Responding to the CO2 Coalition's "Fact #12" on Sea Level Rise

CO2 Coalition's "Fact #12" asserts that global sea levels rose before CO2 levels started to increase. They say, "The evidence shows that the global warming causing the rise in sea levels and the retreat of the glaciers began long before any significant man-made CO2 increases could have influenced either. ... That glacial 'tipping point' occurred around 1800, with full-on retreat by 1850. Thus began more than 150 years of worldwide glacial retreat and sea-level rise that continues at about the same rate today as 150 years ago." They cite Jevrejeva et al 2014[1] in support of this, and they produced their own version of a graph supposedly taken from that paper, complete with a picture of a rock hammer. Why not?

So of course I tracked own Jevrejeva et al 2014 to see if their claims are backed up by the paper they chose to use to support them. Here's the graph from the paper below. As you can see, the paper goes back to 1700, and the 1-sigma confidence intervals are pretty large up through about 1850. CO2 Coalition ignored all the data from 1700-1800 as well as the confidence intervals. If you include them, though, the perceived "tipping point" that CO2 Coalition imagines in 1800 disappears; in fact, I'm not at all confident that they plotted this graph correctly. There's no hint of any "tipping point" in the graph or in the text of the paper.

Now there is a significant amount of variability in rates of sea level rise, but the overall upward slope to the trend in SLR rates shows that there indeed acceleration, despite CO2 Coalition's claim that "sea-level rise that continues at about the same rate today as 150 years ago." The paper says, "Results from the analysis of a 300 year long global sea level using two different methods provide evidence that global sea level acceleration up to the present has been about 0.01 mm/yr^2" beginning near the end of the late 18th century, though the also state that "a significant increase does not occur until much later in the 19th century." This may well have been due to natural variability early on, but the paper understands that this does not continue to be the case. In fact, their findings suggest that the IPCC may well be underestimating the amount of SLR that will occur by 2100. Their words:

We show that sea level rose by 28 cm during 1700–2000; simple extrapolation leads to a 34 cm rise between 1990 and 2090. The lowest temperature rise (1.8°C) IPCC... use is for the B1 scenario, which is 3 times larger than the increase in temperature observed during the 20th century. The IPCC sea level projection for the B1 scenario is 0.18–0.38 m. Our simple extrapolation gives 0.34 m. The mean sea level rise for B1, B2 and A1T is below our estimate. However, oceanic thermal inertia and rising Greenland melt rates imply that even if projected temperatures rise more slowly than the IPCC scenarios suggest, sea level will very likely rise faster than the IPCC projections.

So the paper they cite in favor of their views is actually saying that the IPCC's projections are very likely too conservative. Apparently neither this paper nor the IPCC share CO2 Coalition's confusion over causation. Just because there was some warming prior to 1850 doesn't mean that recent warming can't be driven by increases in CO2 concentrations from human carbon emissions. But I will say this in the favor of the CO2 Coalition's favor. At least they cited the paper that proves them wrong.

For those interested, I have a longer post summarizing major studies evaluating acceleration of sea level rise that will support Jevrejeva finding of a 0.01 mm/yr^2 figure for acceleration.

References:

[1] Jevrejeva et al (2008) Recent global sea level acceleration started over 200 years ago? Geophys. Res. Lett., 35, L08715, doi:10.1029/2008GL033611