Permafrost conditions and processes
Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, USA
ABR, Inc., Fairbanks, Alaska 99701, USA
Osterkamp, T.E., and Jorgenson, M.T., 2009, Permafrost conditions and processes, in Young, R., and Norby, L., Geological Monitoring: Boulder, Colorado, Geological Society of America, p. 205–227 doi: 10.1130/2009.monitoring(09). For permission to copy, contact email@example.com. ©2009 The Geological Society of America. All rights reserved.
Permafrost and the Rationale for Monitoring
Permafrost is ground (soil or rock and included ice and organic material) that remains at or below 0 °C for at least two consecutive years. Permafrost terrain consists of an “active layer” at the surface that freezes and thaws each year, underlain by perennially frozen ground. The top of permafrost is at the base of this active layer. The base of permafrost occurs where the ground temperature rises above 0 °C at depth (Osterkamp and Burn, 2002). In some cases, temperature measurements over a period of two years are required to determine the presence or absence of permafrost. Temperature measurements are also required to determine the status of the permafrost. Permafrost that is warm and/or warming is in danger of thawing.
Approximately 25% of the exposed land area of Earth and ~80% of Alaska are underlain by permafrost. Mountain permafrost occurs at high elevations in western North America and on Mount Washington in New Hampshire. Permafrost has also been found near the summit of Mauna Kea in Hawaii.
Permafrost is a product of cold climates. The fi rst permafrost on earth must have existed prior to or formed coincidentally with the first glaciation, ~2.3 billion years ago. Permafrost occurrences, distribution, and thicknesses must have increased during periods of cold climates and decreased during warm intervals. Permafrost may have disappeared in the Arctic ~50 million years ago. The current permafrost in Alaska appears to have been initiated during the climatic cooling that began ~2.5 million years ago. During the past million years, there is evidence of repeated glaciations at ~100,000-year intervals, and permafrost thicknesses varied significantly in response to them (Osterkamp and Gosink, 1991). The last glacial period ended ~12–14 thousand years ago. About 8–10 thousand years ago, the climate may have been slightly warmer than present. During the last millennium, there was a warm period in the medieval era, followed by a “little ice age.” Permafrost is currently responding to the global warming since then.
Global air temperatures have increased since the mid-1800s (Hansen and Lebedev, 1987). Increases in air temperatures have resulted in an increase in permafrost temperatures. However, other climatic factors, especially timing, duration and accumulation history of the annual snow cover and site wetness impact permafrost temperatures. These factors modify the effects of changes in air temperatures (Zhang et al., 1996).
The climatic changes of the past century coupled with recent observations of warming and thawing permafrost have caused concern about the future of permafrost (PCCGR, 1983; McBeath, 1984). Thawing permafrost in natural settings has been observed in Alaska (Osterkamp, 1994, 1995; Osterkamp et al., 1998, 2000; Jorgenson et al., 2001a). Warming of the permafrost has continued into the twenty-first century in Alaska, Europe, Svalbard, Canada, Russia, China, and Mongolia (Phillips et al., 2003). There are increasing reports of thawing permafrost and thermokarst terrain (an irregular topography resulting from thawing permafrost containing excess ground ice) in Alaska (Osterkamp et al., 2000; Jorgenson et al., 2001a; Jorgenson and Osterkamp, 2005). Thin permafrost is thawing from the bottom
up at some sites (Osterkamp, 2003a; 2005). Ice wedges are thawing in the Alaskan Arctic where temperatures were thought to be too cold for this to happen (Jorgenson et al., 2006).
Global circulation models predict that air temperatures will increase up to 5 °C in the next half century (Maxwell, 1992). Since continuous permafrost (a region where permafrost occurs everywhere beneath exposed land surfaces) is typically colder than −6 °C, no widespread thawing is expected, although some areas may experience localized thawing, slope instability and ice wedge thawing. Discontinuous permafrost (a region where some areas are free of permafrost) and mountain permafrost (permafrost existing at high altitudes) at low latitudes are much warmer, so any climatic warming will cause thawing. A warming of just a few degrees would cause most of it to begin thawing. Thawing proceeds from the top downward and, eventually, from the bottom upward. Thawing rates are slow, initially on the order of 0.1 m per year near the surface, and theoretically less than 0.02 m per year at the base (Osterkamp, 1983). Thus, times ranging from decades to millennia are required to thaw discontinuous permafrost.
