Severn Estuary Partnership - Working in partnership for the future of the Severn Estuary

Weather & Climate Change

The marine environment of the Severn Estuary is one of the most dynamic in Europe, with changes in sea level, waves and storms all playing their part in shaping this coastline. Historical records show that severe storms have caused much coastal erosion and flooding and modern research suggests such events are likely to occur again in the future. Perhaps one of the biggest threats to the Estuary is from rising sea levels which, coupled with a significant storm event, could exacerbate the effects of coastal erosion and flooding and cause significant issues for coastal planners, engineers and local communities. The Severn Estuary coastline is already considerably defended, especially around the Wentlooge and Caldicot levels and the Somerset Levels, with several small scale localised defences existing elsewhere.

Changes to the local climate of the Severn Estuary will have impacts on its social, economic and natural characteristics and it is therefore important to understand these changes for effective adaptation. Current research and evidence from the Estuary suggests that air and sea temperatures are rising in line with global trends¹. Local impacts of particular concern include coastal flooding and erosion. A significant proportion of critical infrastructure such as roads, rail tracks, power stations and industrial developments are located on low lying land within the Estuary and as such, are potentially at risk from these impacts¹.

Sea level

A recent comprehensive review² of tide gauge data for the Severn Estuary and Bristol Channel conducted over 15 years (1993 to 2007) concluded that there had been a rise in mean sea level. The review suggested a rising trend in sea level of 2.4mm/year-1, which is in line with other climate change research results³ and UK Climate Projections 20094. Local research estimates that by 2080, sea levels at Cardiff will be 30-40cm higher than 1990 baseline level¹ (see Fig. 2).

Wave Climate

Seasonal average and extreme waves are generally projected to increase slightly to the south west of the UK, including the Bristol Channel and Severn Estuary, which is consistent with rising sea levels⁵. Models suggest that coastal squeeze (see Fig. 3), habitat loss, coastal erosion and the steepening of intertidal (beach) profiles will all increase in the future due to both sea level rise and possible changes in wave conditions⁵.

Storms

Numbers of severe wind storms in the UK have increased since 1960; however they have not increased above levels recorded during the 1920s^4,6 (a particularly stormy period). Recent local research in 2008 has supported this trend⁷. Projections are uncertain and there is no consistent indication of any change in storms near the UK in either Met Office or other weather models. Such changes are seen as relatively modest, and the potential for substantial changes appears to be small⁷.The Bristol Channel and Severn Estuary are likely to see an increase in storm surge height of about 0.8mm/year⁴. Although only a small increase, when taking into account sea level rise, this could exacerbate the impacts of flooding and erosion.

Isostatic adjustment

The landmass of the UK has been attempting to achieve equilibrium since the last ice age, in a process known as ‘isostatic adjustment’. On the Severn Estuary and Bristol Channel, this has resulted in sinking of the land at a rate of 0.6-0.9mm/year for the past 4,000 years⁸. This trend is expected to continue for the next 100 years⁹.

Rainfall

The nature of rainfall over the Severn Estuary has changed over the last 30 years and is likely to continue to do so, with seasonality of rainfall becoming increasingly important^10,11. This has consequences for river and groundwater levels and possible issues related to future land use and catchment management. However, the UK exhibits highly variable storm and rainfall patterns¹² and, as of yet, there are no clear trends of change in annual mean rainfall levels¹³. Some research suggests that between 2040 and 2069, average summer rainfall levels may decrease by about 18% and average winter precipitation may increase by around 12%¹³.

Temperature

UK marine air and sea surface temperatures have increased by about 0.7°C per decade since the 1980s, with strong regional variations¹³. By 2080, the average temperature in winter is projected to increase by about 2.8°C and the summer average temperature by about 3.9°C¹³. By the mid century the Severn Estuary may have warmer drier summers and milder wetter winters.

Impacts of climate change

There are many potential direct and indirect impacts of climate change, as illustrated in Fig. 5.

Impacts on the human environment

Socio-economic losses - Significant urban development’s and critical infrastructure on low-lying parts of the Estuary need protecting from flooding and the erosive impacts of severe storms; in conjunction with the continued effective maintenance of current coastal defences¹⁴.

Tourism - There may be potential for increased tourism within the Severn Estuary region in the summer months; however this could put strain on existing resources unless adequately managed (see section on Tourism & Recreation).

