Three key messages from a Yale Climate & Energy Institute workshop

Black Carbon’s Grey Areas

Does an overly simplified perspective on black carbon, one of the most important contributors to climate change, risk society’s missing an important opportunity for managing climate warming? The first of a two-part series on black carbon helps pave the way for a better understanding of this critical issue.

A recent black carbon workshop co-organized by the author under the aegis of the Yale Climate and Energy Institute brought together scientists, policymakers, and development experts to discuss black carbon and how to control it.

Some 20 experts sat for a full day in a small room and tried to communicate across disciplines. Development economists said they think that improving cook-stoves is much more challenging than media and policy accounts usually allow for. EPA policymakers asked for certainty and scientists unspooled more uncertainties than one might think possible. They emphasized unknowns about how black carbon interacts with ice-clouds and uncertainties about what happens when it mixes with other aerosols in the atmosphere, especially given the range of particle sizes.

Black carbon, a major factor in global carbon footprint.

Amidst all this murkiness, one near-certainty arose: Unlike the black carbon emissions from cook-stoves, which have a range of uncertainty around them, black carbon emissions from diesel clearly have a warming effect on Earth’s climate.

Workshop participants helped clarify three points that just might fuel a new and possibly more effective way of telling the black carbon story. In turn, these points could change how policymakers decide to approach the issue. Put simply, the three points of this new story are:

1. Stop throwing cook-stoves at the problem.
2. Target diesel.
3. Be very careful about comparing black carbon with carbon dioxide.

The Familiar Story

Black carbon, long known to be one of the leading global killers of children under five, has gained an even more tarnished name recently.

The dark-colored particles of black carbon, which reporters and policymakers and even some scientists incorrectly call “soot,” find their way to the atmosphere every time someone lights up a traditional cook-stove or switches on an older-model diesel engine.

Black carbon has been reviled by the public health community for decades, but atmospheric scientists now suggest it may also be the second most important contributor to anthropogenic climate change, after carbon dioxide (see Note 1). A few prominent scientists have even suggested that black carbon may have contributed about one-sixth of the warming of Earth’s climate since the pre-industrial age. These findings have been published since release of the Intergovernmental Panel on Climate Change’s 2007 Fourth Assessment Report, but they are sure to be discussed in the Fifth, due by 2014.

Increasingly, journalists and policymakers have placed black carbon in their sights, helping to foster a dominant version of the story. A flurry of articles, for instance, has suggested how easy it would be to eliminate inefficient cook-stoves in developing countries. On Earth Day last year, unexpected bedfellows Senator James Inhofe (R-Ok), Barbara Boxer (D-Ca), and John Kerry (D-Mass) asked EPA to conduct a year-long study of black carbon’s climate impacts.

In March 2010, a group of eminent scientists (including Professor V. Ramanathan of the Scripps Institute for Oceanography, and Professor Tami Bond of the University of Illinois at Urbana- Champagne) and activists (such as Conrad Schneider of the Clean Air Task Force) testified before Congressman Ed Markey’s Select Committee on Energy Independence and Global Warming about black carbon. Moreover, a recent U.S. Agency for International Development report found that funding improved cook-stoves in Asia is by far the most cost-effective method of reducing black carbon, and that a whole suite of reduction methods was “cost-effective compared to most large-scale greenhouse gas abatement options presently considered.” The report hints at some of the uncertainties that this article explores, but does not factor them into the cost-benefit analysis.

In addition, the American Power Act, which Senators Kerry and Joseph Lieberman (Independent-Conn) have introduced in the U.S. Senate, proposes introduction of low-emission cook-stoves for developing countries to reduce black carbon by 60 percent in 20 million homes over five years. Their bill also would lead to improved diesel particulate filters domestically and abroad.

All of this is in line with the standing story line about black carbon, which goes something like this:

Black carbon, the product of incomplete combustion of biomass and fossil fuels, arises from agricultural biomass burning, “primitive” cook-stoves, and diesel fuel.

In contrast to carbon dioxide, which stays in the atmosphere for at least a hundred years, if not considerably longer, black carbon remains there for only a week or two. During that period, the tiny particles convert sunlight into infrared heat. To make matters worse, when the particles descend upon the Arctic and the Himalayas, they darken the ice and snow, absorbing sunlight, speeding up melting, and warming up the air directly above until rains rinse them away.

