The world's leaders need to reach a consensus on medium-term carbon emissions targets if the climate change summit in Copenhagen in December is to succeed, Australian Prime Minister Kevin Rudd said on Friday.
The warning came after the G8 agreed to a long-term "goal" of reducing global emissions by 50 percent by 2050.
"The key challenge is what can developed and developing nations do in terms of medium-term targets by 2020 and how can we reach agreement on that by the time we reach Copenhagen," Rudd said before attending the major economies forum meeting, to be chaired by U.S. President Barack Obama.
Both the Rudd government and the opposition have promised cuts of between 5 and 25 percent by 2020, the higher targets conditional on the ambition of any global deal.
The G8 climate change communique had a mixed reception with British Prime Minister Gordon Brown hailing the deal as historic, and American officials calling it a "step forward".
However, G8 member Russia quickly undermined it when President Dmitry Medvedev's economic adviser said the 80 percent emissions reductions would be impossible for Russia to achieve.
Both China and India have also insisted the developed countries need to put medium-term cuts on the table before the developing countries commit to anything.
Conservation groups said the aspiration to limit global warming was weakened by the absence of medium-term targets.