All of the national parks and preserves on mainland Alaska are at least partially underlain by permafrost. Those south of the Yukon River and on the south side of the Seward Peninsula are underlain by warm, discontinuous permafrost, typically within a few degrees of thawing. Recent sparse measurements indicate that much of it is within a degree of thawing (Osterkamp, 1983; 1994; 2005; 2007). These measurements also show that it has warmed significantly during the last quarter century (Osterkamp and Romanovsky, 1999; Osterkamp, 2003a; Osterkamp, 2005; 2007). Thawing permafrost, landslides, and thermokarst terrain have been observed in and around these parks (Osterkamp et al., 2000; Jorgenson et al., 2001a; Jorgenson et al., 2006). An estimated 100,000 km2 of mountain permafrost occurs in the contiguous states at elevations as low as 2200 m (Péwé, 1983). Many of the parks in the western United States have mountainous areas with higher elevations. While there is little information available, mountain permafrost in the contiguous states is also thought to be very warm (within a degree or two of thawing). Because ice helps bond slope deposits, landslides and thaw slumps are expected to occur when these areas thaw (Huscroft et al. 2004). In addition, permafrost impedes subsurface drainage, creating wet to moist conditions that provide good habitat for many alpine plant species. Thawing of the permafrost would increase drainage and dry the soil, thus impacting the vegetation.
The combination of the above observations and conditions and the predicted climatic warming of the twenty-first century are cause for concern about the future condition of permafrost in national parks and preserves in Alaska and in the mountains of the contiguous states. Thus, it is important to determine what changes have already occurred, to determine the current status of the permafrost, and to monitor changes that may occur in the future.
Cause for Concern
Why should there be concern for thawing permafrost? Current climatic scenarios predict up to 5 °C of additional warming of the air temperatures in Alaska and the Bering Sea regions over the next century (Weller et al., 1995). At the low end of the predicted warming (2–3 °C), most of the discontinuous permafrost in Alaska would thaw, creating many attendant problems (Osterkamp, 1983; PCCGR, 1983; Nelson et al., 1994; Osterkamp et al., 1998). Thawing of ice-rich permafrost and creation of thermokarst terrain has been identified as one of the primary problems facing northern ecosystems as a result of climatic warming (Osterkamp et al., 2000; Jorgenson et al., 2001a). While smaller changes are predicted for the contiguous states, any warming there would cause some of the mountain permafrost to thaw.
Boreal forests typically cover discontinuous permafrost in interior Alaska below elevations of 700–1000 m. Sparse data around these parks indicates that permafrost there is usually
within 1–2 °C of thawing. The edges of isolated permafrost bodies are already at the thawing point. If the observed warming of the permafrost underlying boreal forest ecosystems in Alaska continues, then additional permafrost will thaw. Where the permafrost is ice-rich (roughly 50% ice), thawing changes the ice to water, creating a mud slurry that cannot support the weight of overlying soil or vegetation, thereby degrading the physical foundation of terrestrial ecosystems. The observed effects are that the ground subsides, and landslides and thermokarst terrain develop, consisting of channels, pits, troughs, potholes, ponds, lakes, and “drunken forests” (trees leaning in random directions). In addition to these broad-scale climatic effects, thermokarst terrain can also be produced locally by disturbances associated with fires, floods, and human and animal disturbances.
Thermokarst drastically modifi es and remolds the ground surface and alters surface and groundwater hydrology. This process can modify or totally change ecosystems, human activities, infrastructure, and the fluxes of energy, moisture, and gases across the ground surface-air interface. Plant species composition and distribution, plant community productivity, soil chemistry, biological activity, and nutrient supply for plant use can be substantially altered by this geological phenomenon. Drainage conditions determine whether standing water will be present. The affected trees usually die, and vegetation changes significantly (Fig. 1). These changes in the flora have a direct impact on fauna. In lowlands or relatively flat areas, a shift from boreal forests to shrub swamps and wet meadows often occurs with concurrent changes in bird and animal populations (Osterkamp et al., 2000; Jorgenson et al., 2001a, 2006). The new ecosystems often favor aquatic birds and mammals.
Thus, the result of thawing ice-rich permafrost in a boreal forest ecosystem is not just a slight shift in the nature of the ecosystem, but rather partial or total destruction of the ecosystem and replacement with a new ecosystem.
Time scales to create thermokarst terrain are around a decade, but can range from several years to centuries. Time scales for recovery of the ecosystems appear to range from centuries to millennia although, in many cases, recovery may be impossible because of permanent changes in relief, drainage, and other factors.
Need for Monitoring
Although little can be done about the terrestrial and ecological changes associated with natural thermokarst, knowledge about the patterns and processes of thermokarst development is essential for anticipating the potential impacts and for developing rational responses to them. Detailed observations and measurements of the thermal regimes and physical conditions of the permafrost are needed.
Permafrost conditions in the parks and preserves of Alaska and the mountains of the contiguous states depend on a number of factors. Thus, monitoring sites need to be developed in each park to span the range of conditions found there. Accordingly, this chapter makes recommendations for standard methods for observations and measurements that can be applied across a network of parks to determine what changes have already occurred, document current conditions, and monitor future changes.