Ireland can suddenly slip into ice land

THERE are not, despite the blarney, 365 little islands in Clew Bay but certainly there are enough to impress: a maze, an archipelago of whalebacks (actually rather dull grassy wedges, each enough to feed a couple of cows).

The islands, as every bayside schoolchild learns, are drumlins - a word, like eskers, garbled from the Gaelic 130 years ago to describe certain landforms remaining from the Ice Age. Drumlins are smooth backed hillocks moulded from boulders, gravel and silt beneath a surging icesheet, like sand beneath waves. Eskers are long, sinuous ridges of the same sort of debris, shaped within melt water tunnels.

There are thousands of drumlins spread across the northern half of Ireland, often bunched in "fields" or "swarms" aligned with the thrust of the ice and spilling out, finally, into the big bays of the western seaboard. I always imagined this as an infinitely slow, creaking process as the ice moved on, but now it seems events may have been more dramatic than that - even, by geological standards, quite a rush.

Indeed, the Clew Bay drumlins turn up in the evidence for what might be called Gaia's Plan B, the scenario in which global warming precipitates us, prematurely, into another 100,000 years of glacial cold. It takes on extra poignancy as world leaders argue about their CO2 emissions, and whether we should chill our soft drinks in cans that squirt out greenhouse gases.

"The Chilling Facts About Global Warming" is the heading in this month's Technology Ireland on an article by Dr Pete Coxon, senior lecturer in geography at Trinity College, Dublin. He is also a palynologist, drilling down into Ireland's ancient peats for clues to environmental changes and vegetation patterns in and around the Ice Age. His work on the fossil pollen record has greatly helped Frank Mitchell and others in "reading" the Irish landscape.

Altogether, it seems, this island has spent some 90 per cent of the past 750,000 years in periods of bitter cold. Coxon has been probing and analysing the interstadials" - the brief intervening periods of warmth. Each has been about 10,000-15,000 years long, compared with the freeze ups which last 100,000 years. And the more he learns about the Quaternary cycles, and cycles within cycles, the more concerned he gets about the abrupt changes global warming could provoke.

The overall timing of cold and warm cycles is fixed by variations in the earth's orbit and angle to the sun. But extreme instability set in from about 2.6 million years, ago, at the start of the Quaternary period. The first plunge into cold may have been triggered by volcanoes creating veils of dust in the atmosphere, or by the rise of mountains - the Rockies and Himalayas - that changed wind flows and pressure systems.

As the global ice caps expanded, Ireland was covered by ice many times - the last time with a sheet up to 500 metres thick and covering the northern two thirds of the island. The idea that this Ice Age ended 10,000 years ago in Ireland is part of most people's general knowledge. But that was just when it got warm and stayed warm. Before that, the North Atlantic climate had done some extremely dramatic switching back and forth. Somewhere between 19,000 and 14,500 years ago, for example, the Irish ice sheet seems suddenly to have melted, surging across the island and collapsing into the Atlantic. The drumlin swarms such as those in Clew Bay and Donegal Bay are, says Pete Coxon, "ample proof of this rapid deglaciation and of how the ice was pulled offshore". This event has been radiocarbondated from sediments dropping down from the icebergs in the bays.

After the ice had gone, around 14,500 years ago, Ireland stayed cold for another five centuries or so, then suddenly, in a mere seven years, warmed up to today's temperatures. The following 500 year period of warmth brought in plants and animals, including the famous Giant Irish deer. For about 1,000 years the climate swung about between warming and cooling, then plunged into cold again, as glacial meltwater poured from the north into the Atlantic and shut out the warmth of the Gulf Stream.

During this period, about 12,500 years ago, little glaciers formed in Irish mountain corries and the giant deer became extinct. But summer solar radiation was increasing, and climate shifted again, in an amazing nine years, into the 10,000 year warm phase we have been enjoying up to now. These incredibly abrupt shifts in climate - seven years, nine years - are part of what worry Coxon.

It's now thought the big climate changes are triggered by the breaking up of the ice caps by warmth that raises sea level. First the floating ice shelves break up, then the ice cap collapses behind them. As the Atlantic fills with a raft of icebergs, climate is pulled into a "flickering" cycle of warm cold changes (actually severe climatic jumps), each about 1,000 years long. These and other interim flickerings are caused by changes and reactions in atmospheric and oceanic circulation systems - "negative feedback loops".

The most important of these systems - certainly for Ireland - are the great pumps" in the North Atlantic (up near Iceland) which draw the warm water of the Gulf Stream down to the ocean bed and send it back south for recirculation. This part of the global conveyor belt can be disrupted by drastic melting of ice which sends torrents of cold, non salty water flooding out into the ocean.

THE past 10,000 years of our climate have been remarkably stable. By the theory of solar radiation that Pete Coxon favours (worked out by the Serbian, Milankovitch), natural solar warming should grow slowly for another 2,000 years before it breaks up the polar ice caps. But our current global warming is not proceeding at a natural pace.

"Geological evidence from the past," says Coxon, "cannot tell us what will happen in a populated and polluted world, but one thing is certain: if the planet we live on kicks in with an effective negative feedback reaction to global warming, we can forget the idyllic scenarios of warm summers and mild winters so often quoted by the media, and look forward to 100,000 years of glacial conditions at our latitude."