Sea levels
Overview
Sea level rose by 1.5 mm per year during the 20th century. Climate change may affect sea levels.
How is our sea level changing? » What is causing sea level change? » What can happen when sea level rises? » Find out more
How is our sea level changing?
Sea levels around the UK are now about 10cm higher than they were in 1900. Four sites in England show average sea level increases ranging from 2.3 to 4.6mm per year since 1900 (Indicator: Sea level change - click to enlarge). Scientists predict that sea levels will rise by between 2 and 9mm per year (and could rise as much as 86cm in southern England by the 2080s) as a result of climate change.
Britain lies at the edge of the continental shelf. Its coastline geology shows us that sea level in the past was about 100 to 150m below its present level. After the last ice age, the sea rose and flooded low-lying land, forming the North Sea and English Channel. The rate of sea level rise then slowed down and has remained constant in the last several thousand years.
What is causing sea level change?
Several factors cause sea level to rise and fall. These factors act over different time scales.
- ‘Eustatic’ changes are caused by long-term variations in ocean volume. For instance when ice sheets and glaciers melt, they add water to the sea and ocean volume increases. In glacial periods, ice sheets and glaciers grow, storing large amounts of water as ice and therefore decreasing ocean volume.
- ‘Steric’ change in sea level is due to water density changing. Density varies as water temperature and salinity goes up and down.
- Storm surges, river run-off and waves reflecting in bays and harbours can cause very localised, but often quite large changes in sea level.
The land surface can move vertically as well, causing isostatic sea level change.The British Isles are still adjusting to the removal of the ice sheet which covered most of the land surface about 20,000 years ago.
THis post-glacial rebound has caused the North and West of Britain to rise, but the South East of England to sink. Isostatic changes therefore add to see level rise in the South and East and work against sea level rise in the North and West.
In the South East of England the combination of the global rise in sea level and the local sinking of the land results in sea level rises of 5.4 mm a year.
What can happen when sea level rises?
Rising sea levels:
- put low-lying land at greater risk of flooding. Around 17 million people live within 10km of the sea, and much of our manufacturing industry is based at or near the coast. More frequent or serious coastal flooding in low-lying areas will put more of these people and businesses in danger.
- increase the chances of storm surges breaking through sea defences and causing flooding;
- can change sedimentation patterns in estuaries and inlets which may mean more harbours and ports need dredging;
- can cause ‘coastal squeeze’ in areas with hard flood defences. This occurs when sea level rises against a hard barrier such as a sea defence or rocky cliff. Any areas of beach, mud or marshes will be lost;
- can cause some species that can't adapt to disappear. For example, it may be impossible for saltmarsh plant species to migrate onshore.
Environment Agency information on:
- Flood
Around 5 million people live in flood risk areas in England and Wales. We warn people about flooding and build flood defences.
Further information:
- Proudman Oceanic Laboratory (POL)
POL is a leading world centre in tidal prediction (with related interests in earth tides and storm surges) and a leading European centre in modelling and forecasting shelf sea dynamics. - Hadley Centre for Climate Prediction and Research
The Hadley Centre provides the United Kingdom with an up-to-date expert assessment of natural changes in global and regional climate.
