Study suggests abrupt climate change during the last ice age was controlled by CO2 levels

Large jumps within the last ice age between cold and warm climate periods in the Northern Hemisphere may have occurred because the climate system became unstable when atmospheric CO₂ levels were between approximately 190 and 225 parts per million. That is the conclusion from a study, published in Nature Geoscience by Guido Vettoretti et al. at the Niels Bohr Institute (NBI), University of Copenhagen, Denmark. The result is in agreement with data of past temperature and CO₂ concentration in ice cores retrieved from Antarctica and Greenland. The work is part of the European TiPES project on Tipping Points in the Earth's System.

Dansgaard-Oeschger events (D-O events) occurred more than 25 times during the last ice age and were first described 40 years ago. These abrupt, global-scale climate changes had large impacts in the Arctic where the average temperatures jumped up to 16 degrees Celsius sometimes within a couple of decades. Afterwards, over centuries to millennia, temperatures gradually dropped and returned back to the cold ice age conditions.

A long-standing puzzle

What caused D-O events, their duration and strength, and why they reappeared in an almost regular manner have remained unanswered questions in the field of climate science for decades.

This new study completes more pieces to the long-standing puzzle by combining ice-core data and results from both a comprehensive climate model and a simple mathematical model. The two models show the same overall behaviour, which also agrees with the ice-core data, which suggests that the models capture the essential physics of the climate system. The insights from the models allow a new theory for the D-O events to be formulated and show how the occurrence of the abrupt climate changes was controlled by the atmospheric CO₂ levels.

In the study, D-O events turned out to be controlled by levels of CO₂ in the atmosphere in the following way: with CO₂ levels above 225 ppm, the North Atlantic Climate was in a relatively warm stable state. Below 225 ppm, the system entered a ”window of CO₂” where the climate was prone to tipping into an unstable state, where jumps back and forth between warm and cold climate periods would appear naturally in the climate system. As soon as CO₂ levels dropped below 190 ppm and out of the ”window of instability”, the ice age climate system would tip into another stable state, with very low temperatures in the North Atlantic and D-O events would no longer take place.

The results fit the historical observed levels of temperature and CO₂, which are known fairly precisely from measurements of tiny air bubbles trapped in the ice sheets of Greenland and Antarctica. The Niels Bohr Institute pioneered ice-core drilling and retrieved and analysed the Greenland ice cores that provide the records of the D-O events.

A message for our times

The existence of a CO₂ window allowing abrupt and unexpected changes to the climate bears a message for our own 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," says Vettoretti.

The TiPES project is an EU Horizon 2020 interdisciplinary climate science project on tipping points in the Earth system. 18 partner institutions work together in more than 10 countries. TiPES is 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 received funding from the European Horizon 2020 research and innovation program, grant agreement number 820970.