This is part 7 of a multi-part series about “Testing the robustness of semi-empirical sea level projections,” Rahmstorf, et. al., Climate Dynamics, 2011. You can see an index of all parts here. I frequently refer to this paper as R2011.
Let’s talk a little more about the irony of using the Jevrejeva’s 2008 sea level data, which I will refer to as JE08, to confirm Rahmstorf’s sea level projections for the 21st century.
As I have already explained, Rahmstorf claims in his 2011 paper (which I will refer to as R2011), that his model is “robust,” meaning that variations of historical 20th century input sea level data yield essentially the same sea level rise projections for the 21st century. R2011 graphically presents seven sources of sea level data (while ignoring others) and implies their similarity by overlaying the same quadratic fit for all of them. R2011 leads us to believe that the model is robust with, specifically, the input of these various sea level data sets.
R2011 presents the results of the model using only three of the seven sea level rise inputs. Two of the three are by the same authors, Church and White, who clearly believe their later version of the sea level data (CW11) is an improvement over their earlier version (CW06). Then, R2011 cynically rejects the model results from Church’s and White’s better set of data because those results testify against R2011′s desired conclusion of extremely high sea level rises for the 21st century.
Which brings us to Jevrejeva
The third data set that R2011 used is Jevrejeva’s. So after all the blathering about the “robustness” of their model under a broad variety of inputs, R2011 is left with just two sea level data sets that they are satisfied with: Church’s and White’s earlier data set, CW06; and Jevrejeva’s 2008 data, JE08. Figure 1, below shows R2011′s figures 1 and 9, with my annotation.
Keep in mind that R2011′s objective in their claim of robustness was to prove that their earlier results , based on the CW06 were realistic. So, in effect, after all the hand waving JE08 is the only one of the seven sea level data sources that fulfills that purpose. That is why we are taking a little closer look at JE08.
Let’s start by looking at an overlay of JE08, CW06 and CW11 in figure 2. If Rahmstorf’s model were “robust,” as R2011 claims, then all three of these data sets as input to the model should yield very similar sea level rise projections for the 21st century. But one of them yields much lower results than the other two. The amazing thing is that the outlier is CW11, which is nearly a twin to CW06, at least compared to JE08. How can that be?
Let’s suspend our higher cognitive functions for the moment and agree with R2011′s reasoning. That is, we will agree that the sea level rise projections for the 21st century based on CW11 input data must be rejected because they are much lower than the projections based on CW06 input data. Inversely, we will agree that sea level rise projections for the 20th century based on JE08 input data must be accepted because they give high 21st century projections, just like the projections based on CW06 input data.
A closer look at JE08 sea level data
Since we have decided to mindlessly accept the usefulness of JE08 to back up Rahmstorf’s high sea level rise projections for the 21st century, then we should also accept some other interesting features of JE08. So let’s take a closer look.
JE08 says their version of sea level data was in “good agreement with estimates of sea level rise during the period 1993–2003 from TOPEX/Poseidon satellite altimeter measurements.” Figure 3, below, shows an overlay JE08 and the satellite altimeter data,…
It is quite striking that according to JE08 and the satellite data that the sea level rise rate for the middle third of the 20th century (1933 to 1966) is exactly the same as the sea level rise rate at the end of the 20th century and beginning of the 21st century. How can this possibly be!? How can this data that indicates no increase in the sea level rise rate for 80 years cause tremendous increases in the sea level rise rate for the 21st century when used as input to Rahmstorf’s model?
Stefan the Dart Thrower
Consider Stefan Rahmstorf the Dart Thrower. He holds forth at the pub as the best thrower in the kingdom. He brags about his precision, claiming “I can hit high numbers every time! My talent is robust!” Challenged by another annoyed pub patron to “put up or shut up,” Stefan grabs a handful of darts and goes to work. He throws seven, but only three hit the board. Two are on high numbers and one is on a low number, the rest are stuck in the wall. “See!” he says triumphantly, pointing at the two darts on the high numbers.
The other patron points out the projectiles stuck in the wall. “Bad darts” Stefan replies.
“What about this dart on the low number - it is identical to one of the darts on a high number” the incredulous patron points out. “Same length, same material, same weight, same manufacturer.”
“Obviously a bad dart, nevertheless” sniffs Stefan. “If if were a good dart it would have landed on a high number.”
 Jevrejeva, S., et. al. “Recent global sea level acceleration started over 200 years ago? ,” Geophys. Res. Lett., 35, 2008
 Rahmstorf, S., et. al., “Testing the robustness of semi-empirical sea level projections” Climate Dynamics, 2011
 Church, J. A., and N. J. White, “A 20th century acceleration in global sea-level rise“, Geophys. Res. Lett., 33, 2006
 Church, J. A. and N.J. White, “Sea-level rise from the late 19th to the early 21st Century“, Surveys in Geophysics, 2011