The amount of coal and other fossil fuels the world can burn without unleashing dangerous climate change that will undermine the livelihoods of hundreds of millions of people and all but wipe out the Great Barrier Reef is "very small", according to a major climate report.
The Intergovernmental Panel on Climate Change's special report on a 1.5-degree hotter planet, released on Monday, said limiting warming to that amount remains possible, but only with "rapid, far-reaching and unprecedented changes in all aspects of society".
For Australia, the report - based on some 6000 research references and compiled by dozens of scientists - found it likely the continent was already experiencing warmer days and fewer cool nights. Eastern coastal regions were among areas experiencing reduced rainfall and run-off.
The report's projections, though, are what the authors hope policymakers will address. It was prompted by the 2015 Paris climate conference when almost 200 nations including Australia signed up to keep warming to between 1.5-2 degrees compared with pre-industrial times.
'Virtually all' reefs gone
The Great Barrier Reef, already hammered by unprecedented coral bleaching in a world that has warmed about 1 degree, faces annihilation if warming continues apace.
Globally, coral reefs would decline by a further 70 to 90 per cent under global warming of 1.5 degrees and "virtually all" would be lost with 2 degrees.
"We're currently heading towards about 3 degrees or 4 degrees of warming by 2100," said Mark Howden, director of the Climate Change Institute at the Australian National University and one of the review's editors.
"Limiting global warming to 1.5 degrees is not impossible but would actually require major transitions in many aspects of society, and to do those transitions, the next 10 years are critical."
Many of those transitions will mean curbing if not halting entirely the release of greenhouse gas emissions from the burning of fossil fuels, land-clearing and other human activities.
Average temperature rises mask extreme events. Temperatures of hot days are forecast to increase three degrees in a 1.5 degree warmer world, and by four degrees if mean temperatures rise by 2 degrees.
Carbon budget
According to Ove Hoegh-Guldberg, another of the report's authors and director of the University of Queensland's Global Change Institute, the remaining carbon budget means just 5-10 per cent of fossil fuel reserves can be burnt. "The rest will have to stay in the ground," he said.
Professor Howden said global carbon emissions are about 42 billion tonnes a year. To give the world a two-in-three chance of keeping temperature increases to 1.5 degrees, the remaining budget was about 420 billion tonnes - a figure that would be burned through in a decade at current rates.
Emissions will need to fall 45 per cent from 2010 levels - or 58 per cent from 2015 totals - by 2030 to limit warming to 1.5 degrees, the IPCC said.
"If [the temperature] overshoot is to be minimised, the remaining equivalent CO2 budget available for emissions is very small, which implies that large, immediate, and unprecedented global efforts to mitigate greenhouse gases are required," the report said.
The electricity sector, the largest source of emissions in Australia and globally, would have to be all but decarbonised by 2050 to cap warming at 1.5 degrees. That means coal-fired power phased out entirely and the share of gas cut to about a third, with renewable sources providing 70-85 per cent by mid-century, the report said.
While all of Australia's major coal-fired power plants are due to be shut by 2050, some 1380 new plants globally are in the development phase, a German environmental group Urgewald, said last week. Many of the nations eying new power stations have little or no coal-fired plants now and their construction could lock-in demand for decades.
'Doomed'
The report was signed off on by the IPCC delegates on Saturday afternoon in the South Korean city of Incheon after a marathon six days of talks - including an overnighter to end the event.
One delegate who asked not to be identified said the process looked to be "doomed" after delegates from Saudi Arabia objected to the draft report and began "bashing the desk". "A lot of chicken was being played," the delegate said, before discussions settled down.
Climate policies
Australia is the world's largest coal exporter, with thermal and coking coal shipments last year totalling 382 million tonnes.
The findings of the report are at odds with the Morrison government's insistence that coal will play an essential role in the nation's economic future.
The government is without a policy to reduce carbon emissions from the electricity sector following the dumping of the National Energy Guarantee, and has strongly backed a proposal by Indian mining giant Adani to build what would be Australia's largest coal mine in regional Queensland.
Labor has voiced concerns over the mine, but its position has shifted several times and it has not committed to cancelling Adani's approvals if it wins office. It wants to cut 2005-level emissions 45 per cent by 2030.
The IPCC report will inform the next major international climate conference in Katowice, Poland, in December. Governments will assess the Paris climate treaty and are likely to be pressed to lift their national carbon-cutting ambitions.
On current national pledges, there is a two-in-three chance that global average temperatures will rise about 2.9-3.4 degrees above pre-industrial levels by 2100, the report said.
Prime Minister Scott Morrison has indicated his government would not increase Australia's target under the Paris accord. The goal is to cut 2005-level emissions 26-28 per cent by 2030.
The latest quarterly emissions data show Australia's emissions have been rising for the past four years, or roughly from the end of the Gillard-Rudd government's carbon price.
Impacts
The IPCC report identified a range of ecosystems that will be at risk even at 1.5 degrees. These include the kelp forests off the Australian coast that are already shifting because of warming waters and the influx of herbivores such as urchins.
Globally, alpine environments are also changing as are high-latitude regions such as the tundra, and tropical rainforests - including Australia's wet tropics.
The report confirmed the grave risks that sea level rises pose to coastal communities around the world, including Australia's Pacific neighbours. Increasing warming will amplify the exposure of small islands and low-lying coastal areas to saltwater intrusion, flooding and infrastructure damage.
In terms of sea-level rise, the difference between 1.5 degrees and 2 degrees was about 10 centimetres by 2100. That reduction, though, would spare about 10 million people from exposure to climate change-related impacts.
For the first time in a IPCC report, the authors included social and economic impacts. That marked "the end of magical thinking" that sustainable development goals and poverty reduction could be divorced from climate action, said Bronwyn Hayward, a report author and associate professor at New Zealand's University of Canterbury.
"Limiting global warming to 1.5 degrees instead of 2 degrees could result in around 420 million fewer people being frequently exposed to extreme heatwaves, and about 65 million fewer people being exposed to exceptional heatwaves, assuming constant vulnerability," the report said.
On notice
Some processes, such as melting of the world's ice sheets will continue even with temperature increases capped at 1.5 degrees, raising the prospect future generations will need to extract carbon dioxide and other greenhouse gases such as methane and nitrous oxide directly from the atmosphere.
"Instabilities exist for both the Greenland and Antarctic ice sheets that could result in multi-metre rises in sea level on centennial to millennial timescales," the report said, adding there was "medium confidence that these instabilities could be triggered under 1.5-2 degrees of global warming".
Governments everywhere have been put on notice that global warming was already serious and much more needed to be done, with legal moves in prospect if they didn't act, environmental groups say.
"If governments fail to drastically and urgently reduce emissions, they are knowingly contributing to the dangers of a world that is at least 1.5 degrees warmer," Tessa Khan, co-director of the Climate Litigation Network.
"This has clear legal consequences and governments will inevitably be held accountable for knowingly putting people in harm's way," Ms Khan said, noting citizens in the Netherlands, Ireland, Switzerland, Norway, Colombia, the US and New Zealand have already taken their governments to court.
"This litigation is only just picking up speed."
