Fifteen Years of Soil Climate Monitoring on the North Slope of Alaska.

Nine soil climate monitoring stations were established on the North Slope of Alaska starting in 1996 as part of a USDA Global Climate Change research program and Circumpolar Active Layer Monitoring (CALM). The objectives were to study the effects of climate change on the active layer and upper permafrost. The stations (located in Toolik, Happy Valley, Prudhoe Bay, Sagwon Hills, Barrow, and Atqasuk) record hourly measurements of soil moisture and temperature at various depths down to 120 cm. Air temperature was measured at most of the sites. Average maximum active layer thicknesses (ALT) ranged from 41 to 65 cm. Annual variations in maximum ALT ranged from about 8 to 16 cm. There was no significant increasing or decreasing trend in the maximum ALT over the monitoring period. Thaw settlement confounded the effect of a thickening active layer. Soil moisture contents of the active layer were wet (at maximum water contents) throughout the monitoring period. There was some drying in the upper part of the active layer at the Barrow site. At all sites, soil freezing occurred from the bottom up and from the surface down in the active layer, resulting in long periods where the soil remained at 0°C while water underwent a phase change. The mean annual soil temperature (MAST; 45 cm depth) ranged from -4 to -8°C. The mean annual air temperature (MAAT), where measured, was 4 to 5°C cooler than the MAST. The tundra vegetation and surface organic layer was providing a buffer. There was no significant increasing or decreasing trend in MAST or any of the seasonal average soil temperatures. The average summer (0.9 to -1.4°C) and fall (0.15 to -1.5°C) soil temperatures were similar and the average winter (-5 to -14°C) and spring (-8 to -15°C) soil temperatures were similar. Upper permafrost temperatures (50 to 80 cm into the permafrost) ranged from near zero (-0.4 to -1.8°C) in late summer to maximum winter temperatures near -17°C that varied considerably from year to year. In conclusion, the active layer and soil climate were relatively stable over the monitoring period (1996 to 2011).