Agriculture - There are possible adaptation issues for the agriculture industry within the Estuary, in terms of soil quality and types of crops that will grow in a new warmer climate with a longer growing season^15,16.

Impacts on the natural environment

Changes in habitats, ecosystems and ecology - Higher air and sea surface temperatures across the Estuary, along with variations in salinity and chemistry, could affect coastal and marine ecosystems as species try to adapt. This in turn can lead to habitat loss and changes to migratory species. Reduced sediment supply, (due to coastal defence construction associated with climate change adaptation), may further exacerbate such habitat losses¹⁷. A knock-on effect of such changes in habitat, could be a review of protected area designations at national and international levels.

Intertidal salt meadows - Saltmarsh areas within the Severn Estuary have been retreating inland since 1980 due to sea level rise¹⁶. Current predictions suggest that over 1,000ha of inter-tidal habitat will be lost in the Severn Estuary by 2100.

Intertidal mudflats - Mudflats comprise a large proportion of the Severn Estuary intertidal habitats and can be closely interlinked with salt marsh habitats and the protection from the erosive power of wave energy these can provide. Climatic drivers are changing the landforms of open coast sediment systems via increasing wave height, storm surges and increased water depth¹⁷. However, it is not yet possible to accurately identify the various effects of climate change, such as temperature and sea level rise, from other factors such as damage caused by human activities (e.g. scallop fishing gear and bait digging) which also impact negatively on mudflat habitats¹⁷.

Seagrass beds - The species Zostera noltii prefers a low salinity environment for germination. Increased freshwater runoff in recent years may have been the driver for the increased success of this species in Welsh seagrass beds¹⁷ (including the Severn Estuary and Bristol Channel).

Major limitations in data and information

It should be noted that there are significant issues associated with gaps in the information base in predicting impacts from climate change; particularly with respect to the social and economic characteristics of the Estuary. Severn–specific scientific predictions of climate change impacts are also limited by monitoring deficiencies and information gaps; as noted in recent meetings of the Severn Estuary Climate Change Research Advisory Group.

For more information see the following links:

References

Hovey C, (2010) Cardiff University Marine and Coastal Environment Research Group (MACE), as part of the IMCORE project (Innovative Management for Europe’s Changing Coastal Resource)
Phillips M R and Crisp S (2010 – in press) Sea level trends and NAO influences: the Bristol Channel/Severn Estuary, Global and Planetary Change, Wiley & Sons Ltd
Antunes, C., Taborda, R. (2009). Sea Level at Cascais tide gauge: data, analysis and results, 1-5
Lowe, J.A. et al., (2009) UK Climate Projections science report: Marine & Coastal Projections, Met Office Hadley Centre, Exeter, UK © Crown copyright
Wolf J and Woolf D K (2006) Waves and climate change in the North-east Atlantic, Geophysical Research Letters, 33, in MCCIP Annual Report Card 2007-2008 Scientific Review – Storms and Waves
Allan R, Tett S, Alexander L (2008) Fluctuations in autumn-winter severe storms over the British Isles: 1920 to present, International Journal of Climatology, Vol. 29, Issue 3, 357-371
Shennan I and Horton B (2002) Holocene land- and sea-level changes in Great Britain, Journal of Quaternary Science, 17 (5-6) 511-526, Wiley & Sons Ltd
MCCIP (2010) Marine Climate Change Impacts Partnership Annual Report Card 2010-11, Summary Report, MCCIP, Lowestoft, 12pp
MCCIP (2010) Marine Climate Change Impacts Partnership Annual Report Card 2010-11, Summary Report, MCCIP, Lowestoft, 12pp
Hovey C, & Rodgers N. (2010) Cardiff University Marine and Coastal Environment Research Group (MACE), as part of the IMCORE project (Innovative Management for Europe’s Changing Coastal Resource)
NEODASS (2006), in MCCIP (2008) Marine Climate Change Impacts Annual Report Card 2007-2008, Summary Report, MCCIP, Lowestoft, 8pp
Severn Estuary Climate Change Research Advisory Group (SECCRAG) (2010), Citations Database, Cardiff University
Allen JRL, (1990). The Severn Estuary in southwest Britain: Its retreat under marine transgression, and fine sediment regime. Sedimentary
Geology, 66, 13-28
Mieszkowska, N. (2010) Intertidal Habitats and Ecology in MCCIP Annual Report Card 2010- 11, MCCIP Science Review,19pp

Last Updated 2011