The most distinctive feature of polar regions is the large seasonal variation in incoming solar radiation—from very little in winter to 24 hours of continuous sunlight in summer. Although the poles receive less solar radiation annually than equatorial locations, near the time of the solstices they receive more per day. The high albedo of the snow- and ice-covered polar regions, together with the large loss of long-wave radiation through the very clear and dry atmosphere, ensures a net loss of radiation in most months of the year. These losses of radiation are particularly large during the long polar night and help create a deep, stable boundary layer of very cold, dry air at the surface.

These processes ensure sustained, very low temperatures in winter. Even in the summer, many areas—especially in the Antarctic—remain below freezing. Thus, much water remains frozen throughout the year. In Antarctica, snow and ice have continued to accumulate on the continent for millions of years. On the other hand, there are large volumes of liquid water in polar regions in the Arctic and Southern Oceans, although their surface is partially covered with sea ice, whose extent varies seasonally over vast areas. Sea ice insulates the underlying water from heat loss and, because of its high albedo, reflects much of the incoming solar radiation.

Formation of sea ice has important oceanographic consequences in that much latent heat is released, and highly saline, dense water is created. The dense water sinks in the north Atlantic and Southern Oceans, helping maintain the ocean conveyor belt and carrying nutrients and carbon. Production of deep ocean water is a consequence of processes that are operating in both polar regions. There is little evaporation from the vast tracts of sea ice and glacier ice. Extensive areas of the polar regions have very low precipitation; most comes from cyclonic storms that penetrate from the surrounding belt of subpolar depressions.

The polar regions incorporate important environmental thresholds, many of which are associated with water-phase changes. Together with the large seasonal changes in solar radiation, polar regions stimulate important geophysical and biological processes with high sensitivity to impacts. Sustained warming or cooling across the freezing threshold brings dramatic physical changes over land and sea. As a result, the physical environment, biota, and socioeconomic factors are all vulnerable to climate change. The climate and vast areas of ice make the polar regions very inhospitable and marginal for many species, including humans. However, specially adapted species thrive in some terrestrial and marine polar ecosystems. Consequently, the Arctic and Antarctic are characterized by the presence of highly distinctive biomes and are important places for many migratory species. Although the Antarctic continent essentially is a pristine wilderness, the great whales and some species of seals in the Southern Ocean were commercially exploited to virtual commercial extinction in the 19th and 20th centuries. Human activities on many sub-Antarctic islands have altered their biota dramatically.

The Antarctic has limited resource use, apart from growing fishing and tourism industries. There is a multinational approach to natural resources and environmental management, with mineral exploration and exploitation banned by international agreement. The area is managed by the Consultative Parties to the Antarctic Treaty to the dedication of science and peace (UNEP, 1997). By contrast, the Arctic has been populated for thousands of years. There is considerable economic activity, based on fishing, herding, and shipping. Recent decades have seen the establishment of urban areas and resource developments, based on the petroleum, gas, and mining industries. The extreme environment requires unique cold-region engineering and infrastructure solutions. There is a distinct contrast—and sometimes conflict—between the developments of modern society and indigenous peoples. The Arctic lies within the political boundaries of some of the world's richest and most powerful nations. During the Cold War, it was a critical strategic area, and substantial defense establishments remain in the region.

There has been substantial climate change during the past 2 million years in the Arctic and the Antarctic. These changes are well documented in several natural archives, such as ice cores and marine sediments. The quasi-periodic sequence of glacial-interglacial periods and corresponding changes in GHGs is shown clearly in the record of atmospheric composition and climate derived from the ice cores for the past 420,000 years (e.g.,White and Steig, 1998; Petit et al., 1999). Longer term climate changes are not discussed in this chapter, which focuses on observed 20th-century changes as well as those predicted for the 21st century. The following subsections provide a brief discussion of the main climatic changes in the 20th century. More details are provided in TAR WGI Chapter 2.

IPCC Homepage