1. Introduction
Over half of the world’s water is extracted from rivers, which are often fed by glaciers [
1]. Therefore, the world’s glaciers are considered to be of high importance to ensure water supply for millions of people [
1]. Because of their sensitivity to changes in temperature and precipitation, they are often referred to as “natural thermometers” [
2]. Worldwide, the glaciers have lost 275 Gt per year in the period between 1993 and 2009, whereas this trend has been accelerating since 2005 [
2]. These changes are particularly relevant for hydrological systems, as glaciers are a major reservoir of freshwater. Due to their capacity to store vast amounts of water, which is released in the summer months, glaciers are crucial for water supply in downstream areas, particularly in (semi-)arid regions [
3,
4]. In river basins, where large areas are irrigated and rainfall is little, agriculture strongly depends on the glaciers’ contribution to river runoff [
5,
6,
7]. Due to the major importance of glaciers for fresh water availability in combination with global climate change and increasing population growth in large parts of the world, it is essential to measure and monitor possible changes. Only when the dimension of changes and the implications for the downstream areas are known can people and governments take measures to adapt to and manage possible consequences. However, monitoring data for river discharge are not available or even not existing for many remote regions, as is the case for the upper Ayeyarwady River in Northern Myanmar.
The headwaters of the Ayeyarwady River (also referred to as Ayeyarwaddy or Irrawaddy River) are located in the northern parts of Myanmar and the adjacent Tibet Autonomous Region of China in the syntaxis of the Eastern Himalayas and the Tibetan Plateau. The glaciers located in the headwater region are of a summer accumulation type and are predominantly fed by summer monsoon precipitation. Thus, this is a region where precipitation and river discharge peak in the summer, and one can hypothesize that the glaciers do not play a crucial role in water supply [
6]. However, about 1.7 million people live in the Myanmar river source area [
8], without counting the people living in the Chinese part of the headwaters, depending on river discharge also during weak monsoon years when precipitation amounts are low and meltwater input might be more relevant. The Ayeyarwady River emerges in Northern Myanmar from the rivers Mali and N’Mai near the city of Myitkyina (
Figure 1). It is of high significance for the country, as it is considered as the lifeline of Myanmar [
9], and it functions as the main transport system and the central water supply system for people and agriculture [
9,
10]. Despite its importance for Myanmar, the Ayeyarwady is the least known river among the largest rivers in Asia, such as the Indus, Ganges, Brahmaputra, or Yangtze Rivers [
10]. For a long time, Myanmar was governed by a military regime that kept the influence of other countries and international organizations at a low level. Due to the long isolation of the country, there is little to no information and international scientific research on human–water issues [
9].
The glaciers in the Ayeyarwady headwaters have already been studied by Bajracharya and Shrestha [
11]. However, the study only assesses the situation in 2009, focuses on the entire Hindu-Kush Himalaya region, and does not examine whether the glaciers have changed over the past decades. The present paper helps to fill a gap that has been shown, amongst others, by Bajracharya and Shrestha [
11]. As the glaciers are located in a rugged and hardly accessible terrain in the Himalaya mountains, we applied satellite imagery to map and analyze glacier changes in the last four decades. Based on the results, implications for possible future changes in river runoff are discussed.
4. Discussion
We identified 129 glaciers in total, compared to 133 glaciers delineated by Bajracharya and Shrestha [
11]. Therefore, four glaciers could already had disappeared by 2015 or it was not possible to map the glaciers, due to possible misinterpretations.
In general, the studied glaciers in the Ayeyarwady headwaters show a retreating trend in all analyzed glacier parameters between 1976 and 2015. Regarding the glacier changes in the four sub-regions, it is obvious that the glacier parameters such as area, volume, mass, length, and terminus position show the same behavior in the sub-regions as in the glacierized area as a whole. The highest losses in all regions occurred between 2002 and 2015 (
Table 5). In general, the changes in glacier area, volume, and mass in Sub-Region 4 increased over time, which means that the total and annual loss rates in every glacier parameter are lowest in the period 1976–1990 and highest between 2002 and 2015. Area and volume changes of the glaciers in Sub-Regions 1, 2, and 3 are following a different trend, as they show the smallest losses, both totally and annually, in the period 1990–2002 and not at the beginning of the investigation period (
Figure 5 and
Table 5).
