Land

V. Romanovsky¹, R. Armstrong², L.D. Hinzman³, N. Oberman⁴, A. Shiklomanov⁵

¹Geophyiscal Institute, University of Alaska Fairbanks, Fairbanks, AK
²CIRES/NSIDC, University of Colorado, Boulder, CO
³International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK
⁴MIREKO Stock Co., Syktyvkar, Russia
⁵University of New Hampshire, Durham, NH

Permafrost

Long-term permafrost temperature data are available only from a few clusters of stations, mostly in North America. Observations from the long-term sites show a general increase in permafrost temperatures during the last several decades in Alaska (Osterkamp and Romanovsky, 1999; Romanovsky et al., 2002; Osterkamp, 2003), northwest Canada (Couture et al., 2003; Smith et al., 2003), Siberia (Pavlov, 1994; Oberman and Mazhitova, 2001; Romanovsky et al., 2002; Pavlov and Moskalenko, 2002), and Northern Europe (Isaksen et al., 2000; Harris and Haeberli, 2003). Uninterrupted permafrost temperature records for more than a 20-year period have been obtained by the University of Alaska Fairbanks along the International Geosphere-Biosphere Programme Alaskan transect, which spans the entire continuous permafrost zone in the Alaskan Arctic. All of the observatories show a substantial warming during the last 20 years. This warming was different at different locations, but was typically from 0.5 to 2°C at the depth of zero seasonal temperature variations in permafrost (Osterkamp, 2005). In 2006, there was practically no change to the mean annual temperatures at the permafrost surface if compared to 2005 (Romanovsky et al., 2006). These data also indicate that the increase in permafrost temperatures is not monotonic. During the observational period, relative cooling has occurred in the mid-1980s, in the early 1990s, and then again in the early 2000s. As a result, permafrost temperatures at 20 m depth experienced stabilization and even a slight cooling during these periods.

Very similar permafrost temperature dynamics were observed in the European North of Russia during the same period (Figure L1). However, there is some lag in the soil temperature variations at the Alaskan sites compared to the Russian sites. This observation is similar to what was discovered in comparison of permafrost temperature dynamics in Fairbanks, Alaska and Yakutsk, Russia (Romanovsky et al., 2007). Relative cooling has occurred in Vorkuta region in the early and late 1980s and then in late 1990s. The total warming since 1980 was almost 2°C at the Vorkuta site.

Data on changes in the active layer thickness (ALT) in the arctic lowlands are less conclusive. In the North American Arctic, ALT experiences a large interannual variability, with no discernible trends. This is likely due to the short length of historical data records (Brown et al., 2000). A noticeable increase in the active layer thickness was reported for Mackenzie Valley (Nixon et al., 2003). However, this positive trend was reversed into a negative trend at the most of these sites after 1998 (Tarnocai et al., 2004). An increase in thickness of more than 20 cm between the mid-1950s and 1990 derived from the historical data collected at the Russian meteorological stations was reported for the continuous permafrost regions of the Russian arctic (Frauenfeld et al., 2004; Zhang et al., 2005). At the same time, reports from several specialized permafrost research sites in Central Yakutia show no significant changes in the active layer thickness (Varlamov et al., 2001; Varlamov, 2003). The active layer was especially deep in 2005 in Interior Alaska. Around Fairbanks the 2005 active layer depth was the deepest observed in the past 10 years. Data from many of these sites show that the active layer developed during the summer of 2004 (one of the warmest summers in Fairbanks on record) did not completely freeze during the 2004-2005 winter. A thin layer just above the permafrost table was unfrozen during the entire winter. Active layer in the summer of 2006 was also one of the deepest on record at most of our observation sites in the Fairbanks area even though the summer air temperatures were close to normal.

Figure L1. Top: Location of the long-term MIREKO permafrost observatories in northern Russia. Bottom: Changes in permafrost temperatures at 15 m depth during the last 20 to 25 years (Oberman, 2007).

