Ireland and Britain could experience a sudden switch to “extreme seasonality” – in the form of far colder winters and much warmer summers – as a result of global warming, scientists from NUI Galway and the University of Maine say.

Such weather changes occurred in the North Atlantic region at the end of previous ice ages, and confirmed the Earth is capable of high-magnitude changes in temperature and rainfall, which can occur within a human’s lifespan.

The prediction follows their evaluation of what happened in Scotland when it was last covered by an ice cap, and challenges previous perceptions of such events.

The scientists evaluated the most recent abrupt climate event know as the “Younger Dryas stadial” which occurred between 12,900 and 11,600 years ago prior to the onset of our current warm Holocene climate 11,000 years ago.

Stadials are periods when the climate is colder than normal and the Younger Dryas stadial is frequently cited as an example of what might happen as a result of global warming caused by human activity.

Up to now, such events were believed to be defined as a 1,300-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.

‘New light’

“Our study sheds new light on the terrestrial impact of so-called ‘stadial’ events in the North Atlantic region, a key area in [STUDYING]abrupt climate change. We reconstructed the behaviour of Scotland’s last glaciers...to explore past changes in summertime temperature,” said Dr Gordon Bromley, lead author of the study who is based at NUIG School of Geography and Archaeology.

“These latest 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 ice sheet over Greenland, will result in a return to a highly-seasonal climate in Britain and Ireland, and maritime Europe.”

Dr Bromley accepts their finding, published in the scientific journal Paleoceanography and Paleoclimatology, is controversial.

“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,” Dr Bromley said.

Working with Prof Harold Borns, from the University of Maine, critical evidence including temperature indications emerged following radiocarbon dating of marine shell remains to determine when the last glaciers existed in Scotland.

Exactly what causes these potentially catastrophic climate change events, however, and their likelihood in the near future, “remains frustratingly unclear” due to uncertainty about how they manifest themselves on land and in the oceans, Dr Bromley explained.

“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,” he added.

Warm current

The research adds to two other studies published this month by the journal Nature, indicating the warm Atlantic current known as the Atlantic Meridional Overturning Circulation (AMOC) – which includes the Gulf Stream – is now at its weakest in at least 1,600 years. Changes in the AMOC have been associated with abrupt changes to climate.

The findings throw into question previous predictions that a catastrophic collapse of the Gulf Stream would take centuries to occur. In addition, the authors suggest such a collapse would see western Europe suffer far more extreme winters, sea levels would rise fast on the eastern seaboard of the US and would disrupt vital tropical rain patterns.

The research shows the current is now 15 per cent weaker than around 400AD – within a relatively short period – while one of the papers suggests global warming caused by human activity is a major factor in this.

The AMOC carries warm water northwards towards the north pole. There it cools, becomes denser and sinks, and then flows back southwards. If an abrupt collapse of the AMOC happened this century, it could result in several feet of additional sea level rise and changes in hurricane activity, among other effects. The Nature studies, however, do not suggest this is imminent.