February 9, 2010

GPS Aids in Sea Level Rise Debate

The Technical Summary of the most recent IPCC reports states that “Over the 1961 to 2003 period, the average rate of global mean sea level rise is estimated from tide gauge data to be 1.8 ± 0.5 mm yr–1.” “The average thermal expansion contribution to sea level rise for this period was 0.42 ± 0.12 mm yr–1, with significant decadal variations, while the contribution from glaciers, ice caps and ice sheets is estimated to have been 0.7 ± 0.5 mm yr–1. The sum of these estimated climate-related contributions for about the past four decades thus amounts to 1.1 ± 0.5 mm yr–1, which is less than the best estimate from the tide gauge observations. Therefore, the sea level budget for 1961 to 2003 has not been closed satisfactorily.”

That is indeed very interesting – the average rate of sea level is around 1.8 mm per year, and the IPCC can account for only 60% of the increase. This uncertainty is compound considering IPCC’s statements that “The global average rate of sea level rise measured by TOPEX/Poseidon satellite altimetry during 1993 to 2003 is 3.1 ± 0.7 mm yr–1. This observed rate for the recent period is close to the estimated total of 2.8 ± 0.7 mm yr–1 for the climate-related contributions due to thermal expansion (1.6 ± 0.5 mm yr–1) and changes in land ice (1.2 ± 0.4 mm yr–1). Hence, the understanding of the budget has improved significantly for this recent period, with the climate contributions constituting the main factors in the sea level budget (which is closed to within known errors). Whether the faster rate for 1993 to 2003 compared to 1961 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear”.

As you might guess, there is much to be done to improve our understanding of sea level rise.

A team of scientists from France, Spain, and the United Kingdom have a very interesting piece in a recent issue of Geophysical Research Letters; the work was funded by Région Poitou-Charentes, which provided a PhD fellowship for C. Letetrel, and by CNES which provided additional support via its TOSCA program”. In case you don’t know, CNES is Centre National d’Études Spatiales which is the French Space agency; there appears to not be any support from any group with an interest in denying sea level rise.

Wöppelmann et al. begin their article noting “Estimates of global-scale sea level rise over the past century are mainly based on long tide gauge records. The range of estimates published in the literature is rather wide (1 to 3 mm/yr), with figures converging towards 1.8 mm/yr”. They point out that “the origin of the differences lies in the methods used to correct the tide gauge records for vertical displacements of the land upon which they are located”. They muddy the waters stating “While most analyses have included corrections for Glacial-Isostatic Adjustment (GIA) effects, many other land motion processes, for example, associated with plate tectonics, volcanism, sediment compaction, or underground fluid extraction, have not been accounted for, except by excluding the possibly affected tide gauge records from consideration in the studies.” Measuring sea level changes is more difficult than one might think, and uncertainties abound!

Wöppelmann et al. use GPS technology (like the GPS in your car, boat, iPhone, etc) to estimate vertical movements of the land in the vicinity of tide gauges. Their current analysis builds from a technique they have been developing for the past several years. A lot of the discussion is highly technical and a bit difficult to follow, but we take their word for it all as they state “GPS analyses have thus reached the maturity to provide useful information for separating land motion from sea level changes recorded by tide gauges, in particular the most underrated and difficult to model effects that are sediment compaction and land subsidence associated with coastal reclamation, development and withdrawal of underground water. Such effects are very site specific, but are sufficiently frequently associated with harbours and tide gauge sites to raise serious concerns on the validity of global averages obtained from uncorrected secular trends.”

A figure in their article (Figure 1) is especially interesting to us at World Climate Report. The figure shows how tide gauges alone can produce amazingly different estimates of sea level changes. The GIA (Glacial-Isostatic Adjustment) data look more realistic, but still show remarkable differences in the estimated rate of sea level rise. The GPS-corrected data look the best to us, and if you are wondering, Wöppelmann et al. note that when compared to the GIA-corrected data, the GPS-corrected data are better “both on the global and the regional scale, leading to a reconciled global rate of geocentric sea level rise of 1.61 ± 0.19 mm/yr over the past century in good agreement with the most recent estimates”.


Figure 1. Time series of annual mean sea-level values from: (left) tide gauges; (middle) tide gauges corrected for GIA (Glacial-Isostatic Adjustment) model predictions; and (right) GPS-corrected tide gauge records; in (top) Northern Europe and (bottom) North West America. The time series are displayed with arbitrary offsets for presentation purposes (units are in mm) (from Wöppelmann et al., 2009).

Here are three final thoughts from reviewing this research. First of all, the IPCC likes to use the 1.8 mm per year for sea level rise, but Wöppelmann et al. lead us to a value of 1.6 mm per year. Ten percent of the sea level rise just disappeared. Second, as we look a the GPS-corrected data in the figure, we see no evidence of any acceleration in sea level rise that might support the TOPEX/Poseidon satellite estimate of 3.1 mm per year since 1993 (a rate that is actually in decline). And third, the earth warmed from 1910 to 1940, cooled from 1940 to 1975, warmed from 1975 to 2000, and neither warmed or cooled from 2000 to present. The sea level rose rather monotonically throughout all four periods?

We’ll leave that observation for another day!

Reference:

Wöppelmann, G., C. Letetrel, A. Santamaria, M.-N. Bouin, X. Collilieux, Z. Altamimi, S. D. P. Williams, and B. Martin Miguez. 2009. Rates of sea-level change over the past century in a geocentric reference frame, Geophysical Research Letters, 36, L12607, doi:10.1029/2009GL038720.




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