Polar regions as key regions for monitoring climate change

Changes in the Polar atmosphere-ice-ocean system observed in recent years have sparked intense discussions as to whether these changes represent episodic events or long-term shifts in the Arctic environment. Late 20th century concerns about future climate change mainly stem from the increasing concentration of greenhouse gasses in the atmosphere. Existing knowledge on Quaternary climate and Global Climate Models (GCMs) predict that the effect of any ongoing and future global climatic change should be amplified in the polar regions due to feedbacks in which variations in the extent of glaciers, snow, sea ice and permafrost as well as atmospheric greenhouse gases play key roles. In addition, variations in the thickness of sea-ice tend to reinforce surface atmospheric temperature anomalies by altering the heat and moisture transfer from the ocean to the atmosphere. Thus, during the last 15 years the Arctic has gained a prominent role in the scientific debate regarding global climatic change.

The alleged enhanched temperature increase at high latitudes is mainly due to two theoretial greenhouse mechanisms:

• Firstly, atmospheric carbon dioxide (CO₂) has its greatest absorption of infrared radiation (IR) at sub-zero temperatures, as its absorption bands lie in the 12-16 micron wavelength band, corresponding to the wavelength of strongest IR surface emission from polar ice and snow. At higher temperatures, the typical wavelength of the strongest IR surface transmission is less than 12 microns, and therefore less affected by CO₂. At temperatures near the average surface temperature of the Earth (c. 15°C), the strongest emission wavelength is around 10 microns, a wavelength which is largely unaffected by greenhouse gases. This is the so-called `radiation window' of the atmosphere where IR radiation from the surface escapes freely to the space.

• Secondly, by far the most powerful atmospheric greenhouse gas is water vapour. Water vapour shares many overlapping absorption bands with CO₂ and therefore an increase or decrease in atmospheric CO₂ has limited effect on the overall rate of IR absorption in those overlapping regions, if water vapour is present in sufficient quantity. In the Polar Regions , the air is dry due to prevailing low temperatures, allowing CO₂ to exert a much greater influence than would be possible in warmer and moister air masses at lower latitudes. Here water vapour saturates the absorption wavebands to the point where changes in CO₂ have little effect. In addition to the enhanced greenhouse effect, Arctic climate is influenced by a powerful positive feedback mechanism, the temperature-albedo feedback, tending to amplify any initial temperature change. Rising temperatures will usually increase melting of snow and sea ice, reducing surface reflectance, thereby increasing solar absorption, which raises temperatures, and so on. Conversely, if climate cools, less snow and ice melts in summer, raising the albedo and causing further cooling as more solar radiation is reflected rather than absorbed.

For the above reasons, an important enhanced greenhouse surface ‘fingerprint’ is usually considered to be enhanced warming in the polar and sub-polar regions, less warming in the tropics and sub-tropics, and least warming in equatorial regions. This is the basic reason for much renewed research interest in Arctic regions, and recent sub-continental scale analysis of meteorological data obtained during the observational period apparently lends empirical support to the alleged high climatic sensitivity of the Arctic (Giorgi 2002). Analyses by different GCMs specifically of an enhanced greenhouse effect all suggest that the Polar Regions now should be experiencing a much larger warming than registered in lower latitudes. Polyakov et al. (2002a, 2002b), however, recently presented updated observational trends and variations of Arctic climate and sea ice cover during the 20th century, which questions the modelled polar amplification of temperature changes observed by surface stations at lower latitudes. The cryosphere is a prominent feature of the Polar Regions, represented by snow, glaciers, sea ice and permafrost. The physical properties of snow and ice include high reflectivity, high latent heat required converting ice to liquid water, and the low thermal conductivity of snow and ice; these factors all contribute significantly to the characteristics of Polar climates.

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