The US and China have set ambitious greenhouse gas emissions targets in a move that threatens to overshadow the G20 leaders' summit and heap pressure on Australia to do more to tackle climate change.
As world leaders converge on Brisbane for a summit focused on growth and jobs, the world's two biggest polluters on Wednesday announced a pact to reverse emissions and a determination to conclude a global deal in Paris next year.
Despite G20 countries representing 76 per cent of global carbon emissions, climate change is not listed on the summit's official agenda.
Neither does a late draft of an "energy efficiency plan" to be released on Sunday mention the words "climate change".
The G20 Brisbane Action Plan for Growth and Jobs - a compendium of action plans on topics from workforce participation to energy efficiency - lists Australia as leading talks on energy-efficient buildings, Japan on energy generation, Britain on networked devices and the US on vehicle standards.
In China, US President Barack Obama said the joint announcement on the two countries' emissions targets was a "historic agreement" and a "major milestone in the US-China relationship".
Chinese President Xi Jinping said: "We agreed to make sure that international climate change negotiations will reach an agreement in Paris."
China set a target for its greenhouse gas output to peak around 2030 or earlier, which Mr Obama commended as an effort to "slow, peak and reverse the course" of its emissions and boost non-fossil fuel energy use.
Mr Obama, who faces a sceptical US Congress, set a goal for the US to cut its emissions by 26-28 per cent from 2005 levels by 2025.
Growth remains the chief focus of major and emerging economies at the G20, with Prime Minister Tony Abbott and Treasurer Joe Hockey calling for the group to add two per cent to global growth over five years.
But Opposition Leader Bill Shorten said the US-China deal builds momentum to address climate change.
"Climate change is not just an environmental issue. It is a security issue and it is absolutely an economic issue," he said.
"Tony Abbott's failure to recognise this represents a failure of leadership."
The Australian Greens warned Australia risked being sidelined on climate change.
"The global economy is changing around him," Greens leader Christine Milne said.
Prominent environmental economist Bjorn Lomborg was less glowing, saying "nice sounding promises" to tackle climate change had been made in the past with very little action.
"There's a little bit of that risk happening all over again," he said.
Environment Minister Greg Hunt said the government would take into account action by Australia's major trading partners when considering post-2020 emission reduction targets.
Meanwhile, a report commissioned by Origin Energy challenges Australia's reputation as one of the world's worst emitters on a per capita basis.
It points to the nation's performance in terms of emissions per unit of gross domestic product, which is better than the average.
This implies it's one of the most efficient economies in converting emissions into wealth and economic growth.
The G20 represents 66 per cent of the global population, 85 per cent of global GDP and 76 per cent of global carbon emissions.
The two-day G20 leaders' meeting gets underway on Saturday.