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| Permafrost active layer
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| Definition |
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The vertical distance between the surface and the permafrost table.
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| Rationale |
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Permafrost areas at high latitudes and high altitudes are among the regions most heavily affected by continued or accelerated global warming. Active layer changes are the direct/undelayed response of permafrosts to the changing surface conditions. Thaw destabilisation of perennially frozen ground has major effects on the hydrology, vegetation, slope stability, surface albedo etc., but the processes involved are complex and not well understood. The thawing of organic layers could also release additional amounts of methane.
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| Users |
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Change detection analysts, climate impact modellers, regional analysts/planners (hydrology, engineering), and plant ecologists in cold areas (polar and high mountains).
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| Assessment method |
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Tiers 1-4: field measurements (supported by remote-sensing images).
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| Units of Measure |
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Depth to permafrost table (cm) if possible combined with soil temperature (˚C) at 15 cm depth.
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| Frequency of measurement |
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100 points at weekly intervals after snow melt at staffed sites. If possible these measurements should be combined with temperature recordings at 15 cm depth of the active layer. Statistically representative measurements/soundings in late summer (maximum thaw depth) or installation of thaw tubes, at non- staffed sites.
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| Spatial resolution |
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Tiers 1, 2, 3, and 4.
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| Accuracy/precision required |
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0.01 to 0.1 m for a statistically significant number of regularly arranged points following recommended standards of the IPA (Circumarctic Active Layer Monitoring (CALM) protocol), or 0.01 m if using permanent thaw tubes.
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| Associated measurements |
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Permafrost thermal state (temperature measurement at 15 cm), snow cover area and snow water equivalent, vegetation structure, land cover, soil moisture, soil bulk density, ground-water storage fluxes, precipitation, radiation, and topography.
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| Present status |
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Despite its importance to a wide variety of physical and biological processes related to climate change, the development of the active layer has rarely been documented in a systematic, standardised fashion over large areas. The majority of the historical records on thaw depth have been obtained by physically probing of the permafrost table. These were carried out using low-cost methods of non-destructive and geographically extensive data collection in peaty soils and shallow active layers. Geophysical prospecting and/or shallow drilling (for temperature monitoring or frost tube insertion) must be used for coarse material and deeper active layers. A protocol for CALM has been prepared by the IPA and has already been implemented at 20 sites. To take advantage of complementary measurements, the CALM protocol should be initiated at most of the 25 International Tundra Experiment (ITEX) stations and at some of the Arctic long-term ecological research (LTER) and TEMS sites. Where soil conditions permit, frost tubes should be installed. To assure circumpolar distribution some 10 additional new sites should be established; it is thought that existing permafrost sites in Canada and in the Russian Federation can be enhanced for this purpose. Several new sites are required in the Southern Hemisphere including Antarctica.
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| R and D needed |
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- Surface energy balance, especially with respect to snow cover effects and vertical/horizontal heat and mass flux between the surface and the permafrost table, need to be better understood.
- Institute protocols and recommendations of the IPA at key existing sites and at selected international programme sites (e.g., ITEX);
- Select or establish new sites where geographic gaps exist. It is expected that many of the gaps will be in high-altitude areas.
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