They had been eagerly awaited for several weeks. The work of group 1 of the 6th IPCC report on the physical sciences of climate and past, present and future climate variations was published on Monday, August 9. Unsurprisingly, this first part, which includes the evaluation of more than 14,000 scientific articles, concludes that a further rise in the thermometer would lead irreparably to a global warming of 1.5 ° C during the 21st century. And underlines the increase in intensity and number, in the years to come, of extreme climatic phenomena such as floods and droughts. Enough to feed the future negotiations of the COP26, which will be held in Glasgow, next November. While waiting for the publication of the parts of groups 2 and 3 which will be unveiled in 2022.
The climatologist Christophe Cassou, research director at the CNRS and specializing in modeling the climate and its global change, took part in the work of group 1 of this new report. For him, “what we are experiencing today corresponds to the trajectories described in previous IPCC reports”. And that’s not a good thing …
If there was still, if only a very small doubt about the anthropogenic origin of climate change, this new report from group 1 of the IPCC sweeps them away … Was it also categorical in previous reports ?
On the level of attribution of climate change to human activities, there has been an evolution in the various IPCC reports. Today, in this new report, the attribution of observed climate change to human activities is said to be “unequivocal”. In the calibrated language of the IPCC, “unequivocal” means that it is an established fact. And the contribution of natural factors like solar activity and volcanism are rated as negligible. This time, it is well stated! This claim is now possible thanks to improved methods that deal with the attribution of climate change to the various factors that can induce changes in global temperature. A set of indications and evidence based on models, observations and a better knowledge of physical processes. On the other hand, the warming level for the last decade is 1.1 ° C, and the human factor attribution of warming is 1.09.
Whether it is the rise in sea level, the acidification of the oceans, or the retreat of glaciers, the findings and projections seem to reassess the increase in global temperature. Can the 1.5 ° C of the Paris Agreement, despite everything, be respected?
In this new report, we show that this threshold of 1.5 ° C, estimated over an average of 20 years, will clearly be crossed in the next 20 years. So it’s earlier than in
the IPCC special report published in 2018. We estimate this 1.5 ° C on the average between 2021 and 2040. We even expect it to be crossed during the 2030s! This date revision is explained by a more sophisticated approach to estimate the rate of global warming. We no longer rely solely on climate models, but on a set of indicators including the most recent observations, a better understanding of the physical processes of the climate and a set of statistical and mathematical methodologies. This imposes this physical constraint on us, which is carbon neutrality, even more quickly.
In this 6th report of group 1 of the IPCC, a part relates for the first time to extreme weather events. Can we say with certainty today that these phenomena are indeed linked to global warming, and are they likely to intensify?
By definition, the extreme event is rare. So to determine what an extreme event is, you need to have as much time hindsight as possible in terms of observation. There is therefore the observational hindsight which is essential. For each big extreme event, attribution methods, combining observations and models, allow us to see how human influence has modified the probability or risk of this extreme event occurring. Today’s extreme weather events are a foretaste of our future. An increase in frequency has already been observed over the past 50 years. And you should know that the extremes change according to four statistics: frequency, intensity, duration and seasonality. In hot extremes, especially heatwaves, everything is on the rise. And regarding precipitation, torrential rains are also increasing on a global scale. It is expected, for example, that the monsoons will be amplified with, also, regional specificities. If we take the heatwave episode of 2019 in France, that year, it had a one in 50 chance of happening. For a future warming of 1.5 ° C, its probability drops to one in ten, and for 2 ° C, to one in four.
One of the novelties in this report is the improvement of scientific knowledge of climate change assessment at regional level. How is this a breakthrough?
Thanks to a larger number of climate simulations allowing a better understanding of the regional climate, it is now possible to assess the effect of human activities on the local climate, and to estimate how the internal variability of the climate system modulates this effect. For example, this summer in France is colder and rainier than last summer, as the global warming trend continues. A difference due, from one year to the next, to this natural variability. The latter can, moreover, be stronger in certain regions of the world, and it can, over a few years or a decade, amplify global warming or even mask it. But it has very little impact on the global warming that we will reach at the end of the century.
Five possible future climate scenarios were evaluated with, as always, one worse and one less worse. Is the least catastrophic scenario really possible?
Before, the scenarios were based on energy balances of the climate system. We translated CO2 emissions into energy imbalances which then led us to different levels of temperature increase depending on our emissions. In this 6th report, there are now scenarios of socio-economic trajectories which correspond to different histories of the evolution of future societies according to what is done or not, according to technologies, political or geostrategic choices, of social justice. and energy, among others. Scenario five, for example, which uses fossil fuels, also uses technologies to offset its emissions. Conversely, the first scenario is the one in which fossil fuels are used the least. But it takes into account a reduction from today, at time T. And cannot be envisaged without reducing greenhouse gas emissions which are immediate, strong, sustained and on a very large scale. .
Our “future emissions” are not those in ten years, but those of now. And they determine the additional warming of the next few years.