But this cheerless chiaroscuro should not obscure the particles of hope gleaming deep within the black carbon story. Even if all carbon dioxide emissions were to be halted now, the global climate would still require several hundred years to re-adjust. On the other hand, by curbing black carbon emissions now, the global climate would reap impacts almost immediately. The quick response can buy time for reducing CO2 emissions over the longer run.

The New York Times helped popularize this version of the story last April. Since then, the Worldwatch Institute, The Yale Forum on Climate Change & the Media, and many others have reported on black carbon from a similar angle.

Invariably, such articles cite climate scientists V. Ramanathan and G. Carmichael, whose 2008 paper in Nature Geoscience suggested that “emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions” and that “in the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide in the melting of snowpacks and glaciers.”

But that simplified story may exist in part because of difficulties of communicating major uncertainties in scientific and political consensus. A clearer look at the grey areas in black carbon research can perhaps lead to a more productive understanding.

A Closer Look at Soot

Soot generally refers to a climate-influencing aerosol — an airy association of microscopic particles lingering in the atmosphere. Black carbon is a component of soot, but soot can consist of other particles, such as organic carbon and sulfates, which often are emitted at the same time as black carbon. NASA’s Nadine Unger calls these co-emitted particles black carbon’s “siblings.” If so, then black carbon is the hated cousin in the family, because organic carbon and sulfates seem to reflect heat away from Earth, which may be creating a cooling effect on the global atmosphere. This cooling effect may already be offsetting at least part of black carbon’s warming effect (see Note 2).

University of Illinois scientist Tami Bond explained this situation in testimony before a congressional panel as follows:

Any action to reduce black carbon will also affect any co-emitted pollutants from the same source. Any emission source produces warming pollutants (black carbon and some gases) and cooling pollutants (sulfates and organic carbon), and the result is like mixing hot and cold water in a faucet. The mixed water can be very warm, very cold, or in between depending on the amount of each flow. Sources with high emissions of warming pollutants are the most promising targets for reducing black carbon warming.

Moreover, black carbon itself is hardly uniform, an important point often missing in media reports. Bond estimates, for instance, that the particles can vary in size from 0.038–0.32 μm from diesel vehicles and from 0.1–1.3 μm from cook-stoves, and these order-of-magnitude differences may have substantial implications for how black carbon works in the climate.

In a February 10 paper in Atmospheric Chemistry and Physics Discussions, NASA’s S. Bauer and several co-authors pointed out that smaller particles stay in the atmosphere longer, travel longer distances, and behave unlike larger particles when they mix with other aerosols. The modelers found that eliminating black carbon from diesel emissions could cool the climate, but purging it from cook-stoves could actually increase current levels of global warming by 10 percent by also eradicating the organic carbon particles that are currently helping cool the atmosphere.

Elusive Political Consensus

Political consensus on black carbon has been as elusive as scientific consensus. Professor Ramanathan used to characterize the haze of soot pollution which hangs over Asia for several months each year as the “Asian Brown Cloud.” Some Indian scientists and policymakers, however, perceived a blame-game at work. J. Srinivasan and Sulochana Gadgil, from the Indian Institute of Science, for instance, had a public disagreement with Ramanathan about the validity and significance of Ramanathan’s study. They published their report critiquing Ramanathan in Current Science in 2002, and Ramanathan defended his experiment in the same publication.

More recently, India’s Environment Minister, Jairam Ramesh, said that he was unwilling to discuss black carbon at the international negotiations in Copenhagen in December 2009 because he thought the scientific link between black carbon and glacier melt remained inconclusive.

Nonetheless, immediately before Copenhagen, India took important steps to promote an improved cook-stove program. At the Yale Climate and Energy Institute’s black carbon workshop, development economists and others working to implement improved cook-stoves on the ground underscored that, despite decades of research into creating improved cook-stoves that would be locally adopted, major uncertainties persist about creating the needed behavioral changes.

Journalists and policymakers in the developed world have generally failed to describe these conflicting perspectives, perhaps contributing to an overly simplistic popular depiction of the black carbon problem.

As a result, one potential but possibly ineffective approach — improved cook-stoves for the developing world — may be overstated as a potential climate solution, while others, such as targeting diesel emissions worldwide, may not get the attention they deserve.

The New Story

1. Stop Throwing Cook-Stoves at the Problem

In December 2009, Burkhard Bilger, writing in The New Yorker, explored the lure of the improved cook-stove quick-fix. He profiled the world’s top stove-designers, who for decades have been trying to surmount the severe challenge of designing an improved cook-stove that won’t simply end up being used as a second trash-can.

