Could the 2C climate target be completely wrong?
The global warming goal that nearly 200 governments have agreed on should be ditched, say scientists writing in Nature.
In a nondescript conference centre five years ago, as temperatures fell to freezing outside in the streets of Copenhagen and protesters gathered, world leaders did something remarkable: they put a limit on how high temperatures should be allowed to rise as man-made global warming takes hold.
It was the first time the nearly 200 countries in the UN climate talks process had put a number on what constituted the limit for dangerous climate change (Germany had done it years before, followed by the EU). And with hindsight, is one of the signal agreements of a summit that was widely derided as a failure.
Since then, the 2C target - or obligation, as some in climate diplomacy circles refer to it - has been repeated like a mantra, mentioned thousands of times in newspaper articles and most recently uttered aloud repeatedly last week by heads of state in New York for a climate summit organised by Ban Ki-moon.
But two academics in the prestigious journal Nature now argue that the 2C target has outlived its usefulness. They say it should be abandoned and replaced with a series of measures - "vital signs" of the planet's health.
Under the headline, "Ditch the 2C warming goal", they argue the 2C limit is "politically and scientifically ... wrong-headed", it is "effectively unachievable" and it has let politicians off the hook, allowing them to "pretend that they are organising for action when, in fact, most have done little."
David G Victor, the University of California professor who co-wrote the comment along with former Nasa associate administrator Charles F Kennel, said he felt compelled to speak out after watching climate diplomacy efforts and working on the latest blockbuster report by the UN's Intergovernmental Panel on Climate Change.
"All diplomacy was focused around this goal and yet it struck me as obvious that the emissions trajectories, even if governments made a big effort at controlling emissions, were way off track for 2C." Working on the IPCC report, he told the Guardian, made him realise the 'climate establishment' was "entirely geared to supporting 2C even though nobody had a serious plan for meeting it".
For some in international climate politics, Victor and Kennel's message of reality, as they call it, is tantamount to heresy. And it has provoked a strong reaction.
"The University of California should realise 2C is a fact, not a target," said Lord Deben, former UK environment secretary and now chair of the UK's statutory advisers on climate change. "Go above it [2C] and you say something about the world that is intolerable. 2C is dangerous but at least we have some understanding of what that means. To abandon that would seem a most peculiar thing to do."
The 2C mark is often described as the level beyond which disastrous impacts including flooding and heat-waves - and potentially runaway warming as natural 'feedbacks' kick in - would take place.
Michael Jacobs, a special adviser to Gordon Brown when he attended Copenhagen and now an adviser to the recent New Climate Economy report, says the timing of the intervention is far from helpful. "Although derived from science, 2C is not a scientific target due to the uncertainties in our climate system and our ability to model it. It is a political target, whose adoption in 2009 and 2010 indicated a political commitment to limiting climate change as far as possible.
"15 months out from the Paris conference and a week after a successful summit put climate change back on the international agenda is completely the wrong time to consider abandoning that commitment."
Tom Burke, director of the NGO E3g and a former Rio Tinto executive and UK government adviser, is scathing. "Fundamentally he's [Victor] missed the point. Since they took the Copenhagen accord into the framework at the UNFCCC [United Nations Framework Convention on Climate Change, the aegis under which international climate talks take place], 2C is an obligation. It's not a target. They [governments] have defined the boundary of dangerous climate change, therefore countries are obliged to work in order to stay within that boundary."
But given figures published earlier this month, which showed global carbon dioxide emissions at record levels and on an ever-upward trend that puts the world on course for temperatures well above 2C, do Victor and Kennel not have a point when they argue 2C is effectively unachievable?
As they put it: "To be sure, models show that it is just possible to make deep planet-wide cuts in emissions to meet that goal. But those simulations make heroic assumptions - such as almost widespread global cooperation and widespread availability of technologies ..."
