Evrim Yazgin
Cosmos science journalist
Ice core samples collected in Greenland, spanning 120,000 years of climate history, reveal abrupt “tipping points,” related to the Atlantic Meridional Overturning Circulation which plunged Earth into the last ice age.
The last ice age, known as the Last Glacial Period (LGP), spanned the period 115,000–11,700 years ago. It’s believed it was triggered by changes in Earth’s orbit around the Sun.
Much of North America was blanketed in ice, as was Europe with ice reaching as far south as the British Isles, Germany, Poland and Russia. Glaciation was less extensive in the southern hemisphere with local ice caps forming in parts of New Guinea, southern Africa and Patagonia.
The new research, published in the Proceedings of the National Academy of Sciences, reveals the abrupt changes which led to a profound shift in the Earth’s overall climate.
“It is really important to understand such tipping points in the climate, because they may result in catastrophic and irreversible change,” says lead author Christo Buizert, an associate professor at Oregon State University in the US.
The events referred to in the study are known as Dansgaard-Oeschger events. They represent situations in which the climate crosses a threshold that leads to sudden, large-scale change.
The Dansgaard-Oeschger cycle occurred more than 25 times during the LGP.
Previous research has shown the cycle is caused by rapid switching on and off of the Atlantic Meridional Overturning Circulation (AMOC) – the major current which circulates water through the Atlantic Ocean. The Gulf Stream which carries warm tropical waters to the North Atlantic is part of the AMOC.
“The AMOC is fundamentally unstable, and when it collapses, big things happen across the globe. There is significant cooling in Europe and around the North Atlantic, and the Indian and Asian monsoons fail,” Buizert says. “That instability was frequent during the last ice age. It is cause for concern for the future because climate models suggest the AMOC will likely weaken again under global warming, potentially impacting billions of people.”
Buizert’s team analysed tiny air bubbles trapped for thousands of years in the Greenland ice. These bubbles contain valuable data about atmospheric changes.
Among the samples they studied are cores from Greenland’s east coast and south which haven’t been thoroughly analysed previously.
The new data and climate modelling suggests interactions between the AMOC and winter sea ice play a key role in Dansgaard-Oeschger events. Scientists previously thought sea ice from the Nordic Seas north of Iceland was involved. The new research shows sea ice extended much further south – about 40° latitude, the line on which New York and Madrid sit today.
“The model shows that the Nordic Seas alone wouldn’t be big enough to drive a climate change event of this size,” Buizert says. “It just doesn’t pack enough of a punch.”
AMOC has been relatively stable since the end of the LGP. But Buizert warns that this may change quickly.
“We know the AMOC will weaken, but will it collapse? That is the big question. The weakening is likely gradual for the time being, but it could cross a tipping point and become a catalyst for abrupt climate change events like we saw in the past,” he warns. “The climate does not behave in linear patterns; it can change quickly and irreversibly.”
