Data on snow cover

The study area extends over the latitude–longitude domain from approximately 70˚ E to 105˚ E and from 27˚N to 50˚ N. Western China is physiographically divided into two regions: the QXP and northwestern China. The QXP, with an average elevation exceeding 4500 m and an area of >2×10⁶ km², has been acclaimed as the ‘Roof of the World’. The Himalaya, the world’s highest mountain range, provides a natural screen on the southern frontier of the plateau. Northwestern China is an arid region surrounded by high mountains and large basins, such as the Altai Shan, Tien Shan, Pamirs, Karakoram, the Kunlun Shan and the Tarim and Junggar basins.

The meteorological network of western China consists of more than 200 synoptic stations. It was only after 1956 that the QXP network became dense enough to ensure adequate spatial coverage, except for western Tibet. To minimize the defects of station data, a subset of the network was created in northwestern China and the QXP, respectively. The station selection criteria included availability of the longest time series; few missing records; and without site relocation. The station records were used without further adjustment, and no attempt was made to fill in the few missing data (Reference Balling and IdsoBalling and Idso, 2002). Following strict quality control, the station point records were integrated over the snow-cover years (September to August) and space to derive regional time series. With regard to large-scale area averages, biases and errors associated with specific point data were further minimized to such an extent that a homogeneous and meaningful signal could be extracted. Over the QXP, the station network consisted of 60 primary stations, of which 35 are located in Qinghai Province and 25 in the Xizang (Tibet) Autonomous Region. Snow-cover data from 46 stations in Xinjiang and 64 stations in IM-NEC are used to study the long-term trend.

Data on glaciers

Records of directly measured mass balance are obtained at a few logistically ‘easy’ glaciers: Ürümqi glacier No. 1 (UG1), Tien Shan; Xiao Dongkemadi (XDKMD), Tanggula Shan; Meikuang (MK), east Kunlun Shan; July 1st (Qiyi) glacier, Qilian Shan; and Hailuogou glacier, Gongga Shan. The mass-balance data series differ in length, and are discontinuous except for UG1.

Glacier changes are defined as changes of ice mass and glacier size, including thickness, length and width. Ground monitoring of glacier changes in China for recent decades has been carried out at several sites. Continuous observations are operated only on UG1, which is the only glacier in China that is included in the World Glacier Monitoring Service (WGMS; International Satellite Communication Systems–International Association of Hydrological Sciences) network. Other ground-based observations have been operated sporadically on glaciers covering all of the mountain ranges in western China (Table 1).

Table 1.

Ground–based observations of glacier length in China for the past decades

For the statistical estimations of glacial changes over specific regions, studies are based on the 1950–70 topographic map and on Landsat Thematic Mapper (TM)/Enhanced TM Plus (ETM+) and Terra ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) photos taken during 2000–02; glacier changes for the 764 glaciers in western China have been studied (Table 2). The geometrically corrected error of the photos in different areas is usually smaller than two units (30–60m for TM/ETM+, and 30 m for ASTER). The glaciers are not included in the statistics if the length changes are less than the errors.

Table 2.

Statistics of terminal changes of glaciers of western China based on 1950–70 topographic map and Landsat TM/ETM+ and Terra ASTER photos taken during 2000–02

Data on frozen ground

Ground surface temperature is a standardized monitoring parameter at most meteorological stations. It is measured on the ground surface with half of the thermometer buried in the soil and half exposed to the air. Mean cold-season temperatures and mean warm-season temperatures are the average value of monthly temperatures from October to March and from April to September, respectively. SFG depth is defined as the maximum frozen depth in each cold season.

