It's now official: 2024 will certainly become the first calendar year to see the Earth exceed the threshold of 1.5°C of global warming compared to the pre-industrial era. It was the climate change service of the European Copernicus observatory which made the announcement on Monday, December 9.
The objective of limiting warming to 1.5°C – to which States committed by signing the Paris agreement – has, however, not yet been actually exceeded. Because the climate experiences natural variations from one year to the next. To be officially reached, the threshold of 1.5°C will have to be measured on average over several decades. Copernicus, for example, measures current warming at 1.3°C, taking into account the average of the last five years.
Even if the chances of meeting the 1.5°C objective today appear almost zero, the figure is far from being only symbolic. Reporterre returns to some of the reasons which made this threshold crucial.
Average annual warming relative to the pre-industrial threshold since 1940.
Copernicus Climate Change Service / ECMWF
Climatologists usually point out that « every tenth of a degree counts ». In this sense, it is never too late to act because any increase in temperature only increases the risk of climate change and the occurrence of ever more intense disasters. The threshold of 1.5°C, however, remains important because it has been extensively studied by science: research shows how venturing beyond could be dramatic for many beings, humans and non-humans.
10 million more people affected by rising waters
In 2018, the IPCC published a special report on the consequences of global warming of 1.5°C. By 2100, the authors noted, warming limited to 1.5°C, compared to warming of 2°C, would, for example, reduce the rise in ocean levels by 10 cm, exposing 10 million people to less to the risks linked to rising water levels.
Torrential rains, heat waves, drops in cereal yields, loss of biodiversity… All the damage is much greater at 2°C than at 1.5°C. An emblematic case is that of corals, very vulnerable to marine heat waves and which shelter 25 % of known ocean species: losses could range from 70 to 90 % at 1.5°C of warming, compared to 99 % at 2°C.
Month by month anomalies of the average air temperature on Earth since 1940. In orange the year 2023, in red 2024.
Copernicus Climate Change Service / ECMWF
The 1.5°C threshold is particularly important for small island developing States (DON'T). A study published in 2023 in the journal Nature Sustainability concludes that, even limited to 1.5°C, warming will threaten DON'T major damage, « probably leading to forced migrations ». And things get worse as soon as we exceed 1.5°C.
This is also what researchers from the German Climate Analytics Institute point out in a report published in April: « For example, the amount of annual damage due to tropical cyclones in Antigua and Barbuda would increase by almost half if global warming reached 1.7°C in 2050 instead of 1.5°C, and more by three quarters with global warming of 1.8°C in 2050 compared to 1.5°C. »
« Likewisecontinue the scientists, the number of people exposed to heatwaves each year in Senegal would increase by almost a third with global warming of 1.7°C in 2050 compared to 1.5°C, and by half if warming reached 1 .8°C on the same date. »
Irreversible tipping points in the balance
The other major argument for holding the 1.5°C target is the fear that the Earth's climate is about to cross several tipping points. That is to say drastic transformations in ecosystems, triggered by a certain temperature threshold, and irreversible. The disappearance of coral reefs mentioned above, or the melting of the ice cap in Greenland, are among these tipping points to avoid.
An international study published in Science in 2022 estimated that several of these tipping points were likely to be crossed, even at 1.5°C of warming. And the higher the temperature, the more the number of tipping points and the probability that they will be crossed increases.
On the western peninsula of Antarctica, many glaciers are melting at an alarming rate: glaciologists do not know whether, for some of them, the tipping points have not already been crossed, or are on the way. point of being. The objective of limiting warming to 2°C is, in any case, considered far too high there.
Temperature anomalies in the non-glazed oceans in November 2024. In red, abnormally high heat ; abnormally cold areas in blue.
Copernicus Climate Change Service / ECMWF
For island states and coastal populations in particular, the rise in water levels will not stop in 2100 in any case, underlines the IPCC report on warming at 1.5°C. If ice sheets pass these tipping points, they could continue to melt on a scale of « from the century to the millennium » write the scientists, causing a rise in water levels of several meters (compared to a few tens of centimeters anticipated in 2100). These glacial instabilities could be triggered somewhere between 1.5°C and 2°C of warming.
« There is no single tipping point for our climate system butsums up to Reporterre climatologist Kristina Dahl, vice-president of theONG Climate Central, every tenth of a degree of warming above 1.5°C brings us closer to triggering irreversible damage, such as the extinction of species or the release of very warming methane contained in permafrost in the Arctic. »
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