PANALBA

End of the Ice

Dave McNicoll 09/21/2009 03:56 PM

So follows the tale of our wee country as she emerged from the frigid wastes of the last ice age.

The world's climate depends on a series of astronomical events known as the Milankovitch Cycles, which governs whether the Northern Hemisphere will to a greater or lesser extend be covered by glacial ice. It may come as surprise to you, but we are currently living through an ice age (watch out for the sabre-toothed tigers I hear you cry); but what I mean is that ice exists on the planet, which has not always been the case. What we normally consider an ‘Ice Age' is in reality a glacially-enhanced episode. So what are the Milankovitch cycles, and should you be saving up for winter woollies? There are four main cyclic events associated with the orbit of the Earth, and its associated relationship with the sun -

Eccentricy

The shape of the earth's orbit around the sun does not remain constant, but changes from an almost circle shape to a long, sleek, elliptical shape. This means that at times the orbit will create huge differences in the seasons, as the earth is pulled far from the sun and then close again, while at other times the seasonality will be more constant. The more elliptical the orbit, the more likely it is we'll have ice ages.

Obliquity

In relation to the orbital plane of the earth around the sun, the earth is ‘tilted' in its axis of rotation. The planet is currently tilted at 23.5° from the vertical (this is why the Arctic and Antarctic circles are at 66.5° north and south), but this tilt can change over time, bringing the earth closer to the vertical at times, and at others nearer to the horizontal (swinging back and forth between 22.10° and 24.50° off vertical). No-one knows why the earth should be so tilted, but it may have something to do with an ancient collision with another planetary body. The closer the earth is to the vertical the less solar radiation reaches the higher latitudes, which may induce a glacial event.

Precession of the Equinoxes

At present the earth tilts on its axis towards the sun during the Southern Hemisphere summer and the Northern Hemisphere winter (thus, there is an imbalance in the seasons between the north and the south). This is measured against the fixed stars rather than the orbital plane, and in due course the axial position will change and the hemispheres will swap places. Basically, the spring equinox in the north will change from March to September. This ‘wobble' is due to the fact that the earth is not a perfect sphere, and the solid inner core ‘swashes' around in a liquid outer core. With the majority of the world's landmass in the north, tilting the earth in such a way that during the Northern Hemisphere summer the amount of solar radiation is reduced would be instrumental in triggering an increase in ice cover in the north.

Orbital inclination

The earth doesn't always circle the sun at 90° to the solar axis, sometimes it shifts so that it slices the axial plane at a more acute or obtuse angle. These changes dramatically alter the position of the Tropics of Cancer and Capricorn, and the position of the sun directly over-head between the solstices. If this has the effect of reducing solar radiation to the Northern Hemisphere then there will be a major cooling in the north.

If the cooling effects of these cycles happens to coincide then the worlds swings sharply into a major Ice Event; likewise, a misalignment will snap the world back out of an ice age and into a warmer period. If only one or two of these conditions prevail the severity will be much reduced. There are anomalies in the historical record of our glacial past and the Milankovitch cycles, but on the whole they are pretty consistent. There are of course other events that can alter or influence the climate of the earth, such as plate tectonics, enhanced volcanic activity, solar activity such as sun-spot cycles, the shut-down, or failure of oceanic currents such as the Gulf Stream, and possibly human influence.

18,000 years ago these cycles held the British Isles in the thrall of one of the deepest and most profound Ice Ages of them all. The Ice Cap, over 4000ft thick in places stretched from the far north all the way to central England and Ireland, leaving all but the highest peaks, such as Ben Nevis and Ben MacDhui clear from its frozen grip. The main hub of these vast glaciers were Rannoch Moor in the west and Deeside in the east, although in time the ice sheet would become so large that it escaped topographical constraints, and at the height of the ice age the central dome may have sat somewhere over Stirling, or possibly a little further south. Independent ice cores emerged on Skye, in Sutherland, the Southern Uplands, the Lake District and Yorkshire, all merging together to form one gigantic sea of ice.

And yet, by 13,500 years ago this ice was gone. Unlike the vast Laurentide Ice Sheet which covered Canada and the northern USA, the British ice cap didn't initially melt away, but rather decayed due to a process called ‘down-draw'. Inevitably the thaw came, and the rate of retreat gathered a pace, and as the Late Devensian Ice Age came to a close the glaciers retreated back into the high mountains of the Highlands, and finally melted away in the northern corries from whence the ice had first emerged some 20,000 years earlier.

The ice would have one last Swan Song, when the earth took one last plunge into the cold 11,500 years ago, and the glaciers once again spread from the mountains; but nothing in comparison to what had gone before. 10,000 years ago Milankovitch's cog-wheels clicked round again and the world warmed dramatically: and a new Scotland was born.

Despite the rapid rise in global temperatures, this new Scotland would initially look more like northern Alaska or Siberia, with tundra, permafrost and sharp winters. The first vegetation following the lichens would be scrubby trees and coarse grasses, taking the opportunity to colonise the sands, gravels and thin soils covering the exposed and rough bedrock. As this vegetation died, and more organic detritus built up these soils became thicker and more nutrient rich, and this turn laid the foundation for growth of flowers and trees, and so on. As the climate continued to warm and the soil cover thickened, a myriad of new species arrived from the south enveloping the whole country in a blanket of grasses, briars, shrubs, flowers and a whole host of broadleaved trees. There were a few corners of the country such as bare landscapes of Assynt, and the rolling hills of Orkney and Shetland - separated from Scotland before the trees arrived - where the verdant empire never reached, and in some of these places little has truly changed in 10,000 years.

Great Britain remained attached to continental Europe by a slither of land until around 8,000 years ago when the seas rising steadily from the final melting of the Laurentide Ice Sheet in North America resolved once and for all to make us an island. As a result, verdant as our landscape would be, it would miss out on much of the flora and fauna that would come to inhabit northwest Europe. Indeed, only one conifer, one solitary pine species would make it over the isthmus before the sea cut us off - the Scots Pine: a tree that would come to dominate much of our upland areas, and in time become symbolic of our ancient landscape.

7,500 years ago, the climate of Britain, indeed the world, was a lot warmer than today, and the tree-line climbed high into the mountains, covering a vast amount of Scotland with huge forests (pine and birch in the upland; oak, beech, elm and chestnut in the lowlands). The valleys would be a jigsaw of marshland, grassland and meadow, and of course completely uncultivated, but the high peaks would be pretty much the same as to today - heather, grass and thyme up to a frost shattered rock blanket on the tops. The forest and grasslands of the lowlands and of the Highland valleys were full of bears, wolves, lynx, foxes, elk, red deer, wildcats, capercaillie; while the high peaks saw eagles, ptarmigan, hares and reindeer. Most of these animals are gone, while others remain, but it was a newcomer to Scotland, following all these others that would change the landscape forever - it was the arrival of man. And with this newcomer to our virgin world, the Scotland that we know and love began its incredible story.