Kenneth M. Hinkel

The circumpolar active layer monitoring (calm) program: Research designs and initial results 1

Abstract The Circumpolar Active Layer Monitoring (CALM) program, designed to observe the response of the active layer and near‐surface permafrost to climate change, currently incorporates more than 100 sites involving 15 investigating countries in both hemispheres. In general, the active layer responds consistently to forcing by air temperature on an interannual basis. The relatively few long‐term data sets available from northern high‐latitude sites demonstrate substantial interannual and interdecadal fluctuations. Increased thaw penetration, thaw subsidence, and development of thermokarst are observed at some sites, indicating degradation of warmer permafrost. During the mid‐ to late‐1990s, sites in Alaska and northwestern Canada experienced maximum thaw depth in 1998 and a minimum in 2000; these values are consistent with the warmest and coolest summers. The CALM network is part of the World Meteorological Organizations (WMO) Global Terrestrial Network for Permafrost (GTN‐P). GTN‐P observations consist of both the active layer measurements and the permafrost thermal state measured in boreholes. The CALM program requires additional multi‐decadal observations. Sites in the Antarctic and elsewhere in the Southern Hemisphere are presently being added to the bipolar network.

Spatial Extent, Age, and Carbon Stocks in Drained Thaw Lake Basins on the Barrow Peninsula, Alaska

Abstract Thaw lakes and drained thaw lake basins are ubiquitous on the Arctic Coastal Plain of Alaska. Basins are wet depositional environments, ideally suited for the accumulation and preservation of organic material. Much of this soil organic carbon (SOC) is currently sequestered in the near-surface permafrost but, under a warming scenario, could become mobilized. The relative age of 77 basins on the Barrow Peninsula was estimated using the degree of plant community succession and verified by radiocarbon-dating material collected from the base of the organic layer in 21 basins. Using Landsat-7+ imagery of the region, a neural network classifying algorithm was developed from basin age-dependent spectra and texture. About 22% of the region is covered by 592 lakes (>1 ha), and at least 50% of the land surface is covered by 558 drained lake basins. Analysis of cores collected from basins indicates that (1) organic layer thickness and the degree of organic matter decomposition generally increases with basin age, and (2) SOC in the surface organic layer tends to increase with basin age, but the relation for the upper 100 cm of soil becomes obscured due to cryoturbation, organic matter decomposition, and processes leading to ice enrichment in the upper permafrost.

Estimating active-layer thickness over a large region: Kuparuk River Basin, Alaska, U.S.A

Active-layer thickness was mapped over a 26,278-km2 area of northern Alaska containing complex and highly variable patterns of topography, vegetation, and soil properties. Procedures included frequ...

Non-conductive heat transfer associated with frozen soils

Abstract The assertion that pure conductive heat transfer always dominates in cold climates is at odds with decades of research in soil physics which clearly demonstrate that non-conductive heat transfer by water and water vapor are significant, and frequently are for specific periods the dominant modes of heat transfer near the ground surface. The thermal regime at the surface represents the effective boundary condition for deeper thermal regimes. Also, surface soils are going to respond more quickly to any climatic fluctuations; this is important to us because most facets of our lives are tied to earths surface. To accurately determine the surface thermal regime (for example, the detection of climate change), it is important to consider all potential forms of heat transfer. Gradients that have the potential to alter the thermal regime besides temperature include pore water pressure, gravitational, density, vapor pressure and chemical. The importance of several non-conductive heat transport mechanisms near the ground surface is examined. Infiltration into seasonally frozen soils and freezing (release of latent heat) of water is one mechanism for the acceleration of warming in surficial soils in the spring. Free convection due to buoyancy-induced motion of fluids does not appear to be an important heat-transfer mechanism; estimates of the Rayleigh number (the ratio of buoyancy to viscous forces) are generally around 2, which is too low for effective heat transfer. The Peclet number (ratio of convective to conductive heat transfer) is on the order of 0.25 for snowmelt infiltration and up to 2.5 for rainfall infiltration for porous organic soils. In mineral soils, both vertical and horizontal advection of heat can be neglected (Peclet number is approximately 0.001) except for snowmelt infiltration into open thermal contraction cracks. The migration of water in response to temperature or chemical gradients from unfrozen soil depths to the freezing front, and the redistribution of moisture within the frozen soil from warmer depths to colder depths, can also result in heat transfer although this effect has not been quantified here. Many of these processes are seasonal and effective only during periods of phase change when the driving gradient near the ground surface is relatively large.

Patterns of soil temperature and moisture in the active layer and upper permafrost at Barrow, Alaska: 1993–1999

Soil temperature has been monitored continuously at hourly intervals to a depth of 1 m since 1993 at a site near Barrow, AK. Time series of soil moisture from the active layer and upper permafrost have been collected since 1996 at the same location. These records are supplemented by meteorological data from NOAAs Barrow Climate Monitoring and Diagnostics Laboratory facility and detailed description of depth-dependent soil properties at the site. Soil sensors are situated within a low-centered ice-wedge polygon characterized by meadow tundra vegetation. A thin (7 cm) organic layer grades into reworked marine silts at depth. The soil temperature and moisture are used in a site-specific, multiyear thermal analysis of the atmosphere/snow/active-layer/permafrost system. Fusion retards soil freezing during early winter as soil water is converted to ice. Soil heat transfer is dominated by conduction in winter. Infiltration of snow meltwater in spring produces a series of thermal pulses in the active layer, causing rapid warming of the upper several decimeters by about 1°C. The thermal impact is limited because the soil tends to be nearly saturated at the time of freezeback. Volumetric soil water content in summer is generally around 35–40% at a depth of 15 cm, while the base of the thawed zone remains saturated near 50%. The near-surface soil exhibits drying from evapotranspiration and rewetting from precipitation events. During the period of thaw, the apparent thermal diffusivity is around 2–3×10−7 m2 s−1 and increases with depth to reflect the greater soil water content. The maximum thaw depth at the site is typically around 35 cm. However, end-of-season thaw depth has been monitored near Barrow since 1994 and has increased between 1994 and 1998. This warming trend is also reflected in the thawing degree days calculated for the thawed soil volume. A strong correlation exists between maximum annual thaw depth and annual thawing degree days at this site over the period of record.

