotiation.”

He said developing nations could not “just sit and wait to see what the internal debate in the US resolves”. He insisted the meeting chaired by Barack Obama under the aegis of the Major Economies Forum this week had made progress in accepting common responsibility for the crisis and for the need for carbon emissions to peak.

“Climate change is no longer seen as a north-south issue,” he said. “It is no longer a donor recipient relationship.

“The most important message is that assuming individual responsibilities to fight climate change can start immediately, and by doing it immediately it will be easier to reach an ambitious agreement at Copenhagen.”

De Alba said Mexico had already come up with its own carbon reduction programme, and he expected other developing nations to do the same over the coming months.

It was acknowledged at the summit that science dictates world temperatures must not rise more than 2C degrees above pre-industrial levels.

The negotiators hope this acknowledgement will drive the coming negotiations in the run-up to Copenhagen.

The talks include three UN sponsored meetings in Bonn, Bangkok and Barcelona as well as another meeting of the G20 in September.

World leaders from the G8 group of nations attend a round-table discussion during their summit in L’Aquila, Italy. Photograph: Oli Scarff/Getty Images

Barack Obama said today there was still time to overcome cynicism and close the gap with developing powers on climate change, after slow progress towards an agreement on how to cut carbon emissions across the planet.

World leaders are racing to meet a deadline of December when the UN climate talks in Copenhagen are due to conclude a crucial deal designed to set a carbon cutting framework to cover 2012-2050. At a meeting in L’Aquila, the G5 group of emerging economies – Brazil, India, China, Mexico and South Africa – refused to back a specific target for developing countries to cut emissions.

In a small step forward yesterday 17 industrialised and developing countries, which account for about 80% of global emissions, agreed to set an aspiration that world temperatures should not rise by more than 2C on pre-industrial levels. It is the first time India, China and the US have agreed to such a goal.

Obama said: “We have made a good start, but I am the first to admit that progress is not going to be easy … every nation in this planet is at risk, but just as more than one nation is responsible for climate change no one nation can solve it alone.

“Developing nations want to make sure they do not have to sacrifice their aspirations for development and higher living standards, yet with most of the projected growth in emissions coming from these countries their active participation is a prerequisite to a solution.

“Developed countries like mine have a historic responsibility to take the lead with our much larger carbon footprint per capita. I know that in the past the US has sometimes failed to meet its responsibilities so let me make it clear those days are over.”

Ed Miliband, the climate change secretary, said: “Now we have the 2C goal, that can act as a yardstick to drive up ambition, which is what we need to do over the next six months.”

But Ban Ki-moon, the UN secretary general, criticised all sides for not being more ambitious. The world had to agree a long-term target, a cut of at least 50% by 2050, he said. “But more importantly, the leaders of industrialised countries should agree on a mid-term target.”

On Wednesday the G8 industrialised nations committed to cutting emissions by 80% by 2050, the first time the US, Canada and Russia had agreed to such an ambitious target. But the G8 balked at setting interim targets for 2020, partly because of Obama’s belief that he would undermine support in the US Congress for his climate change bill if he went for tough short term targets.

Obama hit another obstacle yesterday when Democratic leaders in the Senate, under criticism from Republicans for trying to rush through sweeping reforms, abandoned plans to produce a first draft of the bill before the summer recess in August.