Prof Ole Humlum sea level – en superforsker eller en bedrager...

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 over time. However, it is still not a precise tool for estimating changes in global sea level due to a number of assumptions made when interpreting the original satellite data. See also note below on using ground true data to correct 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 rhythmic waxing and waning of the large Quaternary ice sheets in North America and North Europe. This enormous mass transfer causes rhythmic 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 altimetry 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 assumed. 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, or more.

On a global basis, glacial isostatic adjustment (GIA) is one of the most modelled of the geophysical signal present in tide gauge data. The plot to the right shows Prof. Richard Peltier's prediction for tide gauge rates from the one degree resolution, ICE-5G (VM2) solutions given below. This map shows a number of features of GIA predictions for relative sea level (RSL; the sea level as measured by tide gauges) rates, and the colour bar was chosen to highlight far field features.

Data from tide gauges all over the world suggest an average global sea-level rise of 1–1.5 mm/year, while the satellite record suggests a rise of about 3.2 mm/year, or more. The noticeable difference in rate (a ratio of at least 1:2) between the two data sets still has no broadly accepted explanation.

Jørgen Petersen skrev:
Det er da yderst relevant at påpege, at havniveaustigningerne i følge kystmålingerne kun er på det halve af hvad satellit-målingerne viser, og at der fortsat ikke er nogen brugbar forklaring på denne forskel.

Jørgen Petersen skrev:
Det er ikke havniveauet i sig selv, som er interessant. Det er derimod ændringerne (stigningerne) i havniveauet, som er interessant.

Jørgen Petersen skrev:
Jeg tror i hvert tilfælde ikke på, at havniveauet ude på de store oceaner årti efter årti stiger ca. dobbelt så meget som det gør inde ved kysterne.

Jørgen Petersen skrev:
Skal der ske sådanne forskydninger, så skal der samtidigt ske andre forskydninger. Det kunne f.eks. være ændringer i højtrykkene og lavtrykkenes placering. Det vil så betyde, at lavtrykkene så skulle placere sig mere eller mindre permanent ude over oceanerne.