GMSL (Global Mean Sea Level)

Global mean sea level is the area-weighted average height of the ocean surface, the single number that summarizes ocean response to warming. The satellite-altimetry record shows it rising about 3.4 mm per year and accelerating, splitting into a thermal-expansion (steric) part and a mass part from melting land ice measured by GRACE-FO. The IPCC AR6 projected roughly 0.4 to 0.8 m of rise by 2100 across emission scenarios, with high-end ice-sheet instability adding tail risk.