How High Will the Waters Rise? Part Two

by Ian Angus

The IPCC’s estimates are not the worst case scenario. Far from it.

In a previous article, I noted that attempts to compare the IPCC’s forecast of rising ocean levels with those offered by Al Gore in The Inconvenient Truth involved a severe case of apples and oranges.

Most importantly, Gore’s figure included the impact of melting the Greenland and West Antarctic Ice Sheets. The IPCC report provided a much lower estimate, because it excluded most of the effect of such melting, apparently because the Panel could not reach consensus on how fast it will occur.

How much difference would that make? According to climate scientist Stefan Rahmstorf:

“Greenland ice is good for 7 metres and the WAIS for 6 metres of sea level rise. 20 feet is about 6 metres so either ice sheet alone, or half of each, could lead to a 20 feet rise.” (RealClimate)

Since al Gore’s estimate was 20 feet, it’s clear that criticism of him on this matter is unfounded.

Rahmstorf recently posted an excellent article on The IPCC Sea Level Numbers, on the RealClimate blog, responding to various media claims that the IPCC’s recent report provided a much lower estimate of sea level increase than its previous report, published in 2001.

Canada’s National Post, always a strident anti-science voice, was among the first to assert this, on Feb. 3, 2007, just one day after the IPCC Summary for Policy Makers was released:

“But in some ways, the summary of the Fourth Assessment Report actually tones down warnings seen in earlier editions. Most notably, the absolute worst case rise in sea level by 2100, assuming uninterrupted rapid economic growth and continued heavy global reliance on fossil fuels, is estimated at 59 centimetres in the new document. The analogous number in the Third Report was 88 cm.”

If the Post’s editors had actually read the IPCC report, they would have seen this statement on the appearance of differences between it and the 2001 Third Assessment Report (TAR):

“The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.”

Rahmstorf’s paper provides a clear exposition of the various factors included in the SPM’s various estimates of sea level rise, including thermal expansion (water expands as temperature rises) and melting ice. The most uncertain part of the calculation, it turns out, is how rapidly the Greenland and Antarctic Ice Sheets will melt.

“it is apples and oranges to say that IPCC reduced the upper sea level limit from 88 cm to 59 cm, as the former included ‘ice dynamic uncertainty’ (albeit only for Greenland, as rapid ice flow changes in Antarctica were considered too unlikely to bother at the time), while the latter discusses this ice flow uncertainty separately in the text, stating it could add 10 cm, 20 cm or even more to the 59 cm in the table.”

Before the Climate Change Deniers get too excited, we should point out that “uncertainty” does not mean that the ice caps may not melt — it simply means that the dynamics of the process are not yet well understood, so the IPCC decided not to include its effects in its tables. In fact, as Rahmstorf writes:

“Sea level appears to be rising about 50% faster than models suggest – consistently for the 1961-2003 and the 1993-2003 periods, and for the TAR models and the AR4 models. This could have a number of different reasons, and the discrepancy could be considered not significant given the error ranges of observations and models. It is no proof that models underestimate future sea level rise. But it is at least a plausible possibility that the models may underestimate future rise.”

And, as Rahmstorf also points out, the impact of global warming won’t be limited to this century:

“this discussion has all been about sea level rise until the year 2095. Sea level rise does not end there, as the quotes from the SPM at the beginning of this article show. Over several centuries, without serious mitigation efforts we may expect several meters of sea level rise.”

An article in the current issue of the journal Environment and Urbanization says that 634 million people live in coastal areas that will be affected by rising sea levels — either directly by being submerged, or indirectly by increases in flooding associated with storm surges. Endangered cities with populations over 5 million include Tokyo, New York, Mumbai, Shanghai, Jakarta, and Dhaka.

The peer-reviewed study says that 75 percent of the people in vulnerable areas are in Asia, and that the poorest nations will be hardest hit.

But that’s just a lot of alarmist ranting ….

Posted in Climate Change, Featured, IPCC

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