ZANSKAR GLACIERS SLOW DOWN: A WARNING FOR INDIA'S WATER SECURITY

Why In News?

A new study in The Cryosphere finds that 12 glaciers in Ladakh's Zanskar Basin have slowed down and thinned faster between 1992 and 2023, flagging an early warning sign for the Upper Indus Basin's long-term water security.

What is Glaciological Deceleration?

Glaciological deceleration refers to a measurable slowdown in the rate at which glacier ice flows downslope over time, tracked through surface velocity measurements.

Slowing glacier flow: Glaciers move under gravity, and a drop in this flow speed indicates weakening internal ice dynamics, not just changes at the glacier's edges.

Link between ice thickness and glacier movement: Glacier flow is driven by "driving stress," which depends on ice thickness; as glaciers thin due to mass loss, driving stress falls, and flow slows down — the study identifies ice mass loss and thinning as the dominant control on this deceleration.

Retreat describes the glacier's terminus (edge) moving back, i.e., area/length loss, while deceleration describes a slowdown in the internal ice flow speed — a glacier can decelerate even while its terminus position changes only modestly, making velocity a distinct and complementary indicator of glacier health.

What are the Major Findings?

Decline in average glacier velocity: The study finds the mean central-flowline velocity across all 12 study glaciers fell from 31.1 ± 8.5 m/year in 1992 to 26.2 ± 1.0 m/year in 2023 — a decline of roughly 16%.

Greater deceleration at lower elevations: The slowdown is high at lower elevations, consistent with these zones experiencing greater warming-driven mass loss.

Accelerating surface thinning: Surface elevation loss (thinning) accelerated over the study period — from about 0.22 m/year between 2000–2005 to about 0.57 m/year by 2015–2020, more than doubling the thinning rate.

Researchers use Landsat satellite imagery, processed via correlation techniques (e.g., COSI-Corr), combined with reanalysed elevation data, to reconstruct interannual velocity and elevation change across glaciers with varied size, elevation, debris cover, and terminus type.

What are the Major Impacts of Glacier Thinning?

• Reduced long-term water storage: Thinner glaciers hold less frozen water reserve, weakening their buffering capacity in future dry years.

• Changing river discharge: Discharge patterns shift — potentially higher short-term flows now, but reduced and less predictable flows in subsequent decades.

• Agricultural stress: Farmers dependent on precise meltwater timing for sowing and irrigation cycles face growing uncertainty.

• Hydropower risks: Variable and declining glacier-fed flows threaten the long-term viability of Himalayan hydropower projects reliant on consistent discharge.

• Ecosystem changes: Altered flow regimes disrupt riparian and wetland ecosystems adapted to historical glacier-melt patterns.

• Water security concerns: Combined thinning and deceleration signal that Zanskar glaciers may be approaching or have passed a critical point in their water-supply trajectory, directly threatening downstream water security.

About Zanskar Glaciers 

• High-altitude cold desert: Ladakh's Zanskar region is a cold desert in the Western Himalayas, receiving very little direct precipitation.

• Western Himalayas: The Zanskar range forms part of the trans-Himalayan zone, geologically and climatically distinct from the wetter eastern Himalaya.

• Drainage System: Meltwater feeds the Doda and Lungnak rivers, which merge to form the Zanskar River, a major right-bank tributary of the Indus River.

• Limited summer rainfall: Unlike monsoon-fed river systems, Zanskar's rivers receive minimal monsoon rainfall, making them heavily dependent on frozen water reserves.

• Dependence on snow and glacier melt: Local agriculture, drinking water, and ecosystems in this cold desert rely almost entirely on snow and glacier meltwater, rather than rain, unlike most of peninsular and monsoon-fed India.

Source: THEHINDU