IRELAND is known for its changeable weather, but nothing quite like this. Scientists have discovered that 11,500 years ago, Glendalough in Wicklow changed from Arctic tundra to its present climate in less than a decade.

Using fossil pollen found in local lakes, they reconstructed the climate in Glendalough over the past 15,000 years and found that rapid warming occurred at the end of the last ice age.

The findings add to evidence from ice cores in Greenland and sediment from the Atlantic seabed that dramatic shifts in climate have occurred before with extraordinary rapidity. But while other studies indicate a warming of the earth at the end of the last ice age over the course of a few decades, the Glendalough study has narrowed it down to just seven years.

“At the end of the last ice age our climate was a bit like what it would be in southern Greenland,” said Fraser Mitchell of Trinity College Dublin’s botany department, one of the authors of the study. “But it warmed incredibly quickly and went from Arctic tundra to what it is like today in about seven years.”

Under tundra conditions, such as in northern Siberia, average winter temperatures are about -28C with permafrost on the ground. Temperatures during the summer rise to about 12C, but can often drop to 3C.

“All the lakes were frozen over and it would have been very cold. Suddenly it got a lot warmer with a change in summer temperatures of about 7C,” said Mitchell.

The retreat of the glaciers that covered most of the planet and the subsequent warmer climate resulted in the extinction of mammoths and many other species. The Earth goes through natural cyclical climate changes due to shifts in the angle of the planet’s axis as it orbits around the sun. The next is due in about 5,000 years’ time.

It was previously thought that climate change took centuries, but there is growing scientific evidence that the next shift in climate may also be dramatically fast. This does not take into account the effect of man-made emissions of greenhouse gases such as carbon dioxide.

“We are trying to pin down what is the natural variability of climate,” said Mitchell. “Instrumental records only go back a couple of hundred years and that was the period during the industrial revolution when we may have been affecting [the climate]. So we have to go back further, to a time when we don’t have any written records, and look at other things that give us an indication of climate. In Glendalough we are using pollen grains that are preserved in the sediment in the lake to reconstruct the vegetation.”

Climate change is reflected in changing vegetation that leaves traces of itself as pollen accumulated in the sediment of lakes. The researchers used sediment cores extracted from the lower lake at Glendalough and analysed the pollen content. This was radiocarbon-dated and compared with modern sediments for which climate data is available.

“That area, like most of Ireland, was covered in ice during the ice age, and it melted and water started to flow in the valley,” said Mitchell. “Plants began colonising the land, but as it was very cold you get the sort of vegetation you find growing around the edge of Greenland — Arctic vegetation. There weren’t any trees — just heathers, grasses, some juniper bushes.

“Then it got very hot and there was a dramatic change in the plants that were growing. All the trees started coming in — birch followed by pine, oak and elm, which came in from the Continent, from France and Spain. Pine disappeared about 4,000 years ago, but all the others are still there.”

John Haslett, of Trinity’s department of statistics, developed a new method of analysing the data that allowed more precise predictions of the change. “The speed with which this happened is gobsmacking,” he said. “It may not have happened in 24 hours, like in The Day After Tomorrow, but the science in the film is correct — it’s to do with the Gulf Stream switching on and off.”

After being shut off during the ice age, the Gulf Stream suddenly got turned on again, sending warm ocean water up from the Caribbean. This is why it suddenly got hotter so very quickly.

Haslett’s new model has allowed the scientists, which also include researchers at the University of Durham, a clearer picture of how accurate their predictions are. “What he can do better than anybody before is say how precise the reconstruction is,” said Mitchell. “There is always uncertainty when people are reconstructing climate, in the future or the past. He has managed to reduce the level of uncertainty.”

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