Permafrost thermal snapshot and active-layer thickness in Svalbard 2016–2017 1 HH Christiansen1, GL Gilbert1, N Demidov2, M Guglielmin3, K Isaksen4, M Osuch5, J Boike6,7 1 The University Centre in Svalbard, UNIS, Arctic Geology Department, Longyearbyen, Norway 2 Arctic and Antarctic Research Institute, AARI, Ministry of Natural Resources and Environment, Russia 3 Insubria University, Department of Theoretical and Applied Sciences, Italy 4 Norwegian Meteorological Institute, Research and Development Department, Oslo, Norway 5 Institute of Geophysics, Polish Academy of Sciences, Poland 6 Alfred Wegener Institute for Polar and Marine Sciences, Potsdam, Germany 7 Department of Geography, Humboldt University of Berlin, Berlin, Germany Corresponding Author: Hanne H Christiansen, [email protected], ORCID Number: 0000-0002-6218-3493 Keywords: permafrost, active layer, meteorology, ground temperature, Svalbard 26 SESS Report 2018 – The State of Environmental Science in Svalbard Introduction and objectives This report is the product of international collaboration of several permafrost researchers working in Svalbard. The report aims to provide an overview of ground thermal conditions and active-layer thickness as they are recorded at five sites during the 2016/2017 hydrolog- ical year from 1 September 2016 to 31 August 2017 in Svalbard. We report on this period, as this is when observation variables are available from all sites. This provides the basis for comparison of spatial variations in permafrost thermal conditions and active layer thickness in Svalbard. For earlier summaries of permafrost conditions on Svalbard see Humlum et al. (2003) and Christiansen et al. (2010). The specific objectives of this report are to: (1) introduce the study area and permafrost in Svalbard; (2) describe instrumentation and operation at each of the five sites; (3) character- ize the ground thermal regime and present the active-layer thickness from the last 2016- 2017 hydrological year; (4) provide an overall analysis of the ground-thermal observations with a focus on the implications of changing permafrost on other parts of the cryosphere relevant for the SIOS network; (5) ensure access to the reported data through the Global Terrestrial Network on Permafrost (GTN-P) adhering to the SIOS Data policy and (6) point to potential avenues and geographic locations for future permafrost observation needs in Svalbard. This report builds on the IPY 2007-2008 snapshot of the permafrost thermal state and active layer thickness in Svalbard (Christiansen et al., 2010), but now provides ground tem- peratures from more areas in Svalbard thanks to the international collaboration. The report may serve as a baseline for future regional observation programs and collaborative activities within the SIOS network. Permafrost background Permafrost plays an important role in the Earth System as it underlies 25% of the terrestrial parts of Planet Earth. It is a ground thermal condition occurring in cold regions, and is defined as ground (soil, sediment, or rock) that remains at or below 0°C for two or more consecutive years. Svalbard has the warmest permafrost this far north (Romanovsky et al., 2010). The thickness of permafrost on Svalbard is believed to range from a few meters in coastal areas to several hundred meters in mountain peaks (Liestøl, 1977; Humlum, 2005). Permafrost regions are further characterized by the presence of an active layer – the layer above the permafrost which thaws during summer and refreezes during winter. The active layer is of significance as it is the main zone through which water moves in permafrost landscapes, and in which chemical and biological processes are most active (French, 2013). Permafrost thermal snapshot and active-layer thickness in Svalbard 2016–2017 27 Due to the nature of permafrost being a negative thermal state of the ground that is often close to the freezing point, it can be affected by climatic changes. However, the relationship between air temperature and ground temperatures is mediated by conditions at or near the ground surface (Smith & Riseborough, 2002). Snow cover impacts ground temperatures, through its insulating effect, by reducing heat loss from the ground during the winter sea- son. In addition, variations in the thermal properties of the active layer, between frozen and unfrozen states, reduce the heat flow into the ground. The thermal conductivity (the readiness with which a material conducts heat) is approximately four-times higher for ice than for liquid water. This means that the ground cools more readily when frozen. Finally, the amount of time it takes for the active-layer to freeze-back during the late autumn and early winter has a significant impact on the underlying permafrost. During