Friday, 19 August 2011

The Beacon

Ben Lomond, 974m, 3194ft
Beinn Laomainn, "The Beacon Hill"

Sitting on the peak of Ben Lomond at 974m above sea level[1] it is difficult to imagine how climate change will affect this, the most southerly Munro[2], and the surrounding landscape. The rock beneath me has hardly changed for thousands of years. That does not mean that it has never changed nor ever will by both natural processes and, increasingly, human activity.

The Islands of south Loch Lomond, from the summit of Ben Lomond
The name comes from the gaelic word for "beacon" but the origins of this name are lost in the mists of time. Perhaps fires were once lit on its summit, linking it with a network of similar beacons throughout Celtic Britain, or it may simply be due to its prominence, visible over large areas of the lowlands - a symbol of what lies beyond.

Hundreds of millions of years ago this land was in the southern hemisphere and it was much warmer than now, joined to what is now North America and the Highland Boundary fault formed at the boundary with the tectonic plate that forms southern Britain and much of Europe, dividing the highlands from the lowlands. Over the eons, the Earth’s crust has moved Scotland to its current location changing climate as it moved. In more recent times, geologically speaking, ice has shaped the landscape: a few metres from the summit vast cliffs descend into the northwest corrie, carved out by the motion of a glacier hundreds of metres deep. And to the east, another great glacier, a kilometre or more thick carved a great valley through the hard metamorphic rock before spilling into the lowland plateaux. As the ice melted and the glacier retreated, the depression left by its great weight and scouring remained full of water forming Loch Lomond. The land is still rising 10,000 years later, relief from the great weight of ice[3].

Near vertical crags in the north east corrie

In the ten millennia since the last ice age, the processes acting on the mountain are less dramatic than the carving glaciers but natural erosion has continued: wind and rain, frost and snow. The exposed rock surface is slowly weathered into gravel, then eventually into a course soil which is in turn colonised by hardy plants. These plants contribute further to breaking down the rock, insinuating their roots in to crevices and forcing them open, before finally dying and enriching the soil for their descendants as they decompose.

Walking up the mountain, there is a progression of habitats. The loch shore and the lower slopes are wooded with oak and birch, where it is warmer, more sheltered from the wind, protected from the hardest frosts and where the soil is at its deepest. At a certain altitude the trees rapidly give way to grasses and heather; the tree line follows the contours, until it is crossed by an incised stream bed which provides a measure of shelter allowing the trees to climb further up the mountain. Beyond the tree line, the grasses continue to the summit, becoming shorter and interspersed with alpine flora.

The mountain has been grazed by sheep and cattle for generations shaping the landscape and ecology of the mountain more than climate and contours. The National Trust for Scotland, acting as steward of the area, have erected deer fences around the woodland to prevent grazing livestock and deer from eating saplings and to promote greater natural biodiversity, but no fence can keep out the effects of climate change.

The first thing to change will be this sequence of habitats.  Current changes to our climate are moving the limits of where certain species exist.  Foe example, a study of the range of the comma butterfly (Polygonia C-Album) in the UK has shown it to be moving north by 50 miles per decade since the 1970s, which is equivalent of increasing altitude [4].  Over time this process will allow the tree line will advance up the mountain, the grasses on the upper slope will become more vigorous, as they are on the lower slopes and the alpine flora may be forced out of its niche at the very top.  The relatively high speed of warming may also allow invasive plant species to overtake the indigenous plants if they do not adapt quickly enough.

A side effect of global warming appears to be more erratic weather.  Warmer seas allow greater evaporation and therefore increased rainfall and changing patterns of ocean circulation may lead to harsher winters. Both of these effects will increase the rate of erosion and weathering of the mountains, accelerating the breakdown of rock into soils and aiding the creep of flora up the mountain.

In the longer term, rising sea levels could also impact on this landscape.  Sea levels are predicted to rise by around 5m if the Greenland ice shelf melts, 12m if both the Greenland and West Antartica ice shelves melt[5] and this is on top of the metre or more from thermal expansion of the oceans over the next century.  The banks of Loch Lomond are only 11m above Ordnance Datum; the worst case scenario would see the loch transformed from fresh to salt water.  This would bring about a complete change in the ecosystem, animals that live on many of the loch's islands would no longer be viable and some of the smaller isles could be submerged completely.  

At first glance the mountains and valleys look immutable, unchanging over long periods of time, but on closer inspection we see continual change: the sunlight and cloud shadow, stripes of rain as the weather changes throughout a day; the plant's seasonal cycle of life - birth, growth, reproduction and death; the uphill wander of the trees with the years and the rising sea levels over decades.  Some of this change is now inevitable; the atmosphere has changed and we cannot change it back.  How far the change goes depends on our actions now. Do we continue pumping warming gasses into the atmosphere or not? There are too many variables to determine the final outcome if we cut carbon emissions now but we have a good idea where we will go if we don't.

When you look to the highlands and see the distinctive beacon of Ben Lomond, think not only of the wild places beyond, think of the changes we are making to them.  And when you see trees flourish on its summit, know it is too late to mend our ways. Before then, we can all do our bit.

[1] Ordnace Datum is taken as mean sea level at Newlyn, Cornwall between 1915 and 1921.
[2] A Munro is a Scottish mountain over 3000 feet. The list of such mountains was first published by Sir Hugh Munro in 1891 and is now maintained by the Scottish Mountaineering Club.
[3] A process known as Isostatic Rebound. the ground has been estimated to be rising by 1mm/year.
[4] from Field Notes from a Catastrophe: A Frontline Report on Climate Change , Chapter 4: The Butterfly and the Toad, 2007.

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