The New Yorker article opens with a characterization of black carbon worth quoting in full:

As global temperatures have risen, the smoke from Third World kitchens has been upgraded from a local to a universal threat. The average cooking fire produces about as much carbon dioxide as a car, and a great deal more soot, or black carbon — a substance seven hundred times as warming. Black carbon absorbs sunlight. A single gram warms the atmosphere as much as a fifteen-hundred-watt space heater running for a week. Given that cooking fires each release one or two thousand grams of soot in a year, and that three billion people rely on them, cleaning up those emissions may be the fastest, cheapest way to cool the planet.

By conflating soot and black carbon, and by glossing over the different kinds of black carbon, that description may over-emphasize the role cook-stoves play in climate change. In addition, the comparison between black carbon and carbon dioxide emissions also is misleading.

Bilger, though imperfect here, is in good company. Even the well-regarded Ramanathan, surely one of the world’s leading authorities on black carbon, notes in his 2008 paper that black carbon is an important component of soot — but he then goes on to use the two terms interchangeably. U.S. physicist Steve Garrett does the same: “Black carbon — which is basically the soot from billions of domestic cooking fires — is about 600 or 700 times more potent than CO2 as a climate warmer: It absorbs more heat because it is black,” he told the Australian site Science Alert.

Garrett also did not mention that 40 percent of global black carbon emissions stemmed from fossil fuels, [this sentence edited on 7/17 to change "diesel" to "fossil fuels"] while only 20 percent stemmed from cook-stoves, according to the figures Ramanathan used in his 2008 paper. (The other 40 percent came from agricultural burning and forest fires. Policymakers looking at black carbon usually ignore this category because its impact on the climate is much more complicated: outdoor biomass burning may have a cooling effect globally, but a warming effect regionally.)

Ramanathan concluded his 2008 paper emphasizing the importance of measuring the sibling particles co-emitted with black carbon. He acknowledged that if they have a cooling effect, it is important to see whether efforts to reduce black carbon would also reduce this temporarily beneficial effect on climate. Notwithstanding those concerns, Ramanathan launched Project Surya, an effort to introduce improved cook-stoves into rural India. The project’s website prominently mentions human health benefits, which are far more certain than the climate impacts, but its slogan is “fighting climate change now.”

Earlier this year, Bauer et al, whose paper in Atmospheric Chemistry and Physics Discussions was mentioned above, concluded based on models that reducing black carbon from diesel could cool the global climate by around 6 percent but that “reducing sources with a larger organic carbon component as well, such as bio-fuels, does not necessarily lead to climate benefits.” This uncertainty is the result, again, of black carbon’s sibling particles. The organic carbon component of cook-stove emissions may be reflecting sunlight away from Earth and increasing cloud cover, thereby cooling the climate as much as, or more than, the black carbon component is warming it.

These sibling particles are not clearly a concern, as they do terrible things to human health. But eliminating them may have surprising and troubling effects on efforts to avoid excessive warming of the climate.

The Bauer paper strongly implies that providing improved cook-stoves throughout the developing world may not in itself reduce warming. This conclusion contradicts the black carbon message that some media and some advocacy groups have echoed for more than a year.

When natural scientists offer conflicting perspectives on black carbon and where it comes from, while social scientists underscore the difficulties of bringing about related behavioral change, what should journalists and policymakers itching for a “fast-track solution” do?

Do struggles to accurately represent these complexities risk under-cutting the new groundswell of support for funding cook-stove initiatives, putting or leaving lives at risk? ( “Kill a million and a half people and nobody gives a damn,” a government official told Bilger. “But become part of this big climate thing and everyone comes knocking at your door.” )

Reducing respiratory disease surely is unquestionably worthwhile, but how should policy makers weigh the consequences of removing one menace that may be helping to keep global temperatures lower than they otherwise would be? This is a nuance journalistic coverage of black carbon generally has missed.

Acknowledging the complexities of the black carbon story, and looking more at diesel emissions rather than maintaining a tight focus on cook-stoves, could prove beneficial…not only in terms of achieving faster action on climate change, but also in light of the regional politics involving black carbon.

Part II of this feature will focus on targeting diesel emissions of black carbon.

