Climate4you - prof Ole Humlum

Sea-level from satellite altimetry
Satellite altimetry is a new and valuable type of measurement, providing unique insight into the detailed surface topography of the oceans, and changes of this. However, it is not a precise tool for estimating changes in global sea level due to a number of assumptions made when interpreting the original satellite data.

One of the assumptions made during the interpretation of satellite altimetry data is the amount of correction made locally and regionally for Glacial Isostatic Adjustment (GIA). GIA relate to large-scale, long-term mass transfer from the oceans to the land, in the form of rytmic waxing and waning of the large Quaternary ice sheets in North America and North Europe. This enormous mass transfer causes rytmic changes in surface load, resulting in viscoelastic mantle flow and elastic effects in the upper crust. No single technique or observational network are able to give enough information on all aspects and consequences of GIA, wherefore the assumptions adopted for the interpretation of satellite altiometry data are difficult to verify. The GIA correction introduced in the interpretation of data from satellite altimetry depends upon the type of deglaciation model (for the last glaciation) and upon the type of crust-mantle model that is asumed.As a consequence of this (and additional factors), interpretations on modern global sea level change based on satellite altimetry vary from about 1.7 mm/yr to about 3.2 mm/yr

Data from tide-gauges suggest an average global sea-level rise of 1-1.5 mm/yr, while the satellite-derived record suggest a rise of more than 3 mm/yr. The rather marked difference between the two data sets has still no broadly accepted explanation, but some of the difference is likely due to administrative changes introduced into the raw data obtained by satellites. Se the paragraph below on temporal stability of the satellite-derived data.

Another factor that may explain some of the difference between tide-gauge and satellite data is probably that while any temperature-driven volumen expansion is recorded by the satellites, this change is not affecting tide-gauges at coastal locations, as the water depth here decreases towards zero.

GMSL Rates
CU: 3.4 ± 0.4 mm/yr
AVISO: 3.3 ± 0.5 mm/yr
CSIRO: 3.3 ± 0.4 mm/yr
NASA GSFC: 3.4 ± 0.4 mm/yr
NOAA: 3.2 ± 0.4 mm/yr (w/ GIA)

We estimate the rise in global average sea level from satellite altimeter data for 1993–2009 and from coastal and island sea-level measurements from 1880 to 2009. For 1993–2009 and after correcting for glacial isostatic adjustment, the estimated rate of rise is 3.2 ± 0.4 mm year−1 from the satellite data and 2.8 ± 0.8 mm year−1 from the in situ data.

Firstly, there is a clear increase in the trend from the first to the second half of the record; the linear trend from 1880 to 1935 is 1.1 ± 0.7 mm year−1 and from 1936 to the end of the record the trend is 1.8 ± 0.3 mm year−1. The period of relatively rapid sea-level rise commencing in the 1930s ceases abruptly in about 1962 after which there is a fall in sea level of over 10 mm over 5 years. Starting in the late 1960s, sea level rises at a rate of almost 2.4 mm year−1 for 15 years from 1967 and at a rate of 2.8 ± 0.8 mm year−1 from 1993 to the end of the record. There are brief interruptions in the rise in the mid 1980s and the early 1990s.