One of the final obstacles in the way of a binding agreement at the Paris climate talks comes down to a simple number: 1.5. Limiting warming to a 1.5℃ temperature rise above pre-industrial levels is one of three potential targets on the table as negotiations approach the crucial final days. The other options are a firm limit of 2℃ and a limit of 2℃ with an aspiration to reduce to 1.5℃ in the coming years.

Releasing the latest draft text on Wednesday afternoon local time, the summit’s president Laurent Fabius listed the target as one of three outstanding major issues, alongside finance and the question of how to differentiate the responsibilities of developed and developing countries.

Many industrialised countries have surprised the world at the talks with a new-found fondness for 1.5℃. The target has long been a key demand of most poor nations, particularly small island states and least-developed countries.

Scientists consider that as 1.5℃ is breached, we will risk passing critical tipping points. In particular, sea level rise associated with that level of temperature increase poses an existential threat to low-lying island states.

Despite more than 100 developing nations being firmly in favour of a 1.5℃ limit, countries came to a political agreement in 2010 to collectively set themselves a 2℃ threshold.

Kicking the carbon habit

If a temperature limit of 1.5℃ is fixed in the new Paris agreement, that raises the question of what countries will need to do to stay below that level of warming.

The UN’s top climate science body has shown that carbon dioxide (CO 2 ) emissions are cumulative, with residence times in the atmosphere of thousands to tens of thousands of years. Temperature rise has a linear relationship with carbon emissions, so we can estimate the remaining amount of CO 2 that can be emitted before we risk passing any temperature limit with some probability. For a 50% probability of staying below 1.5℃, there was a remaining carbon budget of 550-600 gigatonnes CO 2 in 2011. At current annual global emissions of around 36 gigatonnes CO 2 , this budget will be used up in less than two decades.

What does staying below 1.5℃ mean in practice? Nothing less than full decarbonisation of the global economy by 2050. We must stop burning all fossil fuels before the middle of the century, along with a massive effort to keep forests standing and protect biodiversity. That is no small feat.

While some say limiting warming to less than 1.5℃, or even 2℃, is out of reach, ultimately 1.5℃ is a political signal for greater ambition, and a more serious global engagement in addressing climate change. Late in the day as this signal might be, it is important that a new international agreement does not include a temperature limit that even the UN has recognised is not a safe guardrail.

But what does this mean in real terms – if a temperature goal is agreed that requires rapid decarbonization: who must do what, and by when?

Fair shares

At the heart of the standoff in the climate talks are fundamental differences over who has more responsibility to act, and what a fair approach to drive greater ambition looks like. A broad coalition of NGOs set out to define a methodology to answer that question, taking into consideration the remaining carbon budget, historical emissions (as impacts on temperature are cumulative, historical emissions matter), and differing capacities between countries.

Their numbers show that the ambition – UNFCCC jargon for emission reduction efforts – of all developed countries falls well below their fair share of what is needed to stay within the remaining budget. Top of the list of offenders are Russia and Japan who are making little to no contribution to what would be considered their fair share of effort, while the US and the EU have pledged around a fifth of their fair share.

In this context, we start to understand why many developing countries might not want to commit to a 1.5℃ limit without clear rules on how to divide up the effort. The current emissions trajectories of developed countries will take up far more than their fair share of the remaining budget, seriously impeding the poverty alleviation and sustainable development aspirations of developing countries.

But development trajectories that exceed the global carbon budget will not work for the planet – regardless of who has done what historically, all countries are now bound by the very limited carbon budget remaining. This means that even for countries who have pledged what is basically their fair share of global action, such as China or India, they will need to do a lot more to keep the world anywhere near a 1.5℃ pathway.

For poorer countries (India ranks 135 on the Human Development Index), committing to do more than their fair share needs to be in the context of international commitment to support.

Collective goals

In the end, the 1.5℃ conversation is not the real debate. The real challenge in Paris is to agree on language for emissions reductions that is even remotely compatible with achieving whatever temperature goal is set. This is referred to as the “collective global goal”.

Options for a global goal include peaking emissions, zero emissions, or decarbonisation or climate neutrality. But without a differentiated long-term goal – one that puts increased ambition in the context of more support for developing countries – whether the goal is for peaking or zero, 2050 or 2100, all becomes meaningless.

Ultimately, the call for 1.5℃ must not become a distraction from the real challenge: agreeing a collective goal that includes both ambition and equity. Without a clear sense of who needs to take the lead, who needs support to do more than their fair share, and how this will collectively keep global emissions within a carbon budget – everyone will lose.