There are 119 meteorological stations in the QTP, of which 7 are located within permafrost regions with elevations higher than 4500m and mean annual air temperatures lower than –2.8˚C. Three of the seven stations are free of permafrost (Maduo, Tuotuohe and Zhongxinzhan) due to the influences of high geothermal flux and/or rivers (Reference Zhou and GuoZhou and Guo, 1982; Reference Guo, Zhou, Guo, Qiu, Cheng and LiGuo, 2000; Reference Guo, Zhou, Qiu, Li, Zhou, Guo, Qiu, Cheng and LiGuo and others, 2000). Based on analysis of both mean daily and ground surface temperatures, there are no SFG thickness monitoring data at 29 of the stations (20 of these are located in the southwestern region of the QXP), either because of lack of SFG or because the ground in this area normally freezes for only a few nights during the winter. Freezing depths have been monitored at 86 stations. These stations are distributed around the fringes of the QXP, while most of the inland areas are underlain by permafrost (Reference Li, Cheng and ZhouLi and others, 1996). The data used in this study are available from 1967 to 1997 at 50 stations over the QXP after some minor adjustments and estimations for missing data. SFG data in Xinjiang and the Hexi Corridor of Gansu Province are also derived from routine meteorological stations. Among the stations selected for SFG studies are 10 in north Xinjiang, 9 in south Xinjiang and 17 in the Hexi Corridor.

Sea- and river-ice data

Observation stations surrounding the Bohai Sea were set up in the 1950s. The observing handbook of offshore sea ice was produced in 1959, and routine observations maintained since then. Since 1969, three ice-breaking vessels have been built for sea-ice observation, and ground-based radar detection for sea-ice monitoring was established in the 1970s. From 1973, air photos and satellite images of sea ice were used. Currently the sea-ice observations in the Bohai Sea and the northern Yellow Sea include (1) ground stations, (2) remote sensing, including air photos and satellite images, and (3) icebreaker observations.

River ice was observed at the national hydrological observation network governed by the Ministry of Water Resources of China (MWR). River-ice parameters such as the duration of river ice, freeze-up and break-up dates as well as ice thickness were monitored at these stations. Here we apply data from the Inner Mongolia section of the Yellow River, the Haarbin section of the Songhuajiang river and the Fulaerji section of the Nengjiang river to identify river-ice changes during the past several decades in China.

Changes in glaciers

Mass-balance studies have been operated on several relatively easily accessible glaciers such as UG1, XDKMD, MK, Qiyi and Hailuogou (Fig. 1). Between 1959 and 2002 the cumulative mass loss of UG1 equals a lowering of the glacier surface of up to 10.6 m. This loss of mass has accelerated since 1995. For instance, the cumulative loss of mass between 1995/96 and 2001 accounts for 42% of the total loss since 1959. The mass balance at XDKMD has been negative since 1993, and has continued to display an accelerated loss of mass since then. Mass balance at MK displays a parallel trend to that of XDKMD. Mass balance at Qiyi from 1974/75 to 1977/78 was averaged to be 256 mm; from 1984/85 to 1987/88 it was 4 mm, while in 2001/02 and 2002/03 it was –861 and –360 mm, respectively. It is calculated that the rate of mass loss was 25.2 mma^–1 between 1976 and 1986, and –35.4mma^–1 between 1986 and 2002. Hailuogou glacier, located at the east margin of the Tibetan Plateau, has displayed a continuously negative mass balance since 1960.

Fig. 1.

Changes of mass balance of five glaciers in west China in recent decades.

For the past few decades, terminal changes of China’s glaciers have been directly measured at several sites considered to be logistically and morphologically ‘easy’. Terminal changes of UG1 have been observed since 1959 based on a long-term monitoring program. Changes of other glaciers were based on field investigations by various scientists in different years (Table 1; Fig. 2).

Fig. 2.

Changes of glacier terminus (length) based on ground investigations: glaciers that underwent small terminal retreats (a), large terminal retreats (b) and terminal advances (c). Note that the y-axis unit in (a) is 100 m, while in (b) and (c) it is 500 m.

Sporadic field investigations in the past few decades show that changes of glacier lengths vary in different areas. The breadth of these changes varies because of different climatic conditions, glacier sizes and facings. Relatively small changes were found in the Qilian Shan and in the central QXP and in some large glaciers in the Himalaya. We define these changes of <500m between the 1950s and around 2000 as a small retreat (Fig. 2a).

Larger retreats are occurring on glaciers in the margins of the QXP, the Pamirs and in the western Tian Shan. Between the 1950s and 2000, these glaciers retreated >500m (Fig. 2b).

Although retreat is the overwhelming trend, some glaciers in the central and southeastern QXP are still advancing (Fig. 2c). This is probably due to the response of the glaciers to the present cooling of the local climate (Reference Shi, Liu, Shangguan, Li and YeShi and others, 2006), or due to enhanced transport of moisture to high altitudes (Reference HewittHewitt, 2005; Reference LiuLiu and others, 2006).

