Politique Internationale — When did we start making the link between CO2 and the climate?
Valérie Masson-Delmotte — Our understanding of the physics of the greenhouse effect dates back to the early 19th century, with the pioneering work on radiation transfer in the atmosphere by Joseph Fourier (1822), Eunice Foote (1856) – the first to suggest that CO2 was a greenhouse gas – and John Tyndall (1861), who demonstrated the absorption of infrared radiation by various gases. In 1896, Svante Arrhenius (1) made the first calculations of the sensitivity of the climate to the increase in atmospheric CO2 concentration induced by coal combustion.
During the 19th century, meteorological and oceanographic observation networks were also set up, and in 1938 Guy Callendar was able to show that the Earth’s surface temperature had risen over the last 50 years – a rise he explained by the increase in the atmospheric level of CO2. In 1957, Charles Keeling began the continuous observation of this concentration of CO2 in the atmosphere, and it was quickly established that its increase was linked to the combustion of fossil fuels. In 1967, Suki Manabe (2) was one of the first to model the climatic feedbacks, linked in particular to water vapour, which amplify the response of the climate to CO2, in addition to its direct radiative effects. In 1979, the Charney Report, drawn up under the aegis of the American Academy of Sciences, produced the first collective scientific assessment of CO2and climate, and in particular the first in-depth assessment of the response of global temperature to a doubling of its concentration (‘climate sensitivity’). In the 1980s, Claude Lorius and Jean Jouzel analysed core samples from Vostok in the Antarctic, highlighting the unprecedented rise in atmospheric CO2 concentration since the industrial revolution, and the close links between past variations in global temperature, sea level and greenhouse gas concentration during the glacial and interglacial periods. In the 1990s, Klaus Hasselmann developed attribution methods that enable the human influence on climate to be objectively assessed and quantified in relation to the natural factors of its variability – work that was awarded the Nobel Prize in Physics in 2021, and has now been extended to the attribution of human influence on the probability of the occurrence and the intensity of extreme events. Since the 2000s, satellite measurements have made it possible to discern the increase in the greenhouse effect, both at the top of the atmosphere and at the surface.
These chronological benchmarks are not exhaustive: they bear witness to the tremendous progress in knowledge of climate physics, which has been assessed in the six successive IPCC reports since 1990 (3). It is now unequivocal – it is an established fact – that the increase in concentrations of CO2, methane and nitrous oxide in the atmosphere is the result of human activities – 70% as a result of the combustion of fossil fuels, with the remainder coming from deforestation, the food system and industrial processes – and it is unequivocal that …