To understand what determines the x-intercept (i.e., "base year" or "zero crossing") of the global mean sea level (GMSL) time series, we need to review how the estimation is computed. Each point in the time series plots is the area-weighted mean of all of the sea surface height anomalies (SSHA) measured by the altimeter in a single, 10-day satellite track repeat cycle.  Sea surface height anomalies are the differences of the individual, altimeter-measured sea surface heights (SSH) from a modeled mean sea surface (MSS).

We do not account for sedimentation in the GMSL estimate because it is estimated to be a very small factor.  Unlike the glacial isostatic adjustment (GIA) correction, which is 0.3 mm/yr, the estimated effect of sedimentation on global mean sea level is an order of magnitude smaller at 0.02 mm/yr (Gornitz and Lebedeff, 1987; Milliman and Meade, 1983; Holeman, 1968).

The principal tectonic processes (ridge building, subduction, etc.) responsible for changing the ocean basins are measured in millions of years and are so slow that short-term global satellite records do not consider them. Glacial isostatic adjustment is comparatively a much shorter-term process (although still measured in thousands of years), and it does have a minor effect on ocean basin size.

The time tag is simply YYYY.XXX, where YYYY is the year and XXX is the fractional part of a 365-day year (to be added to January 1, YYYY). This format allows the time series to be plotted easily without date formatting.

We update the sea level data approximately bimonthly (every two months). The altimeter data are released by NASA/CNES as a 10-day group of files corresponding to the satellite track repeat cycle (10 days). There is also a two-month delay between the time the data are collected on the satellite to their final product generation (known as a final geophysical data record (GDR)). We use these final GDR products in the global mean sea level estimates. We are planning to shorten the time between our global mean sea level updates.