Expansion of the crater from 1991 to 2018
In Fig. 3, which shows one of the most essential results of the remote sensing study, the expansion of the Batagaika crater over the last 27 years (from 1991 to 2018) is delineated in detail. This figure has been created using the editor tool in ArcGIS 10.3 software. It is clearly visible that the crater is growing faster towards south-west, where it occupies a substantial area with a round shape, while its north-western extremity remains relatively narrow. The area of slump obtained on 13-08-1991 and on 29-06-2018 was 0.19 km2 and 0.78 km2, respectively, i.e. the area of the crater increased three times during the studied period since 1991. All assessed values of the area and the perimeter of the crater for the timeframe between 1991 and 2018 are given in Table 3. The increase in area of the Batagaika crater in the period from 1991 to 2018 is confirmed by satellite images shown in Fig. 4. The image in 2018 was taken by Sentinel-2A while the other images were taken by Landsat satellite. It may be clearly seen in Fig. 4 that Sentinel-2A satellite provides satellite images of better quality than Landsat one.
Table 3 Area and perimeter of the Batagaika crater between 1991 and 2018 We have also calculated the increase rate of the crater during the studied period Table 4 indicates the increase rate from 1991 to 2018. It turned out that it underwent significant temporal changes. For example, the increase rate of the Batagaika crater between 1991 and 1999 was ca 0.016 km2/year. During the next years, the increase rate of the crater was higher, namely ca 0.039 km2/year during 2010–2014, and 0.022 km2/year during 2014–2018. The average expansion rate between 1991 and 2018 was ca 0.026 km2/year.
Table 4 Increase rate of the Batagaika crater area over 27 years, from 1991 to 2018 Using 3D analyst tool in ArcGIS, the image of the head part of the Batagaika crater with seven interpolate lines was created. It is shown in Fig. 3. These lines are used to figure out the expansion of crater along each line within the period 1991–2018. Table 5 indicates the changes in the width of seven interpolate lines in the head part of the Batagaika crater within five different subperiods covering the timeframe 1991–2018. The obtained results show that the head part of the crater expanded the fastest during subperiod 1991–1999. In Fig. 5 it is also apparently a characteristic parabolic shape of the head part of the Batagaika crater. The maximum width of the Batagaika crater in 2018 was 979 m.
Table 5 Increase in width of the Batagaika crater along each black line shown in Fig. 3 from 1991 to 2018 Profiles of the elevation of the Batagaika crater
In Fig. 5 all calculated profiles of the elevation of the Batagaika crater above sea level (a.s.l.) are shown.
In Fig. 5a the span of the abscissa of the elevation graph is equal to the length of the crater (2.1 km), while in Figs. 5b–j these spans are equal to the widths of the crater corresponding to transversal lines 2–10 in Fig. 2. The ordinates of the diagrams in Fig. 5 indicate the elevation a.s.l. in meters. From Fig. 5b–j it can be easily concluded that crater is clearly the deepest and widest in its main south-western part (the head of the “tadpole”) and quickly narrows and shallows approaching the north-east end (the tail of the “tadpole”). It is also easy to notice that the terrain is lowered towards the north-east, where water from the crater flows down to neighbouring rivers. Thanks to this the crater is not filled completely with water.
The distributions of LST inside the Batagaika crater on the hottest days of the year
According to Table 4, the highest crater expansion occurred for the timeframe 2010–2014, but it is visible that during the next four years (from 2014 to 2018) there was not any significant expansion. Air temperature during this period was high in Batagay region (World Weather Online), so that land surface was also intensively heated. This is also in accordance with measurements of thaw layer depths in Northern Siberia (Global Cryosphere Watch), which show that in the period 2010–2014 thaw layer depths were higher than after 2014.
Increasing ground temperature is the main driving factor for the thawing of permafrost, so it was decided to get deeper insight into the LST spatial distribution inside the crater between 2010 and 2018. Figure 6 shows the examples of such LST distributions on low clouded, and hottest days of the year during the timeframe 2010–2018.
Figure 6 clearly shows the high-temperature gradients inside the crater in those days. All of the images show that the warmest ground areas are located mainly in the centre and in the northern part of the crater, whereas the coldest ground areas, where permafrost is still melting are located mainly in its southern part, as well as on the edge of the “tadpole’s” head. Right there the expansion of the Batagaika crater was the fastest in most of the studied periods. Thus, the LST observations confirm the fact that the expansion of the crater is driven by the permafrost thawing due to ground heating during warm months of the year (comp. Figs. 3 and 6).
Variation in the NDVI value over three decades
The satellite observations of the spatial distribution of vegetation inside the Batagaika crater allow to understand the overgrowing process related to the crater expansion within the last three decades. Figure 7 shows the changes in NDVI spatial distributions inside the Batagaika crater in the period from 1991 to 2018, whereas in Table 6 the main descriptive statistics of NDVI value inside the Batagaika crater are given.
Table 6 Main summary statistics of NDVI inside the Batagaika crater Both from Table 6 as from Fig. 7 it may be concluded that noticeable and constant increase in the mean value of NDVI started in 2010. Before this year the mean values of NDVI inside the crater were negligibly small and fluctuated around zero, which is characteristic of bare soil, barren areas of rock, sand, or snow (Carlson and Ripley 1997). The changes in NDVI after 2010 may be interpreted as the result of natural succession of vegetation inside the crater. Comparing Fig. 7 with Fig. 5c–e it can been seen that the vegetation succession was the most intense in the northern part of the crater and its centre which has maximum depth where most of the permafrost has already melted.
The mean value of NDVI inside the crater in 2018, was equal ca 0.24, significantly higher compared with those in all previous years, which could imply that relatively dense vegetation can be already found in some parts of the Batagaika crater with no permafrost. The obtained mean values of NDVI in 2014, 2010, 2005 were ca 0.09, 0, and ca − 0.16, respectively. It is interesting that the expansion rate of crater between 2014 and 2018 was lower than from 2010 to 2014 when the highest expansion rate was observed. Taking into consideration many possible causes of this slowdown of expansion rate in recent years such as climatic, geological, etc., it cannot be excluded, that one of them is a succession of vegetation, the presence of which reduced the heating of the soil (Osawa et al. 2010). Although this issue requires further research, the results already suggest the possibility of reducing the extent of permafrost thawing by appropriate artificial afforestation of newly created craters, or by activities aimed at maintaining or strengthening the forest cover on the areas prone to permafrost thawing.