Are we experiencing exponentially increasing sea level rise?

Desdemona Despair

Guest post by Alexander Ač
9 October 2015

(Desdemona Despair) – Sea level rise (SLR) is recognized as one of the least adaptable impacts of ongoing climate change. Once a certain area is permanently flooded with saline water, people have to leave. Forever.

Thus SLR projections gain a lot of attention not only in the climate science community. This short article aims to show that we may be experiencing an increased rate of sea level rise, somewhere between 4-5 mm/year. This rate of increase will probably be only faster going forward.

I combined the published data of Church, et al. (2006) dating back to 1870 and combined them with modern satellite-based AVISO data where last data point is from July of this year.

Over the shorter time periods, SLR seems to follow linear trend, as fitted by AVISO and other research groups:

Figure 1.: From 1993, the global sea level rises on average by 3.3 mm per year. (Source: AVISO)

However, if we look at longer time periods, we observe an exponential SLR:

Figure 2: Combined data of the SLR from the past to present. Exponential fit gives a good model description of the observed SLR. (Data from Church, et al., 2006 were plotted using the program Data Digitizer. Satellite data were fitted in order to match absolute SL as published by Church, et al.)*

The rate of SLR was about 0.6 mm/y between 1870-1930, then 1.4 mm/y between 1930-1992 and about 3.3 mm/y from 1993 till present (see Hansen). However, over the past 5 years or so, the sea level rose at rate of about 5 mm/year.

Of course, one might argue that the 5-year period is a short-term cherry-picked period influenced by the ENSO cycle, etc. This is very well true, and it might turn out that the call on increased SLR is premature. On the other hand, given the accelerated (non-linear) heating of the global ocean, and recent abundant evidence on rapidly and increasingly irreversibly destabilizing of both Greenland and Antarctica ice sheets, I am quite confident the rate of SLR will follow an exponential, rather than a linear trend. This would only follow the history from past decades. Certainly, the most conservative estimates of SLR as provided by IPCC turn out to be wrong. In fact, some of them already are.

I will leave it to a critical reader to decide, whether ongoing SLR acceleration is only temporary, or more worryingly, permanent (see Fig. 3).

Figure 3: Extrapolated long-term trend from 1870 projects a SLR of about 1,27 m by 2100. This is very well in line with some of the semi-empirical approaches as provided e.g. by Stephan Rahmstorf and others.

*NOTE for critical readers: This article does not aim to provide estimate of SLR by the end of the century. The author recognizes caveats in the statistical extrapolation – higher data point density in the AVISO data compared to Church, et al., problematic fitting of satellite data, etc. However, it does argue that we will see accelerated SLR in the future.

You can get the data and related graphs here: SLR_Data_exponential.xlsx.