According to satellite measurements, the present level of sea level rise is about 3 mm per year. In hundred years, this will amount to 30 cm. So, when the IPCC predicts a sea level rise of about 30 cm during the 21st century, they actually say that the present rate of rising will continue unchanged. And when they have a lower boundary saying 18 cm, this means that they keep the possibility open that the rate of sea level rise will decrease.This is problematic, because up to now, the rate of sea level rise is faster than what IPCC is able to explain by summing up the known contributions.
    In the most recent (fourth) IPCC assessment from 2007, the scientists have utilised information that was available since the third assessment in 2001, but they have not utilised the very newest information, because they had to go through a time-consuming review process and did not have the opportunity to include the latest evidence. For instance, their predictions  about the extent of melting of the polar sea ice during the arctic summer are already outdated - the extent of melting in 2006 and 2007 has been much larger than predicted by IPCC.
    Another point where IPCC may have underestimated the rate of melting, is the melting of land ice on Greenland and Antarctica. In their calculations, they have included the increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but they have not included the further increases in the ice flow observed after 2003.
    The processes governing the rate of discharge of land ice via glaciers into the sea are not fully understood. Information obtained in recent years is that melt water on the surface of the ice may form socalled `moulins´, that is vertical holes through which the water runs to the bottom of the glacier, where it forms a lubricating layer which allows the glacier to slide faster towards the sea. The effect is similar to what is seen in those glaciers whose bottom is below sea level, which means that during periods of high sea water level (tides), the bottom of the glacier is lifted up, which accelerates the outward movement of the glacier ice.
    During recent years, much new information has been gathered on the movements of certain glaciers on Greenland, which are outlets for about 10 % of the total inland ice there. In the Jakobshavn glacier in west Greenland, the velocity of ice movement increased from 1996 to 2000, and increased further from 2000 to 2005. The extent of melting hit a new record in 2007, and altogether the rate of melting during the latest decade was almost twice that of the preceding decade, see this link. In glaciers in mid east Greenland (Helheim and Kangerdlugssuaq) the volocity of ice movement has increased only after 2000. In the latter glacier, the velocity of ice flow has tripled from 2000 to 2005 (data in an article by Rignot and Kanagaratnam, see this link and also this). Such sudden accelerations of glacier flow have not yet happened further north in Greenland, but may come if temperatures continue to increase.
    The point is that IPCC has not foreseen these events, and not included them in their projections. This means that the projected net losses of ice from Greenland and Antarctica are already lower than the actual losses (see this lecture by R. Bindschadler). IPCC has considered the possibility that melt water may increase ice flow by lubricating the glacier bottoms, but they do not  include this effect in their sea level projections, and only notice that this effect may probably give an extra increase of 0.1 to 0.2 m over the century.
    The point is that you may calculate what will happen if ice discharge into the sea grows linearly with temperature rise. But the sudden drastic accelerations in ice flow that have been observed in Greenland after 2000 cannot be incorporated into the mathematical models. One paper has calculated that the contribution to sea level rise from melting Greenland ice has increased from about 0.23 mm per year in 1996 to 0.57 mm per year in 2005 (Rignot and Kanagaratman), but after proper  reductions due to snow accumulation in the centre of Greenland, the net contribution to sea level rise is maybe more correctly estimated at  about 0.04 mm in 2000 and 0.23 mm in 2005. Some others arrive at a contribution of 0.28 mm per year for 2003-2005 (Luthcke et al. 2006), and still others reach estimates of up to 0.7 mm per year for this period (Chen, Wilson & Tapley 2006; references in Cool it).
    Altogether, what happens when the ice on Greenland starts to melt is so complicated that it cannot be simulated properly by mathematical models. This means that nobody knows for sure what will happen. Important facts are 1) that the system does not behave linearly - we see sudden accelerations in ice discharge. 2) The rate of ice loss from Greenland (and Antarctica) seems already to be larger than projected by the linear models of IPCC. The recent acceleration of ice discharge is more than just a random fluctuation due to unusual weather for a few years, but we do not know if it will last - maybe it stops again when the affected outlet glaciers have retreated from the coast line. However, a continued acceleration of ice flow is a definite possibility.
    An important question is if the projected atmospheric levels of CO2 will cause a temperature rise that is sufficient to destabilise the whole inland ice on Greenland and parts of Antarctica. Existing computer models show that a partial or full deglaciation of Greenland may be triggered by even quite modest future increases in CO2 (see this link).Another important question is, if it happens, will we have sufficient time to react? Time passes from the point when greenhouse gases have increased in the atmosphere till the full effect on melting of Greenland ice is seen. And from the time when this is seen, further time passes until the political system wakes up and reacts. The crucial question is if the effects of greenhouse gas reductions appear soon enough to revert the unwanted melting effects.
    Lomborg is sure that we have time enough to react, should this be necessary. Others are not so sure.