Australia's record-breaking heat last summer was at least five times more likely to have occurred in a world subject to to greenhouse gas emissions from human activities than one without, a study has found.
University of Melbourne researchers drew the conclusion after examining 90 model simulations and climate observations from summers of the past 100 years.
They used nine climate models — those used by the United Nation's Intergovernmental Panel on Climate Change — to test how likely the record temperatures in Australia between December and February would be in a world with and without human-induced global warming.
They found it was very likely — with 90 per cent confidence — that summers such as that of 2012-13 were at least five times more likely to occur due to man-made climate change than in a world facing only natural variation.
The researchers also found the frequency of extremely hot summers would continue to increase due to global warming.
"These results support a clear conclusion that anthropogenic climate change had a substantial influence on the extreme summer heat over Australia and that natural climate variations alone are unlikely to explain the recent record summer temperature," the researchers say in a paper published in the journal Geophysical Research Letters.
The Bureau of Meteorology says temperatures throughout the country last summer were 1.11 degrees above the long-term average, making it the hottest since records began.
The average temperature has risen 0.9 degrees since 1910 — an increase scientists have linked to rising greenhouse gas concentrations in the atmosphere.
Study co-author David Karoly said this year's hot summer was "at least five times more likely to have happened in a world with increasing greenhouse gases than in a world with natural variability".
"That is already a substantial change, and we are talking about climate change which is only less than one degree so far globally, ramping up to four or five degrees towards the end of that century," Professor Karoly said.
Most of Australia's coal reserves will have to be left unburnt, according to a report from the federal government's Climate Commission.
The report puts the advisory body on a collision course with some of the nation's biggest export industries and marks the first time a government agency has endorsed calls for fossil fuel industries to be phased out because of their contribution to climate change.
Its findings suggest most of Australia's known coal, oil and gas reserves - many of which are already subject to minerals production licences held by companies such as BHP Billiton and Rio Tinto - must be left alone if the world is to avoid dangerous climate change.
The commission acknowledged its conclusions were ''sobering'' and that the potential for economic disruption could be serious but said there was no alternative if the world was to avoid dangerous climate change. ''How people react to this is up to the policymakers and governments, as well as investors,'' said Lesley Hughes, who co-authored the report, The Critical Decade 2013 - Climate Change Science, Risks and Responses.
''It isn't our job to reconcile the politics of this with the science; we are simply presenting the facts as best we know them. Just because the facts may be unpalatable to some people doesn't make them any less important.''
Australia's fossil fuel resources are the equivalent of about 51 billion tonnes of greenhouse gases, about one-twelfth of the world's ''carbon budget'' of about 600 billion tonnes - the amount scientists estimate can be burnt by 2050 if the world is to stop temperatures rising more than two degrees.
If that budget is exceeded, it is likely to trigger dangerous global warming that escalates up to four or five degrees this century, the report says. The world is still eating into its budget far too fast, with average global emissions rising at about 3 per cent a year, it says.
''If emissions could somehow peak in 2015 … the maximum rate of emission reductions thereafter would be 5.3 per cent, a very daunting task,'' the report said.
Climate Change Minister Greg Combet said the carbon price had driven down coal use 7 per cent and most countries buying Australian fossil fuels would soon have similar carbon prices. A spokeswoman for the shadow minister for climate action Greg Hunt said he would ''consider the report'' but ''we do not support shutting down Australia's major export industry''.
Scientists have said that the rise in global average temperature needs to be limited to less than 2 degrees Celsius this century to prevent climate effects such as crop failure and melting glaciers, but that would require emissions to be kept to about 44 billion tonnes of CO2 equivalent by 2020.
The IEA said that the world is on a path to an average temperature rise of between 3.6 and 5.3 degrees Celsius.
China's CO2 emissions rose by 300 million tonnes last year, but the 3.8 per cent gain was one of the lowest it has achieved in a decade, reflecting the nation's efforts to adopt renewable sources and improve energy efficiency.
Japan's emissions, meanwhile, rose by 70 million tonnes, or 5.8 per cent, as efforts to improve energy efficiency failed to offset increasing use of fossil fuels after the Fukushima nuclear accident in 2011, the IEA said.
In the United States, a switch from coal to gas in power generation helped to reduce emissions by 200 million tonnes, or 3.8 per cent, bringing them back to the level of the mid-1990s.
Even though the use of coal increased in some European countries because of low prices, emissions in Europe declined by 50 million tonnes, or 1.4 per cent, because of the economic slowdown, growth in renewables and emissions caps on industrial and power companies.
IEA chief economist Fatih Birol told Reuters that he does not expect any EU country to be able to produce substantial amounts of shale gas, as the United States has done, before 2020.
"After 2020, if there are increases in shale use in EU countries such as Britain, Poland and Germany, and it replaced coal like in the US, there would be a greater reduction in emissions," he said.
The IEA urged governments to adopt quickly four policies that would ensure climate goals could be reached without harming economic growth. They are: improving energy efficiency in buildings, industry and transport; limiting the construction and use of inefficient power plants; halving methane emissions; and partially phasing out fossil fuel subsidies.
The agency said that the coal industry in particular needs to take the development of carbon capture and storage (CCS) technology more seriously to ensure cleaner use of the fossil fuel.
CCS buries and traps CO2 underground but it is not yet a commercially viable proposition.
"Realistically, we need to see CCS penetrate within the next 10 years - certainly no later than 2025 - or the fossil fuel industry will be caught unprepared," Birol said.
International negotiators are in Bonn, Germany, until Friday for United Nations talks aimed at getting a new global climate treaty signed by 2015 and in force by 2020.
U.N. climate chief Christiana Figueres said that the IEA report comes at a crucial time for the talks.
"Once again we are reminded that the gap can be closed this decade, using proven technologies and known policies, and without harming economic growth in any region," she said.