September 14, 2007

Sea-Level Slowdown?

We have heard a million times that if we don’t stop emitting greenhouse gases, our inexcusable actions will result in a warmer earth, and the warming of the planet will cause icecaps and mountain glaciers to melt and sea level to rise. Island nations will be drowned, coastlines around the world will go underwater, Florida will cease to exist, and the World Trade Center Memorial could someday be a sight seen only by scuba enthusiasts. The United Nations Intergovernmental Panel on Climate Change says very clearly in the Summary for Policymakers “Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear. There is high confidence that the rate of observed sea level rise increased from the 19th to the 20th century.” The IPCC crew reminds us that “Global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice.” It seems that sea levels fell and rose many times in the past and long before humans had any chance interfere with the natural order of things. There is no reason whatsoever to expect sea level to remain constant – it never has and it never will.

We have covered this sea-level rise issue many times in the past at World Climate Report and we fully agree that sea level is rising – sea level has been somewhat steadily rising for the past 10,000 years. During the last glacial advance, a large amount of fresh water was tied up in ice, and as the glaciation ended, that water returned to the oceans. Furthermore, as the earth warmed up following the last glacial advance, thermal expansion of the ocean water occurred, and sea level rose even more. There is little doubt that the sea-level rise will continue into the future, but the rate of rise is the focus of an interesting paper published recently in Global and Planetary Change by a team of scientists from France and Spain.

Measuring sea-level rise would seem simple at first glance, but the measurement is made quite difficult largely due to vertical movements of the earth’s crust. During an ice age, an enormous volume of ice pushes down on the crust and when the ice melts, the crust rebounds. Scientists have attempted to account for this effect using numerical “Glacial-Isostatic Adjustment” routines in their estimates of true sea-level rise. However, changes in tectonic activity, wind and ocean currents, and gravitational patterns further complicate the measurement of true sea level.

Wöppelmann et al. note that “two important problems arise when using tide gauges to estimate the rate of global sea-level rise. The first is the fact that tide gauges measure sea level relative to a point attached to the land which can move vertically at rates comparable to the long-term sea-level signal. The second problem is the spatial distribution of the tide gauges, in particular those with long records, which are restricted to the coastlines”. The records that we do have contain any number of inhomogeneities related to observer and instrument changes – the IPCC estimates of sea-level rise may be even less accurate that we are led to believe.

Chances are that you now have a hand-held GPS unit for hiking, a GPS upgrade to your cell phone, or a GPS unit in your car or boat. GPS satellites are taking measurements of anything and everything, and data from advanced GPS networks can help resolve questions about sea-level rise. Noting this opportunity, Wöppelmann et al. analyzed data from 224 GPS stations with special interest on the 160 GPS stations located within 15 km of a tide gauge station (see below). The data allowed them to very accurately measure the vertical motion of the crust from January 1999 to August 2005, and although the 7.7 year time span would seem rather short, they effectively argue that vertical motion of the crust is not like the weather – the vertical motion remains the same over long periods of time.


Distribution of the GPS stations processed (up to 224, among which 160 are situated less than 15 km from a tide gauge (stars) and 92 (black dots) are included for the reference frame implementation (from Wöppelmann et al., 2007)

OK – here’s the bottom line. When Wöppelmann et al. factored their measurements of land motion into the estimate of sea-level rise, they determined a global value of 1.31 ±0.30 mm per year compared to the 1.8±0.5 mm per year value given by the IPCC for the recent half century. We understand that the IPCC acknowledges a low-end value of 1.3 mm per year in their estimate, but another way to look at this article is that Wöppelmann et al. just reduced observed sea-level rise by 27%! Perhaps the IPCC should reconsider whether they still have high confidence that the rate of sea level rise has in fact increased from the 19th to the 20th century.

Of course, these results gained absolutely no press coverage whatsoever – imagine the coverage they would have received had their results increased sea-level rise by 27% and suggested that sea level rise was occurring faster then previous research indicated!

Reference:

Wöppelmann, G., B. Martin Miguez, M.-N. Bouin, and Z. Altamimi. 2007. Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide. Global and Planetary Change, 57, 396–406.




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