Abrupt climate changes due to atmospheric CO2 levels

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Huge jumps during the last ice age between cold and warm climatic periods in the Northern Hemisphere may have occurred due to the instability of the climate system when atmospheric CO2 levels were approximately between 190 and 225 parts per million.

DO events have been difficult to understand. Vettoreti et al. show that the rapid and sudden warming could have been controlled by a “CO2-the window.” Image credit: TiPES/HP

This is the conclusion of a study published in the journal nature geoscience by Guido Vettoretti et al. at the Niels Bohr Institute (NBI), University of Copenhagen, Denmark.

The result is in agreement with data on past temperature and CO2 concentration in ice cores recovered from Antarctica and Greenland. The work is considered part of the European TiPES project on Earth System Tipping Points.

Dansgaard-Oeschger events (DO events) occurred more than 25 times during the last ice age and were originally explained four decades ago.

These sudden and global climate changes have had enormous effects in the Arctic where average temperatures have sometimes increased by as much as 16°C in a few decades. Later, over centuries, even millennia, temperatures slowly dropped and returned to Ice Age conditions.

A long-standing puzzle

When it comes to the field of climate science, the unanswered questions are what drove DO events, how strong and how long they lasted, and why they reappeared on an almost regular basis.

Current research helps solve this puzzle by integrating ice core data and results from a large climate model and a simple mathematical model.

Both models tend to display the same overall behavior, which is consistent with ice core data, indicating that the models are capturing vital physics of the climate system. The information obtained from the models allows the formulation of a new theory for DO events and shows how the occurrence of sudden climate changes was regulated by atmospheric CO.2 levels.

In the study performed, DO events became regulated by CO levels2 present in the air as follows: with CO2 levels exceeding 225 ppm, the North Atlantic climate was in a relatively warm and stable state.

Below 225 ppm, the system has entered a “CO window”2where the climate was prone to tipping into an unstable state. Here, the back and forth between warm and cold climatic periods would occur 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 shift into a more stable state, consisting of very low temperatures in the North Atlantic, and the events of ‘OD would no longer occur.

Results correspond to historical temperature and CO levels2, which are precisely known from measurements of small air bubbles trapped in the ice sheets of Antarctica and Greenland. The Niels Bohr Institute developed ice core drilling and recovered and examined ice cores from Greenland which offer the records of DO events.

A Message for the present time

The presence of a CO2 window allowing sudden and surprising changes in climate carries a message for modern times.

The results of this paper show us how past temperatures on Earth reacted quickly and unpredictably under different levels of CO2 concentration in the atmosphere. It is important to understand whether the increase in our current CO2 will create conditions where the Earth’s climate will suddenly shift to a very different and possibly irreversible state.

Guido Vettoretti, Niels Bohr Institute, University of Copenhagen

The TiPES project is known as the EU Horizon 2020 Interdisciplinary Climate Science Project on Earth System Tipping Points. Eighteen partner institutions work together in more than 10 countries. TiPES was coordinated and led 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 research and innovation program Horizon 2020, grant agreement number 820970.

Journal reference:

Vetoretti, G. et al. (2022) Atmospheric CO2 control of spontaneous climatic oscillations from the ice age to the millennial scale. nature geoscience. doi.org/10.1038/s41561-022-00920-7.

Source: https://science.ku.dk/english/

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