Himalayan Glacier Melt: The Real Problem

by Upmanu Lall | June 14, 2010

Every day I hear scientists and media people talking about climate change in a way that is often sensational or misses the real challenges. This will be a series of blogs in which I will discuss a few of these cases. Lets start with:

The Himalayan Glacier Melt

A typical statement that I’ve seen used in the media is that the Himalayan glaciers will vanish by 2035, and that about 1.5 billion people will be affected by lack of water as a result. It is true that if the Himalayan glaciers melt, people will be affected, but not necessarily the ones being referred to, nor in the way they’re currently saying.

In reality, the percentage of the water contributed by glacier melt to these societies is small. What people mix up is that there is an annual snowfall which melts and is then available to use. Most of the water doesn’t come from melting glaciers at all. However, lately many glaciers are melting faster than they had before, and so more water is temporarily available. Once the glaciers melt substantially, this increment will vanish.

Hushe Valley and Masherbrum, Pakistan:

There are issues of concern here. First, at warmer temperatures the winter snow may not come as snow. It may come as rain, especially in the lower elevations. So the time at which water is available in the rivers may shift into the winter and not be in the traditional snowmelt period in the early spring.

This could be a significant change, and would have to be planned for. The total amount of winter precipitation may also increase as a warmer world leads to higher moisture in the air, and then higher rain as the air cools while it is lifted over the mountains. If there are reservoirs to store the water, water release could be controlled so that people can start farming earlier in the year, when it is expected to be warmer. Without reservoirs, farmers may or may not be able to adapt to the change in the timing of water availability.

This needs to be studied further, but is not likely as dramatic an issue as the media is making it out to be. The number of people relying on snowmelt-fed rivers is a tiny fraction of the 1.5 billion that is often cited.

As the glaciers melt, the major problem won’t be lack of water. The problem will be induced by soil saturation and slope failure as the ground is exposed. There will be volumes of water released onto slopes which before had been protected by solid sheets of ice. The underlying soil may be made of rocks and sand without binding clay, and may be mobilized by the melt water, causing landslides.

One way this can happen is illustrated below. Suppose there is a slope (a), which is just a big pile of sand (b). We can say that the sand pile is actually stable at this angle given the ice on it or in the absence of melt water. The sand pile becomes saturated with water once the ice melts (c). That water will tend to come out at the toe of the slope (d). This creates an instability. The toe of the slope will erode, causing the whole slope to fail.

Diagram by J A Hitz

When the slope fails, the material goes into the water channels and accumulates, causing natural dams to form. As the water builds up behind, the dams fail, and all the water rushes downstream. If there is a reservoir in the path of this wave of water, it may be destroyed in the process. If not, one natural dam can fail, or worse, several can fail in a chain reaction. Either way, the communities along the water path could be completely wiped out.

Tsho Rolpa Glacial Lake, Nepal:

These glacier lake outburst floods are more of a concern for the communities likely to be most affected by the Himalayan glacier melt. Such events could impact not just mountain communities, but also those living in the plains.

The river Kosi in India is an example of what can happen. In 2008, the embankments of the Kosi were breached and the river moved 180 kilometers laterally. Several million people became homeless when the water moved over to its new path. That event was caused by an embankment failing because of a rainfall induced flood, not a glacier lake outburst, but my argument is that the water released from a glacier lake outburst flood could lead to a similar or more catastrophic outcome.

Kosi River Flood, 2008:

What places are the most vulnerable to this type of event? It’s the South Asia side of the Himalayas, not the South Indian side, which is what most people focus on. The South Asian mountain range goes from about 500 feet above mean sea level to 30,000 feet above mean sea level in the span of about 50 kilometers, so it’s a very steep change.

The Yangtze River and the Mekong River plains are equally vulnerable, so one could identify a relatively large population in the flood plains of the major Himalayan rivers in South Asia, China and Indo-china that could have a serious impact.

Next blog: ‘Predicting the future with Climate Change Models’



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