Note 1: Nadine Unger, of NASA’s Goddard Institute for Space Studies, notes that this figure is quite uncertain. Climate models suggest that the average warming caused by black carbon since 1850 ranges from 15-55 percent of the warming caused by carbon dioxide during the same period. Even 15 percent is significant, of course, but the uncertainty means that black carbon’s role might be overemphasized. Return to article.

Note 2: This is a relatively minor form of what climate scientists call “masking.” Sulfate aerosols are spewed by several sources including coal-fired power plants and the global shipping industry. Stanford’s Mark Jacobson and other scientists estimate that without the presence of cooling aerosols, the temperature increase attributable to anthropogenic climate change would have been more than 1 degree Fahrenheit larger over the past century. As campaigns targeting conventional air pollutants begin to clean up those aerosols, more of the warming will be unmasked; Jacobson’s graph, below, illustrates this:

Return to article.

Bidisha Banerjee is a Masters candidate at Yale School of Forestry & Environmental Studies. E-mail: bidisha @

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5 Responses to Black Carbon’s Grey Areas

  1. Gabriel Mejias says:

    Excellent compilation! waiting for part II

  2. Namrata Kala says:

    Comprehensive, very informative and accessible- an excellent read!

  3. Adrian A says:

    I find it interesting that while part of your rhetorical strategy is to decry the technical misuse of the word “soot,” you incorrectly cite Ramanathan as saying 40% of BC emissions stem from diesel fuel. Reading the article, he says that “fossil fuels” are responsible for 40% of global BC emissions and that “biofuel” combustion is responsible for 20%. I need not remind you (but I will) that “fossil fuels” include more than diesel and “biofuel” does not necessarily translate to cookstove use. Furthermore, looking at the Bond et al. (2004) paper that Ramanathan referenced in his 2008 article, transportation (both road and non-road) accounted for 25% of global BC emissions. This figure for emissions can more honestly be attributed to diesel combustion. Perhaps it is more harmful to use incorrect data to support a claim than it is to conflate a colloquial term with a technical one.

    That being said, I do agree that programs to distribute improved stoves are not the silver bullet for curbing BC emissions, but should be utilized in concert with international regulations on diesel engine emissions and behavioral and infrastructural changes targeted at reducing demand for diesel transport.

    That being said, I do agree that programs to distribute improved stoves are not the silver bullet for curbing BC emissions, but should be utilized in concert with international regulations on diesel engine emissions and behavioral and infrastructural changes targetted at reducing demand for diesel transport.

  4. meander says:

    Thanks for clarifying some of the issues around the different types of black carbon, even though the clarification ends up making the situation a bit more complicated.

    One cook-stove-related area that might be worth exploring is the climate impact of deforestation caused by use of inefficient cook-stoves. Theoretically, if cook-stove efficiency could be dramatically increased, less wood or wood-products (e.g., charcoal) would be needed per stove, and that could reduce deforestation or improve forest health. Could that have any meaningful impact on climate?

  5. Nikhil says:

    In lay terms, could the Jacobson figure you have be interpreted as follows (crude figures, in W/m-2)? Of the total GHG warming of 1.6, about 1.0 is from CO2 directly (the rest from non-CO2 gases, some of which oxidize to CO2). Of the CO2 warming, fossil CO2 contribution is about 0.7. Add to it the fossil non-CO2 contribution, so the total fossil GHG contribution is around 1.0 or so. Similarly, of the 1.2 cooling, fossil contribution is around 1.0 or so. Overall, fossil contribution to warming is 0.2, or some 25% of the total net warming.

    Among fossil fuel sources, power generation contributes both warmers and coolers and has perhaps a net contribution of zero. Coal cooking and heating on the other hand has a high net contribution due to GHGs as well as BC.

    Over the past thirty years, China and South Africa have made rapid progress in transition from small-scale, inefficient use of coal for residential and commercial cooking and heating to to natural gas and/or electricity. Arguably the net effect is climate-positive – i.e., lower RF. (Assuming that the sulfur and organic carbon emissions from coal power plants partially offset those that could have resulted from continuing direct combustion for cooking and heating.)

    If that is the case, yes, stop throwing cookstoves at the problem; throw gas and electricity instead.

    That’s for coal use baseline. What about woody biomass and dung? Perhaps LPG? (No organic carbon but then much greater efficiency.) Don’t throw stoves; throw different stoves instead, with cylinders.

    Deforestation would occur anyway. If poor people use less wood, rich people would use more (for timber, bioliquids, biopower).