Active‐layer thickness in north central Alaska: Systematic sampling, scale, and spatial autocorrelation

Active-layer thickness was determined in late August 1995 and 1996 at 100 m intervals over seven 1 km 2 grids in the Arctic Coastal Plain and Arctic Foothills physiographic provinces of northern Alaska. Collectively, the sampled areas integrate the range of regional terrain, soil, and vegetation characteristics in this region. Spatial autocorrelation analysis indicates that patterns of active-layer thickness are governed closely by topographic detail, acting through near-surface hydrology. On the coastal plain, maximum variability occurs at scales involving hundreds of meters, and patterns were similar in the two years. Substantially less spatial structure and interannual correspondence were found within the foothill sites, where high variability occurs over smaller distances. The divergence in patterns of thaw depth between the two provinces reflects the scale of local terrain features, which predetermines the effectiveness of fixed sampling intervals. Exploratory analysis should be performed to ascertain the scale(s) of maximum variability within representative areas prior to selection of sampling intervals and development of long-term monitoring programs.

The N-factor in Natural Landscapes: Variability of Air and Soil-Surface Temperatures, Kuparuk River Basin, Alaska, U.S.A

The n-factor, or ratio of the seasonal degree-day sum at the soil surface to that in the air at standard screen height, has been used for more than 40 yr in engineering studies to parameterize the temperature regime at the ground surface. Conceptually, this index represents the complex energy balance at the surface as a single dimensionless number and has applications in ecology, climatology, and geocryology. Although the n-factor has been used theoretically to represent the thermal regime of undisturbed natural surfaces, lack of empirical data has hindered its widespread implementation. Nine ground and one air temperature series (1995–97) from each of ten 1-ha plots on the coastal plain and Brooks Range foothills of north-central Alaska were converted to thawing (summer) n-factors and analyzed to address within- and between-plot spatial and temporal variability. Although substantial microscale variation exists, n-factors corresponding to natural vegetation/soil classes are discernable. Incorporation of n-factor values in a standard solution for the depth of thaw resulted in significantly improved estimates of active-layer thickness throughout the study area. The n-factor has considerable potential for addressing problems involving near-surface climate dynamics over extensive regions and long time periods.

Estimating seasonal values of thermal diffusivity in thawed and frozen soils using temperature time series

Abstract Various methods have been developed to estimate the thermal diffusivity in soils from temperature time series, but all have limitations. The method described here is designed to obtain a bulk estimate of thermal diffusivity representative of the soil column during a season, or period of around three months. Hourly precision temperature measurements were recorded between August 1993 and August 1994 at eight probes in the active layer and near-surface permafrost at two sites in northern Alaska. Hourly temperature changes were calculated during the period when the soil was thawed, and used to compute the average temperature change rate (°C h−1) at each probe level. The diffusivity was estimated by relating the rates at each level using the ratio amplitude method. Bulk values for the thawed soils above permafrost were 1.5 and 2.1 × 10−7 m2 S−1 at Happy Valley and Barrow, respectively. Several strategies were employed to verify the methodology. The results indicate that this method is most effective in the upper part of the soil column in summer where hourly temperature variations are fairly large. Thermal diffusivity has strong time and depth dependence. By reducing this inherent variability to a single value representing the thawed soil column over the entire season, information is lost. However, it does provide an estimate as input for modeling active layer thaw at high latitudes given similar soils, vegetation cover and topography.

Permafrost Destabilization and Thermokarst following Snow Fence Installation, Barrow, Alaska, U.S.A

ABSTRACT In autumn 1997, a 2.2 km-long, 4 m-high snow fence was constructed east of the coastal village of Barrow, Alaska. A large drift develops each winter on the downwind side of the fence, and a smaller drift forms upwind. To monitor the thermal impact on ice-rich permafrost, nine monitoring sites were installed near the fence in 1999 to measure soil temperature at 5, 30, and 50 cm; an additional three sites were located in the undisturbed tundra as a control. Maximum thaw and snow depth were measured annually. The results of the 6-yr study indicates that soil temperatures beneath the drift are 2 to 14°C warmer than the control in winter due to the insulting effects of the snow. Since the drift persists 4 to 8 wk after snow has disappeared from the undisturbed tundra, soil thaw is delayed and soil temperatures in summer are 2 to 3°C cooler than the control. The mean soil temperature over the 6-yr period of record has warmed 2 to 5°C, and the upper permafrost has thawed. The ground surface has experienced 10 to 20 cm of thaw subsidence in many places, and widespread thermokarst is apparent where snow meltwater ponds. Both direct soil warming and the indirect effects of ponding contribute to local permafrost destabilization.

Active Layer Thaw Rate at a Boreal Forest Site in Central Alaska, U.S.A.

The compound NF4MF6 where M is a Group V metalloid is reacted with a fluoride of sodium to yield tetrafluorammonium bifluoride in solution with hydrogen fluoride and a precipitate of the formula NaMF6. The preferred metaloid is antimony. The formed tetrafluorammonium bifluoride may be converted to NF4BF4 by reaction with BF3.