Consensus-based scientific material for high-risk trading
On Sunday, December 2, the States met for the COP24 in Katowicein Poland. The main issue at stake at this summit meeting is to improve transparency and monitoring of countries' climate commitments, in terms of emission reductions and transition financing. A key point will be to specify the information that will have to be made public. This enhanced transparency is an essential step in building confidence and thus initiating a process to strengthen the commitments of States by 2020, the date of entry into force of the Paris Accord. A document provides the backdrop for these climate negotiations: the "Special report 1.5°C" of the IPCC. This report, which compiles more than 6,000 scientific publications, represents the new state of the art in climate science. It has two components: the first details the impacts of a 1.5°C warming on the climate and ecosystems; the second presents socio-economic trajectories to achieve this goal.
Ahe publication of this report on 8 October last, two months before the COP, is no coincidence: it is precisely a request made to the IPCC by the States at COP21, with the aim of providing consensual scientific material for this COP24.
With hindsight, we propose to revisit some of the main messages of the report, highlighting the extent to which they can influence future international negotiations at COP24 and beyond.
Every half degree of warming counts
1.5°C instead of 2°C Does it make a real difference to humans and ecosystems? The report provides for the first time a quantified and unambiguous answer: yes, every half degree counts.
Prior to this report, there was relatively little scientific material on the impacts of a 1.5°C warming. Previous studies have focused on warming of 2°C or more. And the differences between 1.5°C and 2°C were not obvious, given the complex, non-linear nature of the climate system and ecosystems.
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The announcement of a special IPCC report has prompted a series of new publications, the results of which are edifying. The heat wavess, the intense rainfall and droughts will be as much less damage to human populations if warming is limited to 1.5°C.
Recall here that 1.5°C is an average temperature target, but this masks significant seasonal and geographical variations. The 2003 heat wave, which killed 70 000 people in Europe, could become a new normal by the end of the century if the warming reaches 3°C. (Ouzeau, 2016). A warming of 2°C would also have a much greater impact on ecosystems, with, for example, a virtual disappearance of corals and widespread reductions in the habitat areas of animals and plants.
These estimates of climate change impacts and damage reinforce the political weight of the 1.5°C target. If this temperature is exceeded, they could provide arguments to the most affected countries in the framework of the discussions on the "loss and damage" provided for in the Paris Agreement.
Getting out of the "tragedy of the horizons"
After estimating the impacts, the second major part of the report analysed the trajectories to reach 1.5°C at the global level.
By analysing all the scientific publications, the report provides a robust result: any trajectory compatible with the objective of 1.5°C at the end of the century implies going through carbon neutrality by 2050. It should be noted that this carbon neutrality involves only CO2. Other greenhouse gases are expected to decrease, but not necessarily to reach neutrality by 2050.
This is undoubtedly a flagship result of the IPCC Special Report, which provides a clear direction for the setting and assessment of public policies. Neutrality is a necessary objective to stabilise temperature. The IPCC report gives it a horizon here, which corresponds to the "least cost" trajectories of energy models.
Although it sets the objective of carbon neutrality, the IPCC report says nothing about the distribution of this effort. It is in fact a global objective, which should be disaggregated in a differentiated manner. But according to what criteria? How can we take into account countries' capacity for action, particularly in terms of per capita income or the capacity of ecosystems to absorb CO2 ? Should historical emissions modulate the date of the neutrality objective?
The answer to these normative questions cannot be unequivocal. The Paris Agreement has made it possible to emerge from these insoluble debates on a fair distribution of effort. The corollary is that it is difficult to be able to scientifically affirm that a country or a city is aligned with the Paris Agreement.
Moreover, the 2050 target remains far away in terms of political mandates and corporate horizons. The risk here is to fall back into the "tragedy of horizons", i.e. the gap between the long-term horizon of climate strategies - 2050 or even 2100 - and the short-term operational decisions of governments, businesses and communities.
However, the IPCC report highlights another strong result: in "1.5°C-compatible" trajectories, global CO2 are halved in 2030 (compared to 2017). This result seems to receive less publicity today than carbon neutrality. However, this transition point in 2030 is undoubtedly an essential element to get out of this tragedy of the horizons.
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Neutrality, a tangible objective
On the other side of these stated ambitions, the IPCC report estimates that the current efforts of the States commit us instead to trajectories leading to a 3°C warming. This hiatus justifies the need to renegotiate States' commitments between now and 2020, the date of entry into force of the Paris Agreement.
The 2050 horizon for carbon neutrality should not mask the immensity of the challenge, nor the urgency to act. To halve global emissions within 12 years, incremental changes are no longer enough. While the achievement of such targets is highly uncertain, the direction to be taken remains clear. From now on, the challenge is to redirect the thousands of billions invested each year in the productive system towards fully decarbonized systems, while preparing our societies to adapt to the now inevitable share of climate change.
More and more players - countries, regions, cities - have taken up this objective of neutrality. France and the city of Paris have thus committed to carbon neutrality by 2050 within the Coalition for Carbon Neutralitywith 18 other countries and more than 30 other cities.
This craze reflects an increased awareness but also the strength of the concept of carbon neutrality. In comparison, the concepts of carbon budget or 2°C now appear much less tangible today, in terms of action or direction to follow. Knowing that the remaining carbon budget is 500 Gt of CO2 does not evoke anything concrete, whereas the idea of "zero net emissions" makes it very clear where we want to go: the end of combustion cars, coal-fired power plants and oil heating. However, the objective of achieving neutrality by 2050 is today as tangible as it is difficult to achieve.
Quentin Perrierenergy economist at Cired, Graduate School of Social Sciences (EHESS); Céline GuivarchEconomist at Cired, Research Director, École des Ponts ParisTech (ENPC) and Olivier BoucherResearch Director at the CNRS, researcher at the Pierre-Simon Laplace Institute, Sorbonne University
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