The recent reports in the media and the spin-off commentaries on sea level rise have been enough to confuse anyone. Here, I wish to set the record straight on a few key points.
Last week, a journalist emailed me with the following:
In a recent New Scientist article you’re quoted as saying that sea-level rise is accelerating. What data are you going by? Data from the Univ of Colorado suggests sea-level is rising linearly, but not accelerating.
I reproduce the U. Colorado chart above, which shows the change in global mean sea level (in mm) between 1992 and 2010, as monitored by satellite altimetry. As you can see, the journalist asked a valid question — the straight-line regression fits the data reasonably well, and there even seems to be a slight deceleration since 2006 (although the highest recorded anomalies in the whole chart come from 2010). The CSIRO also has a nice chart of these data, here, which includes regional trends and some nice animations.
They say the following:
Last two decades
High quality measurements of (near)-global sea level have been made since late 1992 by satellite altimeters, in particular, TOPEX/Poseidon (launched August, 1992) and Jason-1 (launched December, 2001) and Jason-2 (launched June, 2008). This data has shown a more-or-less steady increase in Global Mean Sea Level (GMSL) of around 3.2 ± 0.4 mm/year over that period. This is more than 50% larger than the average value over the 20th century. Whether or not this represent a further increase in the rate of sea level rise is not yet certain.
The New Scientist article that was referred to was Shape-shifting islands defy sea-level rise. It reports on a recent scientific paper which has used GIS analysis to show that the majority of Pacific coral atoll islands are not shrinking in the face of sea level rise, and indeed a lot are growing. I also made some comments to the NS journalist for that story, and later for ABC Radio in their coverage of this story, in which I said:
Well it is surprising in that sea levels are obviously rising. I think in the short term it suggests that there may be more time to do something about the problem than we’d first anticipated. But the key problem is that sea level rise is likely to accelerate much beyond what we’ve seen in the twentieth century.
Actually, I said a lot more that went unreported. One major point I made was that due to erosion, land was being lost along some areas and accreted along others — but this was not a ‘like-for-like’ exchange. The new ‘land’ being created was the result of erosion and coral rubble and would be unlikely to be suitable for use by people or fauna/flora for some time. By contrast, the old land being lost or degraded due to regular flooding and salt water intrusion was more likely to have had previous amenity, as beaches, for housing, or even as agricultural land.
There also came the attempts at ridicule from some — I wonder why they bother, or whether they really understand the point being made.
Anyway, how did I respond to the journalist’s question? I said this:
Sea level rise is indeed accelerating. Here is a recent review on the topic that summarizes the data:
Church, J.A. & White, N.J. (2006). A 20th century acceleration in global sea-level rise. Geophys. Res. Lett., 33, L01602.
Multi-century sea-level records and climate models indicate an acceleration of sea-level rise, but no 20th century acceleration has previously been detected. A reconstruction of global sea level using tide-gauge data from 1950 to 2000 indicates a larger rate of rise after 1993 and other periods of rapid sea-level rise but no significant acceleration over this period. Here, we extend the reconstruction of global mean sea level back to 1870 and find a sea-level rise from January 1870 to December 2004 of 195 mm, a 20th century rate of sea-level rise of 1.7 ± 0.3 mm yr−1 and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm yr−2. This acceleration is an important confirmation of climate change simulations which show an acceleration not previously observed. If this acceleration remained constant then the 1990 to 2100 rise would range from 280 to 340 mm, consistent with projections in the IPCC TAR.
This is also true in the Pacific, e.g.:
Gehrels, W. Roland, Bruce W. Hayward, Rewi M. Newnham, and Katherine E. Southall (2008). A 20th century acceleration of sea-level rise in New Zealand. Geophys. Res. Lett., 35, L02717.
Merrifield, M. A., S. T. Merrifield, and G. T. Mitchum (2009). An anomalous recent acceleration of global sea level rise. J. Clim., 22, 5772.
So, the key is that small, short-term fluctuations in the rate of sea level rise are driven by events such as ENSO, whereas the trend in sea level rise must be assessed over the multi-decadal time scale. The 20th century rate of sea level rise averaged 1.7 mm per year, whereas over the last 15 years it has been >3 mm per year. This is an acceleration in the slope, and is not well characterised by a linear regression.
In addition to the satellite data, the Australian Bureau of Meteorology and National Tidal Centre is also measuring sea level in the Pacific, via their SeaFRAME network which uses an acoustic monitoring methodology that includes corrections for vertical land movement due to tectonics, etc. A summary of the recent results from the South Pacific are given here, with separate reports for each island. The net relative trend differs from station to station — around the Australian coastline it varies from as low as 1.3 mm per year to as high as 8 mm per year. Some examples from the Pacific islands include 5.7 mm/yr in the Cook Islands, 8.1 mm/yr in Tonga, 3.3 mm/yr in Kiribati and 5.3 mm/yr in Tuvalu.
In general, mean sea level has risen at relatively high rates in the southwest Pacific region and has fallen in the northeast Pacific, illustrating basin-wide decadal variability in the Pacific Ocean.
There has also been interesting recent work on this historical record which has attempted to eliminate some of the unexplained variation we see in the tidal gauge record of sea level rise. For instance, the 29,000 large dams and reservoirs that have been built since 1930 have stored so much water on land (nearly 11,000 cubic kilometres worth!) that they’ve masked the full extent of recent sea level rise — without them, it would have been ~30 per cent higher! There is a good review of the work on Mongabay.
So, that’s the recent observational record. Sea levels continue to rise, and indeed, on the multi-decadal scale, that rise is not steady, it’s accelerating (but not geographically uniform).
What of future sea level rise? There’s been a lot of work in this area since 2005, and it’s definitely worth another post. Stay tuned for Part 2.