Environment › Climate science
Cryosphere, oceans and sea level
Ice, oceans and the rising sea: glaciers and glacial lakes, permafrost, polar ice, ocean heat and acidity, and sea level. Prelims has asked which rivers rise from which Himalayan glaciers, and about the source and course of the Teesta.
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Himalayan glacier retreat
Copy link to Himalayan glacier retreatPrelims and Mains
Warming does not only take ice off the mountain; it takes away what was holding the mountain together.
- Meltwater widens crevasses and, at the glacier bed, can destabilise large blocks of ice.
- A retreating glacier strips the snow that cushioned the rock walls above it, leaving rock and ice perched on bare slopes.
- Thawing permafrost removes the ice that cements rock and sediment together.
89%Monitored years with a negative glacier mass balance, over five decades
DoubledRate of ice loss after 2000
7 to 38Glacial lake outburst floods per decade in the region, up since 1950
What changed
Background reading: Frontline, 7 Sep 2026: A warning in the waters (opens in a new tab)
UPSC has asked
- Prelims 2019: matching Himalayan glaciers to their rivers
The Nepal flood of August 2026
Copy link to The Nepal flood of August 2026Prelims and Mains
On 26 August glacier ice and bedrock broke away on Langtang Lirung near the Nepal Tibet border, and what followed was a cascading hazard rather than a single flood.
- A glacial lake outburst flood begins when a lake's natural dam fails; this one began instead with a slope failure of ice and rock, which is why it is not called one.
- A debris flow erodes riverbanks and gains mass and momentum as it travels, which is why experts called this one a sediment bomb.
- The flow ran nearly 100 km, and Nepal counted about 1,340 dead by 7 September.
- About a tenth of Nepal's hydropower capacity, 431 MW, was hit by the single event.
Collapse
Glacier ice and bedrock break away on Langtang Lirung.
Temporary dam
Debris blocks the Lhende Khola in Tibet, then fails within minutes.
Bhote Koshi
The surge sweeps back into Nepal, gathering sediment.
Trishuli
The Trishuli rises by up to 9 metres in under 30 minutes.
Into India
The flow joins the Narayani system and enters India as the Gandak.
What changed
1 Sep 2026
28 Aug 2026Updatealso in newsnewly added
- The cause was contested early on: the United States Geological Survey pointed to a glacial collapse, while ICIMOD examined an avalanche of ice and rock. A barrier lake then formed near the border with China, a second and delayed hazard.
- A separate study records a 525 per cent rise in the number of glacial lakes in Himachal Pradesh between 1990 and 2024.
Background reading: The Indian Express, 31 Aug 2026: 'Needles in haystacks': An expert explains the challenge of early warnings in the Himalayas (opens in a new tab)
South Lhonak and moraine dammed lakes
Copy link to South Lhonak and moraine dammed lakesPrelims and Mains
Many glacial lakes are held not by bedrock but by moraine, the loose rock and sediment a glacier leaves behind, and moraine can be made to give way.
- The South Lhonak flood of 2023 destroyed the Teesta-III dam, though a modelling study in 2019 had flagged the valley as at serious risk.
Creeping slope
Parts of a frozen lateral moraine creep for years.
Collapse
Frozen moraine falls into the lake.
Wave
A wave overtops and breaches the frontal moraine.
Outburst
Lake water is released.
Erosion
The flood scours sediment down the Teesta valley.
What changed
UPSC has asked
- Prelims 2017: the source and course of the Teesta
The Glacial Lake Atlas
Copy link to The Glacial Lake AtlasPrelims and Mains
Most of the lakes that threaten Indian valleys are not in India, which makes a warning system a question of what neighbours are willing to share.
- The Glacial Lake Atlas of the National Remote Sensing Centre (NRSC) maps 28,043 lakes in the Himalayan river basins, almost 73 per cent of them outside India.
- The NRSC works under the Indian Space Research Organisation (ISRO), and the Defence Geoinformatics Research Establishment under the Defence Research and Development Organisation (DRDO).
- Data sharing with China and Nepal is therefore part of the warning system rather than a courtesy.
Ladakh3,219
Arunachal Pradesh2,188
Sikkim733
Jammu and Kashmir546
Himachal Pradesh537
Uttarakhand347
What changed
1 Sep 2026
29 Jul 2026Update
- ISRO has built a database of more than 2,000 glaciers in Ladakh and an inventory of 3,219 glacial lakes larger than 0.25 hectare, prepared under the National Hydrology Project.
- Outburst flood risk has been modelled for the high risk lakes, and a satellite based landslide alert system has issued village level alerts in Ladakh since 2023.
Ranking glacial lake risk
Copy link to Ranking glacial lake riskPrelims and Mains
A list of dangerous lakes is only useful once it is ordered by who lives below them.
- The Central Water Commission, the Centre's technical authority on water resources, gives 30 of 100 marks in its risk index to downstream exposure, since risk lies where hazard meets exposure.
- The Union Home Secretary reviewed Himalayan States' and Union Territories' preparedness for outburst floods and snow avalanches, asking for continuous monitoring and risk informed planning rather than lists alone.
- Early warning systems are to be extended from 30 lakes to at least 60.
Monitoring
- Central Water Commission tracks water bodies monthly in the monsoon
Ranking
- Central Water Commission 12 criteria index
- National Disaster Management Authority list of 195 high risk lakes
Mitigation
- National Glacial Lake Outburst Flood Risk Mitigation Programme, ₹150 crore, four States
Dam safety
- Central Electricity Authority: 47 dams that an outburst flood could affect
What changed
3 Sep 2026
1 Sep 2026
- Nepal's warning system issues alerts when it detects rain, and the August flood did not begin with rain.
- The flow's seismic signal was first read as an earthquake, so environmental seismology is proposed as an added layer of warning.
- In worst case models a flood set off by an avalanche in Tibet reaches the Nepal border in about 30 minutes, so warning has to be matched by land use zoning and local capacity.
- Evacuation on the first signs of slope instability held deaths in the 2025 Blatten avalanche in Switzerland to one, against over 200 in the 2021 Chamoli ice and rock avalanche.
Background reading: The Hindu, 4 Sep 2026
Also filed elsewhere
- Hydropower in the Himalaya · on Wind, hydro and other renewables
The mountains hold most of the fall a turbine wants and most of the hazard a project cannot survive.