Thermokarst (Thermokarst is a land surface formed as permafrost melts)

Thermokarst topography forms as ice-rich permafrost thaws, either naturally or anthropogenically, and the ground surface subsides into the resulting voids. The important and dynamic processes involved in thermokarsting include thaw, ponding, surface and subsurface drainage, surface subsidence and related erosion. These processes are capable of rapid and extensive modification of the landscape. Recent analysis suggests that in regions over thin permafrost (~<20 m), surface ponds may shrink and surface soils may become drier as the permafrost degrades (Figure L2). In colder regions with thicker permafrost, as the warming proceeds, near surface ice thaws, the land surface subsides and new water bodies are formed (Hinkel et al., 2007; Jorgenson and Shur, 2007; Jorgenson et al., 2006; Riordan et al., 2006; Smith, et al., 2005; Yoshikawa and Hinzman, 2003).

Figure L2. Numerous tundra ponds near Council, Alaska (64° 51' N, 163° 42’ W) have decreased in surface area over the last 50 years. A probable mechanism for these shrinking ponds is internal drainage through the degradation of shallow permafrost (Yoshikawa and Hinzman, 2003).

Extensive thermokarsting, resulting in the creation of new water-filled surface depressions, has recently been observed on the Beaufort Coastal Plain in northern Alaska (Jorgenson et al. 2006). Analysis of aerial photography indicated that widespread ice wedge degradation had not occurred before 1980. Field observations and sampling showed that ice wedge degradation has been relatively recent, as indicated by newly drowned vegetation. Despite the relatively cold average annual temperature of this northern permafrost, thermokarst was widespread on a variety of terrain conditions, but most prevalent on ice-rich centers of old drained lake basins and alluvial-marine terraces.

The ponds on the Seward Peninsula were examined to determine if recent changes in climate have impacted the dynamics of their development and degradation (Yoshikawa and Hinzman, 2003). Of the 24 ponds studied, 22 have decreased in area between 1951 and 2000.

Smith et al., 2006 demonstrated that similar processes are occurring in the Siberian Arctic. In northern Siberia, where cold continuous permafrost dominates, there has been a significant increase in the number and size of surface water bodies (consistent with the Jorgenson et al, 2006 study). In the more southerly parts of Siberia, where warmer, discontinuous permafrost predominates, there has been a significant decrease in the number and size of surface water bodies (consistent with the Yoshikawa and Hinzman, 2003 study).

Snow Extent

For the Northern Hemisphere winter of 2005-2006, the microwave data indicate negative departures from the long term mean (1978-2006) for every month except March with an average negative departure for the winter months (November through April) of approximately 1.3 million km². For the calendar year of 2006, the NOAA data indicate an average snow cover extent of 24.9 million km² which is 0.6 million km² less than the 37-year average (Robinson, personal communication).

Northern Hemisphere snow cover extent has a mean maximum of approximately 47 million km², typically in February. The minimum usually occurs in August and is less than about 1 million km², most of which is snow on glaciers and perennial snow fields. As a result, snow cover is the land surface characteristic responsible for the largest annual and interannual differences in land surface albedo (Figure L3). Snow covers a much smaller area in the Southern Hemisphere and plays a relatively small role in global climate.

Hemispheric-scale snow cover fluctuations are monitored with satellite data. Since 1966, NOAA has produced snow extent charts (Robinson et al., 1993; Frei and Robinson, 1999). These charts were primarily derived from the manual interpretation of visible band imagery until 1999, when passive microwave and other data sources became available (Ramsay, 1998; NOAA/NESDIS/OSDPD/SSD, 2004, update 2006). Passive microwave data can enhance snow measurements based on visible data alone, sensing the surface through clouds and in darkness. However, passive microwave may not detect some areas of shallow snow that can be seen in visible band imagery. As a result, time series from the two sources can differ. Figure L4 compares a microwave data derived snow cover data set (Armstrong and Brodzik, 2001; Armstrong et al., 2005b) with NOAA snow extent data. Both show similar inter-annual variability and consistently indicate Northern Hemisphere maximum extents exceeding 40 million km². The NOAA time series indicates a decreasing trend of -2.0% per decade (Brodzik et al. 2006). There is a decreasing trend of -0.7% per decade in the microwave snow cover, although it is not significant at the 90% level. Both sources indicate a decreasing trend in snow cover in every month but November and December. The strongest seasonal signal occurs during May to August when both indicate significant decreasing trends. The western United States is among the regions with the strongest decreasing trends, supporting Groisman et al. (2004) and Mote et al. (2005) results using in situ observations. Shallow snow cover at low elevations in temperate regions is the most sensitive to temperature fluctuations and hence most likely to decline with increasing temperatures (IPCC 2007).

