26 April 2018

New study finds glacial evidence for abrupt climate change

Posted by Nanci Bompey

Rise and demise of Scotland’s last ice fields reveals the conflicting impact of abrupt climate change

By Gordon Bromley

Rannoch Moor, in the central Scottish Highlands, was the epicentre of the last ice cap in Scotland. Radiocarbon ages from plant remains preserved in lake sediments and show the ice cap disappeared around 12,500 years ago. Credit: Gordon Bromley

Results from a new study indicate that the physical impact of abrupt climate change in Britain, Ireland and maritime Europe may be markedly different from previous perceptions of these events. The study was published online today in Paleoceanography and Paleoclimatology, a journal of the American Geophysical Union. 

These findings raise the possibility that future weakening of warm ocean currents in the North Atlantic, which some fear will arise due to global warming and melting of the Greenland Ice Sheet, will result in a return to a highly seasonal climate in Britain, Ireland and maritime Europe, with warmer summers and colder winters.

The study investigated how abrupt climate changes such as high-magnitude shifts in average climate has impacted maritime Europe at the close of the last ice age. The ice age is often thought of as something that happened gradually over very long periods, yet previous studies indicate that this is not the case. Existing climate records show that after the peak of the last ice age, 20,000 years ago, warming of the atmosphere and ocean to modern Holocene (current ‘interglacial’ warm period) levels was not gradual and smooth but dynamic, interrupted by rapid returns in as little as a few years to decades, to very cold climates lasting centuries to millennia, which means it was a very bumpy ride for Earth’s climate out of the ice age.

The most recent abrupt climate event is called the ‘Younger Dryas’ and occurred between 12,900 and 11,600 years ago, prior to the onset of our current warm Holocene climate 11,000 years ago. Ice core records from Greenland and palaeoecologic records from throughout Europe are traditionally interpreted as showing the Younger Dryas as a 1300-year period of severe cooling and permafrost in the North Atlantic region, potentially caused by the weakening of warm ocean currents, such as the Gulf Stream, that transport tropical heat to Europe.

“We used radiocarbon dating of marine shell remains to determine when the last glaciers existed in Scotland, what we call a ‘glacial event’,” said Gordon Bromley from the School of Geography and Archaeology at National University of Ireland Galway, and lead author of the new study. “There’s a lot of geologic evidence of these former glaciers, including deposits of rubble bulldozed up by the ice but their age has not been well established. Instead, it has largely been assumed that these glaciers existed during the cold Younger Dryas period, since other climate records give the impression that it was a cold time. To establish the true age of the glaciers we dated shells that were already dead or had been killed as the glaciers advanced into the fjords and shoveled up seafloor sediments.”

The shells ended up inside the glacial deposits, where the scientists found them, and provide a maximum age for the glacier advance. Similarly, to determine when the glacial event was over and the ice had melted, they dated a shell and also vegetation that was the first organic matter to colonize the newly ice-free landscape. This data provided a minimum age for the glacier advance.

Shells were collected by Bromley from National University of Ireland Galway and Harold Borns from the University of Maine at sites on the west coast of Scotland where glacial moraines have been eroded by the sea, rivers, or lakes, affording them access to the shells. Despite being as much as 14,000 years old, the shells were extremely well preserved and some even retained the fragile periostracum (outermost layer of the shell) and membranes joining the valves. Others have been crushed by the weight of ice as the last glaciers bulldozed the seafloor. While all of these shell species are still in existence in the North Atlantic, many are extinct in Scotland where ocean temperatures are too warm.

This new data collected in Scotland by the scientists, challenges the idea that the Younger Dryas was an abrupt return to a glacial (ice age) climate in the North Atlantic, by showing that the last glaciers there were actually decaying rapidly during that period. They interpreted this meltdown as reflecting atmospheric warming and found that the Younger Dryas was actually characterised by extreme seasonality.

This means that although winters in Britain and Ireland were extremely cold, summers were a lot warmer than previously thought, a situation that is 180 degrees from today’s highly maritime climate in this part of the world (mild winters and cool summers). As it is crucial to establish how past abrupt climate change was manifested here in order to prepare for future disturbances, these findings reshape scientific understanding of how the weakening of warm currents in the North Atlantic might impact Ireland’s climate.

“Our new radiocarbon data crucially shows that the glaciers existed before the Younger Dryas and that they were melting rapidly and disappeared during that period,” Bromley said. “As this doesn’t fit with the traditional notion of the Younger Dryas as a uniformly cold event, we found that despite the cold winters, summers had to be warm as it is the intensity of the summer melt season that dictates glacier ‘health’. This finding is controversial and if we are correct, it helps rewrite our understanding of how abrupt climate change impacts our maritime region, both in the past and potentially into the future.”

— This blog post originally appeared as a press release on the NUI Galway website.