Huge jumps within the last ice age between cold and warm climate periods in the Northern Hemisphere may have taken place as a result of the climate system becoming unstable when atmospheric CO2 levels were approximately between 190 and 225 parts per million.
That is the conclusion obtained from a study that has been reported in the journal Nature Geoscience by Guido Vettoretti et al. at the Niels Bohr Institute (NBI), University of Copenhagen, Denmark.
The outcome is in agreement with data on past temperature and CO2 concentration in ice cores that has been recovered from Antarctica and Greenland. The work is considered part of the European TiPES project on Tipping Points in the Earth’s System.
Dansgaard-Oeschger events (D-O events) took place over 25 times during the last ice age and were initially explained four decades ago.
These sudden and global-scale climate changes had huge effects in the Arctic where the average temperatures increased up to 16 °C at times within decades. Later, over centuries to millennia, temperatures slowly dropped and got back to the ice age conditions.
A Long-Standing Puzzle
As far as the field of climate science is concerned, the questions that have remained unanswered are what resulted in the D-O events, their strength and duration, and why they reappeared in a nearly regular way.
The current research helps to solve this puzzle by integrating ice-core data and outcomes from both a wide climate model and an easy mathematical model.
The two models tend to display the same overall behavior, which matches with the ice-core data, indicating that the models capture the vital physics of the climate system. The insights obtained from the models enable a new theory for the D-O events to be formulated and display how the happening of the sudden climate changes was regulated by the atmospheric CO2 levels.
In the study performed, D-O events became regulated by levels of CO2 present in the air in the following way: with CO2 levels going above 225 ppm, the North Atlantic Climate was in a comparatively warm stable state.
Below 225 ppm, the system got into a” window of CO2” where the climate was inclined to tip into an unstable state. Here, jumping back and forth between warm and cold climate periods would take place naturally in the climate system.
Once CO2 levels fell below 190 ppm and out of the “window of instability”, the ice age climate system would tip into one more stable state, consisting of very low temperatures in the North Atlantic, and D-O events would no longer happen.
The outcomes fit the historical noted levels of temperature and CO2, which are known accurately from measurements of small air bubbles that have been trapped in the ice sheets of Antarctica and Greenland. The Niels Bohr Institute developed ice-core drilling and recovered and examined the Greenland ice cores that offer the records of the D-O events.
A Message for the Present Times
The presence of a CO2 window enabling sudden and surprising changes to the climate bears a message for modern times.
The results of this paper show us how past temperatures on Earth responded quickly and unpredictably under different levels of CO2 concentration in the atmosphere. It’s important to understand whether increasing our current CO2 levels will create conditions where the Earth’s climate suddenly jumps into a very different and possibly irreversible state.
Guido Vettoretti, Niels Bohr Institute, University of Copenhagen
The TiPES project is known as an EU Horizon 2020 interdisciplinary climate science project on tipping points in the Earth system. Eighteen partner institutions work jointly in over 10 countries. TiPES has been coordinated and headed by the Niels Bohr Institute at the University of Copenhagen, Denmark, and the Potsdam Institute for Climate Impact Research, Germany.
The TiPES project has been financially supported by the European Horizon 2020 research and innovation program, grant agreement number 820970.
Vettoretti, G., et al. (2022) Atmospheric CO2 control of spontaneous millennial-scale ice age climate oscillations. Nature Geoscience. doi.org/10.1038/s41561-022-00920-7.