Himalayan glaciers losing ice by thinning

The response of the Himalayan glaciers to climate change is very puzzling in many ways. Despite being subjected to similar climate changes, some of these glaciers appear to be stagnant as their fronts (or mouths) appear to be stationary. However, appearances can be deceptive and these glaciers are in one stage of development where they are losing ice by thinning, as revealed by a study published in the Journal of Glaciology.

The paper is authored by Argha Banerjee, now at IISER, Kolkata, and R. Shankar of the Institute of Mathematical Sciences, Chennai.

Three aspects of the puzzle are interesting: First, despite experiencing similar climatic changes, such as warming, many glaciers appear not to be retreating — in other words, they appear to be stagnant.

Second, there is a marked difference between the average behaviour of extensively debris-covered glaciers and sparsely debris-covered glaciers. Third, there is a large variation in the retreat rates of the “fronts” of glaciers, which is the point where the glacier begins.

“Selecting the problem to analyse was perhaps the most challenging aspect of the study,” says Prof. Shankar. Of the several variables connected with ice loss, a careful study by the researchers indicated that debris cover played a significant role. For instance, of the 128 glaciers with sparse debris cover, only 18 per cent are stationary/advancing and 82 per cent are known to be retreating. In the case of the glaciers with extensive debris cover, as much as 48 per cent are stationary/advancing while only 52 per cent are retreating.

This is all the more puzzling as the two types of glaciers are not geographically separated and do experience similar climates.

“They [the extensively debris-covered glaciers] respond differently from the bare glaciers. They tend to thin and then retreat. So their retreat rate alone may not indicate the ice-volume loss,” he adds.

The group’s numerical investigations reveal that the extensively debris-covered glaciers contain two significant time scales.

During the first period, which can last as long as a century, these glaciers maintain a stationary front while the shape changes and ice is lost by thinning. After this period, it starts to retreat.

The study thus helped in reclassifying the glaciers with extensive debris cover as retreating despite the front appearing stationary. As a result, the fraction of shrinking debris-covered glaciers shot up to 73 per cent.

Thus the percentage of glaciers that are retreating was nearly the same immaterial of whether they were extensively debris covered or sparsely covered.

The group has also obtained the warming rate in the Himalayas from the bare glacier retreat data.

“The glaciated regions in the past 40-50 years experienced an average warming rate roughly the same as the global average warming rate, but with a wide variability,” says Prof. Shankar, referring to local differences in warming rate.