Shrinking of the Cryosphere

The only natural surface which cannot react to a temperature increase caused by an enhanced greenhouse effect of the atmosphere with higher temperatures is a melting snow or ice surface. It will increase the melting rate. Especially during the recent few decades a nearly global shrinking of the cryosphere became obvious also to the laymen. Days with snow cover shrank at most places, permafrost has started to melt in large areas of Alaska and Siberia but also in the European Alps at high altitudes, sea ice cover and extent showed lowest values since observations began in recent years in the Arctic (see also section 2.2), mountain glaciers retreated both by accelerated melting and less snow fall in nearly all mountain ranges, first net mass balance estimates of the Greenland ice sheet using satellite altimetry point to the dominance of melting at the margins over the increased net accumulation in the centre.

The modelling of the cryosphere in climate models is a very difficult task because for sea ice not only thermodynamic processes like freezing and thawing of a salt solution have to be correctly handled but also the drift due to ocean currents and wind forcing as well as deformation by convergent or divergent flow (ice rheology), often leading to packed ice with so-called pressure ridges. Modelling the three-dimensional flow of an ice-sheet is the second major challenge for cryosphere modelling within climate models. The flow field is a function of the geothermal heat flux, trace substance (dust) content, temperature and bedrock orography. First such fully coupled ice sheet models exist, used for assessments of ice sheet geometry in the forthcoming millennia depending on greenhouse gas emissions.

As figure 3.7 already has shown, Arctic sea ice cover will undergo a major retreat in all emission scenarios and multi-year sea ice will disappear in scenarios with CO2-emissions like A1B or higher. This would eradicate sea-ice eco systems or at least endanger species depending on the existence of sea ice like Arctic seals and polar bears. Also snow cover for very large parts of Europe and inner Asia will no longer exist into March.

The simulations with an Earth system model including a three-dimensional ice sheet model for Greenland and Antarctica (Vizcaino, 2006) show for the first time that - depending on the scenario chosen - the Greenland ice sheet could melt away nearly completely in the coming few millennia leading to a sea level rise of several meters. This threat for coastal cities and populations can only be avoided if major emission reductions are implemented within the coming few decades.

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