Sea levels are a combination of tidal level, surge level, mean sea level and waves and their interaction. Any change in mean sea level affects sea level directly but also modifies tide, surge and wave propagation and dissipation by changing the water depth. Local sea level is affected by ocean circulation and by geographical variations in the temperature and/or salinity of the water column. These regional influences are also likely to change under global warming. Local changes in sea level are thus a combination of global mean changes and changes in the patterns of sea level relative to the global mean. Rising sea levels during the Holocene transgression from 15,000 to 5,000 years BP caused the Solent River valley to become drowned.
In 2019, the Met Office and the Environment Agency published 'Exploratory sea level projections for the UK to 2300.' Key findings include:
0.6 - 2.2m (low emissions scenario)
0.9 - 2.6m (medium–low emissions scenario)
1.7- 4.5m (high emissions scenario)
There is limited consensus on how waves will be affected by climate change. The research indicates there may be a reduction in average offshore wave height, but extreme offshore wave heights may increase. The sea level rise element of climate change is expected to be a greater threat to coastal defences than changes in offshore waves. Higher sea levels will cause waves to carry greater energy to the shore, which will have an impact on sea defences. Nearshore waves will be higher and break later, increasing flood water volumes in areas already affected by coastal flooding. This will have implications for the expected lifetime and continued performance of coastal defences, likely requiring greater investment in flood and coastal erosion risk management to maintain current defence lines and standards of protection. Coastal flooding is seen as one of the most serious threats to major cities and coastal populations around the UK. Increases in future extreme sea levels and flooding will be driven by mean sea-level changes, rather than changes in storm surges.
Data from the National Oceanography Centre shows that sea level rise has accelerated in the UK over recent years. The rate of sea level rise has been 1.5mm/yr from the start of the 20th century, however, for the period 1993-2019 it has increased to over 3mm/yr.
NASA’s Sea Level Change Team has created a sea level projection tool that makes extensive data on future sea level rise from the Intergovernmental Panel on Climate Change (IPCC) easily accessible to the public – and to everyone with a stake in planning for the changes to come.
National Oceanography Centre data shows that the average sea-surface temperature in 2020 for near-coast waters around the UK was 11.9°C, 0.5°C above the 1981-2010 long-term average. This was the eighth-warmest year for UK near-coast sea-surface temperatures in records dating back to 1870. In the most recent decade (2011-2020) sea-surface temperatures have been 0.7°C warmer than the 1961–1990 average, and nine of the ten warmest years in near-coast sea-surface temperatures have occurred since 2002.
Sea Level Rise Data for Portsmouth. Source: PSMSL