Ninth International Conference on Permafrost ... - IARC Research

Historical Changes in the Seasonally Frozen Ground Regions of the Russian ArcticOliver W. Frauenfeld, Tingjun Zhang, Andrew J. Etringer, Roger G. BarryCIRES National Snow and Ice Data Center, University of Colorado, Boulder, Colorado, USADavid GilichinskySoil Cryology Laboratory, Institute of Physico-Chemical and Biological Problems in Soil Sciences,Russian Academy of Sciences, Pushchino, Moscow Region, RussiaIntroductionSeasonal freezing and thawing processes in cold regionsplay an important role in ecosystem diversity, productivity,and the Arctic hydrological system (Hinzman et al. 1991,Kane et al. 1991, Woo 1992, Osterkamp et al. 2000, Nelson2003). Long-term changes in seasonal freeze and thaw depthsare also useful indicators of climate change (Frauenfeld etal. 2004). However, only sparse historical measurements ofseasonal freeze and thaw depths are available for permafrostand seasonally frozen ground regions. In previous work, weapplied mean monthly soil temperature data for 1930–1990at 242 sites located throughout Russia, and employed asimple interpolation scheme to determine the depth of the0°C isotherm based on soil temperature data measured at 13depths between 0.2 m and 3.2 m. The relationship betweenavailable observed annual maximum freeze and thaw depthsand our interpolated values indicates a perfect correlation,and thereby verifies our methodology (Frauenfeld et al.2004).In this analysis, we improve on our previous work byemploying a greatly expanded station database with soiltemperatures for 423 sites with updated observations throughthe year 2000. These 423 stations are located throughoutRussia (Fig. 1) and can be obtained from the Frozen GroundData Center (http://nsidc.org/fgdc/). The addition of 181sites throughout the Russian Arctic combined with 10more years of observations allows for a significantly morecomprehensive evaluation. Furthermore, the addition of1991–2000 allows us to quantify changes in the soil thermalregime during a decade when accelerated climate warminghas potentially occurred.Data and MethodsDetailed descriptions of soil temperature measurementsin the former Soviet Union were provided by Gilichinskyet al. (1998), by Zhang et al. (2001), and in the instructionmanuals of the State Committee of the U.S.S.R. forHydrometeorology and Environmental Control (1985).Observations are available for the 0.2 m, 0.4 m, 0.6 m, 0.8m, 1.2 m, 1.6 m, 2.0 m, 2.4 m, and 3.2 m depths. We linearlyinterpolated the depth of the 0ºC isotherm throughout the0.2 m–3.2 m temperature profile (recognizing that this isnot necessarily the same as the “true” freeze/thaw depth).The 423 stations were first classified as either permafrostor seasonally frozen ground, depending on soil temperatureat the 3.2 m depth. If, for the entire record, a station’s soiltemperature at 3.2 m was positive, that station was classifiedas a seasonally frozen ground station; 387 of the 423 stationsFigure 1. Location of the 428 soil temperature observing sites inRussia.qualified. We also employ kriging to improve data quality andin-fill missing observations, and produce nearly continuousstation time series for 1930–2000. For the seasonally frozenground stations, the freeze depth was interpolated betweenthose layers where the temperature switched from negativeto positive. The maximum depth of freezing was selectedfrom the months of March, April, and May only. An averagetime series was generated by averaging all available stations’maximum annual freezing depth departures (with respectto each station’s long-term mean) in the seasonally frozenground region of the Russian Arctic. Linear least-squaresregression was then applied to the time series to quantify itslong-term changes.Results and DiscussionIn our previous work the long-term trend for the 1930–1990 period based on 211 seasonally frozen groundstations indicated a decrease in seasonal freeze depths ofapproximately 4.4 cm decade −1 , or 27 cm overall (Fig. 2).This trend is statistically significant (95%-level). However,because prior to the mid-1950s there were too few stationsto produce a robust trend (Fig. 2), this 27 cm change had tobe interpreted cautiously. Based on the 1956–1990 period,when 100 or more stations contribute to each year’s meanvalue, the overall change is approximately -34 cm (notshown). Note that while a total of 211 stations were availableto generate the time series, a maximum of 158 stationscontribute at any given year, as most individual station timeseries are incomplete and have gaps due to missing data.With the addition of 176 new sites in seasonally frozenground regions, more data for the original 211 sites, and75