“Stop throwing stoves at black carbon”

Can this…

…fix this?

A reputable  scientist commenting on the recent flooding in China and Pakistan might tell a reporter:

“There’s a chance the flooding could be connected to the accelerated melting of the Tibetan Plateau’s glaciers.  There’s also the possibility that the warming effect of Asia’s Brown Cloud might be responsible for the glacier’s rapid disappearance. And the odds are that black carbon may be playing a role in accelerating the melting of the glaciers of the Greater Himalayas.”

The sloppy reporter, applying the journalistic saw of simplify and exaggerate, might report the story like this:

“The floods in China and Pakistan are caused by the soot from the burning of fossil fuels across Asia, which in turn is melting the glaciers on the Tibetan Plateau.”

As the narrative gets repeated, collective wisdom concludes:

“Hey, improved cookstoves reduce the black carbon that causes global warming. That means clean cookstoves can be an effective tool in combating Climate Change!”

(Editorial note: The Charcoal Project reported a similar story in an earlier post.)

Whoa! Hold it right there, partner!

According to a recent workshop on black carbon held at Yale University, the rush to promote improved cookstoves as a quick-fix to global warming is based on a number of perilous misconceptions and risks ascribing them a silver-bullet status they may not deserve.

This is just one of the recommendations contained in a thoughtful, well-written article reporting on the proceedings.

We thought the workshop’s other conclusions were worth repeating below.

I. Stop Throwing Cookstoves at the Problem

Black carbon itself is hardly uniform, an important point often missing in media reports. (Some scientists) estimate, 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.

Conclusion: 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.

II. Target diesel.

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.

III. Be very careful about comparing black carbon with carbon dioxide.

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

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.

The workshop’s conclusions leave many questions unanswered. For example, what types of improved cookstoves are we talking about here? Gassifiers that can efficiently combust all the components of biomass? Rocket stoves? What about the fuels? Briquettes made from char? Does that make a difference?

Whatever the answers, the conclusions do not mean improved cookstoves have no impact on climate change. Clearly they do, if only because they help reduce deforestation. What’s more, better cookstove provide many other benefits to society and the environment (a point mentioned in the article).

But the graph below (from a USAID report about Asia’s black carbon problem) clearly shows inneficient cookstoves  are the largest contributors of black and organic carbon across Asia, whereas poor diesel combustion makes up only a small part.

We hope additional science will provide greater clarity on the role cookstoves can play in abating global warming.