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Environment and Disaster Management › Climate science
हिन्दी — Read in HindiCryosphere, oceans and sea level
The frozen and ocean parts of the climate system: Himalayan glaciers and glacial lakes, sea ice and ice sheets, permafrost, ocean warming and acidification, and sea level rise. Prelims has asked which rivers rise from which Himalayan glaciers, and about the source and course of the Teesta.
Foundation note: Climate science and the greenhouse effect
Glaciers and outburst floods
Himalayan glacier retreat
Copy link to Himalayan glacier retreatPrelims and Mains
Himalayan glacier retreat is the loss of ice from the glaciers of the Hindu Kush Himalaya. That arc of mountains spans eight countries and feeds about ten major basins, including the Ganga, Indus and Brahmaputra, and two billion people.
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
The water tower
- The monsoon brings 70 to 80 per cent of its rain, and winter snow is its seasonal water buffer.
- Himalayan glaciers are losing mass faster each decade.
How warming loosens the mountain
- Warming does not only take ice off the mountain; it takes away what held the mountain together.
- Meltwater widens crevasses and lubricates the glacier bed, so large blocks of ice can slide.
- 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.
- The result is more glacial lakes, more unstable slopes, and floods that begin with a slope failure rather than a lake.
What changed
1 Sep 2026
- A survey after the Nepal flood drew on a March 2026 assessment and a review of recorded outburst floods; the figures are in the chart.
Frontline, 1 Sep 2026: India can see the danger in the Himalaya. Can it manage the risk? (opens in a new tab) · 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
The Nepal flood of August 2026 was a debris flow that began on 26 August, when glacier ice and bedrock broke away on Langtang Lirung near the Nepal Tibet border and ran down the Bhote Koshi.
- It was a cascading hazard, not a glacial lake outburst: no lake dam failed, and a slope failure of ice and rock started it.
- The flow eroded banks and gained mass and momentum as it went, a sediment bomb.
- A barrier lake formed behind it as a second, delayed hazard.
- Nepal counted 1,410 dead and over 6,000 missing by mid September, and 431 MW of hydropower, a tenth of the country's capacity, was hit by the one 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
26 Aug 2026New
- The cause was contested early, the United States Geological Survey pointing to a glacial collapse and a regional centre to an avalanche of ice and rock.
- The question asked in India was whether a country that can see the same danger in its own valleys can manage the risk.
- A separate study records a 525 per cent rise in glacial lakes in Himachal Pradesh between 1990 and 2024.
Frontline, 1 Sep 2026: India can see the danger in the Himalaya. Can it manage the risk? (opens in a new tab) · Frontline, 28 Aug 2026: A sudden flood on a Bhote Koshi tributary kills over 780 people in Nepal and Tibet, and the cause is still contested between a glacial lake burst and an ice-rock avalanche. (opens in a new tab)
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)
Glacial lake risk in India
Copy link to Glacial lake risk in IndiaPrelims and Mains
LeadHimalayan hazards: lake outbursts and avalanche cascadesSeptember 2026
Why in news
Uttarakhand began installing its first glacial lake early warning system at Vasundhara Tal in Chamoli on 29 September 2026. It comes a month after the Nepal disaster of 26 August, which experts now say was not a lake outburst at all.
- 195high risk glacial lakes on the national list
- 30 of 100marks the risk index gives to downstream vulnerability
- about 73 per centof mapped Himalayan glacial lakes that lie outside India
Background
- A glacial lake forms where a retreating glacier leaves a hollow that fills with meltwater, held back by bedrock or by moraine, the loose rock and sediment the glacier leaves behind.
- A moraine dam can fail by slow erosion, by the melting of ice buried inside it, or when an avalanche or landslide falls into the lake and sends a wave over it.
- In Nepal on 26 August 2026, ice and rock broke away on Langtang Lirung and ran down the valley as a debris flow, leaving 1,410 dead and over 6,000 missing.
Two hazards, not one
- A glacial lake outburst flood (GLOF) has a known source, the lake, which can be modelled, monitored or drained.
- A rock and ice avalanche cascade can begin on any of hundreds of thousands of steep slopes where permafrost is thawing, so it is far harder to plan for.
- Chamoli in 2021 and Rasuwa in Nepal in 2026 were both cascades, and in both many of the dead and missing were hydropower workers in the path of the flow.
What India has built
- The National Disaster Management Authority keeps a list of 195 high risk lakes, and a National GLOF Risk Mitigation Programme covers Arunachal Pradesh, Himachal Pradesh, Sikkim and Uttarakhand.
