"Facing climate change, a timely investment in measures protecting the Dutch lowlands from flooding seems a prudent course of action. Such measures would include the building of higher dikes and the application of sand suppletions all along the Dutch coast.

Most strikingly, the Delta-committee also adviced to raise the water table of Lake IJsselmeer by 1.5 meters. This highly invasive measure will be very costly. Many harbours, dikes, sluices and other infrastructure along Lake IJsselmeer will have to be rebuild completely.

Why propose such an extreme elevation of the water table, by no less than 1.5 m? The Delta-committee believes that the Netherlands should anticipate a sea level rise of 65 to 130 cm in the year 2100, and of 2 to 4 meters in the year 2200. On which data are these numbers based?

During the past 100 years the sea level rose at a much slower pace. Figure 1 presents the measured rise in global sea level during the period 1880 to 2005. The rising trend is beyond dispute. However, the total rise in sea level was ‘only' 15 to 20 cm per century. Thus, the rate of sea level rise in the past 100 years was much lower than the rate predicted by the Delta-committee for the next 100 years.

The recent 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), in 2007, provides predictions on the rate of sea level rise for the next 100 years. The IPCC is a highly respected scientific institute that, together with Al Gore, received the Nobel Prize 2007 for its climate research. The IPCC is generally seen as the most reliable source for climatic predictions in the world. Therefore, it comes as no surprise that the Delta Committee claims that its predictions are based on predictions of the IPCC. But is this really the case?

The IPCC used 6 different climate scenarios in its model calculations of sea level rise. Figure 2 present the predicted sea level rise in 2100, for each of the 6 IPCC scenarios. The horizontal bar in the gray box indicates the expected sea level rise, while the top and bottom of the box indicate the uncertainty of this expection using a so-called 95% uncertainty interval.

• Climate scenario B1 is the mildest scenario; it predicts an average sea level rise of 28 cm in the year 2100 (with a 95% confidence interval of 28-38 cm)
• Climate scenario A1F1 is the most extreme scenario; it predicts an average sea level rise of 42 cm in the year 2100 (with a 95% confidence interval of 26-59 cm)

Therefore, we must conclude that even the most extreme scenario of the IPCC predicts a maximum sea level rise of 59 cm. In addition, lowering of the soil in The Netherlands must also be taken into account. Therefore, we add another estimated 10 cm/century to the IPCC estimate of maximum sea level rise. And on top of that let us add an additional 15 cm as safety margin to what is already the most extreme scenario of the IPCC, to cover any unforeseen circumstances such as the Greenland ice sheet melting at an even higher rate than the IPCC scenarios would take into account. Hence, incorporating these extra values into IPCC's most extreme scenario, one obtains an expected sea level rise of 52 cm in the year 2100, with an uncertainty ranging from 36 to 84 cm. This predicted sea level rise is already much higher than the measurements over the past 100 years. However, it is still substantially lower than the 130 cm predicted by the Dutch Delta Committee.

Accordingly, on what underlying information does the Delta Committee base its prediction of a 130 cm sea level rise? The most likely answer is ‘educated guesswork', as explained in the Appendix of the report. The committee identifies several determinants of sea level rise. These include the thermal expansion of sea water as well as melting of glaciers on Greenland and Antarctica. The Delta committee adds some extra safety, by adding another 10-15 cm, or so, to the IPCC estimates for each of these individual factors. Simply as an extra precaution, based on interviews with various experts. Of course, such an approach, simply adding up extra decimeters for 6-7 individual factors separately will yield an extraordinarily large estimate for the predicted sea level rise. With this type of ‘educated guesswork' anyone can predict an extreme sea level rise.

Why does the prediction of the committee differ so much from that of the IPCC? Sea level rise is measurable (Figure 1), it is a fact rather than fiction, but why make such an exaggerated prediction? Does the committee want to make The Netherlands look like the best horse in town? Do they want to rouse the Dutch fear for rising waters? Do they aim at creating room for negotiations within the political arena? What other interests play a role?

The composition of the committee is interesting. The committee is not made up of renowned climatologists, oceanographers or earth scientists that have the trained expertise to interpret climate scenarios. Instead, there is a substantial contribution from other areas of society. Several members are involved in economic developments in The Netherlands, in large-scale dredging activities, or coastal engineering. And, quite interestingly, several committee members have a background in agriculture.

In summer, the Dutch agricultural sector is in need for irrigation water. Perhaps in the future much of this irrigation water could be drawn from the enlarged Lake IJsselmeer. I already feel much safer behind the high dikes of our new freshwater basin. "

Jef Huisman

Full Professor of Aquatic Microbiology
Institute for Biodiversity and Ecosystem Dynamics (IBED)
University of Amsterdam