The world’s leading climate scientists have set out in detail for the first time how much more carbon dioxide humans can pour into the atmosphere without triggering dangerous levels of climate change and concluded that more than half of that global allowance has been used up.
If people continue to emit greenhouse gases at current rates, the accumulation of carbon in the atmosphere could mean that within as little as two to three decades, the world will face nearly inevitable warming of more than 2°C, resulting in rising sea levels, heatwaves, droughts and more extreme weather.
This calculation of the world’s “carbon budget” was one of the most striking findings of the Intergovernmental Panel on Climate Change (IPCC), the expert panel of global scientists who on Friday produced the most comprehensive assessment yet of climate change at the end of their four-day meeting in Stockholm, Sweden.
The 2,000-plus page report written by 209 lead authors also found it was “unequivocal” that global warming was happening as a result of human actions, and that without “substantial and sustained” reductions in greenhouse gas emissions, the world will breach the symbolic threshold of 2°C of warming, which governments around the world have pledged not to do.
UN Secretary-General Ban Ki-moon urged world leaders to pay heed to the “world’s authority on climate change” and forge a new global deal on cutting emissions.
“The heat is on. Now we must act,” he said.
British Secretary of Foreign Affairs William Hague backed the call, saying: “Unless we act now to reduce carbon emissions, [the effects of climate change] will continue to worsen. Governments, businesses and individuals all have a responsibility. The longer we delay, the higher the risks and the greater the costs to present and future generations.”
The Met Office, Britain’s national weather service, said the report had strong implications for the UK’s future weather and was likely to bring more extreme rainfall and warmer winters, while the hottest days of summer could grow hotter still. Although natural variation will mean there are still exceptional cold snaps, heatwaves and other familiar vagaries of British weather, there could also be duller, wetter summers if new research is borne out that suggests they may be associated with Arctic melting.
The IPCC also rebuffed the argument made by climate change skeptics that a “pause” for the past 10 years to 15 years in the upward climb of global temperatures was evidence of flaws in their computer models.
In the summary for policymakers published on Friday morning after days of deliberations in the Swedish capital, the scientists said: “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. In the northern hemisphere, 1983-2012 was likely the warmest 30-year period of the last 1,400 years.”
Thomas Stocker, co-chair of the report working group, said measuring recent years in comparison to 1998 — an exceptionally hot year — was misleading and that temperature trends could only be observed over longer periods of about 30 years.
Natural variability was cited as one of the reasons for warming being less pronounced in the past 15 years and the role of the oceans in absorbing heat, which is still poorly understood.
“There are not sufficient observations of the uptake of heat, particularly into the deep ocean, that will be one of the possible mechanisms that would explain this warming hiatus,” Stocker said.
However, the most controversial finding of the report was its “carbon budget.” Participants told the Guardian that this was the last part of the summary to be decided and the subject of hours of heated discussions in the early hours of Friday.
The scientists found that to hold warming to 2°C, total emissions cannot exceed 1,000 gigatonnes of carbon. Yet by 2011, more than half of that total “allowance” — 531 gigatonnes — had already been emitted.
To ensure the budget is not exceeded, governments and businesses may have to leave valuable fossil fuel reserves unexploited.
“There’s a finite amount of carbon you can burn if you don’t want to go over 2°C,” Stocker told the Guardian. “That implies if there is more than that [in fossil fuel reserves], that you leave some of that carbon in the ground.”