In my post last week about what if anything had changed since I started taking an interest in Anthropogenic Global Warming I made a brief mention of the threat of sea-level rises that might threaten coastal cities. I thought six years ago, and still think, that the data just don’t have the accuracy that would support almost any statement about sea levels, other than we have a lot still to learn.
The current dispute is about whether or not the sea levels are rising, and if they are, whether or not the rise is accelerating, and if it is, by how much. If the fine details of the dispute are new to you, a quick summary is that a warmer sea will expand in volume, and a warmer land mass and atmosphere will help to melt glaciers and other land-based ice masses, which will also increase the volume of the oceans. How do we know what is happening? There are tide gauges, and some of them have been about for quite a long time. And the orbiting satellites are able to measure the height of the oceans. Do the tide gauges and the satellites agree? No, they don’t. Oh, and to further complicate things, the land masses of the earth are not stable. Some are rising, and some, like the land around Venice, are subsiding. So it is not always clear what is being measured.
And we are talking about tiny differences: about 1.7mm a year, across 70 per cent of the surface of the globe. The IPCC claimed in its Fourth Assessment Report that by 2100 the sea levels could be between 18 and 59 cm higher, which on average means annual increases of between 1.8mm and 5.9mm. Our Climate Commission, an organisation whose scientific acumen as revealed in its published work does not impress me, has been much more gloomy, seeing global warming produce seas that could be a metre higher than they now are by the end of the century.
Well, a very recent paper has been published by Nils-Axel Moerner and Albert Parker that looks closely at the Climate Commission’s estimates, and they weren’t impressed either. Moerner is a controversial figure in sea-level studies, notably for his straightforward rejection of any suggestion that the Maldives are under threat through sea-level rises — he has been studying the Maldives for a long time. Parker, who seems in the past to have been been someone called Boretti, has published on oceans, and is at the University of Ballarat. As always, I look at the data and the argument, not simply at the authors, or the journal in which their paper has been published (in this case an Indian journal).
The paper has a most useful introductory section on the use of tidal gauge records, from which I learned a good deal, including the need to keep cyclic changes in the ocean levels in mind (for example, the Pacific Decadal Oscillation). As we know ENSO pushes water around in the Pacific to a surprising degree, and that needs to be considered too. Their view, which is also mine, is that ‘the situation is far from conclusive and straightforward’ — but then, I think that about most aspects of ‘climate change’!
What conclusion did Moerner and Parker come to? They used all the tidal gauges that have a decently long history, not the small set of gauges referred to by the Climate Commission, which have a short time frame. Some of these go back a long way — Fremantle, for example, has a continuous 113-year history. Sydney’s is comparably long. The 86 tidal gauges used by Moerner and Parker produce ‘no evidence [of] any significant changes in the rate of sea-level change’. Further, ”[t]he average sea-level rise over the period 1990-2010 is a negligible 0.1 mm/year for the 70 stations not used by the Climate Commission’ [my emphasis]. If you put them all together you get a general average of 1.5mm per year. One argument is that things have been like that for centuries.
The authors then compare what they have done with the standard measurements used around the world, including satellite altimetry. Their calculations, they argue, are in broad agreement with the average rates of global tide gauge networks around the world. Their Figure 3 shows that the official Australian claim put forward by the Climate Commission is wildly higher than anything that has been measured elsewhere.
Moerner and Parker may be wrong. But they are using publicly available data, and arguing with caution and commonsense. And if they are wrong, then the opposing arguments too must be put forward with caution and commonsense. It is worth remembering that the IPCC has reduced its estimates of sea-level rise in each of its four assessment reports, beginning in 1990 and concluding in 2007. My expectation is that in its fifth report, due next year, the estimates will be somewhat lower still.
But in the meantime we will hear further scary talk about how rising sea levels will threaten coastal settlements. I can live with environmental lobby groups doing this. I find it quite unacceptable that a government Climate Commission can do so, without setting out the ‘ifs’ and ‘buts’ that any educated Australian would want to see..[Readers who followed the discussion in the Comments here will see that I have taken seriously John Hunter’s criticisms of the Moerner/Parker paper. I accepted that paper at face value, because it seemed to me based on good data. Plainly I need to do more work on it, and will do so, returning in due course with a supplementary post. ]