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Paint it Black

February 13, 2013

Many years ago, one of my colleagues was working on a project involving photoacoustic actuation. The essential idea is that a laser pulse striking an optically absorbing surface will generate a small shock wave by heating the air in contact with the surface, thereby producing an acoustic wave in the solid. As an example, if this optically absorbing surface is on a microphone, you'll hear a "pop" at each laser pulse.

What's needed for an efficient actuation is a material that's very black at the laser wavelength. His first thought was to use lamp black, which is a traditional blackening material in optics laboratories, but this didn't work that well. The best shock waves are produced in porous absorbing layers, since the pockets of air in these media are better at absorbing the heat, and the candle black layers were smooth and non-porous. However, soot formed by burning a particular organic liquid worked well. Note that the fumes from most burning chemicals can be toxic.

Soot from burning of fossil fuels will absorb solar energy, and a recent study by a huge international team of scientists has revealed that carbon soot, also known as black carbon, is the second largest man-made contributor to global warming, and the influence of black carbon on climate change has been greatly underestimated. In fact, black carbon has twice the direct climate impact previously thought. As bad as this may sound, it also means that a reduction in black carbon production will immediately slow global warming.[1-8] Reduction in black carbon was among several short-term mitigation strategies published in Science early in 2012.[9-10]

This 31-author, 234-page study, led by scientists from the International Global Atmospheric Chemistry Project, estimates the total effect of black carbon to be about 1.1 watts per square meter (W/m2). This is about two-thirds the effect of carbon dioxide, the largest man-made contributor to global warming.[1-2, 8] The study was recently published online by The Journal of Geophysical Research.[1,7]

The previous best estimate of black carbon's influence on climate was the 2007 Intergovernmental Panel on Climate Change Assessment.[2] Unfortunately, that assessment significantly underestimated the black carbon emissions for some regions.[2] As a result, this 2007 assessment underestimated the extent that black carbon reduction would have on mitigation of global warming.[2]

Co-author of the current study, Piers Forster of the School of Earth and Environment at the University of Leeds, had this to say about the pursuit of black carbon reduction.
"There are exciting opportunities to cool climate by reducing soot emissions but it is not straightforward. Reducing emissions from diesel engines and domestic wood and coal fires is a no brainer, as there are tandem health and climate benefits. If we did everything we could to reduce these emissions we could buy ourselves up to half a degree less warming--or a couple of decades of respite."[2]

Black carbon sources
Black carbon emission sources, from Bond et al., 2013.[1] Click for larger version with additional information. (Image: American Geophysical Union/D. W. Fahey, 2013).

One interesting thing about environmental black carbon is that its effects are not always warming. As expected, warming arises from black carbon's absorption of solar energy, whether its in the atmosphere or on land surfaces, and on ice and snow. Black carbon also promotes cloud formation, which can have either a warming, or cooling, effect. Some processes that produce black carbon will also produce products with a cooling effect. In the end, it's difficult to estimate the net effect of black carbon just as a function of its quantity[2]

Addressing the benefits of reducing black carbon production will depend on what process produces it. The study found that maximum impact would derive from a reduction in diesel engine emissions. Agricultural burning and household wood fires and coal fires would be the next important targets. Ninety percent of black carbon comes from diesel-fueled vehicles, biomass for household cooking and heating, small kilns and industrial boilers, and open burning, such as fires for forest clearing.[5] Reductions in black carbon from these sources would have an immediate cooling impact.[2-3, 8]

Black carbon's effects are regional as well as global. It adds significant warming to the cool industrialized belt of the Northern Hemisphere at mid- to high- latitudes. This encompasses the northern United States and Canada, northern Europe, and northern Asia. One global effect is in changing the rainfall pattern of the Asian monsoon.[2,5-6] Black carbon emissions have been declining in Europe and North America because of regulations on diesel engine emission, but they have been increasing in the developing world.[6] Major black carbon emission in eastern and southern Asia seems to produce a warming effect from soot that's an order of magnitude greater than the global average.[5]

