The overwhelming characteristic of polar regions, in terms of both intensity and duration, is the cold. The long winter night ensures very low temperatures in winter. However, warm North Atlantic water flows into the Arctic Ocean at about 500 m water depth at a temperature of +2 to +4�C in the Fram Strait area and remains near 0�C even after reaching the other side of the Arctic. The polar regions show large seasonal variations in incoming solar radiation, from none during the winter to 24 hours of sunlight at mid-summer. The poles receive less solar radiation annually than the equator, but in their respective mid-summers the daily totals are greater than at other places on earth. The high albedo of polar regions, from the persistent snow and ice and the large loss of long-wave radiation due to the exceptional clarity and dryness of the atmosphere, is a key factor in the surface energy budget and ensures a net loss of radiation in all or most of the months of the year. The loss is compensated through transport of sensible and latent heat from lower latitudes, usually within cyclones, and by heat carried within ocean currents. Because of the lack of transport of warm current to the Southern Ocean and the pressure of strong westerlies, which blocks heat supply over the Antarctic, the Antarctic is colder than the Arctic. Summer temperatures in most of the Antarctic continent remain well below freezing. In the Arctic, however, rapid and strong snowmelt produces a large influx of fresh water to the rivers and Arctic Ocean in the spring and summer and supports a burst of life during a brief and intense summer. Important circulation systems of the world's oceans are driven by sinking cold water at the periphery of polar regions.
After nine months of snow, ice, cold, and relative darkness, there are a few brief weeks of thaw when much of the Arctic ground is awash with water and boggy. Overland travel is easy when surfaces are firmly frozen but becomes more difficult in summer when they are not. Surface transport over ice in seas, lakes, and rivers much of the year must give way to transport over water in summer. This marked seasonal contrast provides two dramatically different environments, which are a challenge and constraint to traditional and modern human systems (Sugden, 1982).
The freeze-thaw threshold of 0�C is crucial in polar regions. Large changes in physical, biological, and human systems occur when temperature crosses this threshold. Therefore, any climate change that shifts the freeze-thaw line, whether in space or time, will bring about important impacts.
Temporary incursions of cold air at lower latitudes have their source in polar regions. Antarctic storms sometimes strongly affect South America and southern New Zealand and exert some influence on the weather and climate of Australia and much of the Southern Hemisphere. In the Northern Hemisphere, the northern part of the subtropical zone and the southern part of the temperate zone in winter are the most vulnerable. Episodes of extreme cold and blizzards are major climate concerns for circumpolar countries like Russia and Canada.
Ice cores from the Arctic and Antarctic provide a particularly valuable archive of past climate and are direct evidence for the amount of increases in carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). There is evidence from these records of rapid warming ~11,500 years ago, at the end of the last glacial period. Recent results (Cuffey et al., 1995; Johnsen et al., 1995) suggest that the temperature changes were larger than previously thought; that the coldest parts of the last glacial period could have been as much as 21�C colder than the present temperature in central Greenland; and that temperatures increased by more than 10�C in a few decades. There is evidence of an even more rapid change in the precipitation pattern, rapid reorganizations in atmospheric circulation, and periods of rapid warming during the past 20,000 years. Surface water salinity and temperature have exhibited parallel changes that resulted in reduced oceanic convection in the North Atlantic and in reduced strength of the global conveyor belt ocean circulation (IPCC 1996, WG I, Section 3.6.4).
There also are indications of rapid warm-cold oscillations during the last glacial period in the central Greenland records. Rapid warmings of ~10�C in a few decades were followed by periods of slower cooling over a few centuries and then a generally rapid return to glacial conditions. About 20 such intervals, each lasting between 500 and 2,000 years, occurred during the last glacial period (IPCC 1996, WG I, Section 3.6.3). However, the great ice sheet of Greenland has changed little in extent during this century (IPCC 1996, WG II, Section 7.4).
The exact sequence of events leading to rapid climate events within the glacial period is not well understood, but it has generally been believed that the mechanisms are related to the existence of large Northern Hemisphere ice sheets. Although there is some indication that a cooling event may have occurred within the last interglacial period (Maslin and Tzedadakis, 1996), the evidence from paleorecords for rapid and catastrophic events in interglacial periods remains a topic in need of further study.
