InSAR Observations Revealed Surface Subsidence Over Permafrost in Northern Alaska

by Lin Liu¹, Tingjun Zhang², Kevin Shaefer², and John Wahr¹

1. Department of Physics and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA.
2. National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA.

The Arctic climate has experienced more rapid warming than anywhere else on Earth over the past hundred years and this trend is expected to continue over the next century [Intergovernmental Panel on Climate Change (IPCC), 2007]. About 80% of Alaska is underlain by permafrost, which is soil at or below 0°C for at least two consecutive years. The active layer is the surface layer of soil that thaws in the summer and freezes in the winter. Observations indicate the active layer and permafrost are already responding to the Arctic warming. For instance, the Beaufort Coastal Plain on the North Slope of Alaska is underlain by ice-rich permafrost that contains ground ice up to 70% by volume (Figure 1). In this region, permafrost temperatures have increased by 2º to 3°C since the mid 1980s in response to the rapid warming of the Arctic (Osterkamp, 2007). It is generally hypothesized that climate warming causes permafrost degradation, i.e., thickening of the active layer, warming, and thawing of permafrost. However, ground-based measurements show no significant trend in the active layer thickness (ALT) in northern Alaska (Brown, et al., 2000).

Permafrost degradation has profound effects on mechanical stability of ground, on biological, biogeochemical, hydrologic, and landscape processes, on the flux of greenhouse gases, and on human infrastructure. It is critical to conduct comprehensive measurements to monitor changes in permafrost and the active layer at regional scales. The International Permafrost Association’s Circumpolar Active Layer Monitoring (CALM) program has conducted sitespecific and grid (one km by one km) measurements of active layer thickness over the past two decades and has produced valuable data and information (Brown, et al., 2000). However, the existing CALM network is still underpopulated and does not represent the full range of climatic and physiographic variability.

Satellite remote sensing using the Interferometric Synthetic Aperture Radar (InSAR) technique has the potential of providing regional-scale monitoring of the active layer and near-surface permafrost dynamics across a broad climatic and physiographic spectrum. The InSAR technique was applied to the European Remote Sensing Satellite-1 (ERS-1) and ERS-2 SAR images taken during summer seasons between 1992 and 2000 to measure surface deformation over the permafrost area on the North Slope of Alaska. The SAR data was archived at the Alaska Satellite Facility (ASF). Surface subsidence of 2 to 4 cm was found in each thaw season between June and September (Figure 2a). In addition, a relatively small, but significant long-term trend in surface subsidence at rates of one to 4 cm per decade was found (Figure 2b). The physical mechanism of surface deformation is linked to soil water phase change within the active layer and near-surface permafrost. The active layer on the North Slope of Alaska is generally saturated with water content greater than 40% by volume. When pore ice melts in the active layer in summer, its volume reduces by about 9%, resulting in surface subsidence. Therefore, the seasonal subsidence is directly related to the volume of melted water in the active layer. A retrieval algorithm was developed that estimates ALT and its change from the InSAR-measured seasonal subsidence using the vertical distribution of water and ice within the soil. As an alternative method, remote sensing of ALT, using InSAR, could be particularly useful for filling the spatial gaps between ground-based measurements in remote permafrost areas.

At a longer time scale of a few years, melting of ground ice beneath the active layer can cause substantial thaw settlement across a broad spectrum of spatial scales and landforms. If enough heat transfers through the active layer to the underlying permafrost, ice-rich permafrost thaws and ground ice melts. Meltwater drains into lowlands, river channels, and thaw lakes, resulting in surface subsidence. Thawing of ice-rich permafrost near the permafrost table offers a possible explanation both for the InSAR-measured secular surface subsidence and for the negligible trends in ALT from ground-based measurements despite an observed increase in permafrost temperatures. This is because little soil material adds to the overlying active layer upon thawing of ice-rich permafrost.

Overall, InSAR is uniquely suited for monitoring surface deformation at high spatial resolution over large permafrost areas. Combinations of ground-based and remote-sensing measurements such as InSAR are valuable for monitoring changes in the active layer and permafrost, identifying scales of spatial variability, detecting temporal trends, and validating permafrost models.

References:
Brown, J., K. M. Hinkel, and F. E. Nelson (2000), The Circumpolar Active Layer Monitoring (CALM) Program: Research designs and initial results, Polar Geography, 24(3), 166–258, doi:10.1080/10889370009377698.
IPCC (2007), Climate Change 2007: The physical science basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press.
Liu, L., T. Zhang, and J. Wahr (2010), InSAR Measurements of Surface Deformation Over Permafrost on the North Slope of Alaska, J. Geophys. Res., 115, F03023, doi:10.1029/2009JF001547.
Osterkamp, T. E. (2007), Characteristics of the Recent Warming of Permafrost in Alaska, J. Geophys. Res., 112, F02S02, doi:10.1029/2006JF000578.