Correspondence to: L. Han, PhD, assistant professor at State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China. E-mail: firstname.lastname@example.org
Climate change can influence soil freeze–thaw cycle, which is an annual occurrence in temperate zones that significantly affects Earth's physical and ecological systems (Smith et al., 2003; Juergen et al., 2008). The timing of freezing and thawing of soil regulates the length of potential growing season for plants and determines the time when significant decomposition of soil organic matter can occur (Kimball et al., 2006). The intensity and frequency of soil frost depend mainly on regional climatic conditions and on the thickness of the insulating snow cover (Zhou et al., 2011). Where global climate change leads to decreased winter precipitation, snow cover will decrease, thereby enhancing the development of soil frost in some regions, whereas warming in other regions may decrease the fraction of seasonally frozen ground at mid-latitudes (Zhang and Armstrong, 2001).
Response of frozen ground to climatic change differs variously among latitudes. Permafrost at high latitudes changes in active layer thickness, while seasonally frozen ground at mid-latitudes may change its distributions (Zhang et al., 2004). As a typical mid-latitude phenomenon, seasonally frozen ground is affected by temperature and surface moisture conditions. As an exogenous factor, lower temperature provides a higher probability of seasonally frozen ground occurrence. Our previous research has documented that seasonally frozen ground is temporally occurred at northern China and Mongolia (Han et al. 2011a, 2012) where temperature varies with regions. Seasonally frozen ground boundary is a transit zone with temporally ground frozen occurred or not, which can be utilized to indicate climatic change. However, research on the relationship between climatic change and seasonally frozen ground distribution are still rare with regard to climatic change studies due to the lack of seasonally frozen ground distribution records. Different from the traditional methods, remote sensing provides an efficient way in monitoring near-surface soil freeze/thaw events/distribution, which could enhance our understanding of climatic change's impact on the near-surface Earth physical system.
Northern China and Mongolia (Figure 1) lies between 31°N and 54°N and 73°E and 136°E and includes humid, semi-humid, semi-arid, and arid zones. There are strong regional climatic differences within the area and a diverse range of vegetation and land surface. Seasonally frozen ground has a high heterogeneity in this region and is generally greater in spring than in autumn (Han et al., 2010). And areas with and without freezing events (transitional zone) are both founded in this region (Han et al., 2011a). Temperature, surface moisture conditions, and other unknown factors might be the factor that changes the spatial pattern of seasonally frozen ground, however, little was known about the drivers of these differences in the seasonally frozen ground. We therefore selected this region for our study of frozen ground boundary shifting responses to temperature variations.
2. Materials, methodology, and boundary classification
In this research, we utilized our previous springtime soil thaw result which has been generated for northern China and Mongolia with QuikSCAT SeaWinds level 2A product during 2000–2007. Our proposed multi-step method, with considering of backscatter time series signatures, was adopted to identify the frozen ground boundary (Han et al., 2011a). Slope of the first 180 d QuikSCAT backscatters was utilized to identify the boundary of seasonally frozen ground (Equation (1)).
where δi represents the QuikSCAT backscatter on day i. In the signature analysis, we found that trends of daily backscatter over time decreased in all areas in which thaw events happened, while they increased in areas in which no thaw events happened, therefore, slope larger than 0 identifies an area of no thaw event, and slope less than 0 identifies an area in which a thaw event occurred; and the boundary was identified where the slope changes from negative to positive.
Station based air temperature records from Global Summary of the Day product were employed to track the temperature changes in the transitional zone. Meteorological stations within the transitional zone and less than 100 km to the northern and southern boundary were selected for the correlation analysis. Those stations' temperatures were utilized from 2 weeks before the average thaw date to 2 weeks after the average thaw date.