There has been a tendency toward warmer temperatures in parts of the Arctic
and Antarctic over the past half-century (Raper et al., 1984; Jones et al.,
1986; Chapman and Walsh, 1993). Figures 3-1
and 3-2 depict winter temperature anomalies from 1961-1990
means for the Arctic and the Antarctic Peninsula (60� and higher latitude),
respectively, for this century. The former depicts evidence of long-term fluctuation
in winter temperatures, with about a 2�C warming trend over nearly 100 years
of recorded data. This trend is supported by the annual temperature record of
Figure A-5 in Annex
3-2 is less definitive in depicting a clear warming trend in the Antarctic,
but it should be noted that the data base for this record is more limited, consisting
mostly of data from near the Antarctic Peninsula. Borehole temperature measurements
show that permafrost is warming in some areas, though not everywhere. Later
freeze-up and earlier break-up dates for river and lake ice are observed in
some tundra and boreal lands. These latter events are each at least a week different
compared to the last century (IPCC 1996, WG II, Chapter 7 Executive Summary).
|Figure 3-2: Observed annual winter (JJA) temperature anomaly over Antarctica during the period 1900-96.|
There is growing evidence of recent atmospheric warming on the Antarctic Peninsula. A warming trend of 0.056�C/yr-a total increase of ~2.5�C since 1945-has been recorded for Faraday Station on the west coast of the Antarctic Peninsula (King, 1994), and a 2.1�C increase in decadal average of the mean annual temperature between 1931-1940 and 1981-90 has been recorded from Orcadas Station on South Orkney Island (Hoffman et al., 1997). The glacier retreat (Skvarca et al., 1995) and the very recent collapses of the northernmost Larsen (Rott et al., 1996) and Wordie Ice Shelves appear to be consistent with a warming trend in this region.
From the main part of the Antarctic continent, there have been discharges of enormous icebergs from the Filchner and Ross Ice Shelves (Rott et al., 1996; Vaughan and Doake, 1996). However, the great ice sheets of Antarctica have changed little in extent during this century. The dynamic responses of the different ice sheets are influenced largely by whether they are marine (e.g., West Antarctic Ice Sheet) or land-based (e.g., East Antarctic Ice Sheet). Accumulation on the Antarctic continent has increased significantly (by as much as 5-10%) in the past few decades. The dynamic response times of land-based ice sheets (East Antarctica and Greenland) to climate change are on the order of thousands of years, so they are not necessarily in equilibrium with current climate. The response times of marine-based ice sheets (West Antarctica) probably are much shorter because they may be directly influenced by sea level and other environmental effects such as salinity, temperature, and currents. Observational evidence is insufficient to determine whether they are in balance or have decreased or increased in volume over the past 100 years (IPCC 1996, WG II, Section 7.4).
In the circum-Arctic, there has been a tendency for negative mass balances in ice caps and glaciers over the past 30 years or longer (IPCC 1996, WG II, Section 7.2.2). A recent Canadian study, however, measured no significant changes in either the mass balance of snow accumulation or ice melt over the past 32 years in the Canadian Arctic (Koerner and Lundgaard, 1995).
An increase has been found in the numbers of cyclones and anticyclones over the Arctic between 1952 and 1989 (IPCC 1996, WG I, Section 126.96.36.199). Station measurements indicate that annual snowfall has increased over the period 1950-90 by about 20% over northern Canada (north of 55�N) and by about 11% over Alaska. Total precipitation has increased in all of these regions (IPCC 1996, WG I, Section 3.3.2).
Projections of future polar climate face several difficulties. The reliability of the simulated climate change scenarios is not high, and there are considerable model-to-model differences. However, all or most general circulation models (GCMs) show the following features: greater warming over land than sea; reduced warming, or even cooling, in the high-latitude Southern Ocean and part of the northern North Atlantic Ocean; maximum warming in high northern latitudes in winter and little warming over the Arctic in summer; increased precipitation and soil moisture in high latitudes in winter; a reduction in the strength of the North Atlantic currents; and a widespread reduction in diurnal range of temperature (IPCC 1996, WG I, Section 6.2.5).
Other reports in this collection