Is Sea-Level Rise Accelerating?

A few months ago a paper by Jim Houston and Bob Dean in the Journal of Coastal Research (JCR) cast doubt on whether global sea level rise has accelerated over the past century or so. As things go these days, ‘climate sceptics’ websites immediately heralded this as a “bombshell”. A rebuttal by myself and Martin Vermeer has now been published in JCR.

The keystone of the argument by Houston & Dean is the fact that a prominent global sea level reconstruction (Church & White 2006) shows no acceleration since 1930. Which raises the question: why 1930, given the sea level data set starts in 1870? The reason becomes immediately evident when looking at the acceleration starting from any arbitrary date (Fig. 1).

Figure 1. Acceleration of sea-level rise (i.e., twice the quadratic coefficient) from different starting years up to 2001 in the global tide gauge data set of Church and White (2006; red line with uncertainty band). Note that after ~1960 the calculation gets excessively ‘noisy’ because the time interval gets too short to robustly compute acceleration. I graphed this right away after reading the Houston & Dean paper, and a few days later Tamino independently came up with a similar plot – it’s the obvious thing to do. The blue line shows the same quantity from the sea-level hindcast of Vermeer & Rahmstorf (2009) computed from global temperature data.

Around 1930 we see a unique minimum in the acceleration curve – I will explain the cause of this shortly. Other start dates either before or after this minimum show positive acceleration. Picking 1930 for this analysis is thus a classic cherry-pick, and according to the authors that is no accident. They write in the paper: ‘Since the worldwide data of Church and White (2006)…appear to have a linear rise since around 1930, we analyzed the period 1930 to 2010.’ The interval was thus hand-picked to show a linear rise rather than acceleration.

Connection to temperature

Houston & Dean use their result to question the future acceleration of sea level rise predicted by Vermeer & Rahmstorf (2009) for the 21st Century as a consequence of global warming. They argue that the 1930s acceleration minimum calls into question the semi-empirical link between global temperature and global sea level proposed by us in that paper. However, it is clear they never bothered to check this, because quite the opposite is the case: our semi-empirical formula predicts this acceleration minimum, as the graph above shows. As it turns out, this is an expected outcome of the mid-20th-Century plateau in global temperature.

If one subtracts out the non-climatic sea level change due to water stored in artificial reservoirs on land, as we did in Vermeer & Rahmstorf (2009), then the agreement between the acceleration curve predicted from global temperature with the actually observed curve is even better (graph below). Thus, the 1930s acceleration minimum pointed out by Houston & Dean supports our approach and projections rather than challenging them.

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