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.