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Just doing what comes naturally

The popular press was quick to blame global warming for the recent rockfall at Beachy Head, but the experts are not so sure.

On the evening of January 15, a landslide caused the derailment of a passenger train in the north of England. The area had been subjected to prolonged and intense rainfall in the weeks before the accident in which, thankfully, nobody was seriously injured.

Just a few days earlier a very different example of ground failure would have meant anyone contemplating jumping from Britain's favourite suicide spot would have had the job done for them. On 10/11 January, a section of cliff roughly 17m thick, 70m wide and 50m high collapsed into the sea, effectively joining the unmanned Beachy Head lighthouse to the mainland.

The popular media were quick to blame global warming for the Beachy Head rockfall and many articles warned of the increased potential for future coastal crumbling. Dry weather will increase cracks, allowing more rainwater to penetrate, wrote Nick Nuttall in The Times. And this, he claimed, combined with rising sea levels, 'will bring havoc to vulnerable areas'.

The suggestion that global warming and increased wave heights caused the rockfall, as many papers did, was quickly dismissed by geologists. 'This analysis misunderstands the nature of erosion of the Chalk. The cause of the fall is uncertain,' said a team of engineering geologists from the British Geological Survey.

They felt it was likely the rockfall was triggered by increased pore pressures in the Chalk resulting from a period of prolonged rain. 'The Chalk at Beachy Head is naturally bedded and jointed, and rainfall levels are reported to have been above average in the two to three weeks immediately preceding the rockfall, with most days being wet.'

However, with little or no research having been carried out into the natural long-term variation of pore pressures in the Chalk, the connection between rain at the surface and pore pressure within the rock mass remains unquantified.

Despite dismissing global warming as a cause of the rockfall, Martin Culshaw, manager of the BGS Coastal and Engineering Geology Group, admits that it - or rather climate change -'may have altered the rate at which things are happening'.

The cliffs are actively retreating as part of a long-term general erosive process, he explained, 'aided by gravity and the fact that the Chalk is naturally fractured'. A number of factors are known to influence erosion rates, including the amount, intensity and duration of rainfall, cliff undercutting by wave action, removal by the sea of previously fallen material and changes in the form of the beach and the abrasion platform.

Culshaw concedes that if global warming were to result, in the long term, in increased winter rainfall and enhanced wave action, then erosion rates 'might be expected to increase', but this would simply represent an enhancement of an existing process. Short-term variations in climate and wave energy impinging on the abrasion platform seaward of the cliff may change erosion rates on an annual basis, he said, but there is no published evidence to suggest that the overall rate of retreat has been increasing.

'Such is the intermittent nature of cliff retreat, in this area, that long-erm monitoring over decades would be needed before it could be confirmed that retreat rates were changing.

'And no such long-term monitoring programme is in place to look at either coastal erosion rates or landslide activity.'

Culshaw rejects suggestions that frost wedging could have triggered the rockfall: 'There were few sub-zero nights in southern England beforehand,' he said, 'so it's unlikely that temperatures were low enough for long enough for penetrative freezing to take place.'

If climate models are to be believed, frosts are likely to be rare occurrences in 21st century southern Britain. The Met Office predicts large increases in autumn and winter precipitation, not to mention a greater frequency of hot summers, and an increase in severe storms.

These, together with predicted sea level increases - caused by the fact that much of southern England is sinking in isostatic adjustment to end of the last glaciation, as well climate change - will ensure that our coastline continues to receive the type of battering that sculptured its distinct topography.

Fiona McWilliam is a freelance writer specialising in earth sciences.

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