What it means: the average height of the sea over each year, measured at Newlyn since 1915, where the UK’s official height datum was set. Single years go up and down with winds and air pressure, so the long trend matters more. Seas rise as the oceans warm and expand and as ice on land melts; in the south of England the land is also sinking slightly. Gaps are years with too few readings. Shown for context.

Source: Permanent Service for Mean Sea Level, Newlyn (National Oceanography Centre), annual revised local reference data.