Mean rise in sea level is only part of the story

Image of the month - March 2001

Sea level trend in the Mediterranean for January 1993 to July 2000. (Credits Legos).

All the data now in our possession indicate that sea level is rising across the globe. But the global mean sea level conceals many local disparities. For example, sea level in the Eastern Mediterranean basin has risen significantly in recent years, apparently due to warmer water temperatures (observed by in-situ measurements). But if we look at the Ionian Sea off the tip of Italy, data acquired by Topex/Poseidon in its first seven years of operation show that sea level in fact fell.

Topex/Poseidon has supplied scientists with a long-term series of data of exceptional quality for the study of global sea level variations. However, this signal is very weak (about 1-2 mm/year) and we must take great care in interpreting results and monitoring instrument drifts. We also need to acquire altimetry data covering several decades to minimize measurement uncertainties. One of the goals of the Jason-1 satellite is to observe sea level variations with millimeter accuracy.