HR: 11:50h
AN: C12A-06    [Abstracts]
TI: Role of Fire in the Permanent Loss of Permafrost under a Changing Climate
AU: * Shur, Y
EM: ffys@uaf.edu
AF: University of Alaska Fairbanks, PO Box 755900, Fairbanks, AK 99775 United States
AU: Jorgenson, M T
EM: tjorgenson@abrinc.com
AF: ABR, Inc., PO Box 80410, Fairbanks, AK 99708 United States
AB: Climate conditions can be described as favorable (1), neutral (2), or unfavorable (3) for permafrost stability. When climate is favorable to permafrost, it takes only a few years to turn the soil below the active layer to a perennially frozen state (way 1). Permafrost thickness will grow with time until it reaches the maximum set by the geothermal gradient. Under the cold temperatures in the continuous permafrost zone, permafrost formation occurs independent of ecological processes. With climate neutral to permafrost, permafrost formation can occur in special topographic situations, such as north facing slopes. More commonly, however, permafrost formation is a result of ecosystem development and its effect on reducing soil temperatures (way 2). Permafrost in the discontinuous permafrost zone is highly dependent on type of soil, vegetation, and soil moisture. Therefore, permafrost is a product of landscape evolution, not just a product of climate, and it takes to an ecosystem hundreds of years to develop conditions that are favorable to permafrost under appropriate topographic conditions. Permafrost initially formed under a favorable climate can also persist under neutral or unfavorable climates because of surface conditions created by ecosystem development. Permafrost forms a drainage barrier that increases soil moisture and peat accumulation under anaerobic conditions, and influences vegetation succession in a direction favorable to moss growth. Thus, mosses, peat, and soil saturation produce an important positive feedback to permafrost. As a result, permafrost that formed in a favorable climate, such as during the Little Ice Age, can remain relatively stable in a neutral climate as long as it protected by other ecological components. Removal of vegetation and soil by natural or human disturbance typically leads to permafrost degradation. After fire, the permafrost table decreases for hundreds of years and can be stabilized only if ecosystem development after fire creates favorable conditions (way 2). The 6 million acres of the boreal forest burned in central Alaska during summer 2004 is a vivid illustration of the importance of the widespread impacts of fire. If permafrost was formed when the climate was favorable and remains protected in an unfavorable climate (way 3), it cannot recover after disturbance. Observations in West Siberia and Alaska show that restoration of permafrost after fire under the current climate of the discontinuous permafrost zone is unlikely under most conditions. Vast areas of boreal forest with evidence of permafrost existence in the recent past now are free of permafrost or have a permafrost table below 5­V10 m as a result of fire. It is typical for areas which are well-drained due to coarse soil or relief, and the changed thermal properties and successional pathways make recovery unlikely. Restoration of permafrost probably can occur only in areas of the flat relief and fine soil, where poor drainage makes possible permafrost development in way 2 and it takes more than 100 years. Such conditions exist in the Copper River Basin where soil is glacial-lacustrine clay. Even under the most conservative scenarios of climate change, which predict an increase in mean annual air temperatures of more than 3,aC in next 100 years, the climate will become unfavorable to permafrost. It means that in areas affected by fire during the last 20-30 years, permafrost in the boreal forest of the discontinuous permafrost zone will not recover. Permafrost degradation will be started by fires and concluded by climate change.
DE: 3322 Land/atmosphere interactions
DE: 3344 Paleoclimatology
DE: 1823 Frozen ground
DE: 1863 Snow and ice (1827)
DE: 1615 Biogeochemical processes (4805)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting