“Mean sea level is virtually certain to continue to rise during the 21ste century “ensures the sixth report of the Intergovernmental Panel on Climate Change (IPCC), published in 2021. “virtually certain”, this means that IPCC scientists estimate that there is a greater than 99% chance that this prediction will come true. Over the last century, the average sea level has already risen by about twenty centimeters. In 2050, it would be 15 centimeters more, in the intermediate scenario of the IPCC where the average temperature on the surface of the globe would have increased by 2 degrees compared to the pre-industrial era.
This type of assertion is based on scientific work on a titanic scale. The prediction of sea level rise “it is one of the greatest contemporary scientific challenges”exclaims Benoit Meyssignac, oceanographer at the Legos laboratory in Toulouse. “And that has enormous societal interest. This is undoubtedly one of the rare subjects whose concrete impact is directly related to the vertigo that science can give. » Because behind this rising water level hide many intertwining factors, effects of scale, technical challenges or even theoretical difficulties. “To model the climate, you need a model of the ocean, a model of the atmosphere, a model for hydrology… And all these models interact with each other”explains Mélanie Becker, geophysics researcher at the Liens laboratory and research director at the CNRS.
Concretely, to establish these forecasts, the scientists rely on observations to identify how the sea level has evolved in the past, and how it is evolving today. They then seek to understand the causes of these changes by proposing theoretical models based on known physical equations. By comparing the results provided by their models with observations made in the past, they confirm or invalidate their hypotheses and refine the parameters. It is then these models, capable of reproducing data from the past, that are used to predict the future according to different scenarios of global warming.
Acceleration of ice melt
On a global scale, the main processes that influence the mean level of the oceans are now well known. First, under the effect of global warming, the ocean is expanding. “Greenhouse gas emissions limit infrared radiation from Earth to spaceexplains Benoit Meyssignac. Consequently, the Earth and its atmosphere receive more energy than they emit. However, more than 90% of this energy surplus is stored in the ocean, because water has a heat capacity [capacité à absorber l’énergie] much larger than air. » Consequence: the ocean warms up, and its immense volume of water expands.
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