Glacial Evidence for Abrupt Climate Change

NUI Galway Scientist Finds Glacial Evidence Abrupt Climate Change
RannochMoor.jpg: 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. Photo: Gordon Bromley

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

Results from a major study carried out by scientists from NUI Galway and the University of Maine have indicated that the physical impact of abrupt climate change in Britain and Ireland and maritime Europe may be markedly different from previous perceptions of these events. The study was published today (26 April 2018) in the international journal, Paleoceanography and Paleoclimatology.

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 and Ireland and maritime Europe, with warmer summers and colder winters.

NUI Galway Scientist Finds Glacial Evidence Abrupt Climate Change

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.

NUI Galway Scientist Finds Glacial Evidence Abrupt Climate Change

Lead author of the study, Dr Gordon Bromley from the School of Geography and Archaeology at NUI Galway, said: “We used radiocarbon dating of marine shell remains to determine when the last glaciers existed in Scotland, what we call a ‘glacial event’. 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 shovelled up seafloor sediments.

“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. 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.”

NUI Galway Scientist Finds Glacial Evidence Abrupt Climate Change
Photo Clam_October2016.jpg: A well-preserved Chlamys islandica (Icelandic scallop) shell retrieved from glacial sediments near Loch Lomond, far south of the species’ modern range. Radiocarbon ages from such subfossils provide crucial information on the timing of past climate change events. Photo: Gordon Bromley

Findings from the data collected

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° 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.

Findings from the study

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 colonise the newly ice-free landscape. This data provided a minimum age for the glacier advance.

Shells were collected by Dr Bromley from NUI Galway and Professor 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.

To read the full study in Paleoceanography and Paleoclimatology, visit: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2018PA003341


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