Why is it important?
Storms transfer energy from the atmosphere to the ocean, driving the mixing of the surface layers. Changes in sea level are also associated with the passage of storms (storm surge), causing extreme water levels and currents. Higher waves during storms also cause greater coastal erosion. Storms can affect coastal and marine habitats and species through direct physical disturbance by waves and currents which then have an impact on the life cycle of species (e.g. damage to nesting sites). They can also have an impact on coastal and marine infrastructure and operations, as well as human health and well-being, with the potential for loss of life.
Many storms reaching Scotland form over the Atlantic and move eastwards (the North Atlantic storm track). In Scotland, the majority (78%) of the coastline is considered ‘hard or mixed’ (where the intertidal and hinterland are non-erodible) and is therefore unlikely to experience significant coastal erosion (Masselink, Russell, Rennie, Brooks & Spencer, 2020; and Coastal erosion and flood risk management assessment).
What is already happening?
The North Atlantic Oscillation (NAO) and its variability are an important driver for the inter-annual and decadal variability of storms and waves in the UK (Wolf, Woolf & Bricheno, 2020; see Wave climate assessment). The NAO describes the relative pressure changes between the high pressure centred over the Azores and low pressure centred over Iceland. It has a strong influence on winter weather and climate patterns in Europe. Storminess (frequency and intensity of storms) is very variable, and a lack of data (particularly prior to 1970) makes it difficult to provide clear assessments of changes over long time scales (in the order of centuries) (IPCC, 2014; Wolf et al., 2020). Observations provide strong evidence that storm activity in the North Atlantic has increased since 1970 (Wolf et al., 2020). From visual observations of wave height from Voluntary Observing Ships (VOS), wave heights in the last 50 years have also shown a significant increasing trend of the order of 14 cm per decade (Wolf et al., 2020).
In Scotland, 19% of the coastline has potential to erode, 78% is considered not to erode at perceptible rates (threshold of 1 mm/yr), and 3% has artificial defences (Masselink et al., 2020). The National Coastal Change Assessment (NCCA; Hansom, Fitton & Rennie, 2017) (see also the Coastal erosion and flood risk management assessment) compared the coastline, as defined by Mean High Water Springs (MHWS) at three time points: 1890s (1898 - 1904), 1970s (1956 - 1995 with most data from the 1970s) and present day (2003 - 2016). Since the 1970s, a little more than three quarters (77%) of the erodible coast has not experienced significant erosion or accretion, with 11% accreting (strand plain development where a broad belt of sand develops along the shore) and 12 % eroding (Masselink et al., 2020). When compared with the historical baseline (1890s to 1970s), data show that the proportion of accreting coastline has fallen (a 22% reduction), that the proportion of retreating coastline has increased (a 39% increase), and that the average rate of erosion has doubled (to 1.0 m/yr; Masselink et al., 2020; Hansom, Fitton & Rennie, 2017). Sea-level rise has increased coastal erosion and caused a landward migration of the coastline, which has an impact on low lying communities and infrastructure. The switch to relative sea-level rise in Scotland (from previous relative sea-level fall due to “post-glacial rebound”; see Sea level and tides assessment) will cause a significant change from accretion to coastal erosion (Masselink et al., 2020).
Coastline adjustment often happens in an episodic manner, as storms cause significant, episodic changes to the coastal landscape. For example, the storm surge in the North Sea, associated with Storm Xaver in December 2013, caused significant coastal erosion to the Norfolk and Suffolk coast (Spencer, Brooks, Evans, Tempest, & Möller, 2015). While a small portion of Scotland’s coastline is at risk, a significant portion of coastal infrastructure lies within these erodible sections, placing buildings, roads, rail and water network at risk of future erosion (Hansom et al., 2017).
What is likely to happen in future?
Climate models have limited accuracy in their representation of the processes influencing storminess (mainly due to issues with downscaling wind fields from coarse-resolution climate models to higher resolution regional models). This means there is considerable uncertainty in the prediction of how the North Atlantic storm track will respond to future climate change. Climate projections (as expressed in the ensemble mean of the 5th Coupled Model Intercomparison Project; CMIP5) suggests a reduction in the frequency of storms due to changes in the northern hemisphere storm track (IPCC, 2014; Wolf et al., 2020).
Climate models predict that warmer sea surface temperatures in the tropical Atlantic Ocean in future are likely to increase the number and intensity of tropical cyclones (e.g. hurricanes), and that these could be more prone in reaching Europe as storms (extra-tropical cyclones; Wolf et al., 2020).
Climate models with a future high greenhouse gas emissions pathway predict that mean significant wave heights will be lower by the end of the 21st century, but that the most extreme waves will increase in height (Wolf et al., 2020). The reduction in Arctic sea ice is also likely to increase wave heights to the north of the UK as the open sea will promote swell to develop (Wolf et al., 2020).
Sea-level rise and wave climate changes will have an impact on coastal erosion in future, and national assessments predict that coastal erosion and coastal flooding will increase (Masselink et al., 2020). Conservative figures by the NCCA estimate that at least 50 buildings, 1.6 km of railway, 5.2 km of road and 2.4 km of clean water network would be affected by coastal erosion by 2050 (Hansom et al., 2017). As coastal erosion is a localised process (due to interaction between the local coastal morphology and physical environment), future adaptation strategies will need to be site-specific (based on local or regional predictions of coastal response). This should consider how best to “work with nature” rather than against it (nature-based solutions; Masselink et al., 2020).