Although sea-level changed dramatically when we came out of the Ice Age and in the following millennia, historical sea-level change is a relatively recent thing. From the time of the Romans until about 200 years ago there was very little change in sea level. But then something happened, we know not what, that caused the seas to start rising. It increased at a steady level for about a hundred years and then around about a hundred years ago the rate of change increased. This can be seen in Fig 1 that was for many years part of the Wikipedia page on recent sea-level change, until someone considered it inappropriate and replaced it with something less clear.
Since mankind’s influence on the climate became apparent, according to most sources around the mid-1950s, it is important to see if there is any change in sea-level rise at that time or subsequently. Based on Tidal Gauge data it seems there is not.
Tidal Gauge data has its limitations. It is only available at the edges of the oceans, it is long-term data and has to be corrected for vertical land movement, of which there is plenty in many parts of the world as a result of surface relaxation after the ice overburden was lifted. Satellite data – which obtains sea level by bouncing microwaves off the ocean surface – was provided firstly by TOPEX/Poseidon, then Jason-1 and later OSTM/Jason-2. It should have some advantages, but it has not been entirely straightforward to calibrate it and to compare it to tidal data.
A new paper by Watson et al (2015) published in Nature Climate Change called “Unabated global mean sea-level rise over the satellite altimeter era,” presents data showing that the sea level rise started a century ago has continued. No one would argue with that.
The paper looks at the calibration of the sensors onboard the three sea-level measuring satellites that have been used and concludes that because of sensor degradation, especially for TOPEX/Poseidon, means that its observations need correcting. They find that applying this “bias drift” correction has two consequences. It reduces the rate of sea-level increase seen since 1993 from 3.3 +/- 0.4 mm per year, to around 2.6 – 2.9 (+/- 0.04) mm per year. The second consequence, and this is the one that has produced the dramatic headlines, is that in contrast to the previously reported slowing in the rate of sea-level rise in previous decades, there is in fact an acceleration.
Personally I have never been very enthusiastic about claims of changes in the rate of sea-level rise. Too much noise combined with timespans that are clearly too short to be meaningful have always dissuaded me. But it is interesting to look at the figures, before and after the correction applied by Watson et al (2015).
When Zero Is Not Zero
The previously reported deceleration was – 0.057 +/- 0.058 mm per year squared. The new figure, now an acceleration is +0.041 +/- 0.058 mm per year squared. Even the researchers themselves state the obvious fact about these numbers; they say, “Neither of these figures is significantly different from zero.” You would think that would be the end of the matter as both results are statistically unimportant being not statistically different from zero. But you would be wrong.
The researchers take the view that there is a difference between the before and after results because the initial figure is smaller than zero (but not statistically different from it) and the corrected figure is larger than zero (but still not statistically different from it) so that means there has been an acceleration! They say “…the revised estimate is significantly different from the earlier estimate derived from data unadjusted for the effects of bias drift.” To reach such an important conclusion based on such statistics is not supportable, in my opinion.
Not that the media noticed such scientific details such as the flimsy base for claiming an acceleration in sea-level rise. The vast majority headlined it without any qualification. It is a clear case of widespread poor science reporting. No financial journalist would not look at the figures behind a headline. Very few covering science ever do, and many would not even think to ask! It is yet another mismatch between what many in the media report, and what is in the peer-reviewed literature.
One of the reasons why sea-level rise is important is because of the effect it might have on low-lying islands, especially those in the Pacific. A new study however suggests that such islands can cope with some measure of sea-level rise. Funafuti atol has been subject to 30 cm of sea-level rise over the past 60 years, and it has increased in size!
Funafuti atol: gets larger as sea-level rises
The paper’s abstract reads: “The geological stability and existence of low-lying atoll nations is threatened by sea-level rise and climate change. Funafuti Atoll, in the tropical Pacific Ocean, has experienced some of the highest rates of sea-level rise (∼5.1 ± 0.7 mm/yr), totalling ∼0.30 ± 0.04 m over the past 60 yr. We analyzed six time slices of shoreline position over the past 118 yr at 29 islands of Funafuti Atoll to determine their physical response to recent sea-level rise. Despite the magnitude of this rise, no islands have been lost, the majority have enlarged, and there has been a 7.3% increase in net island area over the past century (A.D. 1897–2013). There is no evidence of heightened erosion over the past half-century as sea-level rise accelerated. Reef islands in Funafuti continually adjust their size, shape, and position in response to variations in boundary conditions, including storms, sediment supply, as well as sea level. Results suggest a more optimistic prognosis for the habitability of atoll nations and demonstrate the importance of resolving recent rates and styles of island change to inform adaptation strategies.”