- The Central Water Commission scores lakes on twelve criteria, giving 30 of 100 marks to downstream vulnerability, because risk is hazard meeting exposure.
- The Vasundhara Tal system tracks water level, weather, ground movement and discharge, and sends data by satellite to a dashboard that raises an alert at set thresholds.
The gaps
- Detection is not warning: the alert has to reach districts, project operators and villages in time, and people act on it only if they trust it.
- The Nepal flow crossed the border with Tibet, and there is no shared data feed across it, so a warning system that stops at the border misses half the hazard.
- Black carbon from brick kilns, cookstoves and crop burning darkens snow and speeds melting, and accounts for about a third of Himalayan glacier mass loss.
A lake outburst and an avalanche cascade
| Glacial lake outburst flood | Rock and ice avalanche cascade | |
|---|---|---|
| Source | one known lake | any steep, deglaciating slope |
| Trigger | the moraine dam erodes, or a wave from a falling mass breaches it | a slope of rock and ice collapses |
| What can be done | the lake can be modelled, monitored or drained | hard to plan for; siting and land use must change |
| Cases | South Lhonak, 2023 | Chamoli, 2021; Rasuwa, 2026 |
The way forward
- Site hydropower and settlements with the cascade hazard in mind, since many Himalayan dams were planned decades ago without knowledge of climate change.
- Share warning data across borders, which India can lead with early warning data from ISRO and a regional risk and insurance facility.
- Cut black carbon at the scale of the airshed, taking kilns, cookstoves, transport and crop residue burning together.
Prelims facts
- Permafrost is ground that stays frozen for at least two consecutive years.
- Peak water is the point at which glacier meltwater reaches its maximum, after which river flow declines.
- Vasundhara Tal is in Chamoli, Uttarakhand; Ladakh has the most glacial lakes of any State or Union Territory.
- The Hindu Kush Himalaya could lose up to 80 per cent of its present glacier volume by 2100.
Many glacial lakes are held not by bedrock but by moraine, the loose rock a glacier leaves behind, and India ranks them by the risk they pose to the valleys below.
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, four States
Dam safety
- Central Electricity Authority: 47 dams that an outburst flood could affect
The hazard
- A moraine dam gives way to an avalanche wave, to melting of ice inside it, or to overtopping.
- South Lhonak in Sikkim in October 2023, which destroyed the Teesta III dam after a 2019 modelling study had flagged the valley, is the Indian case.
- Risk is hazard times exposure: a dangerous lake above an empty valley is a low risk.
- Warning is half of it: in worst case models a flood set off in Tibet reaches the Nepal border in about 30 minutes, so zoning and local evacuation decide the toll.
- Blatten in Switzerland in 2025, with one death after evacuation, against Chamoli in 2021, with over 200, shows it.
What India does
- The Central Water Commission's twelve criteria index gives 30 of 100 marks to downstream exposure.
- The National Remote Sensing Centre's Glacial Lake Atlas finds almost 73 per cent of the lakes in the Himalayan basins outside India, so data sharing with China and Nepal is part of the warning system rather than a courtesy.
- After South Lhonak the National Disaster Management Authority approved the National Glacial Lake Outburst Flood Risk Mitigation Programme, across four States, and listed 195 high risk lakes.
What changed
30 Sep 2026LeadHimalayan hazards: lake outbursts and avalanche cascades
UPSC has asked
- Prelims 2017: the source and course of the Teesta
Oceans
INCOIS El Niño bulletins for the seas
Copy link to INCOIS El Niño bulletins for the seasPrelims
The Indian National Centre for Ocean Information Services (INCOIS) links El Niño to warmer Indian Ocean waters, which bring more coral bleaching and marine heatwaves.
- Catches of sardine and mackerel fall as stocks migrate or recruitment falls.
- The Bay of Bengal stays rough in the monsoon with more east coast erosion and flooding, while the Arabian Sea and west coast are likely calmer.
What changed
23 Jun 2026BriefINCOIS released its first specialised El Niño bulletin for maritime sectors on 22 June; the event is expected to peak in November 2026 to January 2027. PIB, 23 Jun 2026: INCOIS Launches Specialized El Niño Bulletins to Safeguard India's Maritime Sectors (opens in a new tab)
See also: The 2026 monsoon and El Niño
Also filed elsewhere
- Hydropower in the Himalaya · on EIA, clearances and pollution control law
Hydropower in the Himalaya means dams and power projects built on the rivers of a young, steep and seismically active range.
- The UN declaration on sea level rise and statehood · on Law of the sea
The United Nations General Assembly's declaration on sea level rise affirms that a State remains a State even if the sea takes its land, and that its maritime boundaries stay where they were fixed.