There is a 1-in-10 chance of the world being 6C warmer than it is today by 2100 - a temperature rise that would lead to cataclysmic changes in the global climate with unimaginable consequences for human civilisation, leading climate researchers have warned.
The risk of hitting the highest upper estimate for global warming based on current levels of carbon dioxide emissions is now so high that it is equivalent to tolerating the risk of 10,000 fatal aircraft crashes a day, according to 17 "Earth League" scientists and economists who have signed a joint "Earth Statement" published yesterday.
The experts have drawn up a three-page summary of the action that needs to be agreed upon at the United Nations meeting in Paris this December, which is widely seen as the last chance for the world's political leaders to agree on a binding treaty to prevent the global climate from slipping into a dangerously precarious state. Their statement lists eight areas- an eight-point-plan - of action.
Scientists calculate that the world has already warmed by an average of about 0.85C over the past 120 years, and that a further increase of no more than 2C is the lowest that could be tolerated without running the risk of dangerous climate "tipping points" leading to further, accelerated warming.
"We should aim to stay as far below [2C] as possible, since even 2C warming will cause significant damage and disruption. However, we are currently on a path to around 4C warming by 2100, which would create unmanageable environmental challenges," it says.
"If we do not act now, there is even a 1-in-10 risk of going beyond 6C by 2100. We would surely not accept such a high risk of disaster in other realms of society. As a comparison, such a 1-in-10 probability is the equivalent of tolerating about 10,000 airplane crashes every day worldwide."
The Earth League researchers - who include economists Jeffrey Sachs and Lord Stern, as well as renowned climate scientists from Europe, Brazil and India - warn that time is running out for a climate deal that binds countries to a process of "deep decarbonisation", where fossil fuels are largely replaced with cleaner sources of sustainable energy by 2050.
"2015 is potentially one of the most decisive years in modern human history on Earth when it comes to determining our future prospects for wellbeing and prosperity for 9 to 10 billion people over the next century," said Dr Johan Rockstrom of the Stockholm Resilience Centre in Sweden, who chaired the group.
"The key element of this statement is that a window is still open, but just barely. There is still an opportunity to make the transition to a safe, reasonably stable climate in the future, and the decisions in 2015 may be decisive for that opportunity.
"If we follow the current trajectory of 'business as usual', it would have a 1-in-10 probability of leading to 6C by the end of this century, and 6C, I think even the climate sceptics would agree, is a place the world does not want to be in," said Rockstrom.
He added: "A 1-in-10 probability of a catastrophic outcome is a very high number. It's a probability level that we would never ever accept in any other sector of society ..."
The eight-point plan to save the world
Governments must limit global warming to below 2C in order to limit unprecedented climate risks.
2. The limit of future CO2 emissions must be well below 1000 gigatonnes of CO2 to have a reasonable chance to hold the 2C line.
3. Countries must commit to deep decarbonisation, beginning immediately and leading to a zero-carbon society by 2050.
4. Every country must formulate an emissions pathway consistent with deep decarbonisation. For the sake of fairness, rich countries and progressive industries can and should take the lead and decarbonise well before mid-century.
5. Targeted research, development, demonstration and diffusion of low-carbon energy systems and sustainable land use are prerequisites to unleash a wave of climate innovation.
6. A global strategy to limit vulnerability, build resilience and deal with loss and damage of communities from climate impacts.
7. Countries must agree to safeguard carbon sinks and vital ecosystems, such as forests.
8. Governments must urgently encourage new sources of climate finance for developing countries to enable rapid transition to zero-carbon, climate-resilient societies.