Himalayan Warning From Nepal

Warming of mountains creates conditions for dangerous rain-and-ice events, glacial lakes multiply, and get filled up to the point of triggering outburst floods. Add to this China’s activities in Tibet. Also, Bihar is at the sharp end of all of this

China has warned that a newly formed lake higher upstream at the Tibet-Nepal border is at risk of breaching, which might worsen the ongoing flood-related crisis. But the mechanics of this disaster are, on their own, typical for the region. A warming-induced glacial outbreak sent a debris-laden surge down the Trishuli River system.

It has killed hundreds, with more than a thousand missing. It also put Bihar districts that border Nepal on high alert, as Gandak River rose. Still, what makes the event worth a longer look is not the flood itself. The key thing is it’s sitting on a curve that’s been tracked and measured for years, and scientific papers have been published.

A mountain range warming faster than the planet around it | ICIMOD’s 2019 Hindu Kush Himalaya Assessment found that even if the world warms to 1.5°C, this region will still warm by at least 0.3°C more than that.

Under moderate emissions assumptions, the region is headed for 2.5°C (±1.5°C) of warming by the end of the century; under high emissions assumptions, 5.5°C (±1.5°C).

The same assessment found the region losing roughly one cold night and half a cold day every decade, while warm nights are becoming 1.7 nights more common each decade and warm days 1.2 days more common. None of that is a forecast anymore. It’s a trend already showing up in station data.

Warmer air holds more moisture, and destabilises the permafrost and snowpack that used to hold Himalayan slopes together. This is the physical link between a warming baseline and the kind of compound rain-and-ice event that hit Rasuwa. It’s also the reason for a tremendous rise in glacial lakes.

Lakes are multiplying | An ICIMOD inventory carried out between 1999 and 2005 counted 8,790 glacial lakes across Hindu Kush Himalaya, covering about 802 sq km, and flagged 203 of them as posing an outburst risk.

A more recent mapping exercise in 2018 counted more than 25,000 glacial lakes across the region’s five major river basins, and its 2020 follow-up ranked 47 of these as dangerous enough to warrant close monitoring. Some of this data variation reflects better satellite resolution and more thorough surveying.

But the direction both point in, more lakes, more of them growing, is consistent with what glaciologists expect, as ice retreats and meltwater pools behind loose moraine dams that were never built to hold a lake.

IPCC’s special report on oceans and cryosphere confirms that glacial lake outburst floods are expected to become more frequent as warming continues, because the same warming that shrinks glaciers, is what fills and destabilises lakes downstream of them.

Sikkim got a preview of the failure mode in Oct 2023, when the South Lhonak glacial lake burst and tore down Teesta valley. Whether the Bhote Koshi event will be formally classified as a GLOF or a combination of glacial events is still being worked out by the region’s scientists and authorities, but different hazards increasingly overlap in exactly this kind of terrain.

A monsoon that is drying on average and sharpening at the edges | An Indian Institute of Tropical Meteorology assessment has found that the Indo-Gangetic plain has witnessed its mean southwest monsoon rainfall decline by roughly 0.5 mm per day per decade between 1951 and 2024, a genuine drying trend across the belt that includes Bihar. The same body of research points towards sharper, more concentrated bursts even as the seasonal average falls.

Put together, the physically consistent picture for a river system like Gandak or Kosi is not steadily more water year-round. It’s the same or slightly less water spread across fewer, more violent pulses, arriving from a catchment where glaciers and permafrost, which used to buffer runoff, are doing that job less reliably than they did 30 years ago.

Why Bihar absorbs the sharp end of this | Roughly 73% of its land area lies within the floodplains of rivers some of which rise in Nepal: Kosi, Gandak, Bagmati, Burhi Gandak, and Mahananda.

The state carries 22.1% of India’s entire flood-affected population, and by govt’s own estimate roughly three-quarters of north Bihar’s residents live under a recurring flood threat. The 2017 flood alone killed 514 and displaced or affected more than 17mn.

The engineering response to this exposure has been embankments, more than 3,400km of them by 2004. But their record is genuinely uncomfortable, with the flood-prone area of the state growing from 2.5mn ha in 1954 to 6.89mn ha in 2004.

Embankments raise riverbeds through sediment deposition and concentrate whatever breach does occur into a much larger, faster failure. Kosi did exactly that in 2008 at Kusaha, cutting a new channel more than 100km from its embanked course.

Layer a warming-driven shift towards glacial instability, and sharper monsoon bursts, onto a river system already primed to fail catastrophically rather than gradually, and the implication is Bihar’s flood risk is being reshaped by forces upstream, in a catchment that is warming faster than the global average, filling with more unstable lakes, and increasingly delivering its rainfall in the kind of short, intense pulses that overwhelm infra built for a gentler hydrology.

The response that scales to the challenge? Glacial lake monitoring, automated telemetry that will give information, which can be analysed for threats, and which should be shared across the border in real time, rather than after the water has already moved. Building the next embankment higher won’t cut it.

China’s recent activity on the Tibetan plateau highlights how the catchment flows through three countries. A flood originating upstream does not respect a border, and neither can the response constructed to address it.

Author: Anjal Prakash