The importance of the meltwater contribution to the runoff of the Ayeyarwady River relative to other inputs such as monsoon rainfall and snow is unknown and unstudied. The relevance of the net melt input to gauging stations downstream is also still not known and should be investigated in future studies. In general, the river is predominantly fed by monsoon rains, but meltwater could play a crucial role at least in the upper basin, where the two source rivers originate. Bookhagen and Burbank [
25] showed that the Eastern Himalaya river basins receive less than 20% from meltwater and ~80% from monsoon rains during the summer months. They stated that meltwater contribution plays a significant role during pre- and early monsoon season [
25]. For our study region, we argue that meltwater contribution is likely to be important also during weak monsoon years. In another study from the Tamor River basin in Nepal, Panday et al. [
26] came to the finding that ~30% of annual discharge is generated by meltwater, which may entail societal consequences. In our study, Sub-Region 3 lost the highest amount of water annually (~1 m) compared to the other sub-regions. We conclude that particularly the glaciers in Sub-Region 3 and their contribution to river runoff should be investigated in greater detail.
Glacier changes are considered to be one of the first signs of a changing climate, and glaciers in the study region exhibit a clear retreating trend. It is well known that the reaction of the glacier tongue is a delayed reaction to the climate signal, and it is possible that the glaciers right now are reacting to climatic changes that already happened in the past and that they continue to retreat even if current trends can be reversed [
27]. Increased ablation due to higher temperature can immediately result in negative mass balance, and it is the persistently negative mass balance that leads to glacier retreat. Temperature in the HKH region has risen faster than the global average [
28,
29]. The Ayeyarwady catchment is strongly influenced by the Indian summer monsoon and precipitation patterns here change due to the global warming [
30]. The regional glaciers belong to the summer accumulation type and gain mass from the summer monsoon snowfall [
31]. This specific glacier type is especially sensitive to temperature change because increasing temperatures lead not only to the extension of the melting period but also to the increase in the share of rainfall and the reduction in the share of snow and, consequently, mass gain by the glaciers [
31], enhancing glacier melt [
29].
Figure 6 illustrates the past change of air temperature in the upper Ayeyarwady River basin at the Myitkyina station [
31]. The glacier area changes in our study region and the global annual land–ocean temperature index for the latitudes equator to 24° N [
32]. The local annual mean temperature follows the global trend of warming during the last decades, and it can be seen that the glaciers lost more than a half of their area. The warming was highest from the beginning of the 1980s when glacier retreating rates were comparatively low. The glacier lost has intensified around 2000, when the temperature showed a high variability, but no clear trend. The observed glacier changes in our study region could also be a response to a negative anomaly in monsoon precipitation (
Figure 7). Pan et al. [
33] studied changes in the glacier area of the monsoonal influenced Gongga Mountains, located on the southeastern margin of the Tibetan Plateau, about 400 km east of our study region. They found out that the glaciers shrank by 11.3% since 1966 and that they showed the highest loss rates during the period 2005–2009, which corresponds to our results. The majority of the glaciers of the Gongga Mountains are small (<1 km
2), and Pan et al. [
33] concluded that the observed retreat is mainly due to rising temperatures because the monsoon precipitation even shows a slight positive trend, similar to the observed rainfall amounts at the Myitkyina station located in the upper Ayeyarwady River Basin (
Figure 7). Studies by Liu et al. [
34] indicate that temperature changes are the main controlling factor of glacier changes of the Gongga Mountains.
The highest rates of warming occur between an altitude of 4800 and 6200 m a.s.l. in the study region [
30]; therefore, glaciers within this range are experiencing higher recession rates, but the most vulnerable glaciers are the ones below an altitude of 5700 m a.s.l., as they react exceptionally fast to climatic changes [
3]. As all glaciers in the Ayeyarwady headwaters are located below that altitude, they are especially prone to changes in area, volume, and length. Glacier changes in the study region can further be influenced by glacier size and aspect. It is very well documented that small glaciers are retreating faster than larger glaciers [
3,
35]. This behavior can also be observed in the study area; glaciers <1 km
2 have retreated between 1976 and 2015 more than twice as much as glaciers >1 km
2. The differences in glacier recession rates in the four sub-regions are probably also caused by different glacier sizes. Particularly, the overall smaller recession rates in Sub-Region 3 are probably caused by the different glacier sizes as the glaciers in Sub-Region 3 are larger compared to other sub-regions. This result can also be supported by findings from Sub-Regions 1 and 4. These sub-regions show the smallest total glacier area, but they are experiencing the highest losses in individual glacier area and volume/mass.
