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Changes to below 1,000 m in the Arctic Ocean have been observed, with less sea ice and freshening of the Beaufort Sea mixed layer (Maslanik et al., 1996; and McPhee et al., 1998). Atlantic water inflow to the Arctic Ocean has warmed (Carmack et al., 1995). The halocline, which isolates the surface from this warmer Atlantic water, has grown thinner (Steele and Boyd, 1998). Data gathered from submarines indicate that sea-surface temperature (SST) in the Arctic basin increased by 1°C over the past 20 years, and the area of warm Atlantic water in the polar basin increased by almost 500,000 km² (Kotlyakov, 1997). Field measurements in 1994 and 1995 showed consistent Arctic seawater warming of 0.5-1.0°C, with a maximum detected in the Kara Sea (Alekseev et al., 1997). It is not yet clear whether these changes are part of low-frequency natural variability or whether they represent the early impacts of long-term climate change.

Thickening of the active layer and changes in the distribution and temperature of permafrost are further important indicators of warming in the Arctic. Permafrost thawing has led to major erosion problems and landslides in the Mackenzie basin (Cohen, 1997a) and has caused major landscape changes from forest to bogs, grasslands, and wetland ecosystems in Alaska, affecting land use. In central and western Canada and in Alaska, contraction in the extent of permafrost has occurred during the 20th century (Halsey et al., 1995; Weller, 1998; Weller and Lange, 1999; Osterkamp et al., 2000). However, a cooling trend has been noted for permafrost in eastern Canada (Allard et al., 1995; Allard and Kasper, 1998). Permafrost degradation during the 20th century in subarctic and boreal peatlands of Quebec is discussed by Laberge and Payette (1995).

Temperatures at the top of permafrost along a several hundred kilometer north-south transect in central Alaska warmed by 0.5-1.5°C between the late 1980s and 1996. Thawing rates of about 0.1 m per year were calculated for two sites (Osterkamp and Romanovsky, 1999). On the north slope of Alaska and in northwestern Canada, the temperature-depth profile indicates a temperature rise of 2-4°C over the past century (Lachenbruch and Marshall 1986). In the western Yamal Peninsula, temperatures at a depth of 10 m in ice-rich permafrost increased 0.1-1.0°C between 1980 and 1998; the largest increases in Siberian permafrost temperatures have occurred in relatively cold permafrost in the continuous zone.

Palaeoclimate reconstructions based on dendrochronology also indicate a steady increase in temperature during the past 150 years (Bradley and Jones 1993; Szeicz and MacDonald 1995; see TAR WGI Chapter 2). Growth rings in Larix Sibirica from northern Siberia show that mean temperatures during the 20th century were the highest in 1,000 years, exceeding even those during the Medieval Warm Period (Briffa et al., 1995). Changes in diatom assemblages in lake sediments in the High Arctic over the same period also are thought to be a consequence of climatic warming (Douglas et al., 1994).

Other changes in terrestrial ecosystems include northward movement of the treeline, reduced nutritional value of browsing for caribou and moose, decreased water availability, and increased forest fire tendencies (Weller and Lange, 1999). There are altered plant species composition, especially forbs and lichen, on the tundra. In the marine ecosystem, northern fur seal pups on the major Bering Sea breeding grounds declined by half between the 1950s and the 1980s. In parts of the Gulf of Alaska, harbor seal numbers are as much as 90% below what they were in the 1970s. There have been significant declines in the populations of some seabird species, including common murres, thick-billed murres, and kittiwakes. Comprehensive interviews by Gibson and Scullinger (1998) have revealed notable impacts on food sources and natural environments of native Alaskan communities. Warmer climate has increased growing degree days by 20% for agriculture and forestry in Alaska, and boreal forests are expanding northward at a rate of about 100 km per °C (Weller and Lange, 1999).

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