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