Science & Environment

Is black carbon affecting the Asian monsoon?

Fishermen in India
Image caption Many people are now uncertain about the timing of the monsoon

Although a normal monsoon has been forecast for South Asia this year, and rains have begun normally in many parts of the region, people are still anxious about the rainy season that lasts for four months.

Their anxiety has to do with the uncertainties surrounding the timing of the monsoon in recent years.

While the debate continues over the role of climate change, scientists have also been looking at the possible role of soot and urban smog pollution in disrupting this weather system.

The uncertainties surrounding the monsoon have mainly affected agriculture, resulting in a rise in food prices.

In the past decade, a number of monsoon seasons saw lower than average rainfall in some places. Some areas were hard hit by droughts while other areas were flooded with unusually heavy and torrential rainfall in a short span of time.

The variability and the erratic pattern has begun to emerge in some parts of the region this year already.

Soot includes particles of so-called black carbon from the incomplete combustion of fossil fuels, wood and biomass burning.

Smog consists of air pollutants in the lower atmosphere, including troposphere ozone, a powerful greenhouse gas.

Several scientists have over the years said that increased concentrations of black carbon and troposphere ozone could be disturbing monsoon patterns.

Carbon controversy

A recent report by the United Nations Environment Programme (Unep) and World Meteorological Organisation (WMO) has further stressed that both black carbon and ground level ozone can be factors that disrupt monsoon rains.

"They disturb tropical rainfall and regional circulation patterns such as the Asian monsoon, affecting the livelihoods of millions of people," the latest report read.

"They can change wind patterns by affecting the regional temperature contrasts that drive the winds, influencing where rain and snow fall.

"While some aspects of these effects are local, they can also affect temperature, cloudiness, and precipitation far away from emission sources."

There have been several reports on both troposphere ozone and black carbon in the past but it is the latter that has at times generated some controversy.

The Indian Network for Climate Change Assessment (INCCA), a government-launched network of several scientific bodies, pointed out that there were some conflicting scientific statements on the impact of black carbon on the monsoon.

It quoted the US space agency (Nasa) scientist William Lau and his team's findings that the "absorption of solar radiation and consequent warming by aerosols over the Tibetan plateau (elevated land) acts like an elevated heat pump which draws in warm and moist air over the Indian sub-continent leading to advancement and subsequent intensification of the Indian summer monsoon."

But not everyone subscribes to this theory. INCCA has pointed to another finding by atmospheric scientist Veerabhadran Ramanathan, of the Scripps Institution of Oceanography at the University of California.

Converging opinion

Ramanathan's result suggested a large reduction of solar radiation at the Earth's surface simultaneous with the warming of the lower atmosphere increases atmospheric "stability". It also slows down the hydrological cycle and reduces rainfall during the monsoon.

"The consequence of these contrasting processes needs to be understood before arriving at conclusions on the aerosol impact on a regional climate system," the INCCA said in its statement.

But one of the experts in the recent UNEP/WMO report, Chien Wang of the Massachusetts Institute of Technology (MIT) said there was no confusion on the issue.

"I have to indicate that the basic conclusion that black carbon aerosol forcing over South Asia is large enough to perturb the monsoon system is reached by all the studies so far, therefore there is no different opinion here," he told BBC News.

His own recent research, he said, showed that the heating of the air can change the large-scale atmospheric stability and basically cause the monsoon rainfall to shift towards the north and west, leading to reduced rainfall in large areas of India.

"Note that the total rainfall of the monsoon system might not necessarily be reduced, but it only experiences a pattern change," he explained.

"Black carbon aerosols also cool the surface, this would suppress evaporation over the land area, on the other hand causing a change in temperature gradient that might weaken the circulation."

Changing patterns?

Drew Shindell of Nasa's Goddard Institute for Space Studies who headed the research team to prepare the UNEP/WMO report said it was a step ahead in assessing black carbon's possible influence on the monsoon.

"We are not determining details like whether monsoon comes early or late or what direction does it change, but what has been confirmed is that there is disruption in the rainfall," he says.

Some scientists in India agree that monsoon patterns are changing.

The director of the National Atmospheric Research Laboratory under the Indian government's department of space, Professor A Jayaraman said: "A clean atmosphere without black carbon and a dirty atmosphere with black carbon are certainly going to behave differently, quantifying that difference is what remains to be done and that is where we are stuck."

While scientists take time to figure out how black carbon actually affects monsoon rains, South Asia's major player India and its regional rival China continue to see a steep rise in energy consumption.

"By 2035, China will account for 22% of the world energy demand, up from 17% today," the International Energy Agency said in its World Energy Outlook 2010.

"India is the second-largest contributor to the increase in global demand to 2035, accounting for 18% of the rise."

Both the countries derive energy mainly from fossil fuels - one of the main sources of black carbon.

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