In the time period from 1971 to 2009, the glaciers all over the world lost around 226 Gt of their ice mass per year. The mass losses of the Ayeyarwady glaciers are a tiny fraction of global losses, but it shows that the volume and mass losses are in accordance with global trends. Furthermore, the examination of global glacier changes shows that the losses in all three considered glacier parameters have increased toward the end of the 20th and the beginning of the 21st century. The exact same behavior can be observed in the glaciers of the Ayeyarwady headwaters. One example for the regional variability of glacier changes is the HKH region. Glaciers in the northern and western parts of the HKH region behave differently compared to the southern and eastern glaciers. For example, the glaciers in the Himalaya mountain range lost 5–55% of their area over the past 30 years, which happened much faster than in the Hindu Kush and Karakoram at the western side of the HKH region [
3]. Moreover, the area changes of the Ayeyarwady glaciers have lost more than the average glaciers in the Himalaya mountain range. In Southeast Asia, however, annual glacier area losses around 0.55% between 1975 and 2005 could be observed [
2]. Over the period between 1970 and 2009, annual glacier area changes in Central Asia of approximately −0.8%
a−1 could be detected [
2]. With an annual relative glacier area change of −1.39 ± 0.03%
a−1 for the glaciers in the Ayeyarwady headwaters, it seems that the area losses in the study area are exceeding the mean annual glacier changes in Southeast and Central Asia. This compares with annual glacier area loss rates in the Indian Himalayas (0.2–0.7% year
−1 for 1960s—2004), the Garhwal Himalayas (0.12% year
−1 for 1968–2007), and in Bhutan (0.3–0.6% year
−1 for 1963–1993) [
27,
29]. The retreat rates for the Indian Himalayas and the rates of the Garhwal Himalayas, however, should be considered carefully, as they are located in the western part of the HKH region, which is less dominated by monsoonal precipitation and experiences advances at some glaciers. Glaciers that might show a similar climate are located at the southeastern Tibetan Plateau. Studied by Yao et al. [
30], the glaciers in the Parlung valley show mean annual area losses of 0.57% year
−1, which is higher than other retreat rates observed in India but still half of the annual area retreat rates calculated for the Ayeyarwady glaciers. Therefore, it can be concluded that the glacier area changes in our study region are occurring at a faster and more extreme rate compared to High Mountain Asia, which could be explained by the generally small glacier sizes of the Ayeyarwady glaciers and changes in temperature and precipitation. The comparison between the glacier losses in the Ayeyarwady headwaters with other regions should be considered carefully because the glaciers differ widely in size and volume. Nevertheless, the study provides information about the general dimension of glacier loss in our study region.
In the Himalayan region, the annual mass balance values typically range between −0.32 m year
−1 (1992–2000) and −1.60 m year
−1 (2001–2006) WE for individual glaciers [
35]. Comparing these values with the glacier changes of the Ayeyarwady headwaters for the period of 1976–2015 (−0.87 ± 0.035 m WE), they fit into the regional trends. Admittedly, the glacier changes in the Ayeyarwady catchment for shorter time periods show a different behavior. For the three selected time periods, the annual changes are higher (up to >3 m WE,
Table 6) than the typical range presented above. This indicates that some glaciers in the study region are losing volume/mass faster than the regional trends would imply. The glaciers in our study region are part of the GAMDAM (Glacier Area Mapping for Discharge in Asian Mountains) Inventory and Sakai and Fujita [
36] found out that the mass losses in that Himalaya region are predominantly due to warming temperatures and/or decreasing summer precipitation and they stated that the mass balances of the glaciers are highly sensitive to temperature changes. The retreat rates of the glaciers in the upper Ayeyarwady region are highest in the period 2002–2015, when the regional temperature does not increase, but is sustained at a higher level than 1976–1995 (
Figure 6). This glacier retreat is likely a response to both the regional temperature increase in the late 1990s and to low precipitation amounts in the beginning of the 21st century (
Figure 7). Glaciers in the Indian Himalayas, for example, vary in their response times between 4 and 60 years depending on glacier size and mass [
37]. Since smaller glaciers are more sensitive to climate change, the response time of the glaciers in the study region is assumed to be rather short.
