New rapid variations in atmospheric carbon dioxide concentrations over the last 500,000 years, known as ‘’CO2 jumps‘’, have been identified by the IGE’s Hotclim project (CNRS/Inrae/IRD/UGA/Grenoble INP). In association with the LSCE (CEA/CNRS/UVSQ), the international study shows that these major events are ten times lower than the current rate of increase in man-madeCO2 concentrations.
© S.Kipfstuhl/IGE
Drilled to a depth of 3,260 metres in 2004 by the Concordia research infrastructure, in association with the LSCE, and tracing the Earth’s atmospheric archives over 800,000 years, the EPICA Dome C Antarctic core continues to reveal new information. An international collaboration led by the IGE, as part of the Hotclim project funded by the Make our planet Great Again programme, has carried out new isotopic analyses of air bubbles trapped in the ice.
By increasing the number of samples measured, the researchers obtained a new high-resolution record of carbon dioxide that enabled them to discover 7 ‘CO2 jumps’ and to identify the 15 previously measured over the last 500,000 years. These events correspond to increases of around ten ppm (number of CO2 particles observed among the millions of particles in the air) over a few decades.
Note: the current rate of increase in CO2 concentration caused by human activities is around 2.5 ppm/year, ten times higher than these ‘jumps’.
Disturbed ocean circulation and high obliquity
How can these phenomena be explained? By comparing their data with previous studies, the palaeoclimatologists have shown that 18 of the 22 CO2 jumps occurred during periods of high obliquity, i.e. when the inclination of the Earth’s axis of rotation in relation to the plane of its orbit around the Sun was high.
Obliquity, which follows cycles of 41,000 years, would therefore appear to be a necessary context for the occurrence of these events, without however being the primary cause: ‘ these jumps are linked to a disturbance in Atlantic ocean circulation, the origin of which is still uncertain, which if it occurs during a period of high obliquity can trigger a massive release of CO2 into the atmosphere from terrestrial carbon reservoirs’, points out Etienne Legrain, researcher at the UGA and first author of the study.
The researchers used new climate model simulations to demonstrate that the various terrestrial carbon reservoirs are particularly sensitive to climate change induced by major changes in Atlantic ocean circulation. ‘ The massive release of CO2 by the oceanic reservoir is directly caused by the reorganisation of ocean circulation. Whereas release by the terrestrial reservoir, particularly vegetation, appears to be a response to climate change induced by oceanic disturbance ’, explains Amaëlle Landais, a researcher at the LSCE.
What’s the link with current global warming?
The Earth is currently in one of these periods of strong obliquity. Researchers have therefore developed different scenarios for disrupting ocean circulation, such as the Atlantic Meridional Overturning Circulation (AMOC). This consists of artificially adding more freshwater to the North Atlantic in the models to bring about a change in density that stops the surface waters sinking to the depths.
Result: in the scenario of a collapse of the AMOC as a result of climate change caused by human activities, a quantity of carbon equivalent to four years of global anthropogenic emissions (at the rate for the 2010-2019 period) could be released into the atmosphere in the space of a few decades, superimposed on current anthropogenic emissions. ‘ However, there is still considerable uncertainty about the future of AMOC in response to current global warming ’, say the researchers.
https://www.cea.fr/Pages/actualites/environnement/sauts-passes-CO2-atmosph%C3%A9rique.aspx