If you're going to link to nothing but a chart and say that it's of any worth whatsoever, it should at least have all its units specified. What are those anonymous numbers on the left supposed to mean?
Judging by the graphics, the numbers on the left most likely represent the net change in the number of satellites in orbit, as measured every six months by NASA. Notice that this chart appears to begin in 1993. This is because the Mt. Pinatubo eruption of June 1991 perturbed the atmosphere with its stratospheric aerosols, thus making it impossible to properly track the satellites until December of 1992. It is still unknown what caused the Great Satellite Decline of 1994, but it is quite possible that Al Gore was involved.
On a serious note, the graphic is from As Sea Level Rises, Beaches Shrink
, an article from '03. The article really doesn't explain the pretty graphics very well, but they were probably attempting to refer to sea level change in mm, with 0 as some base of what they think it should've been at some point in time. If you're interested in the newer, more accurate data on this, check out GRACE
or get your own GRACE TELLUS data