The French government is currently preparing the 3rd National Plan for Adaptation to Climate Change (aka PNACC3). As adapting means anticipating, this work has been preceded by the publication of new global warming scenarios: the "Trajectoire de référence pour l'adaptation au changement climatique" (reference trajectory for adaptation to climate change), or TRACC. In this article, we look at what these scenarios are and how the corresponding climate projections are produced.
👋 If you're more interested in the results than the details of the methodology, we offer a free application for easy access to TRACC projections anywhere in France
TRACC and the French "4°C scenario" for dummies.
Let's be honnest, the French administration loves to create arcane acronyms, and even among well-informed French nationals, few people know what the PNACC or TRACC is. However, most have heard of the "4°C scenario".
In February 2023, the French Minister for Ecological Transition announced the creation of this new scenario: ‘a four-degree trajectory’. Previously, the French adaptation strategy had anticipated a much more limited level of warming. The announcement raised a number of questions and controversies.
Global warming of +4°C, but where? Compared to when? And isn't the prospect of such warming an admission that the fight against climate change failed? That's what we're going to find out.
To sum up the debates very quickly: the communication surrounding these new reference scenarios has been riddled with ambiguity and confusion. So let's forget what you might already have heard ans start again.
In reality, there are two scenarios :
A trend scenario leading to global warming of 3°C compared with the period 1850–1900.
According to the UN's latest assessment, the emission reduction commitments made by governments so far would probably lead to global warming of 3 to 3.5°C by the end of the century compared with the pre-industrial era (more on this subject here).
A “Paris Agreement" scenario with two variants: optimistic (global warming of 1.5°C compared with 1850-1900) and pessimistic (warming of 2°C).
Indeed, the objective of the Agreement signed in 2015 is to hold "the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels".
It's pretty simple, isn't it?
So where is the infamous "4°C scenario" ?
A little too simple, even... But we can fix that: as the temperature is not rising uniformly across the planet's surface, as long as we know how to apply a mask to the climate projections, we can translate the level of global warming into an equivalent level in a sub-region. Take mainland France, for example: how many degrees warmer will France be when the planet has warmed by 3°C?
To make things a little more complicated, we can also take a different reference period: a warming of 3°C compared with the average for 1850-1900 corresponds to a warming of how many degrees compared with... say... 1900-1930?
Now let's do both at the same time, and you have the origin of the famous 4°C:
The trend scenario, which results in global warming of 3°C compared with 1850-1900, gives an average temperature increase of 4°C in France compared with 1900-1930.
Do you think this is still a little too easy? Add the fact that the variations in temperature and precipitation in the TRACC projections are not calculated in relation to 1850-1900, nor in relation to 1900-1930, but in relation to 1976-2005.
So we end up with this oddity: according to the projections published by Météo France, in a scenario of 4°C average warming in mainland france, the average warming in mainland france would be ... 3.4°C.
Why TRACC's projections seem optimistic?
Beyond the confusion, this tangle of reference periods has a more problematic effect: even if the new reference scenario for the adaptation takes a distinctly realistic, even pessimistic turn, one can get the impression that the resulting projections are rather reassuring.
For example, in this 4°C warming scenario, the average temperature rise in Paris would only be 3.0°C (according to the median projection). Phew... Not so bad after all! Who will notice that the first figure refers to warming compared with the average climate of 1915 and the second to warming compared with the average climate of 1990?
All the more so as, although the French "4°C warming" (one again, in mainland France compared with 1900-1930) reference scenario is often presented as worst case scenario, in reality it is not. As we have seen above, the continuation of current policies would probably lead to a higher level of warming.
In reality, the TRACC's new ‘four degrees’ scenario is not pessimistic: it is what we can expect if current commitments are kept and we are in the low range of warming anticipated by climate models...
What is the relationship between the new French scenario and IPCC projections?
The IPCC scenarios are based on greenhouse gas emissions pathways projected over the coming decades. For example, there is a rapid reduction in emissions for the RCP2.6 or SSP1-2.6 scenarios, and intermediate emissions levels for the RCP4.5 and SSP2-4.5 scenarios. Climate models are then used to describe possible future weather patterns based on these emissions pathways. The projections are made under an evolving climate: the same emissions scenario will result in different levels of warming and different climate outcomes depending on the time horizon considered.
The TRACC approach is different, as it directly references specific warming levels achieved, regardless of emissions pathways. This method is known as projection under a fixed climate. It aims to anticipate the consequences of a globally warmed climate—by 1.5, 2, or 3°C—without regard to the emissions trajectory leading to this result or the timeframe in which it is reached.
Making projections for the TRACC scenarios involves asking: how do we transition from the IPCC's evolving climate approach, with its abundance of available climate projections, to a fixed climate framework?
The method used involves taking a conventional projection under evolving climate and isolating the period when the average climate corresponds to the desired level of warming.
To obtain a projection under a fixed climate, the following steps are followed:
Selecting an IPCC projection: Since the goal is to study warming of up to 3°C, a projection from a high-emissions scenario (RCP8.5 or SSP5-8.5) is used. Less pessimistic scenarios may not reach this warming level within the projection period.
Calculating the rolling température average: The global average temperature is calculated over a sufficiently long rolling period, typically 20 or 30 years. In the French projections, a 20-year period is used.
Determining warming relative to the preindustrial level: The warming relative to the reference period of 1850-1900 is then calculated.
Identifying the pivot year: At some point, this value reaches the desired warming level. The 20-year period centered around the year in which this level is achieved (known as the "pivot year") represents a climate that has, on average, warmed by the desired warming level.
This process must be repeated for each climate model. This is because projections made with different models may reach the desired level of warming at different times, for example because of different climate sensitivity or simply because of the natural variability of the climate.
The pivot years in the TRACC projections
To produce the TRACC projections, Météo France used this method with the DRIAS 2020 projection sets (actually biais-corrected CMIP5/EuroCordex projections) for the RCP8.5 emissions scenario.
Météo France has not shared the pivot periods for each projection, but they can easily be deduced from the time series published on DRIAS website:
Pivot periods for each pair of models and TRACC scenario
For example, in the first pair of models, the CNRM-CM5/Aladin63 projections show that the climate of the "4°C scenario" (a +3°C global average) corresponds to the period 2070-2089 under the RCP8.5 scenario, giving a pivot year of 2079.
Similarly, the "optimistic Paris Agreement" scenario (+1.5°C global average) corresponds to the 2031-2050 period in the same scenario, with a pivot year of 2040.
These "pivot years" are important because identifying the periods corresponding to specific warming levels enables us to make projections compliant with the new French reference scenarios.
For instance, we can produce indicators that Météo-France has not yet developed or make projections for regions beyond mainland France.
By the way, do you need tailor-made climate projections under the French "4°C scenario", for example indicators and regions of the world specific to your business ? Callendar can help!
As pioneers of climate risk assessment in France, we help companies implement a science-based adaptation approach with local, actionnable projections aligned with public policies. Do you have a question about climate change and its impacts? Contact us to discuss it!