One of the study co-authors, physicist David Fahey of the National Oceanic and Atmospheric Administration (Boulder, Colorado), is quoted in Nature as saying that the magnitude of black carbon climate forcing surprised many of the study members. Fahey also acknowledged the uncertainty in the result, as shown in the figure.[8] One study co-author, Sarah Doherty, is quoted by BBC News as saying,
"The large conclusion is that forcing due to black carbon in the atmosphere is larger... The value the IPCC gave in their 4th assessment report in 2007 is half of what we are presenting in this report - it's a little bit shocking,"[6]
Black carbon climate forcing statistics
Final statistics for the quantitative estimates of climate forcing from black carbon in the industrial era (1750-2005).[1] There's a lot of uncertainty in the estimate because of the many separate factors involved. Click for larger version with additional information. (Image: American Geophysical Union/D. W. Fahey, 2013).

There are some sobering statistics about black carbon. Humans put about seven trillion tons of black carbon annually into the atmosphere.[5] Of this, about 270 billion tons, or seven percent of the total, comes from kerosene lamps.[5] California, which is usually at the forefront of environmental policy, was able to halve its black-carbon emission between 1990 and 2008 through regulation of diesel emissions.[5]

"Paint It, Black" is a song by The Rolling Stones. Released on May 13, 1966, it was titled, "Paint It Black," without a comma. Somehow, a comma made its way onto the record label, making the title sound like a call to a program function, years before ubiquitous computing.


  1. T. C. Bond, S. J. Doherty, D. W. Fahey, P. M. Forster, T. Berntsen, B. J. DeAngelo, M. G. Flanner, S. Ghan, B. Kärcher, D. Koch, S. Kinne, Y. Kondo, P. K. Quinn, M. C. Sarofim, M. G. Schultz, M. Schulz, C. Venkataraman, H. Zhang, S. Zhang, N. Bellouin, S. K. Guttikunda, P. K. Hopke, M. Z. Jacobson, J. W. Kaiser, Z. Klimont, U. Lohmann, J. P. Schwarz, D. Shindell, T. Storelvmo, S. G. Warren and C. S. Zender, "Bounding the role of black carbon in the climate system: A scientific assessment," Journal of Geophysical Research-Atmospheres (in Press), 10.1002/jgrd.50171. A PDF file is available, here.
  2. Black carbon larger cause of climate change than previously assessed, Press release of the International Geosphere-Biosphere Programme of the International Global Atmospheric Chemistry Project, January 15, 2013.
  3. Elisabeth Rosenthal, "Burning Fuel Particles Do More Damage to Climate Than Thought, Study Says," The New York Times, January 15, 2013.
  4. Richard Chirgwin, "Soot forces temperatures more than thought: AGU," The Register (UK), January 16, 2013.
  5. Pete Spotts, "Soot is No. 2 global-warming culprit, study finds," Christian Science Monitor, January 15, 2013.
  6. Matt McGrath, "Climate change: Soot's role underestimated, says study," BBC News, January 15, 2013.
  7. Richard A. Kerr, "Soot Is Warming the World, A Lot," Science Now, January 15, 2013.
  8. Jeff Tollefson, "Soot a major contributor to climate change," Nature, January 15, 2013.
  9. Richard A. Kerr, "A Quick (Partial) Fix for an Ailing Atmosphere," Science, vol. 335. no. 6065 (January 13, 2012), p. 156.
  10. Drew Shindell, Johan C. I. Kuylenstierna, Elisabetta Vignati, Rita van Dingenen, Markus Amann, Zbigniew Klimont, Susan C. Anenberg, Nicholas Muller, Greet Janssens-Maenhout, Frank Raes, Joel Schwartz, Greg Faluvegi, Luca Pozzoli, Kaarle Kupiainen, Lena Höglund-Isaksson, Lisa Emberson, David Streets, V. Ramanathan, Kevin Hicks, N. T. Kim Oanh, George Milly, Martin Williams, Volodymyr Demkine and David Fowler, "Simultaneously Mitigating Near-Term Climate Change and Improving Human Health and Food Security," Science, vol. 335. no. 6065 (January 13, 2012), pp. 183-189.

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