T. E. Osterkamp

OBSERVATIONAL EVIDENCE OF RECENT CHANGE IN THE NORTHERN HIGH-LATITUDE ENVIRONMENT

Studies from a variety of disciplines documentrecentchange in the northern high-latitude environment.Prompted by predictions of an amplified response oftheArctic to enhanced greenhouse forcing, we present asynthesis of these observations. Pronounced winter andspring warming over northern continents since about 1970ispartly compensated by cooling over the northern NorthAtlantic. Warming is also evident over the centralArcticOcean. There is a downward tendency in sea ice extent,attended by warming and increased areal extent of theArctic Oceans Atlantic layer. Negative snow coveranomalies have dominated over both continents sincethelate 1980s and terrestrial precipitation has increasedsince 1900. Small Arctic glaciers have exhibitedgenerally negative mass balances. While permafrost haswarmed in Alaska and Russia, it has cooled in easternCanada. There is evidence of increased plant growth,attended by greater shrub abundance and northwardmigration of the tree line. Evidence also suggeststhatthe tundra has changed from a net sink to a net sourceofatmospheric carbon dioxide.Taken together, these results paint a reasonablycoherent picture of change, but their interpretationassignals of enhanced greenhouse warming is open todebate.Many of the environmental records are either short,areof uncertain quality, or provide limited spatialcoverage. The recent high-latitude warming is also nolarger than the interdecadal temperature range duringthis century. Nevertheless, the general patterns ofchange broadly agree with model predictions. Roughlyhalfof the pronounced recent rise in Northern Hemispherewinter temperatures reflects shifts in atmosphericcirculation. However, such changes are notinconsistentwith anthropogenic forcing and include generallypositive phases of the North Atlantic and ArcticOscillations and extratropical responses to theEl-NiñoSouthern Oscillation. An anthropogenic effect is alsosuggested from interpretation of the paleoclimaterecord,which indicates that the 20th century Arctic is thewarmest of the past 400 years.

PERMAFROST DEGRADATION AND ECOLOGICAL CHANGES ASSOCIATED WITH A WARMING CLIMATE IN CENTRAL ALASKA

Studies from 1994–1998 on the TananaFlats in central Alaska reveal that permafrostdegradation is widespread and rapid, causing largeshifts in ecosystems from birch forests to fens andbogs. Fine-grained soils under the birch forest areice-rich and thaw settlement typically is 1–2.5 mafter the permafrost thaws. The collapsed areas arerapidly colonized by aquatic herbaceous plants,leading to the development of a thick, floatingorganic mat. Based on field sampling of soils,permafrost and vegetation, and the construction of aGIS database, we estimate that 17% of the study area(263,964 ha) is unfrozen with no previous permafrost,48% has stable permafrost, 31% is partiallydegraded, and 4% has totally degraded. For thatportion that currently has, or recently had,permafrost (83% of area), ∼42% has been affected bythermokarst development. Based on airphoto analysis,birch forests have decreased 35% and fens haveincreased 29% from 1949 to 1995. Overall, the areawith totally degraded permafrost (collapse-scar fensand bogs) has increased from 39 to 47% in 46 y. Based on rates of change from airphoto analysis andradiocarbon dating, we estimate 83% of thedegradation occurred before 1949. Evidence indicatesthis permafrost degradation began in the mid-1700s andis associated with periods of relatively warm climateduring the mid-late 1700s and 1900s. If currentconditions persist, the remaining lowland birchforests will be eliminated by the end of the nextcentury.

Effects of Climate on the Active Layer and Permafrost on the North Slope of Alaska, U.S.A.

Thermal regimes of the active layer and permafrost and their relations to the present-day climatic conditions on the north slope of Alaska, U.S.A. were investigated by using data collected over six years and by numerical modelling. The thickness of the active layer increases from the Arctic coast to the foothills of the Brooks Range and is directly proportional to summer air temperatures and thawing index. Within about 120 km from the Arctic coast, mean annual air temperature for the period from 1987 through 1992 was nearly constant at about −12.4±0.4°C, while the mean annual ground surface and permafrost surface temperatures increased more than 4°C. Variations in the length of thaw season and thawing index are the major factors which influence permafrost temperatures during the summer. Interactions of wind, microrelief, vegetation and seasonal snow cover and variations of physical (such as density and structure) and thermal properties of snow are the major factors affecting permafrost temperatures during the winter. The insulating effect of the seasonal snow cover could be either positive or negative on a daily basis depending upon the synoptic weather processes and on a monthly basis depending upon the time of year. On an annual basis, seasonal snow cover could increase the mean annual ground surface temperature by 2 to 7°C. Over a decade, snow cover also shows a strong effect on permafrost temperatures. Modelling results show that the depth hoar fraction of the seasonal snow cover varies from about 0.31 along the coast to about 0.57 inland. Higher permafrost temperatures along the foothills of the Brooks Range are the results of warm winters due to the impact of less strong atmospheric temperature inversion.

Characteristics of Changing Permafrost Temperatures in the Alaskan Arctic, U.S.A.

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Permafrost: changes and impacts

Industrial activities and those of stakeholders in northern regions occur in close relationship with permafrost and periglacial processes. The significant role of permafrost relates to the dependence of its mechanical and physical properties on the temperature (especially if the temperature is close to 0 °C). These properties change dramatically if permafrost becomes unstable and starts to melt. All kinds of engineering constructions in Arctic regions are effected by cold climate and permafrost. The permafrost conditions determine the principles of design, construction and use of the engineering works. Permafrost also has direct impacts on the subsistence activities of local residents and their food sources.