It must be taken into account that studying glacier changes using remote sensing includes uncertainties depending on the application of different possible analyzing methods [
38]. Area-related scaling approaches as applied here overestimate glacier volumes in some regions [
39]. However, the aim of our study is to give first insights into glacier changes in the headwater region of the Ayeyarwady River during the last several decades rather than presenting absolute values. Therefore, our results show that the studied glaciers are retreating, independent from the method applied.
4.1. Possible Future Development
Losing more than half of their area and about 60% of their volume and mass leaves the glaciers in the Ayeyarwady headwaters in a critical condition in 2015, left with a total ice reserve of 1.02 ± 0.103 km
3. As presented before, the Ayeyarwady glaciers, as well as glaciers all over the world, have been experiencing an increase in recession rates since the beginning of the 21st century. The overall increase in temperature and changes in precipitation are not about to stop or even turn back in the foreseeable future. For the Kachin state in Northern Myanmar, even an increase in mean temperature of up to 2.5 °C is projected until 2100. Therefore, it is possible that the glaciers disappear. On the contrary, Su and Shi [
40] came to the conclusion that the maritime temperate glaciers in China, close to Northern Myanmar and the southeastern part of the Tibetan Plateau, will lose 75% of their total area by the year 2100. It can be assumed that the glaciers in the Ayeyarwady catchment are strongly threatened by the ongoing climate change and that they will disappear due to their generally small glacier area and volume; particularly, the sub-regions with a very small glacier area (1 and 4) will be more vulnerable toward future climate change.
4.2. Implications for Downstream Areas
The high retreat rates and the possible total disappearance of the glaciers will influence the glaciers’ storage function and likely the general hydrological system in the Ayeyarwady headwaters. This study did not analyze runoff or discharge data of the upper Ayeyarwady River because there are no data available at the moment for this region; therefore, it is difficult to actually determine the glaciers’ contribution of glacial meltwater to the river flow and what implications the glacier changes might have on downstream areas. Compared to other glaciated river catchments originating from the Tibetan Plateau, it becomes clear that glacier contribution to river flow can vary substantially. The Indus and Brahmaputra Rivers, for example, are highly dependent on glacier melt to sustain water supply for the large population in the downstream area [
41]. However, in the Ganges, Yangtze and Yellow Rivers, glacial meltwater is less important and only contributes <10% to river discharge. The small contribution to discharge in these catchments is caused by large downstream areas, limited upstream precipitation, a small glacier area, or a downstream area dominated by a wet monsoon [
41]. As the Ayeyarwady River is also dominated by the monsoon climate in the downstream area and has only a small glaciated area in its headwaters, it can be assumed that the glaciers’ contribution to discharge is even lower due to the existence of only small glaciers in the headwaters. Kaser et al. [
42] investigated the potential relevance of glaciers in the headwater region of the Ayeyarwady River, and they found almost no impact of the glaciers on the Myanmar population. However, their findings refer to the entire population in Myanmar, and they did not distinguish between single regions. Therefore, further studies are necessary in order to assess the impact of glacier loss on the people living in the upper part of the river basin. A continuing loss of glacier ice in the headwater regions is expected to increase the glacial meltwater contribution to river discharge in the short term. For the Hailuogou glacier in the Gongga Mountains on the eastern margin of the Tibetan Plateau, it could be shown that there is a significant increasing trend in annual and summer meltwater runoff [
34], which highlights the potential crucial contribution of meltwater to river discharge in the headwater region of the Ayeyarwady and the issue of water availability for the ~1.7 million people living in the most northern part of Myanmar. In the long term, as glaciers start to completely disappear, the water supply from glaciers will decline completely [
27,
30]. However, this will probably only have consequences for the Ayeyarwady tributaries in the headwater region and the local population in Myanmar’s Kachin State and a small part of the Tibet Autonomous Region of China. The past glacier changes and those happening in the future are probably more severe for the local people in the headwater region and may not even be noticed in the downstream areas of the Ayeyarwady catchment. However, to make assumptions on the implications for glacier hydrology and impacts on the river flow, further research needs to be conducted. Future studies should investigate the contribution of the glacial meltwater to river runoff in the upper Ayeyarwady River by stable isotope analyses or other tracer methods.