So here we go again. Nations are meeting in Paris for their twenty-first attempt to agree decisive action to avoid what the United Nations defines as dangerous climate change.

Climate negotiations have set the extent of this threat at 1.5–2  °C of global warming above pre-industrial levels. With the installation of such a guard rail, the essential components of a ‘successful’ climate deal are more or less in place. A fair chance of achieving 2 °C translates into a limited global carbon budget of about 900 gigatonnes of carbon dioxide from 2015 that must be shared fairly among all countries.

Can the Paris talks lead to an agreement that gives a binding commitment from all countries to meet this outcome? The last time the world gathered for a decisive global agreement on climate change in Copenhagen in 2009, the message was that, yes, world leaders need to do nothing less than decide on a global, legally binding agreement. which fulfills one’s scientific goals. Safe and in the future below 2 °C.

But since Copenhagen, the global discourse has changed. In 2009, it was only possible to show that we need to address the climate challenge; It was not easy to show that it was possible. Today the need for this is clearer than ever. And, more importantly, there is ample evidence that it is possible to grow economically competitive, clean-energy solutions.

Before Copenhagen, economists generally thought that a higher oil price was the best way to enable the transition to a decarbonized future. The surprising reality is that low oil prices appear to be the most effective way to make the transition away from fossil fuels.

Renewable energy systems also compete at low oil prices, which in turn closes the door to exploitation of unconventional, expensive oil, such as offshore oil, and in difficult environments such as the Arctic. It also opens a unique window for introducing a global price on carbon – clearly the most effective policy measure to accelerate the transition to fossil-fuel-free energy.

Experience in the industrial sectors shows that new solutions can become mainstream and become part of the market and society only if they have penetrated at least 15-20% of the market or society. For renewable energy, this penetration has been achieved in only enough countries in the last three to four years.

In this new situation, is it possible to envisage a transition to a carbon-free world by 2050, even if Paris does not reach the ‘perfect’ agreement? the answer is yes. To get there, the threshold of success in Paris must not be at the level of ‘solving the climate problem’ through incremental change, but ‘reassurance that the world is serious about change’.

We need a deal that is decisive enough to move the world rapidly towards decarbonisation. A new treaty need not force nations to comply, but should build trust and send the right signal to investors, businesses and societies – that global political leadership is irreversibly turning towards a new sustainable era .

How ambitious must the Paris Agreement be to decisively support such a trajectory? To meet the 2 °C limit, the world would have to cut carbon emissions by about 6% per year. The national resolutions laid on the table in Paris will not bring us any closer.

From experience, we know that emissions reductions in the 0-2% per annum range are within the scope of incremental policy measures. A range of 2-3% requires ambitious adaptation. Once levels exceed 3-4%, experience indicates that radical measures are needed, such as a carbon tax and phasing out coal power.

These are the kinds of changes needed to decarbonize the world economy, and above all, to send a clear signal of a shift from incremental to transformational change. Thus the success in Paris should be seen as an agreement that matches the pace of emissions reductions of over 3–4% per year starting in the 2015–20 window.

This, in turn, would suggest that Paris should submit to 80% of the national pledges required to stay within the 2 °C guard rail, with at least 20% of countries committing to an average cut of no more than 4% per year, in order to build a bigger one.

A substantial critical mass of nations committed to decarbonization and to influence the global argument (see go.nature.com/1uxlyn). Achieving this goal is ambitious but realistic.

And it comes with a good opportunity that, once nations realize the benefits of decarbonisation, they will step up their pledges. Therefore, it is important that the Paris Agreement permits a repetition of the pledges at least every third or fifth year.

It would be dangerous to allow ‘success’ to be reduced to a low level of political achievement so that the world continues along an incremental policy path that has no chance of supporting the transition to decarbonisation.

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