杏吧原创

THE 20th round of climate talks in Peru鈥檚 capital city Lima this month are seen as a crucial step towards reaching a global agreement in Paris in 2015.

One of the things that is likely to be discussed is whether the 2 掳C target 鈥� the threshold between 鈥渁cceptable鈥� and 鈥渄angerous鈥� climate change 鈥� should remain the world鈥檚 linchpin climate policy goal. In recent months, the validity and appropriateness of this ubiquitous target have been called into question.

Concerns about the 2 掳C target centre on its scientific credibility, its ability to galvanise meaningful action and its political feasibility. In October, for example, the journal Nature published a comment piece urging the world to 鈥渄itch the 2 掳C warming goal鈥�. The authors 鈥� David Victor, a professor of international relations at the University of California, San Diego, and Charles Kennel of the Scripps Institution of Oceanography 鈥� argue that 2 掳C is both scientifically and politically meaningless.

There is no doubt that the 2 掳C target has its shortcomings, but the risks of jettisoning it now are too great for the planet and society to bear. Instead, scientists should learn from the criticisms to give more nuanced, transparent and honest 鈥� and therefore more effective 鈥� policy advice.

鈥淭he 2 掳C target has its shortcomings but the risks of jettisoning it are too great for the planet鈥�

Despite its prominence in the debate, the origins of the 2 掳C target aren鈥檛 clear. One possible source is a 1938 study by pioneering climate scientist Guy Callendar in which he predicted that doubling the global concentration of carbon dioxide from pre-industrial levels would result in around 2 掳C of warming.

Whatever its origins, 2 掳C has proved to be useful when considering the subjective issue of dangerous climate impacts. The 2009 Copenhagen Accord explicitly identified 2 掳C as a target, and the number has since gained considerable political importance as a way of anchoring climate politics to the science. According to many studies, 2 掳C is the point around which many impacts become dangerous.

So why suggest it should be abandoned? Victor and Kennel argue that global temperature change isn鈥檛 a good way to measure planetary health. There are many other physical changes that better demonstrate how our planetary limits are being pushed, they say. The heat content in the ocean, for example, is increasing more steadily than globally averaged surface temperatures. This hasn鈥檛 gone unnoticed by climate deniers; they frequently point to a hiatus in temperature rises since the late 1990s and a lower than expected upward trend in the past decade.

This is a valid criticism. But I argue that dropping the 2 掳C target now would further confuse the already complicated choices that policymakers have to consider.

Victor and Kennel also argue that the target has been accepted without criticism and yet has proved politically influential. This is debatable. Perhaps it has been influential, but this hasn鈥檛 translated into policies to put the world on track to avoid 2 掳C.

Another way of looking at this is that we scientists haven鈥檛 critically and clearly explained what hitting the 2 掳C target would actually entail. This may be because it is politically unpalatable: some analyses predict that avoiding 2 掳C threatens economic growth in wealthy nations.

One scientific framing that might work is to make a link between a certain level of cumulative greenhouse gas emissions and future temperature rises. For too long, this simple but important relationship has been underplayed. It can be used to paint realistic visions of the future and, more importantly, it shifts the focus from long-term targets to the necessity of cutting CO₂ in the next few years.

This brings me to what 2 掳C means, and why I think it should remain centre stage.

The 2 掳C target isn鈥檛 a single target, but a probabilistic one. It tells us how much carbon we can emit (our 鈥渃arbon budget鈥�) if we want a 90 per cent chance of avoiding 2 掳C of warming, or a 10 per cent chance, or whatever probability we choose. In other words, there is already a huge amount of flexibility in aiming to avoid 2 掳C. That doesn鈥檛 imply some sort of get-out clause, but it does mean that if 2 掳C is to remain, then there are plenty of options on the table.

In short, 2 掳C gives policymakers choices. Switching to some other measure, or measures, would just complicate the picture.

It isn鈥檛 scientists鈥� job to dictate how science should be used by policymakers. Instead, we should offer a thorough analysis of carbon budgets aimed at avoiding 2 掳C and the implications for how quickly and deeply we need to cut CO₂, as well as the associated climate impacts that society will need to be resilient to if policies fail to deliver. We should avoid getting into the political feasibility of any required change.

If this approach had been taken a decade ago, perhaps today鈥檚 policymakers would be delivering measures that cut CO₂ in line with a certain probability of avoiding dangerous warming.

Instead, we now require a new research agenda: what does it mean to radically cut CO₂ to give us a decent chance of avoiding 2 掳C? Is it feasible? And how do we adapt society and infrastructure to 4 掳C of warming, to account for the inevitable risk of failure?

Arguably we have been doing the opposite: advising policymakers towards policies linked to at least a 4 掳C rise, while adapting infrastructure to cope with 2 掳C. It is time to turn this on its head.

Now isn鈥檛 the time to rethink the 2 掳C goal. The climate debate is complicated enough as it is. Abandoning 2 掳C, with its simplicity and value as an anchor point for debate, isn鈥檛 a prerequisite for getting smarter with targets, and risks a further delay in delivering change at a point when the planet is already at breaking point.

This article appeared in print under the headline 鈥淭wo degrees of separation鈥