Hopes of keeping global warming below the long-established target of two degrees above pre-industrial levels are rapidly eroding, according to a collection of papers in two Nature journals today.

That might sound like a gloomy backdrop to this week’s climate summit, convened by UN director-general Ban Ki Moon to refocus world leaders’ attention on climate action.

But chalking up the two degrees target as a political failure is a “naive” way to look at climate ambition and could even obstruct future negotiations, the authors argue.

Origin of two degrees

In 1992, the United Nations Framework Convention on Climate Change (UNFCCC) said the objective of global climate policy should be to stabilise humans’ influence on the climate below the level at which it can be considered “dangerous”.

Scientifically-speaking, there’s no definitive threshold beyond which climate change tips the balance into being “dangerous”. But as temperatures rise, so do the risks.

Curbing temperature rise to two degrees above pre-industrial levels has become the most widely accepted point beyond which climate change risks are considered unacceptably high.

As the recent report on climate change impacts from the Intergovernmental Panel on Climate Change (IPCC) puts it:

“Increasing magnitudes of warming increase the likelihood of severe, pervasive, and irreversible impacts. Some risks of climate change are considerable at 1 or 2°C above preindustrial levels â?¦ The precise levels of climate change sufficient to trigger tipping pointsâ?¦ remain uncertain, but the risk … increases with rising temperature”.

A two degree limit has been a symbolic focus of climate ambition for the past two decades – it is the limit recommended by the UK’s Committee on Climate Change, for example.

Blowing the budget

Carbon dioxide – the biggest contributor to global warming – has a predictable relationship with temperature. That means scientists can calculate how much carbon we can emit and still have a reasonable chance of staying below two degrees. This is known as a carbon budget.

Global emissions from fossil fuel-burning and cement production grew by 2.5 per cent per year on average between 2004 and 2013. Source: Friedlingstein et al., (2014)

To have a two thirds chance of staying below two degrees, total emissions from the beginning of the industrial revolution to the time we stop burning carbon would need to stay below 3,670 billion tonnes of carbon dioxide, one paper in Nature Geoscience notes.

When you allow for emissions of methane, CFCs, ozone, nitrous oxide and soot – which also warm the atmosphere – the carbon dioxide budget is an even tighter 3,200 billion tonnes, note Pierre Friedlingstein and colleagues in the paper .

However, global carbon dioxide emissions from fossil fuels and cement production have continued to grow by 2.5 per cent per year, on average, in the past decade.

That means two thirds of the carbon dioxide budget to stay below two degrees has already been spent. And if countries continue to emit at current rates, the budget is likely to be exhausted in 30 years time, the paper adds.

Top down vs bottom up

The two degree target, and a finite carbon budget to accompany it, seems like an appealing way to visualise the scale of climate action needed. At least, from a scientific perspective.

So why haven’t we seen emissions drop in response?

Coming to a global agreement for limiting global warming to two degrees means finding a way to divvy up the remaining carbon budget between countries. And that’s a tough ask, a separate paper in Nature Climate Change explains.

The Australian National University’s Michael Raupach and colleagues consider some potential options, noting that the challenge of resource-sharing between countries cuts across the thorny issues of fairness, governance, institutions and ethics.

This ‘top down’ approach of quota sharing has made little progress in the last two decades of climate negotiations, the authors note.

Over 90 per cent of global emissions come from burning fossil fuels and cement production.

An obstacle to progress

The two degree target may even be more of a hindrance than a driver of progress, argues a Nature Geoscience commentary by David Frame and colleagues, including the University of Oxford’s Myles Allen.

The authors suggest it focuses too much attention on short term fixes at the expense of a long term vision for meaningful change. The paper notes:

“So far, international focus on near term Kyoto style targets has allowed post 2020 (or 2030) emissions to remain largely undiscussed, even though these determine the success or otherwise of climate mitigation.”

A separate comment piece by David Victor in Nature Climate Change has some strong words on the topic, suggesting a “top down” limit on warming is an “abstract”, “naive” and “fictitious” target.

A more realistic perspective of the world, Victor says , is a ‘bottom up’ approach to effort sharing, whereby nations decide for themselves what they can reasonably achieve in terms of carbon cuts by a certain date.

As part of the so-called Durban Platform, an agreement adopted in South Africa in 2011 to forge a new treaty which will come into force in 2020, many countries have already pledged to limit or reduce emissions by 2020, the paper by Friedlingstein notes.

All UN countries are expected to submit pledges by March 2015.

Spurring action

As another long season of climate diplomacy gets underway, hopes for success rest on fresh ideas, momentum and a re-energised sense of collective effort sharing, today’s collection of papers concludes.

The focus for international climate policy should be on policies, architecture and commitment, not specific goals and outcomes, they suggest.

A reality check about the two degree goal is necessary to ensure next week’s UN summit and efforts to negotiate a new climate deal in Paris in 2015 aren’t dismissed in a “fizzle of disagreement and dashed expectations”, Victor warns .

Failing to limit warming to two degrees above pre-industrial levels is no doubt a missed opportunity. And as for whether this week’s negotiations will garner a renewed sense of optimism, all eyes turn to New York to see what countries bring to the table.