Science Spin January 2008

Chilling lessons to be learned from the past

By Tom Kennedy

The only predictable thing about climate is that it will continue to change. Climate has never been stable, and, whether we like it or not, it never will be. Most of our ideas about climate change come from the closing years of the Ice Ages, as distant from us as a geography lesson. We often assume that these were changes over an extended period of time, and that once the ice caps had retreated, we entered into an extended period where season seemed to follow season with predictable regularity.

The stories about our great-grandparents going ice-skating on the Boyne and Liffey, seem like no more than minor variations on a well-worn seasonal theme, yet we now know that Ireland's climate did change even more significantly over the past 10,000 years. There were periods of intense cold, and, during the Bronze Age, there were extended periods of near Mediterranean conditions, and some of those changes appear to have been big enough to influence the pattern of human settlement.

In the same way as we think of the Ice Ages as remote, we often think of climatic change as gradual, yet this impression of a slow drift, explained John Haslett, Professor of Statistics at TCD is false. As he explained, change, when it occurs, can be startlingly abrupt.

One of the main reasons why we assume that changes are slow and gradual is that the available information has usually been averaged out over time. At the recent Science Foundation Ireland research summit, Professor Haslett explained that a closer look at the data reveals that the abruptness of climate change could take us all by surprise.

Glendalough

Prof Haslett has been working with a multi-discipinary team on reconstructing the palaeoclimate of Glendalough. By examining pollen in sediment cores from the lower lake, the team has come up with evidence to show the occurrence of a sharp and relatively sudden fall in temperature. The outer coat of wind blown pollen from plants is amazingly robust, and every species has a distinctive shape and pattern. Thousands of years later the pollen can be examined under a microscope, and by looking at the relative proportion of hazel, juniper, alder, and other species, scientists can determine what sort of climate occurred at the time.

A statistician, who describes himself as part of the mathematics community, Prof Haslett said his main concern is to reduce the uncertainty in analysing the pollen and other associated data. If we can be more precise in interpreting this data, he explained, we can become better in modelling abrupt climate changes in the future. Over the past 10,000 years, he said, there have been many serious changes, yet, he asks: "what do we know about abrupt climate change? The answer is, almost nothing."

At Glendalough the team have been looking at cores covering the last 13,000 or so years, and at one stage, about 10,000 years ago, the growing season for trees was seen to increase by more than two thirds. This is thought to have been the equivalent of going from Arctic tundra conditions, where the upper valley would have been under a thick sheet of ice, to the sort of climate we enjoy today. This change, said Prof Haslett, was big, yet it occurred in just a few years. We do not know, for certain, he said, and while some scientists might take a guess at ten years, Prof Haslett believes the change may have occurred over 20 years. "No matter what sort of time it took", he observed, "it was fast."

Global

Glendalough is just one of over 7,000 sites being examined around the world, and one of the objectives is to see how the data can be co-ordinated to give a broader picture of climate in the past. The cores taken from Glendalough go down 15 metre, and samples from different levels are extracted for examination back in the lab. The ratio of species at any level, explained Prof Haslett, can be related to samples from other sites. "Somewhere in the world," he said, "we would see the same pattern."

To achieve more accuracy, and to gain a broader view, scientists have to look beyond pollen. There are uncertainties with pollen, said Prof Haslett, making it difficult to pin point events in time. Pollen, blown by the wind, may take a while to find its way into lake sediment, and if it comes from a long-lived species, such as oak, there is no way for the observer to see if it comes from new, or old, well established trees. 'Pollen is not the way to go for high resolution," remarked Prof Haslett.

"There are other sources of information, such as tree rings, or the Greenland ice cores." The more data we can look at the better, he said. "We don't really know what the climate was like," he said, "all we can talk about is degree of probability."

The Greenland ice cores go back much further in time, and as Prof Haslett remarked, "our study at Glendalough concerns the last little bit." Even so, the Greenland ice cores show that variability was a significant feature of the more distant past and the present era of stability is highly unusual. "The last 10,000 years," said Prof Haslett, "has been astonishingly smooth, and astonishingly warm."
In the past there was not just one, but a whole series of ice ages, and if the long term pattern is anything to go by, the current interglacial is due for a change. As the study at Glendalough suggests, a swing from one type of climate to another, completely outside human control or intervention, could occur much faster than we expect.

People

Climate has an enormous impact on how and where we live, and we have to look no further than County Mayo to see how an extensive prehistoric settlement dwindled as conditions deteriorated, allowing peat to cover abandoned farmland.

Many of the population movements in the past were probably caused by climate change, and some archaeologists believe that global warming may have been the trigger for the emergence of city states. Brian Fagan, emeritus professor of anthropology at the University of California, in his book, How climate changed civilization, argues that prolonged drought put people from small farming villages in Mesopotamia under pressure to share precious resources, such as water.

This, in turn led to the emergence of bureaucratic structures and centralised power with all the attendant trappings of civilization. Thus climatic change could have led to the emergence of early cities, such as Ur, and the growth of power in Egypt, the Indus Valley, north central China, and the northern coast of Peru.

While climatic change may have had a positive side, forcing people to share and manage resources, the usual impact is more likely to have been negative. For some time, archaeologists have wondered why images of birds and water-loving animals occur deep in the Sahara. Archaeologists from Norwich, investigating one area of desert in the western Sahara, have found lots of evidence to show that a humid climate flora and fauna once existed there, and that traces of human settlement could be traced back 150,000 years.

Under the Western Sahara Project, directed by Nick Brooks, Maria Gaughnin, a PhD student from the University of Edinburgh has been recording outlines of animals cut into flat stones at Sluguilla, a site extending over a length of 26 km. Although the area is now dry desert, the images of giraffes, elephants, rhinos and other animals stand out as evidence that the region must have been a rich hunting ground.

After thousands of years climate change forced the original inhabitants to move or starve. No one yet knows what became of these people, but one of the team, earth scientist, Ann Mather, from the University of Plymuth, hopes to determine a date for their departure by looking for sediments covered by the first drifts of sand.