The polar regions are increasingly recognised as being:

• geopolitically and economically important,

• extremely vulnerable to current and projected climate change,

• the regions with the greatest potential to affect global climate and thus human populations and biodiversity.

Sub-regions of the Arctic and Antarctic have shown the most rapid rates of warming in recent years. Substantial environmental impacts of climate change show profound regional differences both within and between the polar regions, and enormous complexity in their interactions. The impacts of this climate change in the polar regions over the next 100 years will exceed the impacts forecast for many other regions and will produce feedbacks that will have globally significant consequences. However, the complexity of response in biological and human systems, and the fact that these systems are subject to multiple stressors, means that future impacts remain very difficult to predict.

15.1.1 Summary of knowledge assessed in the TAR

This chapter updates and extends the discussion of polar regions in the IPCC Working Group II, Third Assessment Report (TAR, Anisimov et al., 2001). That report summarised the climatic changes that have been observed in the polar regions over the 20th century, the impacts those changes have had on the environment, and the likely impact of projected climate change in the future. The following summarises the key findings of that assessment to which 'very-high confidence' or 'high confidence' was attached (see Figure 15.1 for overview and place names used in this chapter).

Key trends highlighted in the TAR

• In the Arctic, during the 20th century, air temperatures over extensive land areas increased by up to 5°C; sea ice thinned and declined in extent; Atlantic water flowing into the Arctic Ocean warmed; and terrestrial permafrost and Eurasian spring snow decreased in extent.

• There has been a marked warming in the Antarctic Peninsula over the last half-century. There has been no overall change in Antarctic sea-ice extent over the period 1973-1996.

Key regional projections highlighted in the TAR

• Increased melting of Arctic glaciers and the Greenland ice sheet, but thickening of the Antarctic ice sheet due to increased precipitation, were projected.

• Exposure of more bare ground and consequent changes in terrestrial biology on the Antarctic Peninsula were anticipated.

• Substantial loss of sea ice at both poles was projected.

• Reduction of permafrost area and extensive thickening of the active layer in the Arctic was expected to lead to altered landscapes and damage to infrastructure.

• Climate change combined with other stresses was projected to affect human Arctic communities, with particularly disruptive impacts on indigenous peoples following traditional and subsistence lifestyles.

• Economic costs and benefits were expected to vary among regions and communities.

Key polar drivers of global climate change identified in the TAR

• Changes in albedo due to reduced sea-ice and snow extent were expected to cause additional heating of the surface and further reductions in ice/snow cover.

Awareness of such issues led to the preparation of a uniquely detailed assessment of the impacts of climate change in the Arctic (ACIA, 2005), which has been drawn upon heavily in the Arctic component of this chapter. There is no similarly detailed report for the Antarctic.

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