2. Results and discussion

Glaciological mass-balance measurements for Urumqi Glacier No. 1 have been well documented by previous studies (e.g. Liu and others, Reference Liu, Xie and Wang1997). Combining the uncertainty in glaciological measurement evaluated by Xu and others (Reference Xu, Li, Li, Wang and Zhou2019), a total mass change of −0.042 ± 0.002 Gt was observed at Urumqi Glacier No. 1 from 1959 to 2022 and this corresponded to a cumulative mass balance of −21.98 ± 1.2 m w.e. Between 1959 and 2022, the glacier experienced two periods of accelerated ablation (Li and others, Reference Li2020). The net mass balance of Urumqi Glacier No. 1 in 2022 (−1251 ± 100 mm w.e.) was the second most negative value on record (Fig. 2). The lowest observed net mass balance was recorded in 2010 (−1327 ± 100 mm w.e.). Applying the same quantile thresholds as for air temperature, the net mass balance observed in 2022 exceeds the 97.5% quantile (Fig. S2b), and the summer balance observed in 2022 is the most negative on record (Fig. 2).

Figure 2. Annual (a), winter (b) and summer (c) mass balances for Urumqi Glacier No. 1 for the period 1959–2021.

Decadal changes in glacier mass balance versus altitude help put the extreme conditions of 2022 in context. There was no accumulation area in 2022 and the mass balance at each altitude band was more negative than the annual average value of the 1990s, 2000s and 2010s, especially at higher altitudes (Fig. 3a). Compared with the previous 5 years (Fig. 3b), the mass balance of each altitude band in 2022 was especially negative at higher altitudes. As winter balances were not extremely low in 2022 (Fig. 2), we confirm that strong ablation at high altitudes drove substantial glacier mass loss in 2022.

Figure 3. Comparison of the altitudinal distribution of glaciological mass balance on interdecadal (a) and annual (b) scales. Departures are derived from the 1959–2022 average. Normalized elevation is calculated as (h–h _min)/(h _max–h _min), where h is the mid-value of each elevation band (50 m interval), h _min and h _max are the maximum and minimum mid-value of the elevation bands, respectively, for different periods. Note that the normalized elevation is used to have a better comparison of the observed results due to the retreat and the subsequent altitude increase of the glacier terminus.

Summer air temperature and annual precipitation are the main drivers controlling mass balance of Urumqi Glacier No. 1 (Liu and others, Reference Liu, Xie and Wang1997; Li and others, Reference Li2020). The extreme summer air temperatures of 2022 (Fig. 1) are one of the major contributing factors to the extreme glacier mass loss. Precipitation phase is another important factor that affects glacier mass balance (Jouberton and others, Reference Jouberton2022). At the headwaters of the Urumqi River, 99.5% of total precipitation falls as snow when the air temperature is below −2.0°C and <40% of total precipitation falls as snow when the air temperature is above 2.0°C (Chen and others, Reference Chen, Li, Wang, Jia and Jin2023). Daily mean temperatures observed at the Daxigou Meteorological Station were above −2°C for the whole summer of 2022. An automatic weather station (AWS) located at an elevation of 3835 m a.s.l. on a moraine ridge beside Urumqi Glacier No. 1 (Fig. S1) recorded air temperatures between −1.7 and 10.0°C throughout the summer of 2022 (Fig. S3), with average daily temperatures above 2°C. If we adopt the previously used threshold temperature of 0°C for solid/liquid precipitation separation at Urumqi Glacier No. 1 (Zhang and others, Reference Zhang, Wang and Zhou2022), the solid precipitation at the AWS only accounted for 13% of the summer total. The extreme summer temperatures observed in the region resulted in zero summer accumulation at Urumqi Glacier No. 1, and the total precipitation observed at Daxigou Meteorological Station over the 2022 mass-balance year was the lowest since 2010 (Fig. S4). The lower solid precipitation ratio, decreasing total precipitation and continuous high air temperature may jointly reduce glacier accumulation during the summer of 2022 (Jouberton and others, Reference Jouberton2022).

The meteorological records at the Urumqi Glacier No. 1 AWS can theoretically result in a slight summer accumulation of 48 mm w.e. in 2022. However, this cannot compensate for the strong ablation caused by extreme summer air temperatures. A temperature-index model (Zhang and others, Reference Zhang, Wang and Zhou2022) driven by summer air temperatures observed at the AWS produces −2146 mm w.e. of summer ablation at the glacier terminus (3700–3750 m a.s.l.). This contributes to ~83.3% of the annual mass loss and to ~75.6% of the summer mass loss at the terminus. This model uses snow and ice melt degree-day factors of 7.4 and 2.6 mm °C⁻¹ d⁻¹, respectively, based on in situ measurements of summer mass balance at Urumqi Glacier No. 1 between 2001 and 2020 (Zhang and others, Reference Zhang, Wang and Zhou2022). Previous studies have indicated that the temperature-index models have a good performance in simulating the mass balance of Urumqi Glacier No. 1 (Wu and others, Reference Wu, Li and Wang2011).

Glacier surface albedo is another key factor to influence mass balance by governing the quantity of net shortwave radiation. Here we selected six Landsat images on cloudless days to find the spatial distribution of surface albedo during the 2022 ablation season (see Supplementary Table S1; Liu and others, Reference Liu, Xie and Wang1997). The process for deriving glacier surface albedo from Landsat images includes geolocation, radiometric calibration, atmospheric correction, topographic correction, anisotropic correction and narrow-to-broadband conversion, which have been described in detail by Yue and others (Reference Yue2017). The spatially distributed surface albedo presented relatively high values at the early (26 June 2022) and end (6 September 2022) of the ablation season. In the middle of the ablation season, the spatial pattern of surface albedo was characterized by lower values except for 22 July 2022; the proportion of surface albedo less than 0.30 (bare ice albedo value) was 79% on 5 July 2022, 0.91% on 5 August 2022 and 88% on 13 August 2022, respectively (Fig. 4). High albedos observed on 22 July 2022 are related to fresh snow cover, as air temperatures on the eve of the satellite transit were low enough to result in solid precipitation (Fig. S3). Overall, the mean surface albedos in the middle ablation season of 2022 ranged from 0.14 to 0.40, which were lower than the corresponding value for the normal mass-balance year of 2014 (Yue and others, Reference Yue2017). The lower surface albedos, especially at higher elevations, may result in more negative mass balances for Urumqi Glacier No. 1 in 2022.

Figure 4. Spatial distribution of Landsat-derived surface albedo for Urumqi Glacier No. 1 on selected dates of the ablation period in 2022 (date: day/month/year).

The response of glaciers to heat waves is a growing concern. For example, European heat waves contributed to 35% of the overall glacier mass loss in 2022, and 56% of the average summer mass loss between 2010 and 2020 (Cremona and others, Reference Cremona, Huss, Landmann, Borner and Farinotti2022). In the Nooksack River basin, USA, heatwaves enhanced glacier ablation and increased discharge in the North Fork Nooksack River by 24% (Pelto and others, Reference Pelto, Dryak, Pelto, Matthews and Perry2022). Our results indicate that Urumqi Glacier No. 1, and other glaciers in the region, are potentially more sensitive to heatwaves than glaciers in other mountain regions. The increasing intensity, frequency and duration of regional heatwaves around the world (Perkins-Kirkpatrick and others, Reference Perkins-Kirkpatrick and Lewis2020) will pose a substantial threat to mountain glaciers.