Glacier changes for 764 glaciers in western China were studied (Table 2) using digital photos from Landsat TM/ETM+ and Terra ASTER. The changes have varied remarkably in different regions. In total, about 80.8% of the studied glaciers are retreating or have vanished, while 19.2% are advancing (Fig. 3). For each region (mountain range), the number of advancing glaciers is much less than that of retreating glaciers. The percentage of advancing glaciers on the margin of the QXP (e.g. Qilian, A’nyêmaqên and Qomolangma) is higher than in the interior. Because of the resolution limits of the photos, changes of some small glaciers and some stable glaciers cannot be determined; this should be considered when assessing the overall changes of glaciers.

Fig. 3.

Percentages of retreating glaciers among a total of 764 glaciers in China during the past five decades.

Based on the above observations and the typical glacier monitoring, Reference Yao, Wang, Liu, Pu, Shen and LuYao and others (2004) estimated a shrinkage of 3790 km² of glacier areas in approximately the past 40 years, which equals a mean glacier thinning rate of around 0.2 ma^–1.

Changes in frozen ground

Changes in snow cover

Using scanning multichannel microwave radiometer (SMMR) data, covering 2500 cells of a 0.5˚×0.5˚ latitude–longitude grid between 1978 and 1987, Reference LiLi (1999) showed the spatial pattern of average snow depth (cm) during the winter snow maxima (January and February) in west China. The spatial distribution of snow cover is very uneven geographically. Snow depths vary remarkably in altitude, especially between large mountains and basins in northwestern China. The highest snow depth is seen in the Altai Shan, followed by the Tien Shan, Pamirs, Karakoram and Kunlun Shan. Snow cover is also noted in the Elgis and Ili valleys. In contrast, snow cover is rare in the Tarim basin, Lop Nur and Badain Jaran desert. In the Junggar basin, snow is usually light, and frequently of short duration. Long distances to moisture sources, and the blocking effect of the mountains, keep the basins very dry. Snow cover, at about 59% in winter, is far from a pervasive feature over the QXP. Heavy snow cover is present only in the peripheries, including the Himalaya, Pamirs, Nyain-qêntanglha and eastern Tanggula Shan. In the vast interior (e.g. Qaidam basin and Yarlung Zangbo valley), snow cover is rare, and in the northern QXP it is light and of short duration.

Annual variations of snow cover in the three major snow-cover regions (Xinjiang, QXP and IM-NEC) were studied based on station observations (Fig. 7). For the Xinjiang and IM-NEC regions, the results of ground observation coincide with those derived from SMMR–Special Sensor Microwave/Imager (SSM/I) (Reference Li and ShiLi, 2000; Konig and others, 2001), but for the QXP there are biases because of insufficient station data over the plateau.

Fig. 7.

Interannual variability of snow cover over China during the past 50 years: (a) Xinjiang, (b) QXP and (c) IM-NEC region.

Figure 7a shows a time series of the annual and spring ablation season (March and April), number of days with snow cover, and annual cumulative daily snow-cover depth over northwest China from 1951 through 1997. It demonstrates that long-term variability of snow cover is normally characterized by random oscillation. Snow cover fluctuated around the mean, alternating between heavy and light snow cover. Neither abrupt changes nor continuation of snow minima from the late 1980s and early disappearance of spring snow cover were found. From the end of the 1980s, however, a prolonged decrease in snow cover was seen, but not as great as the three previous snow-deficit periods. In the 1960s and early 1970s, measured snow cover was lower than at any other time in the second half of the 20th century.

Over the QXP, long-term variability of snow cover is characterized by large interannual variabilities superimposed on a continuous increasing trend. Furthermore, the annual amplitude of snow-cover variability has increased significantly since the 1980s (Fig. 7b). Both extremely heavy and light snow-cover years occurred more frequently. The anomalies did not appear to be outside the range of natural variability.

In the IM-NEC region, there are several scattered areas with deep snow cover where large interannual variabilities in snow depth were observed. These areas include the Da Hinggan, the Xiao Hinggan and Xilinguole. There are also large seasonal variations in snow depth. The deepest snow cover appears in mid-winter, when the largest interannual snow depth variations also occur. The overall trend in the IM-NEC regions shows a very slight decrease (0.1ma^–1) in snow depth (Fig. 7c).