active-layer freeze-back, temperatures in the active layer are isothermal above the phase-equilibrium temperature, and the permafrost is less directly influenced by the atmosphere (Osterkamp & Romanovsky, 1997). Following the freeze-back of the active layer, the temperatures at the permafrost surface is permitted to decline (c.f. Burn & Zhang, 2009). The duration of active-layer freeze-back is an important variable as it is a derivative of several factors including: active-layer moisture content, snow cover timing and thickness, and autumn air temperatures. Longer freeze-back durations reduce the amount of time available for ground cooling, resulting in increased ground temperatures in the permafrost. Essential climate variables on permafrost in Svalbard The monitoring of essential climate variables, ECVs, for permafrost is delegated to the Global Terrestrial Network on Permafrost (GTN-P) by the World Meteorological Organiza- tion (WMO). GTN-P established permafrost temperature and active-layer thickness (ALT) as ECVs related to two specific monitoring programs: 1) TSP (Thermal State of Permafrost) and 2) CALM (Circumpolar Active Layer Monitoring) (Romanovsky et al., 2010; Shiklomanov et al., 2012). GTN-P was developed in 1999 by the International Permafrost Association (IPA) with active support by the Canadian Geological Survey (Burgess et al., 2000) under the Global Climate Observing System (GCOS) and the Global Terrestrial Observing Network (GTOS). The purpose of GTN-P is to establish an open access early warning system for the consequences of climate change in permafrost regions. The first overview of GTN-P obser- vations and their key results include data from Svalbard (Biskaborn et al., 2015). Ground temperature monitoring sites consist of three elements: a borehole, an encased thermistor string and an automated data logger. As sites were established independently, there are differences in the instrumentation used and the depths at which ground tem- perature sensors (thermistors) are positioned. The density of temperature sensors generally decreases with depth as the temperature signal at the ground surface is rapidly attenuated 28 SESS Report 2018 – The State of Environmental Science in Svalbard moving with depth into the ground. The automatic data loggers in all boreholes are pro- gramed to record the borehole temperature at regular intervals (varying from 1 to 6 hours). The instrumentation used at each site is included in Table 1. Boreholes vary in length from a few meters to upwards of 100 meters. For this report, ground temperatures from within 20 m of the ground surface are included to characterize the upper part of the permafrost profile, the part that is most directly affected by climatic variations. We are aware that for some boreholes the casing might affect the ground temperatures recorded closer to the ground surface. To study the active layer dynamics we have also used data from all parts of the boreholes. The thickness of the active layer is either recorded directly through probing or estimated by interpolating the depth of the 0°C isotherm using borehole thermal measurements (Burn, 1998). Probing is suitable only in fine-grained soils, without gravel and boulders. In addition to point measurements, made at each borehole, two Circumpolar Active Layer Monitoring (CALM) sites are established in Svalbard, one in Adventdalen (UNISCALM) and one near Barentsburg (Fig. 1; Christiansen & Humlum, 2008; Shiklomanov et al., 2012). These CALM sites consist of a grid, measuring 100 m x 100 m or 50 x 50 m with 10 or 5 m grid size, and probe measurements are made at the 121 regularly spaced grid points. Measurements are repeated throughout the thawing season to monitor the thaw progression in the UNIS- CALM site. In Svalbard, systematic measurements of active-layer thickness and ground-temperature profiles are presently limited to sites in the western and central parts in Spitsbergen (Fig. 1). Data from Ny-Ålesund, Adventdalen, Barentsburg, Kapp Linné, and Hornsund are examined. These areas are all located near major settlements and research stations in Svalbard, and have been established independently by scientists based in Norway, Svalbard, Germany, Italy, Russia, and Poland. The results of this report will therefore be of significance to those working in these areas, and form the foundation for a collaborative international long-term permafrost monitoring network in Svalbard as part of SIOS. Permafrost thermal snapshot and active-layer thickness in Svalbard 2016–2017 29 Table 1: Site information and metadata for permafrost boreholes used in this report. Borehole name/ Elevation Borehole Instrument Sensor depths Location Operator Longitude Latitude Landform ID (m a.s.l.) depth (m) manufacturer