How do you measure the level of the sea at the mm precision? It is highly doubtful we are able to do that with any kind of instrumentation, more so around the whole world at the same time.
EDIT: they say they measure everything with satellites, but that does not mean there is no incertitude in the measurement. No measurement is 100% accurate, and I'd certainly like to see the error range of satellites, when even GPS can't do better than +/- 30 cm precision.
EDIT: they say they measure everything with satellites, but that does not mean there is no incertitude in the measurement. No measurement is 100% accurate, and I'd certainly like to see the error range of satellites, when even GPS can't do better than +/- 30 cm precision.