Satellite-derived changes in the permafrost landscape of central Yakutia, 2000–2011: Wetting, drying, and fires

Highlights

• Permafrost landscape in central Yakutia was observed by satellite data (2000–2011).

• Land surface temperatures indicated intense surface warming trend.

• Average warming of 0.12 °C/year, with maximum of up to 0.49 °C/year during September–October.

• 80% of the area showed reduction in snow coverage in spring.

• The total lake area increased by 17.9%, and ranged between 11% and 42%.

Abstract

The focus of this research has been on detecting changes in lake areas, vegetation, land surface temperatures, and the area covered by snow, using data from remote sensing. The study area covers the main (central) part of the Lena River catchment in the Yakutia region of Siberia (Russia), extending from east of Yakutsk to the central Siberian Plateau, and from the southern Lena River to north of the Vilyui River. Approximately 90% of the area is underlain by continuous permafrost. Remote sensing products were used to analyze changes in water bodies, land surface temperature (LST), and leaf area index (LAI), as well as the occurrence and extent of forest fires, and the area and duration of snow cover. The remote sensing analyses (for LST, snow cover, LAI, and fire) were based on MODIS–derived NASA products (250–1000 m) for 2000 to 2011. Changes in water bodies were calculated from two mosaics of (USGS) Landsat (30 m) satellite images from 2002 and 2009. Within the study area's 315,000 km² the total area covered by lakes increased by 17.9% between 2002 and 2009, but this increase varied in different parts of the study area, ranging between 11% and 42%.

The land surface temperatures showed a consistent warming trend, with an average increase of about 0.12 °C/year. The average rate of warming during the April–May transition period was 0.17 °C/year and 0.19 °C/year in the September–October period, but ranged up to 0.49 °C/year during September–October. Regional differences in the rates of land surface temperature change, and possible reasons for the temperature changes, are discussed with respect to changes in the land cover.

Our analysis of a broad spectrum of variables over the study area suggests that the spring warming trend is very likely to be due to changes in the area covered by snow. The warming trend observed in fall does not, however, appear to be directly related to any changes in the area of snow cover, or to the atmospheric conditions, or to the proportion of the land surface that is covered by water (i.e., to wetting and drying).

Supplementary data (original data, digitized version of the maps, metadata) are archived under PANGAEA (http://dx.doi.org/10.1594/PANGAEA.855124).

Introduction

Air temperatures in high latitude areas of the earth have increased by 0.06 °C/year over the last 30 years, which is about twice as much as global temperatures have increased by over the same period (IPCC, 2013). Since these areas are underlain by permafrost to a variable extent (from sporadic to continuous) and to variable depths (ranging from meters to a maximum of about 1500 m; Yershov, 1991), they are susceptible to thaw and thus potentially susceptible to major changes in surface and subsurface conditions that are likely to result in the mobilization of stored carbon (Schuur et al., 2015).

The area investigated for this study is likely to be sensitive to possible future changes. It is transected by the Lena River, which has a catchment area of approximately 2,430,000 km²; this is the largest catchment area of any river in Eurasia and covers a similar area to the Mediterranean Sea. The Lena River discharges about 528 km³ of water per annum into the Arctic Ocean (Liu et al., 2005), with runoff estimated to have increased by 10% over the last century (Berezovskaya et al., 2005). About 90% of the catchment area is underlain by permafrost (Ye et al., 2003). A number of investigations have revealed rapid changes to the landscape in this region, which is particularly sensitive to changes in the underlying permafrost and to ongoing changes in the climate. These investigations, which are summarized below, have provided the motivation for this research.

According to Fedorov et al. (2014a), this part of the Lena River Basin in central Siberia is experiencing a strong warming trend. The middle taiga is the dominant landscape type in this area (Fedorov et al., 1989; Fig. 1; Appendix Fig. 2). Fedorov et al. (2014a) chose three phases of maximum warming (1935–1945, 1988–1995, and 2005–2009) to investigate regional differences in warming and their effects on the permafrost thermal regime. The highest rate of warming occurred over the last of these periods, during which an annual average increase in air temperature of 0.036 °C was recorded at the Yakutsk weather station (Romanovsky et al., 2007). The mean annual ground temperature (MAGT) at the depth of zero annual amplitude (where seasonal variations in temperature can no longer be detected) has been reported to range between 0 and − 11 °C at this site (Fedorov et al., 1989, Romanovsky et al., 2010; Appendix Fig. 2). Regional warming and wetting of the land surface in this region between 1998 and 2007 (as indicated by an increase in lake areas) was also reported by Iijima et al. (2010). Rapid warming of the upper permafrost soil was observed in 2005 and is likely to have been a response to increased summer precipitation prior to fall freeze-back, an early onset of snowfall, and increased snow accumulation (Iijima et al., 2010). Long-term permafrost thaw has been observed at locations in the vicinity of Yakutsk, occurring in conjunction with an atmospheric warming trend (Romanovsky et al., 2010, and references therein). Permafrost thaw in the region leads to various changes in landscape structure such as increased retrogressive thaw slump activity (Séjourné et al., 2015), erosion of riverbanks (Costard et al., 2003), and the formation of thermokarst. Natural and anthropogenic disturbances such as fires and forest clearance have been identified as possible triggers for the permafrost thaw in these areas. Thermokarst development in natural, undisturbed locations is generally evident as thaw along polygonal networks of ice-wedges, and in the formation of depression and lakes (Soloviev, 1973). Further thawing then results in further lowering of the land surface in newly-formed depressions. Permafrost thaw subsidence in central Yakutian thermokarst depressions (alases) has been reported to generally occur at about 4 cm/year (Fedorov et al., 2012), with rates of up to 24 cm/year recorded (Fedorov and Konstantinov, 2003). The melting of ground ice is known to have a substantial regional impact on the water balance of thermokarst lakes by providing up to one third of the water supply to these lakes (Fedorov et al., 2012).

