Yesterday I learned from the vrt news site that sea level rise is accelerating. The last two weeks I was absorbed by a project, so it took me a bit off guard. Since I had a little bit more time now, I was curious what was really said and decided to take a closer look.
Looking at their article Sea level rise accelerated in last decades (Dutch) it became quickly clear that what was found was not an accelerating of the rise in the last decades, as the title suggests, but a different estimation of sea level rise in the the period 1900 – 1990s. It could be traced back to a study published in Nature (abstract here) by Carling Hay (a postdoctoral fellow in the Department of Earth and Planetary Sciences), Eric Morrow (a recent Ph.D. graduate of Earth and Planetary Sciences) and their colleagues at Harvard University.
Their study allegedly shows that calculations of global sea level rise from 1900 to the 1990s had been overestimated by as much as 30 percent. It was assumed that sea level rise in the period between 1900 – 1990s was 1.5 – 1.8 mm per year. But after their reassessment they came to a smaller figure of 1.2 mm per year, therefor the current rate of 3 mm per year is higher than one would expect.
That didn’t impress me by a bit. I think those scientists are seriously overstating their case and/or underestimating the uncertainties.
Sea level rise is a very complex issue. Sure, we can measure the tides and this could get us a rough idea of the rise, but there is much more to it than this. Sea level is not rising in the same way over the globe. In some parts the level is increasing, in others it is staying the same and in others it could even lower.
Our Earth is not really a perfect sphere. By the rotation it is flattened at its poles and has a bulge at the equator. Also mass is distributed unevenly within the planet and the greater a concentration of mass is, the stronger its gravitational pull, creating higher of lower sea levels. There is also the effect of subsiding land (giving the impression of a sea level rise, even if the level stays the same) or rising land (giving he impression of a sea level lowering, even when the level stays the same). There is also a compression of the deep sea beds by the increasing pressure of more water, decadal variations and so on and so on. It is not just add water and see this or that result. There are many parameters and they are interacting with each other in an ever changing world.
Now let’s look at the other side of the equation. Before the 1990s sea level was measured with tide gauges around the coast, so those records are in no way representative for what is measured today with satellites that not only measure sea level at the coasts, but also at the ocean. That means we have two types of measurements. In the past there are only the scarce tide gauge measurements along the coasts where people like to live and since the 1990s we got a high accurate measurement over the whole of the Earth. Looking at tide gauge records in coastal areas doesn’t tell us anything about what actually happened in the ocean.
The investigators now used a more “advanced” way to calculate the historical sea levels and that is where they lost me. From 1900 until the 1990s no detailed measurements were taken and there are gaps in the records. The largest part of the data is not just there and can’t be used to compare with our recent reliable measurements. Whatever (re)analysis done on it will again be at best another estimation. Maybe better, maybe worse than the previous one. We won’t be able to check it anyway.
If this acceleration was actually seen in the satellite record, I would have no problem accepting a 30% increase sea level. But this is not the case here and this makes the title in the VRT article very misleading. There wasn’t an accelerated increase in sea level rise in the last decades, because that is not what we see in the records. The issue is not an acceleration in the last decades, the issue is a re-assessment on basis of scarce and utterly incomplete data. There will be loads of uncertainties involved here, not acknowledged by the authors and certainly not by the journalists.
This remembers me about the statements of Professor Denys about anxiety being fear without an object. Here we have a situation where we have accurate measurements of sea levels in the last decades which don’t show concern, but yet here we are worrying us stiff because of calculations made about a complex system based of scarce, incomplete data that might or might not be correct.
Trust, yet verify 
I am a bit confused by reference 4 as in this from the abstract:-
“Our analysis, which combines tide gauge records with physics-based and model-derived geometries of the various contributing signals, also indicates that GMSL rose at a rate of 3.0 ± 0.7 millimetres per year between 1993 and 2010, consistent with prior estimates from tide gauge records (4).”
Click on the reference and you get
4. Church, J. A. & White, N. J. Sea level rise from the late 19th to the early 21st century. Surv. Geophys. 32, 585–602 (2011)
http://link.springer.com/article/10.1007%2Fs10712-011-9119-1
Church & White begin the abstract with:-
“We estimate the rise in global average sea level from satellite altimeter data for 1993–2009….”
The problem with the satellite data is that they have shown nearly double the rate of sea level rise as the tide gauges.
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Not really sure what is the question exactly.
If it is about the reference 4 itself: the reference is made because Church & White found a similar figure for the sea level rise between 1993 and 2009.
If it is about the difference between satellite records and tidal gauge records, that is something I also wondered about, especially since individual and long term tidal gauges don’t seem to show an increase in trend. I now think it may have to do with how measurements were taken and time frame issues that biases towards a higher trend from 1993.
First, tidal gauge data and satellite data are two different beasts altogether and it is not because they agree between 1993 and 2009/2010 that necessarily the same was true in the past. The more one goes back in time, the more sparse and incomplete the tidal gauge records get.
The satellite record started in 1993 under the influence of the eruption of Mount Pinatubo, therefor probably starting low and increasing the short term slope.
From 1993 until 2009/2010 is 16/17 years which is then compared with a 100+ years period. The long term tidal record is not a clean straight line. It has considerable annual and decadal variation with significant departures from the linear trend. Selecting 16/17 year periods in that the tidal record would show different slopes. In the long run they get averaged out, but this is not the case in the short term satellite data.
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I not very good at explaining things.
The comment in the abstract of the Nature paper is about their modelled results being consistent with tide gauges, but they reference Church and White paper which says the measurements were based on satellites.
On the impact of Mount Pinatubo I believe is small in the satellite record. The detailed satellite data is http://sealevel.colorado.edu/ which shows the 3.2mm per year average rise .
However, even this steady rise over the last 20 years is not corroborated by the tide gauges, which so a lesser sea level rise over the same period. The real test of whether sea level rise has accelerated or (as I think) it is due to the change in measurement from tide gauges to satellites, is to see if the tide gauges show acceleration in the 1990s as the Nature paper claims. I think the authors effectively splice the older tide gauge records onto the satellite data. This would show acceleration where none existed.
http://scienceandpublicpolicy.org/blog_watch/sea_level_is_not_rising.html
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As far as I know the Church & White paper compared the tide gauge record with the satellite record. This is the first sentence complete:
and they came to the conclusion that they agreed rather well:
While Hay et al said:
That is why I think this paper got referenced.
What Hay et al actually did was stating that:
Therefor the acceleration resulting in higher rates over the last 20 years (around 3 mm) is much larger compared to what was the previously estimated rise of the past (1.2 in stead of 1.5-1.8 mm). Et voilà, an acceleration was found, even when nothing of that kind was seen in the actual measurements.
And the media, they just loved it.
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