Frequently Asked Questions
How would remote sensing of these variables improve our scientific understanding or contribute to societal applications?
Data provides information on permafrost landscapes and on subsurface properties that determine the vulnerability of permafrost systems to warming. Ice content is particularly important, because it strongly influences the vulnerability of permafrost to degradation (thaw) and thus the processes that follow that degradation (e.g., surface subsidence, thermokarst formation, and the release of old carbon frozen for decades to centuries and millennia).
The data sets and products thus derived—and necessarily calibrated and validated with field measurements—would allow parameterization and development of more realistic permafrost models than would otherwise be possible. Remote sensing–derived maps of permafrost properties and improved models would advance our understanding and prediction of the state of permafrost landscapes and associated feedbacks to the climate system.
What are important some of the constraints and opportunities of using remote sensing methods to measure these properties and variables?
Permafrost is a challenge to study because it is a subsurface condition of the ground, largely found in remote locations, and vastly distributed. The temporal evolution of permafrost triggers change in many other ecological characteristics, such as variations in micro-topography, local hydrology, and vegetation. The relationship between permafrost and other ecological characteristics provides an opportunity to observe and document changes in permafrost that otherwise would be difficult to detect. Thus, remote sensing techniques can be applied to observe and monitor permafrost using indirect indicators of permafrost evolution.
Measurements of permafrost-related ecological variables provide some crucial information about changes in the relevant ecological characteristics and are used to extract information about permafrost conditions and processes. One innovative example of a permafrost-related ecological variable is the measurement of changes in seasonal micro-topography to estimate ice content in permafrost. Permafrost properties are those characteristics that are inherent to permafrost. Examples include ice content, maximum depth of seasonal thaw (depth to the surface of permafrost), and permafrost temperature.
Currently, there are considerably more permafrost-related ecological properties that can be observed with remote sensing methods than permafrost properties.
What planned or current platforms can meet anticipated needs for observing these variables?
Although there are a number of current remote sensing instruments on various platforms and planned missions for estimating surface variables relevant to indirect inference of permafrost properties, there are few such instruments or missions for direct estimation of permafrost properties.
Current and planned remote sensing observations and derived data sets will be used in innovative ways to map aspects of permafrost landscapes or indirectly infer subsurface properties:
- Polarimetric, InSAR, and LiDAR may be particularly valuable
- The SMAP mission, planned for launch in late 2014, will be valuable for providing frequent (2-3 days) freeze/thaw and soil moisture products, albeit at relatively coarse (~3 km at best) spatial resolution.
- The U.S. L-band polarimetric InSAR mission would be particularly valuable for mapping higher resolution (100 m or better) seasonal freeze/thaw cycles, surface deformation and subsidence.
- P-band SAR, such as that planned for the BIOMASS mission, scheduled for launch in 2019, has the potential to advance remote sensing of active layer thickness and soil moisture content (and, based on studies with GPR, perhaps even ice content). However, BIOMASS P-band satellite will be restricted from transmitting in most of North America, Europe, and northern Eurasia because of spectrum usage conflicts (especially usage by military services). Regardless of the frequency transmit permission, however, the spatial resolution of BIOMASS (200 m or coarser) is not as high as desired, as noted by many workshop participants.
- The ICESat-2 mission will be valuable for providing time series of surface topography that could be used to map surface deformation and thermokarst features.
Advances in permafrost mapping in the near future are likely to come from studies based on the use of airborne instrumentation coupled with field measurements used for calibration and validation efforts and associated model development:
- P-band stripmap SAR data can be acquired across large regions, where military radars and communications are not affected, to retrieve active layer and other subsurface properties. The potential of using multiple-receiver multiple-transmitter airborne radars, at low frequencies (P-band and lower), has also been demonstrated for 3D imaging of subsurface features, and should be exploited here as well, said several participants.
- L-band InSAR data can be acquired over areas where increases in seasonal thaw and long-term permafrost degradation create substantial deformation of the surface because of changes in subsurface moisture/ice content.
- High resolution LiDAR data can be acquired over areas where there is thermokarst activity, often indicative of rapid permafrost degradation.
- AEM can be acquired over areas where boreholes and other field measurements allow the AEM data to be calibrated to map permafrost extent, depth to top of permafrost, permafrost thickness, near-surface soil moisture and ice content, and other variables of interest.
Each of these airborne observations allows upscaling of field measurements to much larger areas while building on the information that can be discerned from current and planned spaceborne missions. This multi-tiered approach, scaling from field to aircraft to satellite observations would allow high-resolution and spatially extensive retrievals and would thus enable the use of satellite observations over regions where no aircraft or field measurements exist.