Figure L3. Mean snow cover extent (grey), 1966-2006, for February (left) and August (right) from the Northern Hemisphere Equal Area Scalable Earth (EASE)-Grid Weekly Snow Cover and Sea Ice Extent data set (Armstrong and Brodzik, 2005). The product includes climatologies of snow average conditions, probability of occurrence, and variance based on NOAA charts as revised by Robinson et al. (1993).

Figure L4. Time series of Northern Hemisphere snow-covered area (SCA) derived from passive microwave (green/blue) and visible (pink) sensors (top), and SCA departures from monthly means (bottom), from NOAA snow charts (orange) and microwave (purple/green) data sets.

Glaciers

Glaciers and ice caps, excluding those adjacent to the large ice sheets of Greenland and Antarctica, can be found on all continents except Australia and have an estimated area between 512 and 540 x 10³ km². The complicated and uncertain processes that control how fast glaciers move make it difficult to use changes in the areal extent of glaciers as a straightforward indicator of changes in climatic conditions. Further, many large collections of glacier photographs are available, but it is only in the last decade or so that remote sensing imagery has provided a means to monitor changes in the areal extent of glaciers. The Global Land Ice Measurements from Space glacier database project, with participation from more than 60 institutions in 28 nations, is working now on a baseline study to quantify the areal extent of existing glaciers (Armstrong et al., 2005a).

Mass balance measurements, or the difference between the accumulation and ablation, are a more direct method to determine the year-to-year "health" of a glacier. Changes in mass balance correspond to changes in glacier volume. These measurements are typically obtained from less than about 0.2 percent of the world's glaciers. Researchers have measured mass balance on more than 300 glaciers since 1946, although a continuous record exists for only about 40 glaciers since the early 1960s. Nevertheless, considerable compilation and analysis has occurred (e.g. Cogley, 2005). These results indicate that in most regions of the world, glaciers are shrinking in mass. From 1961 to 2003, the thickness of "small" glaciers decreased approximately 8 meters, or the equivalent of more than 6,000 cubic kilometers of water (see http://nsidc.org/sotc/glacier_balance.html). Recent mass loss of glaciers and ice caps is estimated to be 0.51+/- 0.32 mm sea level equivalent (SLE) per year between 1961 and 2003 and 0.81 +/-0.43 mm SLE per year between 1993 and 2003 (Dyurgerov and Meier, 2006; http://nsidc.org/sotc/sea_level.html). The greatest mass losses per unit area are found in Patagonia, Alaska and NW USA/SW Canada. However, because of the corresponding large areas, the biggest contributions in total to sea level rise come from Alaska, the Arctic and the Asian high mountains.

River discharge

The river discharge database R-ArcticNet (http://www.R-Arcticnet.sr.unh.edu) was extended up to 2004 for 48 downstream river gauges. The last five years were characterized by an increase of total discharge to the Arctic Ocean mainly due to a contribution from Asian rivers. Mean 2000-2004 discharge from Asia was 110 km³ (5%) higher than over the previous twenty years. The mean discharge to the ocean from North America and Europe for 2000-2004 was practically unchanged relative to 1980-1999. A consistent increase in river discharge is observed from Eurasia for a longer time interval as well. Mean discharge over 2000-2004 for the large Eurasian rivers was 3-9% higher than the discharge over 1936-2004. Thus the contemporary data further confirms the presence of a significant increasing trend in the fresh water discharge to the Arctic Ocean from Eurasia documented earlier by Peterson et al. (2002). The maximum total discharge of the six largest Eurasian rivers over 1936-2004 was observed in 2002, at 2080 km³/year (see BAMS State of the Climate Report, 2006).

References

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Armstrong, R.L., B. Raup, S.J.S. Khalsa, R. Barry, J. Kargel, C. Helm, and H. Kiefer (2005a) GLIMS glacier database. National Snow and Ice Data Center, Boulder, Colorado, digital media. [Available online at http://nsidc.org/data/nsidc-0272.html]

Armstrong, R.L, M.J. Brodzik, K. Knowles, and M. Savoie (2005b) Global monthly EASE-Grid snow water equivalent climatology. National Snow and Ice Data Center, Boulder, Colorado, digital media. [Available online at http://nsidc.org/data/nsidc-0271.html]

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