Rising Sea Levels

DOI: https://doi.org/10.58248/PN555

Global mean sea level (GMSL) has risen 20 cm since 1900, at an average rate of 1.5mm per year during 1901-1990. During 1993-2014 it rose on average 3.2 mm per year. The rate of sea level rise during the 20th century was faster than at any point since reaching near modern-day levels around 3,000 years ago. Higher sea levels increase the likelihood of coastal flooding and speed up coastal erosion, which poses problems for UK coastal communities, businesses, infrastructure and habitats. Current UK annual damages from coastal flooding are estimated at £540 million per year, and will almost certainly increase with future sea level rise.

Human greenhouse gas (GHG) emissions have increased global surface temperatures, which causes sea level rise in two main ways:

• Thermal expansion. The oceans increase in volume as they become warmer.
• Added water. The amount of water in the oceans is increasing as ice sheets and glaciers melt.

From 1993 to 2014 GMSL rose 6.4 cm. 40% of this rise was caused by thermal expansion, 20% by the Greenland and Antarctic ice sheets and 25% by glaciers elsewhere. Satellite observations show that the net sea level contributions from the Greenland and Antarctic ice sheets increased considerably over the past decade. Their contributions are projected to continue to increase because of climate change, but the future rate is uncertain.

Key points in this POSTnote include:

• Global average sea level has risen by 20 cm over the last century. It will continue to rise, but how much and how fast it rises depends on human greenhouse gas emission levels.
• Sea level change varies locally from the global average. Local sea level rise will be higher in southern England than in Scotland.
• Sea level rise increases the risk of coastal flooding and erosion. 50 cm of local sea level rise would make about 200 km of UK coastal flood defences vulnerable to failure.
• The impacts of coastal flooding are highest during extreme sea level events, which will become more frequent as sea levels rise.
• Shoreline Management Plans provide the framework for coastal adaptation planning.

This POSTnote accompanies POSTbrief 25 - Projecting Future Sea Level Rise.

Acknowledgements

POSTnotes are based on literature reviews and interviews with a range of stakeholders and are externally peer reviewed. POST would like to thank interviewees and peer reviewers for kindly giving up their time during the preparation of this briefing, including:

• Prof Andy Shepherd, University of Leeds
• Dr Fernando Paolo, Scripps Institution of Oceanography, San Diego (USA)
• Prof David Vaughan, British Antarctic Survey*
• Prof Tony Payne, University of Bristol*
• Prof Jonathan Bamber, University of Bristol
• Dr Rory Bingham, University of Bristol*
• Dr Tamsin Edwards, Open University*
• Prof Jim Hall, University of Oxford, Environmental Change Institute
• Dr Jeremy Hoffman, Science Museum of Virginia (USA)*
• Dr Ivan Haigh, University of Southampton
• Prof Robert Nicholls, University of Southampton*
• Prof Ian Townend, University of Southampton*
• Dr Sally Brown, University of Southampton*
• Dr Phil Woodworth, National Oceanography Centre*
• Prof Jonathan Gregory, University of Reading*
• Prof Chris Rapley, UCL & London Climate Change Partnership*
• Dr Matt Palmer, Met Office*
• Dr Richard Wood, Met Office*
• Dr Tom Howard, Met Office*
• Daniel Johns, Committee on Climate Change
• Emma Beckles, DEFRA*
• Nick Hardiman, Environment Agency*
• Katy Francis, Environment Agency*
• Andrew Eden, Environment Agency
• Bill Parker, Suffolk Coastal District Council*
• Jeremy Pickles, East Riding of Yorkshire Council*
• Richard Jackson, East Riding of Yorkshire Council*
• Neil McLachlan, East Riding of Yorkshire Council*
• Lisa Marshall, Gwynedd Council*
• Tim Reeder, London Climate Change Partnership*
• Jim Hutchison, Balfour Beatty*
• Kevin Burgess, CH2M
• Christina DeConcini, World Resources Institute
• Dr Stephen Cornelius, WWF-UK*
• Phil Dyke, National Trust
• Tim Collins, Natural England
• Matt Cullen, Association of British Insurers (ABI)