Scientists have warned of a brief and rapidly closing window to prevent global warming exceeding the Paris Agreement, in the latest Intergovernmental Panel on Climate Change report.
The 1.5 degree limit of global warming is out of reach unless greenhouse-gas emission reduction targets currently not being pursued are adopted, the IPCC found.
The report, which was delayed in receiving endorsement by all governments late on Monday, found all countries and all sectors must act faster to reduce emissions, and Australia had massive potential to contribute.
In a post-report address, United Nations secretary-general Antonio Guterres said investing in new fossil fuels infrastructure was moral and economic madness.
"Some government and business leaders are saying one thing but doing another," Mr Guterres said. "Simply put, they are lying."
Referencing more than 59,000 reports, scientists found annual CO2 emissions would need to be reduced by 48 per cent globally by 2030 and reach net zero by 2050 to stay within 1.5 degrees of warming.
Methane emissions would need to be reduced by a third by 2030 and almost halved by 2050 to limit warming to 1.5 degrees, according to the IPCC.
Agriculture and forestry can provide large-scale emissions reductions and also remove and store carbon dioxide at scale. However, land cannot compensate for delayed emissions reductions in other sectors, the report found.
Professor Frank Jotzo, a lead author on the report and based at the Australian National University Crawford School of Public Policy, witnessed the delayed approval process, as one of the presenting experts.
He said the fact that each line was so strongly examined and contested by governments was testament to the importance of the report.
Professor Jotzo said the temperature rise had currently reached 1.1 degrees and any reasonable judgement indicated action was not occurring fast enough to stop the 1.5 degree trajectory.
"So 1.5 degrees is gone, but the report assesses that there are tremendously large opportunities to reduce emissions by 2030," he said.
He said it had been identified from a detailed sector-by-sector, case-by-case analysis that a reduction of 50 per cent was possible by 2030 and 25 per cent reduction was possible with "bearable economic effort".
"The reason is that clean technologies have just become so much cheaper," he said.
"Another reason is that many countries have been implementing policies to constrain greenhouse gas emissions and they're working.
"So the outlook is not a bright one, but very much more positive than what we expected just a few years ago."
Professor Jotzo said, among developed nations, Australia was one of the most vulnerable regarding future climate change.
"Australia has a climate that is highly variable and we're experiencing that with the succession of fires and floods," he said.
"Agricultural growing conditions are precarious in many parts of the country and all of these difficulties tend to be exacerbated by climatic change."
He said the systems Australia had built and location of cities appealed to the existing environment, not some changed future climate.
"Coal really is what needs to be on its way out for any kind of reasonable climate change future, that is relatively easy in our power sector because we have ample opportunities to just replace coal with wind and solar and storage," he said.
"In Australia, the same by and large goes for gas. In the Australian electricity system, gas is expected to play just a minor role in a future decarbonised energy system."
The report found cities and other urban areas offered opportunities for emissions reductions through lower energy consumption, electrification of transport and enhanced carbon uptake and storage.
Jim Skea, IPCC Working Group III Co-Chair, said zero energy or zero-carbon buildings offered mitigation potential, with action required this decade.
"It's now or never, if we want to limit global warming to 1.5°C," said Skea.
Climate Change 2022: Mitigation of Climate Change is the culmination of seven years of work and follows the release of the IPCC Working Group 2 report in February.
Professor Mark Howden, IPCC working group 2 vice-chair and director of the ANU Institute for Climate, Energy and Disaster Solutions, said the latest IPCC report rounds out the key information needed to make informed, rapid and large changes.
Professor Howden said the report demonstrated the technology and governance options were available to cost-effectively reduce emissions.
"It is even clearer than before that taking strong action on climate change is much less costly than inaction."
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