Black carbon landing on snow may be accelerating the melting of Himalayan glaciers more than atmospheric soot or even greenhouse gases such as carbon dioxide (CO2), according to a study.

Himalaya of Mount Qomolangma [File photo]

The effects of this carbon, a key component of soot, could change the timing and intensity of the monsoons; increase the chances of downstream floods and reduce farmers' access to irrigation when they most need it, researchers say.

The Tibetan Plateau has warmed 1.8 degrees Celsius in the last 50 years and the Intergovernmental Panel on Climate Change (IPCC) predicts a four degree warming in the next 100 years, both of which are much larger increases than the average global warming.

Scientists already knew that soot in the air was likely to be an overlooked cause of global warming. And they knew that the amount of surface soot on Tibetan Plateau glaciers has been on the rise, especially since the 1990s.

Now, US researchers have used computer models to see whether this soot might be responsible for the regions' quicker warming.

The model shows soot accelerates the melting of snow and increases the amount of surface runoff in the late winter and early spring, when farmers downhill do not need irrigation water, but reduces runoff in the late spring and early summer when they need it the most.

It also contributes to surface temperature rise, causing the monsoons to start earlier — bringing more rain to northern India and northern and southern China, but less rain to central China.

"We can confidently say that the 'soot on snow' effect is more efficient than CO2 in accelerating snow melting in the Tibetan Plateau," Yun Qian, the lead author of the paper, which appeared in Atmospheric Chemistry and Physics (2 March), and an atmospheric scientist at the US Department of Energy's Pacific Northwest National Laboratory, told SciDev.Net.

But Indian scientists criticised the output of the model for not matching the observed data.

Anil V. Kulkarni, a distinguished visiting scientist at the Indian Institute of Science, Bangalore, said: "Though [such] models show decrease in snow cover in the Himalaya, some of the actual observations are contrary to that".

Kulkarni and colleagues showed, in a study published in The Cryosphere Discussions earlier this month (March 2), that snow cover in fact increased in some regions of the Hindu Kush Himalaya between 2000 and 2010.

And Madhavan Rajeevan, scientist at the National Atmospheric Research Laboratory, India, said that the model's strength in simulating monsoons "is not that great."

D. P. Dobhal, a scientist at the Wadia Institute of Himalayan Geology called for more regional studies to confirm the extent of the effect of black carbon on glacial melt.

Qian admitted that their model only represents the upper limits of the soot's effects and has not taken into account all the other factors that might affect runoff.

But he maintained that the conclusions that soot contributes to snow melting and changing the monsoons are valid, although the exact amounts might be overestimated.

He said better spatial resolution of snow cover in the region, and more data on soot deposition, will be important for future studies that try to quantify the exact effects.