About the Variables

Below is some additional information on each of the permafrost and related ecological variables identified, including some of the rationale for the desirable spatial and temporal resolutions as discussed by committee and workshop participants.

Active layer thickness: ALT is the thickness of the top layer of soil and/or rock that thaws during the summer and freezes again during the following winter. Increase in the depth of ALT indicates the beginning of permafrost degradation and may lead to local permafrost instability if any substantial amount of ground ice is present in the near-surface permafrost. Inversely, a decrease in the maximum ALT may indicate the formation of new permafrost. A desirable resolution for the ALT is 5 cm vertical and 30 m horizontal. The minimum temporal resolution of measurements would be once per year at the end of the warm season (mid-August to late September depending on latitude). However, it is desirable to have several measurements per warm season to establish the rate of thaw depth propagation.
Biomass: Biomass affects energy balance and soil carbon dynamics that are important to permafrost. Techniques for remotely sensing biomass are well developed and current spatial resolution is sufficient.
CO2: Indications of changes in carbon storage can include vegetation change leading to “greening” and “browning” trends, as seen in measures such as Normalized Difference Vegetation Index (NDVI). Changes in vegetation type that may affect carbon budgets (such as shrub expansion) are likely occurring on scales of 1-10 m per decade. A daily repeat cycle would be ideal for looking at seasonal shifts in larger-scale NDVI. Therefore an ideal sensor would have a daily repeat cycle with 1-10 m horizontal resolution.
Ground ice (volume and morphology): Several permafrost properties depend on the amount of ice included and the specific geometric forms of these inclusions. If ice is present, then all properties of permafrost become temperature-dependent with a major threshold at the melting point of ice. Because at some locations the volume of ice in permafrost may exceed 80 percent or even 90 percent of the total volume, dramatic changes in the environment could be expected upon thawing. Permafrost can also be very ice poor, and the volumetric ice content may not exceed a few percent in other locations. In this situation, thawing of permafrost will not produce any significant changes in micro-topography. However, even in this situation, the loss of pore ice can affect the physical and biogeochemical properties of the subsurface substrate. A horizontal resolution of 1 to 10 m is sufficient and the accuracy of ice volume estimations of 10% or more of the total volume would be ideal.
Land cover: Vegetation characteristics and land cover are critical to assessing active layer and permafrost dynamics because they affect the surface energy budget, soil organic matter and thermal properties, evapotranspiration and water balance, and snow cover. Current spatial resolutions are sufficient.
Land surface temperature: Land surface temperature, which is different than air temperature, is the temperature of the upper-most surface of objects on the Earth surface as detected by remote sensors. When in equilibrium, land surface temperature is the result of a balance of upward and downward radiative sensible and latent heat fluxes and is impacted by the vegetation, albedo, and underlying soil properties, such as thermal conductivity.
Longer term surface subsidence: The gradual settling or sudden sinking of the Earth's surface due to subsurface movement of earth materials as a result of permafrost thaw.
Methane: In the climate research community, much of the concern about changes in permafrost actually relate to its role in the global carbon cycle, including uncertainty about whether permafrost-dominated regions will become net sinks or sources of greenhouse gases as permafrost thaws. Therefore satellite measurements of carbon dioxide (CO2), methane, and water vapor should be a top priority. A number of future carbon sensors are proposed by NASA and other agencies that, if built, will have the appropriate resolution to detect changes in carbon fluxes from thawing permafrost (100 m-1 km).
Seasonal heave/subsidence: When water turns into ice underground, it expands. This can make the ground move, causing frost heave. Frost heave lifts up the ground, as well as everything on top of it. Likewise, when ice melts, it contracts causing the ground to develop a localized depression.
Snow characteristics: A critically important driver for the evolution of ground temperature is snow on the ground because of its powerful insulating properties. Key properties of snow for permafrost interactions include its depth, density, snow water equivalent, and thermal conductivity. Snow depth on the ground is highly variable on scales from 10 cm and larger, in part because the pack evolves differently when it interacts with vegetation and wind may redistribute snow on the ground, packing it into topographic and biological crevices. Modeling can estimate snow depth from snow-covered areas, but only during the depletion season, and it is limited by the large pixel size of the snow covered area products (such as 500 m from MODIS). This scale is sufficient for large landscape scale estimates of snow cover, but not for studying snow interaction with individual thermokarst features or ice-wedge polygons (1-10 m scale).
