Sea level in the 5th IPCC report

What is happening to sea levels? That was perhaps the most controversial issue in the 4th IPCC report of 2007. The new report of the Intergovernmental Panel on Climate Change is out now, and here I will discuss what IPCC has to say about sea-level rise (as I did here after the 4th report).

Let us jump straight in with the following graph which nicely sums up the key findings about past and future sea-level rise: (1) global sea level is rising, (2) this rise has accelerated since pre-industrial times and (3) it will accelerate further in this century. The projections for the future are much higher and more credible than those in the 4th report but possibly still a bit conservative, as we will discuss in more detail below. For high emissions IPCC now predicts a global rise by 52-98 cm by the year 2100, which would threaten the survival of coastal cities and entire island nations. But even with aggressive emissions reductions, a rise by 28-61 cm is predicted. Even under this highly optimistic scenario we might see over half a meter of sea-level rise, with serious impacts on many coastal areas, including coastal erosion and a greatly increased risk of flooding.

IPCC_AR5_13.27

Fig. 1. Past and future sea-level rise. For the past, proxy data are shown in light purple and tide gauge data in blue. For the future, the IPCC projections for very high emissions (red, RCP8.5 scenario) and very low emissions (blue, RCP2.6 scenario) are shown. Source: IPCC AR5 Fig. 13.27.

In addition to the global rise IPCC extensively discusses regional differences, as shown for one scenario below. For reasons of brevity I will not discuss these further in this post.

IPCC_AR5_13.19

Fig. 2. Map of sea-level changes up to the period 2081-2100 for the RCP4.5 scenario (which one could call late mitigation, with emissions starting to fall globally after 2040 AD). Top panel shows the model mean with 50 cm global rise, the following panels show the low and high end of the uncertainty range for this scenario. Note that even under this moderate climate scenario, the northern US east coast is risking a rise close to a meter, drastically increasing the storm surge hazard to cities like New York. Source: IPCC AR5 Fig. 13.19.

I recommend to everyone with a deeper interest in sea level to read the sea level chapter of the new IPCC report (Chapter 13) – it is the result of a great effort by a group of leading experts and an excellent starting point to understanding the key issues involved. It will be a standard reference for years to come.

Past sea-level rise

Understanding of past sea-level changes has greatly improved since the 4th IPCC report. The IPCC writes:

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