Results from the Gravity Recovery and Climate Experiment (GRACE) satellite mission suggest an increase in mass for the Lena River catchment area between 2002 and 2007 (Landerer et al., 2010, Muskett and Romanovsky, 2009, Seo et al., 2010), centered on the upper Lena River area to the north of Lake Baikal, in central Yakutia. This trend then reversed to a mass reduction between 2007 and 2011 (Vey et al., 2013). Such variations in mass were postulated by Vey et al. (2013) to be due to hydrological variations (i.e. to variations in surface and subsurface water storage in lakes and groundwater).

Fires are natural phenomena that can occur over vast areas of boreal forest (Cherosov et al., 2010). They are believed to cause major disturbance to permafrost (Jones et al., 2013) by destroying the surface organic layer, thus changing the soil's thermal properties and heat transfer. They can therefore have a substantial impact on permafrost temperatures and result in an increase in active layer thickness (Mackay, 1995, Jafarov et al., 2013) and, in areas of ice-rich permafrost, subsequent thaw subsidence (Liu et al., 2014) with possible infilling of the resulting depressions with water and the formation of lakes (Westermann et al., 2016). Lakes can modify the surface energy balance on a local scale in a variety of ways by, for example, influencing the radiation budget, subsurface heat storage and atmospheric heat fluxes (Boike et al., 2015, Boike et al., 2013, Krinner and Boike, 2010, Krinner, 2003). Changes in the area covered by lakes may therefore correlate with changes in the land surface temperature. Of the 18 permafrost sites in Siberia that were investigated by Kravtsova and Bystrova (2009), the site north of the Vilyui River in central Yakutia showed the greatest increase in the area covered by lakes, which doubled between 1976 and 2000. Fedorov et al. (2014b) reported that the surface area of water bodies at a thermokarst monitoring site on the Lena-Amga watershed (located east of Yakutsk, between the Amga and Lena rivers) had steadily increased from 195 m² to 3135 m² over a period of 15 years up to 2008.

Satellite-derived land surface observations are of great importance for detecting changes in these vast, remote areas where the availability of locally derived data sets is extremely limited. Permafrost is defined by the temperature of the ground and is therefore a subsurface phenomenon that cannot be directly measured by optical, thermal or microwave remote sensing (in contrast to, for example, other elements of the cryosphere such as snow or glaciers). Changes in permafrost temperature, as well as in active layer thickness, can be detected with permafrost models, using remote sensing data or data from general circulation models as input (Anisimov and Nelson, 1996, Anisimov et al., 1997, Langer et al., 2013). However, observations in many permafrost landscapes demonstrate that lateral and vertical movement of water can have a pronounced influence on rates of thaw, creating distinctive landforms such as thermokarst ponds and lakes, even in areas where permafrost is otherwise thermally stable. Permafrost models and land surface schemes used in Earth system modeling have recently started to include novel process parameterizations in order to take into account such phenomena for projections of future permafrost thaw and the resulting climatic feedbacks (Westermann et al., 2016).

While the original focus of our research was on investigating the relationship between the mass signals derived from the GRACE mission and the land cover (i.e. the areas of lakes and the forest biomass), the focus subsequently shifted to detecting changes in the land cover, analyzing these changes, and identifying possible feedback mechanisms. The objective was then to relate the changes in land cover to regional changes in atmospheric patterns, in snow cover, and in the permafrost landscape. We have therefore focused on (i) the quantification of observed changes, (ii) improving our understanding of feedback mechanisms, and (iii) providing regional data sets as a basis for future numerical analysis of land cover changes and permafrost degradation in a changing climate.