Waterworld scenario sinks

Although the recently released fourth assessment report of the Intergovernmental Panel on Climate Change provoked much hand wringing from environmentalists, it contained little that could be considered new. But it does permit those of us without the “end is nigh” mentality which seems to be part and parcel of the environmentalist outlook, to declare an indefinite postponement to one particular nightmare that has been haunting the world for decades.

The nightmare is that of rising sea levels engulfing coastal cities, submerging entire islands nations or forcing mass migrations. It simply is not going to happen - at least not to our grandchildren, who may be in a position to assess whether it is going to happen to their grandchildren. This is not to deny climate change or reject the need to curtail emissions, but to point to the reality of recent work in this area.

It its recent report Climate Change 2007: The Physical Science Basis, the IPCC gave a range of scenarios for average sea levels increases which, broadly, predict that sea levels will rise from anywhere between 0.18 metres to 0.59 metres by 2099, relative to sea heights in 1999.

A little earlier in the report the panel also noted that global average sea level rose at about 1.8 millimetres per year between 1961 to 2003, and at the somewhat faster rate of around 3.1mm a year between 1993 to 2003. The panel also estimated that the total increase in sea levels for all of the 20th century was about 0.17 metres (170mm, or an average increase of 1.7mm a year). Two points to note here are that the figures given in the report have large error bars (that is, plus or minus certain amounts), and that the panel does not know whether the faster rate actually means anything or whether it is part of a natural cycle.

The latest estimates are a climb down from the previous estimates, released in 2001, of anywhere between 0.1 and 1 metre over a century, but even those earlier, vaguer figures were hardly going to make coastal property developers hit panic buttons - certainly not in Australia, and probably not in any of the South Sea islands. Environmentalists trying to put the worst possible light on those figures may say the IPCC’s estimates are cautious, and that additional storm surges brought about by increased temperatures will cause additional flooding and that the IPCC did not take into account recent work on ice melting in the ice caps and so on.

The first point can be dismissed out of hand. The IPCC has always been part of the greenhouse industry and is doing its best, within the limits of known science, to produce forecasts that keep the industry expanding. It is certainly not in the business of declaring a crisis over. However, a much stronger objection, related to the first, is that given the present state of knowledge about climate change in general, and the effect climate change may have on ocean levels in particular, is that the panel’s vague forecasts may be little more than stabs in the dark. Quite so. As noted, the rate of increase in sea levels went up for several years but no-one knew why.

Regular reports by the National Tidal Centre (NTC) which runs projects monitoring sea levels around Australia and some of the South Sea Islands, point to some of the difficulties in interpreting such increases. The NTC report notes that during the El Nino climate cycle one side of the Pacific may be 50 centimetres higher than the other. The ocean has a slope.

Variations in trade winds can also change sea heights in certain areas, as can cycles in water temperature, air temperature and atmospheric pressure. All this and other cycles which scientists have yet to identify make interpreting changes in sea heights at any one location or for an area over a number of years extremely difficult. Although the NTC report says the centre can detect indications of climate change in the results, it does not commit itself to any figure for the increase.

The NTC report says:

Variations in sea level and climate are inextricably linked, with both undergoing interrelated seasonal, interannual and interdecadal fluctuations. Quasi-periodic fluctuations associated with natural phenomena such as the El Niño - Southern Oscillation can be large and cause significant social and economic impacts. Particularly noisy or low frequency variations can conceal the underlying long-term trend in sea level records that are shorter than several decades.

The report The Australian Baseline Sealevel Monitoring Report, Annual Sea Level Data Summary Report July 2005-2006 (found on www.bom.gov.au - the centre is part of the Bureau of Meterology), points to sea level increases around Australia ranging from around 2mm a year along the eastern coast since the stations were installed in the early 1990s, through to 7 and 8mm a year along the northern and western sides of the continent, and 3 to 5mm a year along the southern edge.

There is no comment on these variations in the report but the NTC clearly does not think that 14 years worth of data means much. The sea level increases for the last two years have been either an increase of a fraction of a millimetre a year or an equally small decline, but two years of data obviously means even less than 14. As an indication of the significance of these figures, note that a 3mm a year increase sustained for a century works out to a total increase of 0.3 metres.