Aerosols heating up Himalayan climate, accelerating retreat of glaciers: study

Aerosols are heating up the Himalayan climate and contributing significantly to the accelerated retreat of the glaciers and changes in the precipitation patterns over the Hindu Kush-Himalaya-Tibetan Plateau (HKHTP) region, researchers said.

Aerosols alone account for more than half of the total warming of the region's lower atmosphere, with the remainder coming from greenhouse gases, the researchers from the Physical Research Laboratory, Ahmedabad, and Helmholtz Centre Potsdam and the University of Leipzig, Germany found in a joint study.

According to the study, aerosols will likely remain a key factor driving climate change over the region.

The researchers observed the concentrations of aerosols and the the amount of heat absorbed (radiative forcing) across several locations in the Indo-Gangetic Plain (IGP), the Himalayan foothills and the Tibetan Plateau.

These are "relatively poorly studied regions with several sensitive ecosystems of global importance, as well as highly vulnerable large populations," they wrote in their study.

Pointing out that the current climate models significantly underestimate aerosol-induced heating, efficiency and warming in the HKHTP region, the researchers called for a more realistic representation of aerosol properties.

These findings have been published in the journal Science of The Total Environment.

The Indo-Gangetic Plain covers parts of Pakistan, India, Bangladesh and Nepal and is a densely populated and an industrialised South Asian region.

This region is heavily polluted, the scientists confirmed by determining the aerosol optical depth (AOD) to be over 0.3 at all of their observation sites. AOD is widely used to assess the extent of air pollution with a value of less than 0.1 indicating a crystal clear sky with maximum visibility and a value of 1 indicating very hazy conditions.

Over Nepal's capital city, Kathmandu, which is a metropolitan region in the Himalayan foothills, higher heat-absorbing aerosols were found to be present for most of the study period of a year.

The finding confirmed that not only are the fine aerosol particles, which dominate AOD, more at higher altitudes in the central Himalaya foothills, but also are more absorbing in nature as they are dominated by black carbons, the researchers said in their study.

Their analysis also revealed that the radiative forcing of the aerosols in the atmosphere was 2-4 times more efficient over the IGP and the foothills of Himalayas, with values being greater at higher elevations in the Himalayas. This meant a higher AOD and aerosol heat absorption, they said.

Further, the observed yearly average aerosol heating rates were about 0.5-0.8 Kelvin per day, which the researchers found to be significantly higher than previously reported values.

This implied that the aerosols alone could account for more than 50 per cent of the total warming, with greenhouse gases accounting for the rest of the warming of the lower atmosphere and surface over the region, the researchers said.

They said his was a first-time analysis of its kind, that included ground-based observations, satellite data, and model simulations.