TIL BDR GHALLEY | Thimphu
Bhutan‘s has lost 27 glaciers and 57.69 square kilometres of glacier area over the past nine years, according to the latest national glacier assessment, showing continued retreat of the country’s Himalayan ice reserves that serve as vital freshwater sources for rivers, agriculture, hydropower generation and mountain ecosystems.
The Bhutan Glacier Inventory (BGI) 2026, released by the National Centre for Hydrology and Meteorology (NCHM), recorded 673 glaciers covering 571.86 square kilometres, compared with 700 glaciers covering 629.55 square kilometres documented in the previous national inventory. The decline represents a 9.16 percent reduction in glacier area between 2016 and 2025.
Located in the eastern Himalayas, Bhutan’s glaciers function as natural water storage systems, releasing meltwater into river networks that support downstream communities, agricultural production and the country’s hydropower sector. The Bhutan Glacier Inventory 2018, the country’s first nationally led glacier assessment, identified around 700 glaciers covering approximately 630 square kilometres, equivalent to about 1.6 percent of Bhutan’s total land area.
The latest assessment found that glacier retreat has affected most glacier-fed river basins, with declines recorded in both glacier numbers and overall glacier coverage. The changes reflect continued transformation of Bhutan’s high-altitude environment amid rising temperatures and changing precipitation patterns.
The Punatsang Chhu basin remains the country’s largest glacier-bearing basin, accounting for the highest share of Bhutan’s glacier area. The previous inventory recorded 341 glaciers covering 361.07 square kilometres in the basin.
Within the Punatsang Chhu basin, the Pho Chhu sub-basin remains one of Bhutan’s most glacier-rich areas. The latest inventory recorded 200 glaciers covering 32.49 square kilometres in the sub-basin. Earlier assessments had identified 206 glaciers covering 252.42 square kilometres in Pho Chhu, while the Mo Chhu sub-basin contained 135 glaciers covering 108.64 square kilometres.
The Mangde Chhu sub-basin previously contained 111 glaciers covering 108.26 square kilometres, while the Chamkhar Chhu sub-basin had 90 glaciers covering 68.27 square kilometres. The Kuri Chhu and Drangme Chhu sub-basins recorded 90 and 21 glaciers respectively, covering 55.29 square kilometres and 3.28 square kilometres.
The Wang Chhu basin, which has historically had the smallest glacier coverage among Bhutan’s major river basins, recorded 47 glaciers covering 33.38 square kilometres in the earlier inventory.
Researchers observed an increase in glacier numbers within the Mangde Chhu sub-basin in the latest assessment. However, the increase was not due to the formation of new glaciers. An NCHM official explained that larger glaciers had fragmented into smaller ice bodies, resulting in a higher glacier count.
“Although the number of glaciers increased in the Mangde Chhu sub-basin, this does not indicate the formation of new glaciers. The increase is mainly due to the fragmentation of larger existing glaciers into smaller ice bodies,” the official said.
The largest glacier identified in the previous national inventory was MMagr16_482 (G090443E28024), located in the Mangde Chhu sub-basin. The glacier measured 15.56 kilometres in length and covered 45.85 square kilometres.
Glacial lakes reflect changing Himalayan landscape
Alongside glacier retreat, Bhutan’s northern mountain regions contain hundreds of glacial lakes formed through glacier melting and retreat. These lakes represent important indicators of changes taking place in the country’s high-altitude environment.
The Bhutan Glacial Lake Inventory (BGLI) 2021 mapped 567 glacial lakes covering a total area of 55.04 square kilometres.
These lakes are closely monitored because some have the potential to trigger glacial lake outburst floods (GLOFs), where sudden releases of stored water can affect downstream settlements, infrastructure and river systems.
The Manas basin contains the highest number of glacial lakes, with 331 lakes covering 29.2 square kilometres, while the Wang Chhu basin has the lowest number, with 31 glacial lakes covering 0.6 square kilometres.
In the Punatsang Chhu basin, the Pho Chhu sub-basin contains 157 glacial lakes covering 20.98 square kilometres, followed by the Chamkhar Chhu sub-basin with 131 lakes covering 11.56 square kilometres and the Mangde Chhu sub-basin with 130 lakes covering 11.85 square kilometres. The Kuri Chhu and Drangme Chhu sub-basins contain 61 and nine glacial lakes respectively.
The latest assessment found that Bhutan’s glacial lake systems are changing rapidly, with new lakes forming while some previously identified lakes have disappeared as glaciers continue to retreat and evolve.
“Fifty newly formed glacial lakes have been identified in the latest assessment after meeting the scientific criteria for classification, while 27 glacial lakes recorded in the earlier inventory of around 700 lakes have disappeared, reflecting the rapid changes taking place in Bhutan’s high mountain environment as glaciers continue to retreat and evolve,” NCHM officials said.
The changes highlight the dynamic nature of glacial lakes, which can expand, drain, merge or disappear depending on glacier movement, melt patterns and changing environmental conditions.
The latest glacier inventory introduces artificial intelligence, machine learning and deep learning technologies into Bhutan’s glacier monitoring system.
According to NCHM, the assessment used Sentinel-1 radar and Sentinel-2 optical satellite imagery collected during the post-monsoon season of 2025 to map glacier boundaries.
Scientists applied deep learning models, including DeepLabv3+ with a ResNet101 backbone, to automatically identify glacier areas before conducting detailed manual verification.
NCHM said the integration of artificial intelligence improved the efficiency, consistency and reliability of glacier mapping, particularly in difficult mountain terrain and debris-covered glacier areas.
The researchers estimated glacier mapping uncertainty at 0.374 percent, indicating a high level of confidence in the assessment. The inventory also found that most of Bhutan’s glacier ice remains concentrated between 5,000 and 5,700 metres above sea level.
NCHM Director General Karma Dupchu said glaciers remain among Bhutan’s most important natural assets because of their role in maintaining freshwater systems and supporting ecosystems.
“The glaciers of Bhutan are among the country’s most important natural assets, serving as vital freshwater reserves and providing essential ecosystem services that sustain downstream communities, agriculture, hydropower generation, and biodiversity,” he said.
He said glaciers are highly sensitive indicators of climate change and require continuous monitoring to understand environmental changes and support evidence-based planning and decision-making.
“The adoption of Machine Learning and Deep Learning technologies reflects NCHM’s commitment to embracing innovative scientific methods and keeping pace with the rapidly evolving field of cryospheric research,” Karma Dupchu said.
He added that the updated inventory would serve as an important reference for researchers, planners, policymakers and development partners working on climate change, water resources, hydrology and mountain hazards.
The BGI 2026 provides Bhutan with an updated national benchmark for tracking future glacier changes. NCHM officials said the database will strengthen long-term glacier monitoring, climate research, water resource management and efforts to reduce risks associated with mountain hazards, including glacial lake outburst floods.
As Bhutan’s glaciers continue to retreat, the latest assessment provides updated scientific evidence on changes taking place in the country’s Himalayan environment and strengthens efforts to monitor the future of its frozen freshwater reserves.