The annual snow-cover variations depend both on the amount of winter snowfall and on the winter temperatures. Reference Li and ShiLi (2000) established relationships between snow cover and winter air temperatures and precipitation, respectively for the Xinijing, QXP and IM-NEC regions. They are:

Sd and Sn are the annual daily cumulative snow depth data (cm) and annual number of snow-cover days, respectively, P is the amount of winter snowfall (mm) and T is the mean air temperature during the snow-cover season.

The interannual snow-cover variations based on the above functions coincide well with the observed results, with an error range within ±10%. However, these functions cannot properly reproduce long-term changes in snow cover, probably due to the large errors in measuring solid precipitation.

Changes in sea ice

The Bohai Sea and the northern part of the Yellow Sea are located on the south edge of the Northern Hemisphere sea ice (37˚05^'–40˚55^' N, 117˚30^'–125˚30^' E). Since the early 1950s, sea-ice coverage and thickness over the oceans were monitored for assessment and prediction of harbor disasters. Sea-ice disasters are classified into five grades according to the sea-ice cover and thickness (Reference Zhang, Yu, Tang, Preller and WuZhang, 1993). Heavy sea-ice disasters occurred in 1956/57 and the winter of 1968/69, and less heavy disasters in the winters of 1967/68 and 1976/77.

During the past five decades, the Bohai Sea ice cover has shown a declining trend, while there is no obvious trend over the north Yellow Sea area. For the whole area, however, a general decline in sea-ice coverage has been shown since late 1960s (Fig. 8).

Fig. 8.

Changes in sea-ice area over the Bohai Sea (a), north Yellow Sea (b) and the two regions combined (c).

Reference Zhang, Yu, Tang, Preller and WuZhang (1993) detected the relationships between sea-ice cover and climate, showing that the cycles of solar activities and El Niño events correlate to the anomalous sea-ice conditions. The maxima and minima of solar activities, and the intensity and timing of the El Niño events, play a very important role in the occurrence of severe ice conditions.

Changes in river ice

We discuss the changes in river ice over two key areas – Inner Mongolia and northeast China – where river-ice data are available. River ice of the Inner Mongolia section of the Yellow River has been observed at the Sanhuhekou Shizuishan, Baotou, Zhaojunfeng and Toudaogui hydrological stations. River ice of the Songhuajiang river has been observed at the Haarbin hydrological station, and of the Nengjian river at Fulaerji station.

Changes in river ice can be described as changes in freeze-up and break-up dates, length of frost period as well as ice thickness.

Over the Inner Mongolia section of the Yellow River, freeze-up dates are 5 days later in the 1990s than the 1950s–90 s average, and the break-up dates in the 1990s are 3 days earlier than average. The frost period duration and the maximum ice thickness in the 1990s are 9 days shorter and 0.16m thinner, respectively, than average (Table 3; Fig. 9).

Fig. 9.

River-ice changes in north China since the 1950s: (a) freeze date, (b) break-up date, (c) river ice duration and (d) maximum ice thickness.

At Haarbin, the freeze-up date is 1 day later, and the break-up date 1 day earlier, in the 1990s than the 1950s–90s average. The frost period duration and the maximum ice thickness in the 1990s are respectively 4 days shorter and 0.21 m thinner than average.

At Fulaerji, the freeze-up date is 6 days later, and the break-up date 2.5 days earlier, in the 1990s than the 1950s–90s average. The frost period duration and the maximum ice thickness in the 1990s are respectively 10 days shorter and 0.06m thinner than average.

In three study areas, the frost period and the maximum ice thickness have decreased during the period since the late 1970s, when global and regional climate warming is most prominent. During the period 1959–2002, annual mean air temperature increased by 0.48˚Ca^–1, and winter air temperature increased by 0.72˚Ca^–1 over northeast China (Reference Sun and YuanSun and Yuan, 2005), where the Songhuajiang and Nengjiang rivers are located.

Table 3.

Decadal characteristics of river ice in Ningxia–Inner Mongolian section of Yellow River for the past five decades