To stop temperatures rising beyond a United Nations target that seeks to curb climate-change dangers most known reserves of fossil fuels will need to stay unburned.
About 531 billion tonnes of carbon have been emitted by burning oil, coal and gas, cutting down forests and making cement since 1750, the UN has said.
Bloomberg newsagency reports it added, in its first such estimate, capping greenhouse gas output at 840 billion tonnes gives a 50 per cent chance of meeting the UN target of restraining warming below two degrees Celsius.
“This gives you an indication of the space that’s available to us in terms of how much greenhouse gases we can emit to have a chance of stabilising the Earth’s climate,” Professor Rajendra Pachauri, chairman of the UN Intergovernmental Panel on Climate Change (IPCC), said in an interview in Stockholm with Bloomberg.
The data suggests the bulk of known fuel reserves will need to stay in the ground to meet the limit.
Reserves reported by listed companies, or a quarter of the total, equate to 762 billion tonnes of carbon-dioxide emissions, said a report this year by the London School of Economics’ climate research unit and nonprofit Climate Tracker Initiative.
That amount of CO2 contains 208 billion tonnes of carbon.
Cumulative carbon emissions must be capped at 800 billion tonnes to get a 66 per cent chance of staying below two degrees of warming and 880 billion tons to give a 33 per cent chance, the UN said.
The conclusions are part of a 36-page summary of the most important scientific knowledge on climate change, processed by the IPCC in its widest study into global warming.
“We are not on a path that would lead us to respect that climate target,” said Professor Thomas Stocker, co-chairman of the group that drafted the UN report released in Stockholm.
Professor Stocker teaches climate science at the University of Bern in Switzerland.
The study strengthens the case for world leaders to set limits to future emissions, said Samantha Smith, head of the climate and energy program of environmental group WWF.
“This gives a clear scientific basis for a carbon budget and an indication of how much carbon we have already used and how much we have left before we start breaking planetary boundaries,” Ms Smith said in an interview in Stockholm with Bloomberg.
“This has gone from being an interesting piece of modeling to being something that now has solid foundation in the IPCC report.”
The best way to achieve cuts needed to control warming is by putting a price on emissions, Professor Pachauri told reporters.
“An extremely effective instrument would be to put a price on carbon.” he said.
“It’s only through the market that we might be able to get a large enough and a rapid enough response.”
Scientists including 20 Nobel Prize winners touted the idea of a global carbon budget in 2009.
Bloomberg reports calling themselves the Nobel Laureates Symposium, they signed a memorandum that said a carbon budget that set limits on global emissions was needed for 2020 and 2050.
Professor Pachauri was among the signatories.
Even if emissions from cement making and deforestation were halted, a maximum of 309 billion tonnes of fossil fuel-related emissions would be possible before the 50 per cent chance of meeting the two degree goal is lost, according to Bloomberg calculations based on the UN report’s numbers.
By comparison, cumulative carbon emissions from making cement and burning fossil fuels combined total 365 billion tonnes already.
The UN Environment Program estimates humans emit an annual 50 billion tonnes of carbon dioxide from all sources.
That’s about 13.6 billion tonnes of carbon with oxygen’s mass stripped out.