Science News

Brahmaputra valley pushing 'glacier-melting pollutants' into Himalayas

Subhra Priyadarshini

doi:10.1038/nindia.2012.71 Published online 16 May 2012

Alarmingly high pollution levels in the Brahmaputra river valley in the northeast of India might be resulting in an outflow of pollutants into the Himalayas, probably melting the glaciers and interfering with India's monsoon cycle, a new study warns1.

A team led by a climate change researcher at the Desert Research Institute in Nevada (USA) investigated the black carbon (BC) aerosol pollution levels in the river valley during winter 2011. They were surprised with the results.

Rajan Chakrabarty monitoring air pollution levels of Guwahati.

© RC

"What we found was alarming – Guwahati, the largest city in the valley, has one of the highest BC pollution levels in the world," lead researcher Rajan Chakrabarty told Nature India. The winter-time BC mass concentrations observed in Guwahati were higher than those measured in the mega cities of India and China, and much higher than in urban locations of Europe and USA, he says. The Brahmaputra River Valley has been experiencing extreme regional climate change in recent years.

Chakrabarty, an engineering graduate from the University of Madras and a post doctorate from the University of Nevada-Reno, is currently an assistant research professor and investigator on two NASA-sponsored projects researching the role of BC aerosols in global warming. These aerosols are emitted from vehicles and other combustion sources.

Chakrabarty and colleagues conducted their week-long study using a micro-Aethalometer during January and February 2011 to measure BC aerosol mass concentrations in Guwahati. The daily median values of BC mass concentration were found to be 9–41 micrograms/m3, with a maximum of over 50 micrograms/m3 during evenings and early mornings.

The researchers attribute such high pollution levels in Guwahati to rapid urbanization, irresponsible citizens, and poor enforcement of environmental laws in the Brahmaputra valley. "We calculated that the high level of pollutants in Guwahati is pushing the day temperature up by two degrees Celsius on an average. This is quite significant."

Black Carbon aerosols absorb incoming sunlight and trap heat in the atmosphere. They act like an invisible blanket that warm up the atmosphere. The researchers calculated that the blanket effect of BC on any non-cloudy, winter day in Guwahati, resulted in shooting up the temperature by 2°C more than what it would have been without the aerosols. For example, during January and February of 2011 if the temperature of Guwahati was 25°C with the aerosols, the researchers say it would have actually recorded 23°C without the pollutants. This temperature increase could also hold true during other seasons of the year, Chakrabarty says.

"Unfortunately, this important region of India has gone unnoticed so far when it comes to climate change. The focus has always been on the Indo-Gangetic Plain of India, and not the Brahmaputra River Valley", he laments.

To examine the possible outflow trajectories of these aerosols from Guwahati, they conducted a 7-day study using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model of the National Oceanic and Atmospheric Administration (NOAA) at different altitude levels from the surface.

"The trajectories at all the different heights suggested outflow of pollutants to continental China and Tibet. Such outflow has been suggested to be a major cause of the rapid melting of glaciers and permafrost," the researchers wrote in their research paper.

These pollutants are out-flowing into the Himalayas "where they are melting the glaciers" and interfering with the Monsoon cycle, causing abrupt rainfall and droughts, Chakrabarty says.

He feels that the study points to the need for further research in the valley. Due to the high BC levels, the region has experienced severe droughts (2005-06 and 2008-09) leading to crop failures and significant impact on the economy. Extreme flooding during some recent years, abnormality in monsoon patterns and melting of the glaciers in the Tibetan plateau due to the outflow of these pollutants are of extreme concern, Chakrabarty points out.


  1. Chakrabarty, R. K. et al. Strong radiative heating due to wintertime black carbon aerosols in the Brahmaputra River Valley. Geophys. Res. Lett. doi: 10.1029/2012GL051148 (2012)