Black carbon, a component of soot, and potentially one of the most important contributors to climate change, rises into the atmosphere each time someone fires up a traditional cook-stove or switches on an older-model diesel vehicle. The author recently co-organized a workshop under the aegis of the Yale Climate and Energy Institute (YCEI), which brought together scientists, policymakers, and development experts to discuss controlling black carbon.

That workshop had three key conclusions: stop throwing cook-stoves at the problem; target diesel; and be very careful about comparing black carbon with carbon dioxide. The first part of this article examined the limits of targeting cook-stoves as a bid to slow climate change. Part II looks at the case for phasing out diesel emissions, and urges a more cautious approach to comparing black carbon with carbon dioxide.

The Case for Targeting Diesel Emissions

Unlike the emissions from cook-stoves, the black carbon emissions from diesel vehicles unambiguously warm Earth’s atmosphere. Although diesel vehicles offer lower carbon dioxide emissions and higher gas mileage than gasoline vehicles, some expert scientists, like Stanford University Mark Jacobson, have argued that controlling diesel emissions could be the fastest and “most effective” way to slow climate change.

Diesel black carbon emissions ‘unambiguous’ climate warmer.

For instance, in 2002, many European countries were meeting their Kyoto Protocol obligations by switching from gasoline to diesel vehicles, which can offer better mileage and lower carbon dioxide emissions. Jacobson’s models, published in the Journal of Geophysical Research–Atmospheres, concluded that, even with stringent U.S. and European standards, diesel vehicles could warm the climate more over 100 years than gasoline-powered cars as a result of the relatively higher black carbon content of diesel. “Almost all particles from diesel vehicles are types that warm climate rather than cool climate,” Jacobson later told Congress at a black carbon hearing in 2007.

The pending American Power Act introduced in the U.S. Senator by Massachusetts Democrat John Kerry and Connecticut Independent Joe Lieberman calls for a “black carbon reduction retrofit program” which, if enacted, would authorize EPA to make grants of an unspecified amount for installing diesel filters on vehicles built before 2007.

It’s an approach unlikely to satisfy Jacobson, who expresses little tolerance for diesel, even with filters installed. “The addition of a particle trap to a diesel vehicle increases its fuel use by 3.5-8.5 percent. Assuming a 5 percent increase, diesels with a trap emit even more CO2 per unit distance than do the gasoline vehicles,” he has testified before a congressional committee. Jacobson strongly recommended switching from both diesel and gasoline powered vehicles to renewably-powered alternatives. He also emphasized the need to target other fossil-fuel sources of black carbon, such as aviation fuel and coking coal.

Jacobson is hardly the only scientist in favor of targeting diesel. NASA’s Dorothy Koch, a co-author of the Bauer paper mentioned in Part I, which suggests that reducing diesel emissions would have a cooling effect on the climate, gave the keynote at the Yale Climate & Energy Institute workshop. Koch emphasized that other sub-sectors, such as kilns and coke, also have a high proportion of black carbon emissions compared to cooling organic carbon and sulfate particles. Because up to 20 percent of the melting in the Arctic may be from black carbon, she said, targeting black carbon sources is critical.

Koch has also looked at how black carbon interacts with clouds. Black carbon, which is co-emitted with organic carbon and sulfates, tends to work as a nucleus for cloud condensation — these clouds form in the lower atmosphere, where they play a cooling role. Based on her studies, Koch argues it is important to target “pure” black carbon from diesel, which does not have as much of an effect on cloud cooling.

‘Historic Responsibility’ and Political Traction

One of Koch’s collaborators, University of Illinois scientist Tami Bond, also testified that the black carbon record in Arctic ice-cores peaked in the 1920s, and could be traced back to emissions from U.S. coal. In principle, one of the most loaded terms in the international climate talks — historic responsibility — could also apply to black carbon. Moreover, just as carbon dioxide emissions from developing countries are projected to rise significantly over the coming decades, so are diesel emissions.

The idea of targeting black carbon may appeal to pragmatic policymakers in part because it would be easier to gain political traction on this issue than on carbon dioxide pollution. Switching out cook-stoves in the developing world may seem much easier and less contentious, from the comfortable vantage point of Washington D.C., than swaying Congress on carbon emissions.

But Jacobson’s and Koch’s messages suggest that curbing the warming impacts of black carbon might require more radical across-the-board measures in any case. Gaining support for a world-wide shift from diesel and gasoline to renewably-powered vehicles clearly will not be simple and will not occur overnight — though it may help avoid the blame-games which all along have hampered international climate talks.

For example, Indian Environment Minister Jairam Ramesh’s reluctance to discuss black carbon in the climate negotiations — and also U.S. interest in black carbon — may in part stem from the fact that India and China emit more black carbon than the U.S. and Europe. But few U.S. policymakers who see black carbon as a potential point of agreement between developed and developing countries have openly acknowledged that this is a contentious issue. At the Yale black carbon workshop, Ibrahim Rehman, an Indian scientist from The Energy and Resource Institute who has devoted much time to disseminating more efficient cook-stoves for Project Surya, suggested that focusing on diesel could lead to a genuine breakthrough in the convoluted world of international climate talks — assuming that world leaders don’t start trading black carbon reductions for carbon dioxide reductions. That insistence raises a key point — the difficulties involved in efforts to compare black carbon and carbon dioxide.