Northern China and Mongolia varies in elevation, land cover, and surface dry/humid conditions that were utilized in empirical classification of seasonally frozen ground boundary. The boundary can be classified into five typical regions (Figure 2; Table 1): (1) in northeast of China. The zone locates in the adjacent areas between Northeast Plain—Songnen Plain and Three River Plain—Mt. Xiaohingganling—Mt. Dahingganling, where could be identified as the transition from plain farmland to mountain forest; (2) in northeast of Mongolia, where the zone is the largest, extends from the north of Desert Basin and then extend to the adjacent areas between west Hulunboir Plateau and Mt. Henteyn, where is the transition from dry desert to humid plateau steppe—mountain forest; (3) in northwest of Mongolia, it's in the Mt. Hangayn, the south of Mt. Altayn, and the adjacent area between those mountains, where is the transition between permafrost and dry steppe; (4) in west of China, the zone locates at the adjacent area between dry desert and mountain permafrost, such like from Taklimakan Desert—Gurban Tonggut Desert to Mt. Tianshan; and (5) west of Tibetan Plateau, the boundaries locates on the transitional area from low to high elevation.
Table 1. Characters of five boundary shifting zones at northern China and Mongolia
Finally Pearson correlation analysis was carried out in each of the typical region to understand the seasonally frozen ground boundary shifting responses to temperature anomaly.
In northeast of China (Figure 3(a)), a significant relationship (R = 0.71, P < 0.01) was found between the boundaries' shifting and monthly average air temperature, where the higher the temperature was, the more north the boundary was, vice versa. Similar significant relationship (R = 0.75, P < 0.01) was also obtained in northeast of Mongolia (Figure 3(b)). This indicates that the air temperature serves as one of the dominating factor on the changes of the boundaries' shift in those regions. And the shifting potentially impacts on the weak ecosystem and leads to extreme environmental events. One case is that the shifting of the zone was found to relate with dust outbreak in west Mongolian Plateau (Han et al., 2011b).
In northwest of Mongolia (R = 0.27, P > 0.1), and west of Tibetan Plateau (R = 0.11, P > 0.1), no significant relationships were learned although the boundary also shifted during 2000–2007. As the analysis in west of China, the boundary showed nearly no change (the average change of the boundary was within 2 pixels) whatever the temperature was.
Generally, temperature and vegetation changes are the most popular ways for tracking climate change (Woodward and Lomas, 2004; Hansen et al., 2006; Shen et al., 2012). Temperature studies always focus on a long-term various, and vegetation studies almost on the vegetation productivity (e.g. NPP and GPP) or phenology (e.g. green-up onset) estimation (Shen et al., 2011). In the areas without significant relationships, temperature is not the major factor accounting for the changes in seasonally frozen ground boundary. Relatively low moisture together with higher temperature would drive the boundary shift in mid-latitude areas like the northeast China and northeast Mongolia (Han et al., 2011a). However, no changes were observed on the edge of Tibetan Plateau, which might be also attributed to the permafrost changes with active layer depth (Cheng and Wu, 2000; Wang et al., 2000). In west of China, the topographic difference together with land cover dramatic change, from low to high elevation, from desert to plateau grassland, may shorten the transitional zone, making it difficult to identify the changes with coarse resolution remote sensing records. Further higher spatial resolution remote sensing records are highly recommended to meet this demand.
This study is the first to investigate the shifting of frozen ground boundary in response to temperature variations at northern China and Mongolia. Five typical regions were analysed at the study area. By examining the temperature anomaly and shifts in the frozen ground boundary, we reached the following conclusions.
In typical mid-latitude areas, like the northeast of China and the northeast of Mongolia, frozen ground expands/shrinks with low/high temperature at the transitional zone. However, in the other three areas, the northwest of Mongolia, the west of China, and the west of Tibetan Plateau, no significant relationship was obtained, indicating temperature might not be the major factor driving frozen ground shifting at some mid-latitudes.
The result of this research demonstrated that, seasonally frozen ground boundary may serve as an indicator in tracking climatic change in some areas at mid-latitudes. Our findings also suggested more attention be paid on the winter and winter–spring transitions in the Earth near-surface system at mid-latitudes.
This research was supported by the Japanese Society for Promotion of Science Core University Program and the Global Center of Excellence Program for Dryland Science of the Japanese Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank the following data providers: NASA/JPL for QuikSCAT Level 2A product; NOAA NCDC for Global Summary of the Day product.