Monday 28th September 2015
Blog post 7 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/
Reason 7. Global change is influencing landscape dynamics. Ongoing global environmental change, which includes atmospheric warming and sea level rise, is currently driving landform development, including desert lake desiccation, ice sheet and glacial retreat, and coastline erosion.
Being located in the temperate mid latitudes far from desert lakes, ice sheets and glaciers, some of these landscape dynamics appear to be of little direct relevance to Wales. But sea level rise and coastal erosion is certainly an issue along large parts of the Welsh coastline (see below), and global environmental change is driving other forms of landscape development. For instance, many parts of the Welsh uplands are covered by blanket mires (‘bogs’), which are characterised by peat that is draped across the underlying topography. Mires are major carbon sinks, being formed from partly decomposed plant material that has built up over many thousands of years under typically cool, waterlogged, oxygen-starved conditions. Today, however, many areas of blanket mire are degraded and actively eroding, as seen from Bwlch Y Groes in Gwynedd, one of the highest public road mountain passes in Wales (left photo: eroding mire is visible on the hilltop in the far distance). Mire degradation can be driven by a combination of factors, including overgrazing, overburning, and air pollution but global atmospheric warming is almost certainly playing a role, particularly through subtle changes to upland water balances. A warmer, more variable climate can lead to longer and/or more frequent dry periods, thereby reducing the extent and duration of waterlogging. This enables oxidation of the peat and promotes increased rates of microbial breakdown of the dead plant matter, with some of the solid carbon being converted to carbon dioxide gas. Drier mires are more vulnerable to extreme fires, leading to loss of peat through combustion, a process that also generates carbon dioxide. Drier, fire-affected mires may also be susceptible to gully erosion (right photo), especially during subsequent extreme rainfall events. Gully erosion may be associated with rapid lowering of water tables, leading to further peat oxidation and carbon dioxide generation. Hence, rather than remaining as a carbon sink, extensively degrading and eroding blanket mires may ultimately become a net source of atmospheric carbon dioxide, so adding to the ever-increasing burden of greenhouse gases and providing a positive feedback in an already warming climate (Photos: Stephen Tooth).
While a large percentage of the Welsh landmass lies well above sea level (Reason 1), the country nonetheless possesses a lengthy coastline. Much of this coastline is characterised by headlands formed of resistant rock, and intervening bays formed in more erodible rocks or sediments, as exemplified in Pembrokeshire (Reason 3) or along the Llŷn Peninsula. These erodible rocks and sediments may be vulnerable to increased erosion under rising sea levels, particularly if higher seas are accompanied by an increased frequency of extreme storm events. For instance, in Aberdaron Bay, located near the western tip of the Llŷn Peninsula, coastal erosion is pronounced where the 20 m high cliffs are formed in weakly-consolidated glacial sediments that are comprised principally of sand, silt and clay. Erosion of the base of the cliffs is leading to slumping and collapse of the entire cliff face (upper left photo). Along the eastern part of the bay, the cliff is retreating mainly into agricultural land but farther west, the erosion threatens a nearby road, a church cemetery and other buildings. In response to this threat, an elaborate coastal defence scheme has included sea wall construction and chevron-style drainage on the cliff face (upper right and lower photo) (Photos: Stephen Tooth. Aerial imagery from Google Earth, 0.72 km across, with north oriented to the top).
Did You Know? How long is the coastline of Wales? The answer partly depends on the scale of the map that is used for the measurement, as the larger the map scale, the more ‘wiggly’ the coastline appears to be, and so the longer the length that can be measured. Whether measurements are made at high tide, low tide or somewhere in between could also make a difference. Using 1:10 000 maps and measuring along the mean high water mark, mainland Wales’s coastline is said to be about 1317 miles (2120 km) long, while adding the islands of Anglesey and Holyhead increases the figure to 1680 miles (2740 km) (Source: The British Cartographic Society – http://www.cartography.org.uk/default.asp?contentID=749). But with rising sea levels, even that figure may change over time, as some beaches may narrow or disappear owing to inundation or erosion, while others perhaps get wider as retreating cliffs generate greater sediment supply. Around the Welsh coastline, recognition of a rapidly changing coastline is leading to serious debates about future priorities for coastal defence schemes. While some communities have benefitted from extensive investment in coastal protection works in recent years (e.g. http://www.bbc.co.uk/news/uk-wales-mid-wales-16105700), there is tacit acknowledgement that not everywhere can continue to be protected, with other communities possibly facing the prospect of abandonment as sea levels rise further (e.g. http://www.bbc.co.uk/news/uk-wales-26495935).