Monday, 28 September 2009

Black Carbon Warms the Planet Second Only to CO2 ...

Monday, September 28, 2009 9:13:24 AM

The Institute of Science in Society
Science Society Sustainability

This article can be found on the I-SIS website at


ISIS Report 28/09/09

Black Carbon Warms the Planet Second Only to CO2


Eighty percent of black carbon emissions come from fossil
fuels and biomass burning associated with deforestation;
reducing black carbon emissions may be the quickest,
cheapest way to save the climate Dr. Mae-Wan Ho

New research shows that airborne soot, or black carbon (BC)
aerosols resulting from incomplete combustion, are warming
the earth much more than previously thought [1]. According
to Veerabhadran Ramanathan at the Scripps Institution of
Oceanography San Diego and Greg Carmichael at the University
of Iowa, the warming effect of black carbon is 55 percent
that of CO2, the biggest contributor to global warming.

The annual emission of BC (for year 1996) was estimated at
about 8 Tg (1012g); of which 20 percent comes from
biological fuels (wood, dung and crop residues), 40 percent
from fossil fuels (diesel and coal) and 40 percent from open
biomass burning (associated with deforestation and crop
residue burning). High BC emissions occur in both northern
and southern hemispheres, the former from fossil fuels and
the latter from open biomass burning. BC is often
transported long distances, mixing with other aerosols on
the way such as sulphates, nitrates, organics, dust and sea
salt, to form transcontinental plumes of brown clouds that
extend vertically 3 to 5 km. BC is removed from the
atmosphere by rain and snowfall; that and direct deposition
limits the atmospheric lifetime of BC to about a week.

Major BC sources coincide with atmospheric solar heating and
surface dimming

Until about 1950s, North America and Western Europe were the
main sources of soot emissions, but now developing nations
in the tropics and East Asia are the major source regions.
Field observations and satellite sensors reveal that BC
concentrations peak close to major source regions, giving
rise to regional hotspots of solar heating in the Indo-
Gangetic plains in South Asia, eastern China, most of
Southeast Asia including Indonesia, regions of Africa
between sub-Sahara and South Africa, Mexico and Central
America, and most of Brazil and Peru in South America.

Whereas CO2 heats the earth surface through the greenhouse
effect, BC heats the earth by decreasing its albedo in
several ways. (Albedo is the fraction of solar energy not
absorbed but reflected from the earth back into space.)
First it heats the atmosphere by absorbing solar radiation
reflected by the earth’s surface to the atmosphere. This is
referred to as ‘top of atmosphere’ or TOA heating. Second,
soot inside cloud drops and ice crystals decrease the albedo
of clouds by enhancing absorption of solar energy. Third,
when airborne black carbon particles, or soot, is deposited
over snow and sea ice, it darkens the surfaces and decreases
the otherwise high albedo, contributing to the melting of
Arctic ice.

Ramanathan and Carmichael estimate that TOC heating (the
first pathway), is 0.9 W/ m2 (range 0.4 to 1.2 W/m2), which
is 55 percent of the CO2 warming of 1.66 W/m2; greater than
that due to other greenhouse gases including methane, and
much larger than the 0.2 to 0.4 W/m2 estimated previously by
the IPCC.

BC also absorbs solar energy directly, a heating effect
estimated at 2.6 W/m2. This direct absorption reduces the
solar radiation reaching the earth surface, resulting in a
dimming effect estimated at -1.7W/m2.

The calculations are complicated by the mix of aerosols that
originate from some sources of BC which co-emit organic
carbon compounds (such as benzene, ethane and ethyne from
wood burning, all harmful to human health [2]) and sulphate,
also harmful to human health [3], that tend to have a
cooling effect by direct light scattering and interaction
with clouds.

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