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. Fluctuations in sea levels followed until approximately 3,000 years BP.
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)
UK climate change for sea level rise projections at four UK capital cities by 2100 relative to 1981-2000 have been published. They show a low emission scenario and high emission scenario. UK sea levels have already risen by 16 cm since the start of the 20th century. In London the low emission scenario is for a rise of between 70cm to 29cm. The high emission scenario is for 53cm to 115cm. This rise is likely to be higher in the south of the country in areas like the Solent where the land is subsiding.
Coastal flooding is seen as one of the most serious threats to major cities and coastal populations around the UK, as well as worldwide. Increases in future extreme sea levels and flooding will be driven by mean sea-level changes, rather than changes in storm surges
Sea Level Rise Data for Portsmouth. Source: PSMSL