Australia could do more to combat climate change but its plan to reduce carbon emissions by 5 percent by 2020 was "one of the world's leading" reduction targets, according to Environment Minister Greg Hunt.

Responding to a major report from the UN's Intergovernmental Panel on Climate Change (IPCC), Hunt conceded on Monday "we can all do more" to combat climate change but said the report vindicated the government's 2.5 billion AUD Direct Action policy.

His comments came as a leading climate change scientist from the Australian National University warned that Sydney, Melbourne and other capital cities would soon experience temperatures of 50 degrees, if the government did not start to take drastic action to reduce greenhouse gas emissions.

Dr Elizabeth Hanna, from the Climate Change Adaptation Research Network at ANU's College of Medicine, Biology & Environment, whose research featured in the IPCC report, said "There is a level of urgency that now permeates the report, and we don't have many years left to reduce our levels of carbon in the atmosphere."

"We are definitely on track for more warming, more extreme weather events. It won't be too long, according to these trajectories, that capital cities will start to experience temperatures over 50 degrees. If that happens, we are risking mass death events in Australia, similar to the death tolls due to extreme heat overseas."

In its landmark review, the IPCC said that the planet is unequivocally warming, that burning fossil fuels is significantly increasing greenhouse gas emissions and the effects of climate change - like sea level rises - are already being felt.

It also said most of the world's electricity should be produced from low carbon sources by 2050 and that fossil fuel burning for power should be virtually stopped by the end of the century.

UN secretary-general Ban Ki-moon said the report was the most comprehensive appraisal of climate change yet and that human influence was clear.

"Science has spoken, there is no ambiguity in their message," he said. "The report found that the world is largely very ill- prepared for the risk of changing climate, especially the poor and most vulnerable, who have contributed least to this problem."

"Leaders must act. Time is not on our side."

Australia's leaders, including Hunt and Prime Minister Tony Abbott, were non-committal, however, when asked to respond to the report's key findings.

"I won't be drawn on it either way, it's a fresh report," Hunt told the ABC on Monday.

Asked if Australia needed to do more to contribute to global reductions of carbon emissions, Hunt described the 5 percent 2020 target as "one of the world's leading reductions".

"We are reducing our emissions on a basis that is very, very significant against business as usual - it's one of the world's leading reductions," he said.

"We can all do more," he conceded. "But we have to do this step by step, instead of big talk and just an electricity tax. I would rather have outcomes [and] achieve what we said we'd do," he said.

Hunt said the Government's Emissions Reduction Fund - part of its Direct Action climate change policy which passed the Senate last week - will ensure power stations are cleaner. "The work that the CSIRO is doing is looking at a 30 to potentially 50 percent reduction in the footprint of coal fired power stations," he said.

Direct Action, which parliament passed on Friday with the support of the Palmer United party and crossbench senators, provides financial incentives for big polluters to volunteer to reduce emissions.

The first projects to win money from the 2.23 billion U.S. dollar funding pool through a reverse auction will be known in the first quarter of 2015.

Greens and Opposition MPs immediately denounced the government for failing to heed the warnings of eminent scientists and climate change experts, saying the IPCC report should sound the loudest of alarms for Australia's climate change skeptics.

Greens leader Christine Milne said fossil fuel reserves must be left in the ground or there was no chance of preventing catastrophic warming.

"The game is up prime minister Abbott can no longer engage in high-risk denying behavior, plead ignorance or continue to shirk this," she said.

The Shadow Minister for Environment and Climate Change, Mark Butler, said the government was turning a deaf ear to the panel's warnings of the immediate threats posed by climate change and the need for urgent global action.

He said the Direct Action initiative was a sham. "Tony Abbott's Direct Action policy is nothing more than a token gesture that will do nothing to reduce Australia's carbon pollution," Butler said.

"He has teamed up with Clive Palmer to hand out taxpayers' money to big polluters. What will it take for Tony Abbott to wake up and realize Australia must take meaningful action on climate change?

"When the report calls for direct actions, the IPCC means taking action that will directly reduce carbon pollution, not a dressed up slush fund with a fancy name."

In opening a coal mine in Queensland last month, Abbott courted controversy when he declared "coal is good for humanity".

IPCC vice-chair Jean-Pascale van Ypersele said while Abbott was correct to point out the world could not end its reliance on fossil fuels for energy immediately, change was necessary.

"Energy is important for development but clean energy is important for protecting [the] climate."

Greens leader Senator Milne was less diplomatic about Abbott's stance: "Coal is bad for humanity, and so is Tony Abbott's denial of global warming science," she said.

"The Abbott government needs to take its head out of the sand and face the scientific and economic reality set out in this IPCC report. Global warming is real and urgent, and politicians must act to cut emissions fast. The world has run out of time."

The report warned that greenhouse gas levels were at their highest levels in 800,000 years, with recent increases mostly due to the burning of fossil fuels.

"Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems," the report said.