24 Dec 2009


One hot topic in global warming is its effect on the sea level. Skeptics have questioned the validity of the predictions and the underlying mechanisms. For example, my friend refuted the enormous contribution from melting ice using Aristotle’s principle (See http://ccszeto.blogspot.com/2008/05/melt.html).

At present, the prevailing view still supports the notion that temperature change affects the sea level. Firstly, there are large ice sheets in Greenland and Antarctica. Since these ice sheets are on land, Aristotle’s principle does not apply and any melted ice adds water to the sea, a phenomenon known as eustatic change.

Secondly, materials expand in volume when heated up in most situations. The same holds true for water. Although we are just talking about changes in several degrees Celsius at most, the effect can be considerable since the net volume of the sea is so huge. This is known as thermosteric change.

In last week’s issue of Nature, Robert Kopp and colleagues estimated the sea level 125,000 years ago. That was the last interglacial stage when polar temperature was 3-5 degrees Celsius warmer than today.[Kopp RE et al. Nature 2009;462:863-7] Compared to previous reports, this work used integrated analysis of the most comprehensive data sets from different geographical locations and stringent mathematical modeling. Surprisingly, their conclusion was also one of the most alarming. According to their model, there is 95% probability that the global sea level at that time was at least 6.6 m higher than today. There is also 67% chance that the level was at least 8.0 m higher. In addition, the rate of sea level rise was very likely to be greater than 5.6 m per 1000 years.

In layman terms, if global temperature increases at the present rate, it is unlikely that the sea level can have a major impact during our life time but it is very likely that many predictions by scientists can come true in 1000 years. If the suggestions are sensible, we should support the measures to combat global warming. On the other hand, we must never forget that humans have many other silly methods to destroy the world before the passage of 1000 years.

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