Lake sediment study gives clues to glacier health and Indus valley civilisation

The westerlies play a bigger role than the Indian monsoon in controlling rain, snow and glacier health in the Western Himalayas, a new study has found.

A team of scientists from the School of Environmental Sciences at Jawaharlal Nehru University (JNU) have analysed lake sediments from Chandratal lake in Himachal Pradesh to understand what has been determining glacial retreat, or in some cases glacial advance, in the western Himalayas.

The results, published in Nature journal last month, indicate that summer monsoon dominated precipitation 21% of the time, whereas the mid-latitude westerlies dominated precipitation 79% of the time and hence have been the main source of moisture during the last 1,100 years.

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Glaciers are vulnerable to the impact of climate change, which can lead to several changes in water availability in the region. Understanding past moisture source for rain and snowfall over the Himalayan glaciers can also help understand how climate change will impact them in the future, authors of the study said.

Normally, studies on a glacier’s life cycle are conducted based on ice cores. “Obtaining ice core from the region is difficult and where it has been obtained it hasn’t given much information. So we have retrieved sediments from Chandratal to understand what may have happened. We know now that the monsoon impact has weakened and it’s not bringing as much rain in the northwestern region as it used to be . Climate change has also impacted the westerlies or western disturbances with more cases of extremely heavy rain or snow being recorded. Such changes will impact moisture availability and hence could affect the glacier dynamics and its retreat ,” explained AL Ramanathan, professor, JNU.

India Meteorological Department (IMD) defines western disturbances as a cyclonic circulation/trough in the mid and lower tropospheric levels or as a low-pressure area which occurs in middle latitude westerlies and originates over the Mediterranean Sea, Caspian Sea, Black Sea and moves eastwards across north India.

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A ministry of earth sciences (MoES) report released last year titled Assessment of Climate Change over the Indian Region suggested that there is a significant rising trend in western disturbances and rain extremes. But rain extremes don’t necessarily mean availability of more water or ice, Ramanathan said, as they are not evenly distributed.

Ramanathan said the sediment core records from Chandratal are a very useful tool for archaeologists and paleoclimatologists who are studying the fall of the Harappan civilisation. Some scientists believe that the Harappan civilisation fell due to drying of rivers. “Reconstruction of the dynamics of moisture source changes during the Holocene is of great importance to get an insight into societal development (e.g Indus civilisation) and future climate changes,” he added.

“We haven’t conducted any studies on moisture sources but there is a general understanding that the westerlies could have more influence and this study quantifies that. It is difficult to say how the changing pattern of western disturbances will impact glacier health because there may not be a change in precipitation but timing of snowfall or rain is most important. If that changes we can see greater retreat of glaciers,” said Anil Kulkarni, Distinguished Scientist, Divecha Centre for Climate Change, Indian Institute of Science (IISc), Bengaluru.

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