Short-Lived Black Carbon vs. Long-Lived CO2

As illustrated in Part I of this report, scientists, policymakers, and journalists can all fall prey to treating reductions in black carbon as being equal to reductions in carbon dioxide. Black carbon may be the second most important contributor to global warming based on some accounting methods, but defining “important” turns out to be as much a value judgment as a mathematical calculation.

Journalists and policymakers may want simple ways to compare black carbon’s impact to the impact of other greenhouse gases. Think here of the Burkhard Bilger’s statement (quoted in full in Part I) in his New Yorker piece on cook-stoves — “[a] single gram [of black carbon] warms the atmosphere as much as a fifteen-hundred-watt space heater running for a week.” Clean air advocate Conrad Schneider more abstractly suggested that “over a 20-year period, pound for pound, BC may be 2,000 times more potent a climate-forcing agent than CO2.”

But how can black carbon’s global warming potential best be compared to that of CO2? The space heater, which is responsible for CO2 emissions, is contributing to the long-term buildup of CO2 in the atmosphere. Each pound of CO2 will cause a relatively small amount of warming each year for many years. Each pound of black carbon, by contrast, will warm the planet relatively intensely for the week or two it remains in the atmosphere. Scientists call such short-lived hazards “flow pollutants,” as opposed to “stock pollutants” like CO2 which accumulate over time. (If black carbon lands on a glacier, of course, it may contribute to ice melt until it is washed away, but that is a different effect.)

The value judgment hidden in the word “important” has to do with how short-lived effects are compared to long-lived effects. The IPCC generally uses a “CO2 equivalent” metric (usually denoted as CO2e). Calculating CO2e is relatively simple: a time horizon (usually 20, 50, or 100 years) is chosen, and the cumulative effect of a particular greenhouse gas over that time horizon is compared to the cumulative effect of the same amount of CO2. This method of comparison makes some sense when carbon dioxide is being compared to greenhouse gases such as HFCs, which also accumulate in the atmosphere over long periods of time. But calculations of CO2e may be misleading when comparing stock pollutants like CO2 to flow pollutants like black carbon. Those short time horizons can make the flow pollutants seem more important, whereas long time horizons will emphasize stock pollutants.

A graph of Jacobson’s (below) demonstrates this point elegantly. Wiping out both fossil fuel- and biofuel-based soot immediately would, by 2100, yield only about 0.3 degrees C of cooling, compared to the 1 to 1.4 degrees C saved by eliminating CO2 immediately. But the effects of wiping out soot are seen almost immediately, with 100% of the reduction happening within the first five years. CO2 reductions take a longer time to manifest.

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So which is more important? The answer is that both are, but in different ways. As Tami Bond put it during her Congressional testimony, “reducing black carbon and ozone in the atmosphere is like applying an emergency brake in a car out of control. It will slow the vehicle quickly and give you a little time to think. But the problem will continue if you don’t take your foot off the gas pedal — that is, if CO2 emissions are maintained.”

‘Buying Time’ … ‘Headroom’ … and Tipping Points

The distinction is crucial given the popular perception that curbing black carbon emissions can “buy us time” to reduce carbon dioxide emissions. As NASA scientist James Hanson and University of East Anglia climate modelers Timothy Lenton and Naomi Vaughan caution, time is at a premium in avoiding some of the worst-case impacts. Lenton and Vaughan’s work on climate tipping points — thresholds which cause the climate system to abruptly “flip” to a very different stable state — underscores their message. Reducing black carbon by reducing diesel emissions is crucial because it may buy us “headroom,” reducing the probability of an extreme climate transition, but it is not a good way to buy time. The phrase “buy us time” makes it sound like experts can calculate the bargain society is getting. The problem here is that we don’t really know where the tipping points lie, and we may not know that we’ve crossed one until it is too late. Increasing levels of conventional greenhouse gases could exceed those same unknown thresholds unless across-the-board reductions happen immediately.

‘Buying time’ helpful, but CO2 emissions continue to increase…toward tipping points?

Trading-off black carbon emission reductions for CO2, NASA’s Nadine Unger has cautioned, “might divert attention from the critical need for the reduction of long-lived gases.” It would be particularly problematic in the context of a carbon offset scheme. Unger and her colleagues are working to create another metric, a “radiative forcing index,” and some scientists have suggested a “two-basket” approach, treating short-term pollutants in a different category than long-lived greenhouse gases.

As with most other issues in the complex world of communicating climate change science and policy, there is no black-and-white up/down true/false simplicity in considering black carbon. Because black carbon’s climate effects have received intense scrutiny only recently, there are several grey areas not only in media coverage but also in public and policymakers’ understanding, particularly around the consistent use of scientific terms such as soot, and around cook-stoves vs. diesel emissions, international responsibility, and the comparability of carbon dioxide and black carbon.

Focusing on these scientific uncertainties and complexities may produce a less familiar story, but a more revealing one.

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