There is a news article being circulated with a claim that NASA has established that global sea levels have risen by three inches (8cm) since 1992. Other headlines shout fears of catastrophic rises inundating cities and coastal areas. A look at the NASA website will give you graphs showing a consistent and disturbing upward trend and if that was all you considered then it’s enough to have us all running around in circles crying that the sky is falling.
But it is not quite as simple as that. The earth itself is not a nice fixed stable platform. In fact the whole geology of the crust moves up and down once or twice a day depending on whether the earth tide influences of the sun and moon are diurnal or semi-durnal. It has been reported that at the equator the earth can move up and down by as much as 55cm. We don’t notice it at all because the whole mass is moving as a block but there is some flexing continent wide as that happens.
Add into that equation the rotation in the vertical plane of the continents, or crustal subduction and uplifting. In the same week as the NASA article there was an article from the Geological Society of America about the rate of uplifting of the Pacific seaboard of the United States. Another article in the NY Times stated that the flooding problems in Maine are due in part to the sinking of the eastern seaboard. For the U.K. studies show that Scotland is rising and England is sinking, which shows that whole continents are in vertical motion of some degree or other.
Granted, this geological movement is a hundred times smaller than the reported increases in tidal level, however, the rates are localised and global averages and trends should not be applied in the local environment. The problem with measurement of the sea levels is that everything is moving together. The onshore benchmarks rise and fall with the earth tide and change elevations with the geological tilting. The GPS satellites, that are so heavily relied upon for elevation measurements, also are affected by the same forces that move the tides and the earth.
One thing that is reasonably well fixed is the tidal constituents that were derived after Laplace advanced his theory of tidal movement in 1775. This theory and the associated calculations were refined in the 1860s when William Thomson applied Fourier analysis to the tidal motions as harmonic analysis. Since then the calculations of tidal heights have been quite accurate and we can, even today, rely on tidal height calculations performed a hundred year ago.
So where does that leave with respect to climate change and increasing sea levels? In a position of justified scepticism, or at least a refusal to be driven by emotive news releases that we often see surrounding this subject. The reality is that global or continental averages should not be assumed for local situations.
When a local authority is discussing things like coastal works they should be sure they are relying of information relevant to the locality. Measurement taken in relation to local geodetic benchmarks and confirmed by comparison with those century old calculations that are still proving reliable today.
That is, your local tide gauges.
(Image “Tide Board Bidford” courtesy of acclaimed professional photographer Simon Warner. www.simonwarner.co.uk)