Melting ice likely triggered climate event >8000 years ago

Using geological samples from the Ythan Estuary in Scotland, scientists have identified a melting ice sheet as the probable trigger of a major climate-change event just over 8000 years ago. Their findings, published in the journal Quaternary Science Advances, may hold clues as to how present-day ice loss in Greenland could affect the world’s climate systems.

More than 8000 years ago, the North Atlantic and Northern Europe experienced significant cooling because of changes to a major system of ocean currents known as the Atlantic Meridional Overturning Circulation (AMOC). The change in AMOC also affected global rainfall patterns — it is believed that an influx of a massive amount of fresh water into the saltwater seas of the North Atlantic caused the AMOC to break down.

Geoscientists from four Yorkshire universities took core samples from the sediment in the Ythan Estuary to build up a picture of what was happening to sea levels around this time. From analysing microfossils and the sediment in the samples, they found that sea-level changes departed from normal background fluctuations of around 2 mm a year and reached 13 mm a year, with individual sea-level events resulting in water rising most likely by about two metres in the Ythan Estuary.

Analysis of the core samples provides evidence that there were at least two major sources of fresh water that drained into the North Atlantic, causing the changes to the AMOC, and not a single source as previously thought. The view held by many scientists was that the fresh water had come from a giant lake — Lake Agassiz-Ojibway, which was the size of the Black Sea and was situated near what is now northern Ontario — which had drained into the ocean.

“We have shown that, although huge, the lake was not large enough to account for all that water going into the ocean and causing the sea-level rise that we observed,” said study leader Dr Graham Rush, who holds positions at the University of Leeds and Leeds Beckett University. Instead, Rush and his colleagues believe the melting of the Hudson Bay Ice Saddle, which covered much of eastern Canada and the north-eastern United States, provided the injection of vast quantities of water that was reflected in the core samples.

The disruption to the ocean current had major ramifications around the world. Temperatures in the North Atlantic and Europe dropped by between 1.5 and 5°C and lasted for about 200 years, while other regions experienced above-average warming. Levels of rainfall also increased in Europe, while other parts of the world, such as Africa, experienced drier conditions and extended periods of drought. The authors of the study believe this gives an insight into how current day melting of ice sheets in Greenland may affect global climate systems.

“We know that the AMOC is currently slowing down and, although still debated, some forecasts indicate it could shut down altogether,” Rush said.

“However, by looking at past events we can learn more about what causes these changes and their likelihood. We have shown that rapid ice-sheet retreat, which may occur in Greenland depending on the path of future fossil fuel emissions, can cause a range of significant climatic effects that would have very worrying consequences.”

Image caption: The sediment core being taken from the Ythan Estuary.