| Permafrost temperatures in boreholes displayed a 2 - 4-C increase over the last 50-100 years
on the North Slope of Alaska and there was a concurrent warming of discontinuous
permafrost. Long-term monitoring of deep wells in a north/south transect across the North
Slope of Alaska reveals variable warming with some cooling periods, over the last
twenty five years; this is consistent with the broader-scale trends in air temperatures
observed in northern Alaska. Discontinuous permafrost is warming and thawing and
extensive areas of thermokarst terrain (marked subsidence of the surface resulting from
thawing of ice-rich permafrost) are now developing as a result of climatic change.
Thawing permafrost and thermokarst have been observed at several sites in Interior
Alaska. Thermokarst is developing in the boreal forests of Alaska where ice-rich
discontinuous permafrost is thawing. Thawing destroys the physical foundation
(ice-rich soil) on which forests develop, causing dramatic changes in the ecosystem.
Impacts on the forest depend primarily on the type and amount of ice present in the
permafrost and on drainage conditions. In sites underlain by ice-rich permafrost, trees
die when their roots are regularly flooded, causing wet sedge meadows, bogs and
thermokarst ponds and lakes to replace forests. These new ecosystems favor aquatic birds
and mammals, whereas the previous forest ecosystems favored land-based birds
and mammals. Much of the discontinuous permafrost in Alaska is both warm and
ice-rich, making it highly susceptible to thermal degradation if regional warming
continues.
The active layer is an important factor in cold-regions science and engineering, because
most ecological, hydrological, biogeochemical, and pedogenic activity takes place within it.
The thickness of the active layer is influenced primarily by surface temperature and length of
the thaw season and secondarily by several factors, including vegetative cover, thermal
properties of the surface cover and substrate, soil moisture, and modes of heat transfer. To
date, there has been no conclusive evidence of broad-scale increases in active layer thickness
with increasing average annual air temperatures; however, it appears that the surface of
the soil is experiencing broad-scale subsidence in some areas as the ice-rich soil
at the base of the active layer thaws, which would mask increasing active layer
thickness. |