Sea levels have changed for two main reasons in recent decades (Fig. 6.18). First water has been added to the ocean from other reservoirs. The main contributors are the glaciers and ice caps that have experienced considerable mass losses during the 20th century because of the large-scale surface warming observed over this period (see Section 5.5.3). The melt water flow from Greenland and Antarctica is relatively small on this time scale, and it is not even clear whether the net flow from the Antarctic is positive or negative.
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The second cause of sea level change is related to the ocean density. For a constant oceanic mass, any modification of the density affects the ocean volume and thus the sea level. As the density variations are mainly ruled by the water temperature, this term is often referred to as thermal expansion, although salinity changes can play a non negligible role in some regions. The contribution of this process is similar to that of glaciers and ice caps over the period 1961-2003 but it is clearly the largest contributor if the analysis is restricted to the period 1993-2003. However, this is maybe related to decadal climate variability (Fig. 6.18). Overall, the sea level rise has been estimated at about 1.8 mm yr-1 over the period 1961-2003. This is not very different from estimates for the first half of the 20th century but much less than those for the years 1993-2003. Integrated over the whole 20th century, the total sea level rise is then a bit less than 20 cm.
Over the 21st century, the melting of glaciers and ice caps and thermal expansion are expected to remain the two main causes of rising sea levels. Greenland will likely make a small positive contribution. However some frozen water may accumulate over Antarctica, the additional precipitation over a large area of the continent, related to warming (see Section 6.2.3), being approximately equal to the additional melting close to the shore. Indeed, temperatures in the centre of Antarctica are so low that the warming estimated for the 21st century is far too small to produce melting there.
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Depending on the scenario, the estimates of sea levels at the end of the 21st century range from 20 to 60 cm higher than in the late 20th century in the latest IPCC report (Figure 6.19). However, many uncertainties remain. The ice-sheet models used to obtain these estimates (see Section 3.3.6) do not include an adequate representation of the rapid ice flow changes that occur on relatively small scales (a kilometre or even less, to a few hundreds of metres), which may transport ice to the ocean or to warmer areas where it would melt relatively quickly. These fast ice-flow changes may be high frequency fluctuations that average out when looking at changes over a century or more. However, it has also been hypothesised that they could induce large-scale destabilisation of parts of the ice sheets, with potentially large consequences for the mass balance of the ice sheet and thus for sea-level rises. As a consequence, alternative methods have been proposed, based on simple statistical relationships between the rises in surface temperatures and sea levels. These studies predict that sea-level rises ranging from 75 to 190 cm by the end of the 21st century are not unlikely (e.g., Vermeer and Rahmstorf, 2009).
Even if the concentration of atmospheric CO2 stabilises or decreases after 2100, sea levels are predicted to continue to rise fast (see Figure 6.16). First, the deep ocean will have to come into equilibrium with the new surface conditions, leading to warming at deeper levels, and thus thermal expansion over several centuries (Fig. 6.20).
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Second, the thermal inertia of the ice sheets is very large, taking several millennia to tens of millennia to completely melt, even when the warming is considerable. For Greenland, it has been estimated that a sustained local warming of the order of 3–6°C, which is not incompatible with the values provided by models of several scenarios, may be sufficient to induce a complete melting of the ice sheet. The ice sheet would start to melt on its periphery, and would gradually retreat to the centre of the island, to finally survive only in the eastern mountains (see Figure 6.21). As the Greenland ice sheet retreats, the bedrock will slowly rebound because of the smaller weight on the surface. This will initially cause a series of big inland lakes to appear below sea level. After 3000 years, almost all the initial depressed areas will have risen above sea level again. Such a complete melting of the Greenland ice sheet is predicted to produce a rise in sea level of about 7m.
The melting in Antarctica will be much smaller and slower even than that in Greenland, because of the size of the ice sheet and the very cold temperatures there at present. However, some regions of East Antarctica may experience a significant melting on similar timescales to those of Greenland.
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