A representation of airborne black carbon pollution hanging over east Asia in 2004 to 2005, compiled from various measurements.  (Source: Scripps Institution of Oceanography, UC San Diego)
A new study indicates that while CO2 still is the worst anthropogenic forcing agent, black carbon is not far behind

While skeptics rush to dismiss anthropogenic global warming and point to new studies that offer alternative theories about climate change, a new study led by Scripps Institution of Oceanography at UC San Diego atmospheric scientist V. Ramanathan and University of Iowa chemical engineer Greg Carmichael reveals that a certain type of carbon air pollution from anthropogenic sources may be even more effective in inducing global warming than previously expected.

The new study, to be published in the Sunday, March 23 online version of the journal Natural Geoscience, looks specifically at so-called "black carbon," anthropogenic particulate airborne carbon created from burning biomass, diesel, solid fuel, or other sources.  The new research states that this sooty compound has a warming effect on the atmosphere that is three to four times the previous estimates. 

The researchers state that while CO2 still leads the way in having the greatest effect on warming, black carbon could have as much as 60% of the effect of CO2, despite being present in much smaller quantities.  This would put black carbon ahead of all other carbon emissions, including methane, other than CO2.  The good news, researchers say, is that this means that mitigation of black carbon will be especially effective in reducing potential anthropogenic warming effects.

The new study used mostly data from observation, as opposed to some other NASA and ESA studies, which rely heavily on computer models, which in the past has been a source of criticism.  However Ramanathan suggested their observation based research is very close to agreeing with the current models.  He states, "Observationally based studies such as ours are converging on the same large magnitude of black carbon heating as modeling studies from Stanford, Caltech and NASA.  We now have to examine if black carbon is also having a large role in the retreat of arctic sea ice and Himalayan glaciers as suggested by recent studies."

The study integrates data from aircraft, satellites, and surface instruments, analyzing the forcing effect of black carbon.  The new experimental data shows the forcing from the black carbon to be 0.9 W/m2, as opposed to the previous U.N International Panel on Climate Change (IPCC) estimate, which placed it between 0.4 W/m2 and 0.2 W/m2.

While computer models are often accused of being flawed and alarmist, in this case Ramanathan and Carmichael concluded that the prevalent models are actually overly conservative, failing to account for the amplification effect of the combination of black carbon with other aerosols such as sulfates.  Also the models fail to account for the atmospheric altitude at which the effect occurs, which impacts how much warming occurs.  The study shows that significant black carbon effects occur at 2 kilometers (6,500 feet).  This means that black carbon not only absorbs sunlight, as previously expected, but also absorbs light energy reflected from cloud cover in the lower atmosphere, an unexpected effect.  The black carbon also continues to impact warming upon landfall in snow, covering it in a sooty layer that helps to intensify sunlight absorption through an increase in albedo.

Interestingly, 25 and 35 percent of black carbon comes from China and India; heavy users of coal, cow dung and wood for heat sources.  Europe also is a significant contributor, due to its major use of diesel.  Ramanathan warns not to blame southern and eastern Asia too much, stating, "Per capita emissions of black carbon from the United States and some European countries are still comparable to those from south Asia and east Asia."

In other words, Asia has a far higher contribution to the pollution, but it also holds a far higher percentage of the world's population than North America.  In parts of Asia, such as China the air is so polluted that a thick brown haze known as the “atmospheric brown cloud" blankets the area.  This smog has some dire effects. 

The researchers suggest black carbon may be accelerating the melting of the Himalayan glaciers.  These glaciers provide billions of people in Asia with drinking water.  Secondly, the indoor inhalation of smoke is suspected to cause the deaths of as many as 400,000 women and children in southern and eastern Asia.  Also, this particular type of warming is much more localized than other types of warming.  While contributing slightly to overall global temperatures, the greatest effect is on the local area, leading to the creation of hot spots.  These hot spots over India and China carry with them serious implications for agriculture, housing and weather patterns.

The researchers point out that black carbon, a hazard both from a local health standpoint and from a global warming perspective could be easily and quickly eliminated.  While CO2 stays in the atmosphere for as long as a century, black carbon particles only stay airborne for a week.  Further, mitigation technologies already are widely available commercially, according to the researchers.  As black carbon is the result of incomplete combustion, this also means that it could be eliminated without switching fuel sources.

The researchers are championing Project Surya.  The project, which is seeking corporate sponsors, looks to replace 20,000 homes in India's wood burning stoves with smoke free stoves.  The stoves would come with sensors to help track the effect that squelching the wood burning has on the local air quality.  Carmichael says that he hopes that the paper will help raise awareness and promote efforts such as this project.  He states, "It offers a chance to get better traction for implementing strategies for reducing black carbon."

The review was funded by National Science Foundation, the National Oceanic and Atmospheric Administration and NASA.

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