6.2.3 Changes in the ocean and sea ice

The warming simulated at high latitudes is associated with year-long decreases in the extent and in the thickness of sea ice in both hemispheres. The projected decrease is larger in summer than in winter, and particularly pronounced in the Arctic. As a consequence, both hemispheres are predicted to move towards seasonal ice cover during the 21^st century. The differences between the projections provided by the various models are quite large and so are the uncertainties, but many simulations forecast a totally ice-free Arctic in summer before the end of the 21^st century, although some ice would still be present in winter (Fig. 6.12).

Ocean circulation is also projected to change during the 21^st century. Because of the warming and the increase in precipitation at high latitudes (see Section 6.2.2), the density of the water at the surface will tend to decrease, increasing the stratification in many regions. In the North Atlantic, this would imply less sinking of dense water and a weaker southward transport of dense water. As a consequence, the northward transport of warm surface water will also decrease with potential implications for the heat budget of the North Atlantic and the surrounding regions.

The intensity of this thermohaline circulation is generally measured by the maximum of the meridional overturning circulation (MOC) in the North Atlantic, although the two concepts are slightly different (the overturning circulation also including the contribution from the winds). The scatter of the results for the thermohaline circulation from the different GCMs is very large, both for present-day conditions and for the whole 21^st century (Fig. 6.13). Three simulations show a clear slow down during the 20^th century. This is not related to the forcing during this period, but rather to a slow drift of the model to a state that is inconsistent with observational estimates. All the other models have more realistic predictions for the 20^th century, with the meridional overturning circulation ranging from a more or less stable situation over the 21^st century to decreases of more than 50% compared to the late 20^th century. None of them simulates a complete collapse of the circulation, a state which is sometimes referred to as the off-state of the meridional overturning circulation.

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