Sea-level rise: Where we stand at the start of 2013 — Part 2

This is Part 2 of my thoughts on the state of sea-level research. Here is Part 1.

Sea-level cycles?

A topic that keeps coming up in the literature is the discussion on a (roughly) 60-year cycle in sea level data; a nice recent paper on this is Chambers et al. in GRL (2012). One thing I like about this paper is its careful discussion of the sampling issue of the tide gauges, which means that variability in the tide gauges is not necessarily variability in the true global mean sea level (see Part 1 of this post). I want to add some thoughts on the interpretation of this variability. Consider this graph from my Response to Comments in Science (2007):

Fig. 1: Fifteen-year averages of the global mean temperature (blue, °C, GISS data) and rate of sea level rise (red, cm/year, Church&white data), both detrended.

This graph clearly shows a 60-year-period variation in the Church and White (2006) sea-level data, and it also shows the strong correlation of this with global temperature. (This of course is just another way of looking at the same data already shown in Fig. 3 of Part 1 of this post, and the fact that the data look like this is also the reason why the quadratic acceleration factor is small, as discussed in Part 1 and here). This correlation to me is evidence that the 60-year-variability is real (rather than a sampling artifact), because it suggests a simple physical mechanism for it: sea level tends to rise faster when it is warmer. That is the very idea behind semi-empirical sea-level models. Given the shape of the global temperature curve, semi-empirical models naturally predict this ~60-year “cycle” in global sea level.

But is this a real periodic behaviour, i.e. a climatic oscillation? I don’t think so. The variation in global temperature shown above probably is not an oscillation: it is the familiar time evolution of global temperature that can largely be explained by the history of radiative forcing. E.g., the temperature plateau from 1940-1970 (which in the detrended data shown above is a downswing) is explained by aerosol cooling balancing greenhouse warming, while the following upswing is due to the then dominant increase in greenhouse gases.

And if the sea-level history is a result of the temperature evolution, then the apparent sea-level “cycle” is also not an oscillation. At least not if you understand “oscillation” as a periodic behavior caused by a restoring force that drives the system back towards an equilibrium, which it then overshoots, so that the restoring force starts to work the other way, and so on. There is a simpler explanation: it looks like global temperature basically follows the forcing and global sea level follows the temperature, just as you would physically expect.

So my interpretation of these data is: those two 60-year periods seen in the data may look like an oscillation, but they aren’t. Which has some consequences for prediction: an oscillation would suggest a coming downturn; if the system simply follows the forcing we’d expect the opposite. In 30 years time we will know for sure!

Are process-based sea level models mature?

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  1. D.P. Chambers, M.A. Merrifield, and R.S. Nerem, "Is there a 60-year oscillation in global mean sea level?", Geophys. Res. Lett., vol. 39, pp. n/a-n/a, 2012.
  2. S. Rahmstorf, "Response to Comments on "A Semi-Empirical Approach to Projecting Future Sea-Level Rise"", Science, vol. 317, pp. 1866d-1866d, 2007.
  3. J.A. Church, and N.J. White, "A 20th century acceleration in global sea-level rise", Geophys. Res. Lett., vol. 33, pp. n/a-n/a, 2006.