I wrote a post about sea levels at the end of April, and it received a lot of comments. The most sustained were from Dr John Hunter, who felt that I had taken too much notice of the paper by Moerner and Parker that had prompted my post. He had many objections both to the paper and to the authors. I accepted that I ought to investigate the matter further.
I won’t go into the ad hominem stuff; it’s not my style. But there is no doubt that Moerner has a serious standing in historic sea-level work that was carried out a long time ago, before AGW became the issue of the day. Dr Hunter rather dwelled on a claim by Moerner and Parker that Australian governments had claimed an average 5.4mm sea-level rise, and he couldn’t find any such authoritative statement. I can’t either, and the best I can point to is the NSW Parliament’s Briefing Paper on sea levels, which tells us (on page 2, paragraph 3) that the NSW Government has adopted a planning increase of 90 cm over the 1990 sea-level mark by 2100, which assumes well above a 5.4mm average increase.
Dr Hunter is much more confident about all this than I am. My position is that the data are uncertain, that the satellite estimates don’t mesh well with tidal gauges, and that you can find arguments both ways about whether or not the rate of increase in sea levels is accelerating or decelerating. I don’t think that it is sensible to talk about a global average sea level figure, any more than it is sensible to talk about a global average temperature. In both cases there are rises and falls: the ocean basins, though connected, are different places.
Let’s look at some recent research. Simon Holgate has published twenty papers on sea levels in the past ten years. Their general tendency is that the average rise over the twentieth century was about 1.7 mm a year, and that the century saw periods of both acceleration and deceleration. It is possible that there has been acceleration in the past decade, but we can’t be sure. He notes the differences between satellite altimeter and tidal gauge measurements.
Stefan Rahmstorf is a German oceanographer who won his PhD, interestingly, at Victoria University of Wellington across the Tasman. He is convinced that we are in for a hell of a 21st century, with huge rises in sea levels. He is well published and frequently cited. I would have to say that he is much more impressed with the power of models than I am, and he also sees the oceans rising in sympathy with air temperature, which seems to me to be connected to his belief in the virtue of the models.
Closer to home, there is Phil Watson, a scientist who works for the NSW Department of Climate Change, Environment and Water. In a recent paper in the Journal of Coastal Research, Watson used ‘four very long, continuous tide gauge records, at Fremantle (1897), Auckland (1903), Fort Denison (1914), and Newcastle (1925)’ to see ‘whether there is evidence that the rise in mean sea level is accelerating over the longer term at these locations in line with various global average sea level time-series reconstructions’. The outcome? Watson found ‘a consistent trend of weak deceleration at each of these gauge sites throughout Australasia over the period from 1940 to 2000. Short period trends of acceleration in mean sea level after 1990 are evident at each site, although these are not abnormal or higher than other short-term rates measured throughout the historical record.’
Because I am a bit of a data-monger, I like papers that are based on data, and I like historic data too. So my general reaction to all this is to say first, that we don’t know a lot about sea-levels because the oceans are vast, and the measurements are sparse, and second, that there is no obvious sign of the imagined disaster to come. Indeed, my sense of the debate is that rather than telling us that there will be a steady increase in sea levels, as was the case ten years ago, some of the more extreme calamitists are proposing some kind of exponential rise in the second half of this century. None of us, of course, is likely to be here then to adjudicate.
And that returns me to the post I wrote several weeks ago. On re-reading it, the general argument looks right to me. Maybe Moerner and Parker have been in error here and there, but all such papers have their quantum of error. Certainly our governments have been worrying about high the seas will rise (incidentally, the Briefing Paper I referred to above is a helpful guide to all of this), because they will be blamed if something does happen.
On what we know at the moment, no dramatic rise in seal levels is in the offing. If the tidal gauge and satellite altimetry can be brought into agreement, that will be a start. If the 2011-2020 decade shows a continued increase in sea-level rises, that will give us two decades of information. There has been a pause in the increased warming of the air, and that is likely to reduce the expansion of the seas. If we are going to get to the projected 90 cm increase by 2100, then the rate of increase is going to have to be pretty considerable, and soon.