I’ve always wondered how sea level is measured so accurately. Global sea level changes are measured to high accuracy. The global yearly sea rise averaged over many years is measured to a fraction of a mm. There are two ways sea level is measured. The old system uses tidal gauges, and satellites were launched in 1992 (TOPEX/Poseidon) and 2005 (Jason-1).
Here’s a page explaining how the tidal gauges work: Univ. of Colorado. Ah, stilling well!