THE CIRCUMPOLAR ACTIVE LAYER MONITORING (CALM) PROGRAM: RESEARCH DESIGNS AND INITIAL RESULTS1 J. Brown International Permafrost Association P. O. Box 7, Woods Hole, MA 02543 K. M. Hinkel Department of Geography, University of Cincinnati Cincinnati, OH 45221-0131 F. E. Nelson Department of Geography and Center for Climatic Research University of Delaware, Newark, DE 19716-2541 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 tempera- ture on an interannual basis. The relatively few long-term data sets available from north- ern 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 Organization’s (WMO) Global Terrestrial Network for Permafrost (GTN-P). GTN-P observations con- sist 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. PART I. INTRODUCTION2 Evidence continues to accumulate that climatic change is having a profound impact in the Earth’s cold regions (Morison et al., 2000; Serreze et al., 2000; Anisimov et al., 2001). Long-term environmental monitoring of sea ice extent (Parkinson et al., 1999), glacier mass balance (Dyurgerov and Meier, 1997; Sturm et al., 2001), vegeta- tion (Myneni et al., 1997), ocean temperature and circulation (Morison et al., 2000), 1All lead and contributing authors are named in alphabetical order throughout the report. A complete list of contributors is given, by country or region, in Appendix 3. 2Authors: J. Brown, K. M. Hinkel, F. E. Nelson, and N. I. Shiklomanov. 166 Polar Geography, 2000, 24, No. 3, pp. 165-258. Copyright © 2000 by V. H. Winston & Son, Inc. All rights reserved. POLAR GEOGRAPHY 167 permafrost (Osterkamp and Romanovsky, 1999), and air temperature indicate that concerns about pronounced warming in polar regions raised more than a decade ago by general circulation modeling experiments (e.g., Manabe et al., 1991) were well founded. The Intergovernmental Panel on Climate Change Third Assessment Report recently stated with very high confidence that “regions underlain by permafrost have been reduced in extent, and a general warming of ground temperatures has been observed in many areas” (Anisimov et al., 2001, p. 803). Changes in the ground thermal regime of cold regions have considerable potential for bringing about ecological and terrain disturbances (Moskalenko, 1998a; Forbes, 1999; Garagulya and Ershov, 2000; Osterkamp et al., 2000; Beilman et al., 2001; Jorgenson et al., 2001), and for disrupting existing foundations, roads, and other built structures. These empirical studies indicate that recent climate-induced changes in permafrost environments are widespread and accelerating. Modeling studies indicate that climate warming will continue to decrease the geographical extent of the perma- frost regions (Kane et al., 1991; Anisimov and Nelson, 1996; Vyalov et al., 1998), increase the thickness of the seasonally thawed layer above permafrost (Anisimov et al., 1997), and create widespread hazards for engineered works (Smith and Burgess, 1999; Nelson et al., 2001). The Circumpolar Active Layer Monitoring (CALM) network was developed in the 1990s to address some of these concerns and scientific issues (Burgess et al., 2000). The active layer, defined as “the top layer of ground subject to annual thawing and freezing in areas underlain by permafrost” (Permafrost Subcommittee, 1988, p. 13), plays an important role in cold regions because most ecological, hydrological, biogeochemical, and pedogenic activity takes place within it (Hinzman et al., 1991; Kane et al., 1991). The thickness of the active layer is influenced by many factors including surface temperature, thermal properties of the surface cover and substrate, soil moisture, and the duration and thickness of snow cover (Hinkel et al., 1997; Paetzold et al., 2000). Consequently, there is widespread variation in active-layer thickness across a broad spectrum of spatial and temporal scales (e.g., Pavlov, 1998; Nelson et al., 1999; Hinkel and Nelson, in press). Geocryological investigations have been under way in both polar lowlands and mountainous regions throughout most of the past century. Other than at Russian research stations and a few other notable exceptions, most investigations of active- layer dynamics involved relatively short time periods (e.g., 3–5 years) and did not consider the consequences of long-term climatic change. Well-documented studies of active-layer response to disturbance (fire, drilling, trails, etc.) do exist, however, and these may serve as analogs of future natural changes and responses in soil climate (Walker et al., 1987; Mackay, 1995; Burn, 1998a, 1998b; Moskalenko, 1998a). In general, several decades are required following disturbance for the active layer to recover or stabilize. Prior to the 1990s, many data sets related to the thickness of the active layer were collected as part of larger geomorphological, ecological, or engi- neering investigations, and used different sampling designs and collection methodolo- gies. Moreover, the typical study did not deposit data records in archives accessible for general use (see Barry, 1988). The combined effect of these circumstances made it difficult to investigate long-term changes in seasonal thaw depth or possible inter- regional synchronicity. 168 BROWN ET AL. Measurements of interannual and multi-decadal variations in the seasonal thawing and refreezing of permafrost soils on regional and hemispheric scales are required to understand and refine predictions of the response of cold soils and permafrost to cli- mate change. The hypothesis and existing evidence that warming will increase the thickness of the active layer, resulting in thawing of ice-rich permafrost, ground insta- bility, and surface subsidence require further investigation under a variety of contem- porary environmental settings. Historical Background Several factors and events converged in the late 1980s and early 1990s to encour- age development of long-term geocryological monitoring, and to make the resulting data sets freely available to interested users. These include: (1) publicity about the impacts of climate change followed two decades of unprecedented resource develop- ment in the cold regions and raised concerns about the stability of associated infra- structure; (2) permafrost scientists became increasingly aware of the benefits accruing from free exchange of data (Barry et al., 1995); (3) international agreements were signed and governments became concerned with facilitating data exchanges with interested users; and (4) the global nature of climatic change made apparent the need for widespread cooperation among permafrost scientists, who became increasingly aware of the importance of their subject in the context of recent climate change. In an initial attempt to meet the new needs and to facilitate and coordinate data acquisition procedures, the International Permafrost Association (IPA) developed the Global Geocryological Database (Barry et al., 1995). The GGD is a collection of data from pre-existing and ongoing research projects. The first CD-ROM containing GGD data was compiled and distributed by the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado (IPA, 1998). An international symposium held in Yamburg, West Siberia in 1989 (Melnikov, 1990) set the stage for expanded multi-national cooperation among permafrost scien- tists. During the symposium, a field trip to the Parisento Research Station on the Gydan Peninsula focused attention on the need and opportunities for international cooperation in monitoring the thermal regime and thickness of the active layer. The resulting series of international scientific exchanges and site visits stimulated initia- tion of standardized active-layer sampling designs. By 1993, several 1000 × 1000 m (1 km2) grids in Alaska and western Siberia had been established. Following these early informal international activities, the CALM program was formally established in the mid-1990s as a long-term observational program designed to assess changes in the active layer and to provide ground truth for regional and glo- bal models. The CALM approach was the first attempt to collect and analyze an inter- national geocryological data set obtained according to standardized, international methods. The initial CALM sampling design was developed in 1995 at the 6th Inter- national Tundra Experiment (ITEX) workshop, held in Ottawa, Canada (Åkerman, 1995; Brown et al., 1995). It included thaw measurements associated with the ITEX experimental design (Open Top Chambers–OTC; Henry, 1997). Several types of mea- surement serve as minimum requirements for observing end-of-season thaw depths— probing on grids ranging in