"There are lots of people who disagree with the idea it's unachievable," said Burke. "It depends what effort you put in. Could we get to 2C? Would we wreck economies to do it? Certainly not. Could we get the political will to do that? That's the question. But if you say we shouldn't even try then we definitely won't get there."
One senior western veteran of the climate talks says privately that dropping the 2C goal would be a "really dumb thing to do." Politically, they said, "it translates into 'let's not try so hard'."
One big name in climate diplomacy who did applaud the intervention is the UK's former deputy prime minister, John Prescott, one of the architects of the 1997 Kyoto protocol, the first and still the only international agreement on cutting emissions.
"I welcome it, I didn't when I read the bloody title [of the article in Nature]," said Prescott, who believes the current effort to get a legally-binding deal in Paris is misguided and that a focus on domestic climate legislation is more important. "We're very close to the ceiling on 2C ... we need to have a much more open debate about climate change."
Several scientists were critical of the idea that the 2C limit should be dropped, but did not dismiss Victor and Kennel's argument entirely.
Prof Rowan Sutton, director of climate research at the University of Reading's National Centre for Atmospheric Research, said: "I have some sympathy with it. He's right that temperature is an imperfect measure of climate change, there's no doubt about that. The 2C target is somewhat arbitrary, it always was. But I wouldn't go as far as he does to argue it should be ditched."
Instead of focusing on holding the world's average surface temperature to 2C, Victor and Kennel propose instead measuring a suite of "vital signs" that they liken to the health indicators that doctors track. They propose tracking the stress man is putting on the climate system by looking at CO2 concentrations in the atmosphere, ocean heat content, and temperatures in the polar regions.
"I do agree with him that it could be useful to supplement temperature with a small number of other measures of how the planet is changing," said Sutton. "Having a long shopping list of other measures wouldn't really help people, but I agree with him that a small number, of two or three of four instead of one, that would help wider understanding of what we're talking about."
Prof Kevin Anderson, deputy director of the Tyndall Centre in Manchester, who thinks it is very likely we will fail to keep temperature rises to 2C but believes it is still possible, said there was "some merit" in a suite of vital signs but rejected the idea that 2C should be dropped.
"2C is the best, or perhaps least worst, proxy for a range of impacts that the international community has repeatedly affirmed as an appropriate threshold between acceptable and dangerous climate change," he said. "The [Nature] paper hints at it being the role of scientists to define dangerous - this is, in my view, a mistake that undermines much of their paper." The level of acceptable danger is not a decision for scientists, he says.
Hans Joachim Schellnhuber, Angela Merkel's climate adviser and the man often credited as the inventor of the concept of a 2C limit in the 1990s, says that the arguments have been made by the authors before - what's new is the visibility of the journal, Nature, that they are writing in.
"This will create a little storm I guess. Many lobbyists will celebrate tonight because this provocation will of course try to stop the momentum which was building up last week in New York. For the first time since Copenhagen, people feel a new movement emerging towards a meaningful agreement in Paris and such a piece can serve as an excuse for inaction again. It somehow conveys the message the science is unclear. All in all, it's a provocation at a critical point in time," he said.
He insists the target is achievable. "To say 2C is practically impossible is against all the findings of the IPCC, which are that yes it is ambitious to keep warming to 2C, but technically, scientifically, it is feasible."
"I do believe in human progress, in our innovation capacity, yes sir, I'm optimistic," he said. "We are very close to a social tipping point regarding decarbonisation of the economy."
Schellnhuber, who is preparing a full riposte for publication in a journal, is most damning of all on the alternative system that Victor and Kennel propose - the set of climate change vital signs.
"I am communicating to heads of state and you have to keep it neat and simple. It was difficult enough to commuicate a 2C target ... but it seems to have sunk in. How should I communicate to policy makers who have an attention span of 10 minutes a set of volatility signals ... this is politically so naive," he says.
"How on Earth do I explain to people they should care about joules of energy in ocean heat?"