Soil moisture: Soil moisture is a significant terrestrial factor for controlling the surface energy balance after the presence or absence of snow cover. It is a major factor in permafrost aggradation and degradation because of the thermal properties of water and ice. Soil moisture information is important for estimating the thermal properties of the ground needed for permafrost modeling. At sub-catchment and finer scales in particular, the spatial patterns (e.g., heterogeneity, or relative spatial variations) of soil moisture become as or more important than the absolute value of the soil moisture at every point. Spaceborne or airborne microwave instruments that provide weekly acquisitions at spatial resolutions of 100 m or better and have a vertical sensitivity to the top 10 cm of the soil moisture profile (i.e., vertical resolution) would meet most user requirements. However, determination of soil moisture to greater depths (i.e., ~1 m into the ground; the top of the permafrost table) would be of even greater value to the permafrost community.
Soil organic layer: Properties such as thickness, moisture content, density, and thermal conductivity are important to permafrost dynamics because of their strong controls on soil thermal properties, active layer dynamics, and permafrost stability. High-resolution LiDAR can be used for detecting change in organic layer thickness over time and could provide useful information, especially before and after fire.
Subsurface soil temperature: According to its definition, the presence or absence of permafrost depends on the temperature of the soil. Many of the properties of permafrost, including those important to engineering, also depend on temperature. Currently, no remote sensing techniques allow direct measurement of subsurface soil temperature.
Surface topography: Topographic variables provide essential information on the presence/absence of permafrost via “integrated terrain units” that combine slope, aspect, and elevation along with other indirect indicators. Topographic variables (such as longer-term surface subsidence and seasonal heave and subsidence) are also useful for estimating the topographically influenced distribution and variability of incident radiation across the surface, which can be used for traditional mapping approaches as well as for input in some permafrost models that simulate radiation balance. For developing regional maps of topography, it would be desirable to have 1 to 10 m horizontal resolution; the higher resolution would be more applicable to local-scale studies of thermokarst features. At more global scales, topographic horizontal resolution would be beneficial at 30 m or less with a vertical accuracy less than 10 cm (this also applies to the next 3 variables)
Surface water bodies: Thermokarst lakes and ponds are important indicators of permafrost degradation. They cover very large regions and have a significant impact on hydrology, geomorphology, and biogeochemical cycling in permafrost lowlands. Currently, SAR data acquired at spatial resolutions of ~3-100 m and at weekly to monthly time scales are suitable for monitoring the evolution of floating ice and bedfast ice for lakes of various sizes.
Surficial geology terrain units: Terrain units are used to differentiate unconsolidated deposits with varying soil/sediment textures, depositional processes, and landscape age.
Thermokarst distribution: Because the distribution of ground ice is usually spatially uneven, the subsidence will develop a localized depression. Additional snow will collect during the winter and surface water will accumulate during the summer in this depression. Both of these processes will make the ground below the depression even warmer, and the local thawing of near-surface permafrost will progress more rapidly, developing into larger depressions, ponds, and eventually lakes. This process is called thermokarst formation and is typical within areas of degrading ice-rich permafrost.
Vegetation structure and composition: Vegetation structure and composition are measured by height, species or lifeform density, leaf area index, and species biomass or cover, and they are important both to the surface energy balance and as indicators of permafrost degradation. Higher resolution time series of systematic vegetation indices would be useful for linking productivity changes to variability in permafrost properties decoupled from more regional scale climate change, including those induced by fire disturbance. Coupling these higher resolution image series with LiDAR would be particularly useful for observing changes in vegetation structure and composition. A vertical resolution of 5 cm and a horizontal resolution of 10-30 m are desirable.
Water vapor flux: Current product resolutions probably suffice for remote sensing of large-scale water vapor; however, estimates of water vapor fluxes between permafrost-dominated landscapes and the atmosphere in the form of evapotranspiration are limited to a few in situ measurements.