Black Carbon Larger Cause Of Climate Change Than Previously Assessed

Black carbon is the second largest man-made contributor to global warming and its influence on climate has been greatly underestimated, according to the first quantitative and comprehensive analysis of this issue.

Figure 1.1 Schematic overview of the primary black carbon emission sources and the processes that control the distribution of black carbon in the atmosphere and determine its role in the climate system [Bond et al., 2013].

An International Geosphere-Biosphere Programme news release

Key findings
  • Black carbon has a much greater (twice the direct) climate impact than reported in previous assessments.
  • Black carbon ranks “as the second most important individual climate-warming agent after carbon dioxide”.
  • Cleaning up diesel engines and some wood and coal combustion could slow the warming immediately.

The landmark study published in the Journal of Geophysical Research-Atmospheres today says the direct influence of black carbon, or soot, on warming the climate could be about twice previous estimates.  Accounting for all of the ways it can affect climate, black carbon is believed to have a warming effect of about 1.1 Watts per square meter (W/m²), approximately two thirds of the effect of the largest man made contributor to global warming, carbon dioxide. Co-lead author David Fahey from the U.S. National Oceanic and Atmospheric Administration (NOAA) said, “This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane.”  The study, a four-year, 232-page effort, led by the International Global Atmospheric Chemistry  (IGAC) Project, is likely to guide research efforts, climate modeling, and policy for years to come.

The report’s best estimate of direct climate influence by black carbon is about a factor of two higher than most previous work, including the estimates in the last Intergovernmental Panel on Climate Change (IPCC) Assessment released in 2007, which were based on the best available evidence and analysis at that time.

Scientists have spent the years since the last IPCC assessment improving estimates, but the new assessment notes that emissions in some regions are probably higher than estimated. This is consistent with other research that also hinted at significant under-estimates in some regions’ black carbon emissions.

The results indicate that there may be a greater potential to curb warming by reducing black carbon emissions than previously thought. “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,” says co-author Professor Piers Forster from the University of Leeds’s School of Earth and Environment.

The international team urges caution because the role of black carbon in climate change is complex.  “Black carbon influences climate in many ways, both directly and indirectly, and all of these effects must be considered jointly”, says co-lead author Sarah Doherty of the University of Washington, an expert in snow measurements. The dark particles absorb incoming and scattered heat from the sun (solar radiation); they can promote the formation of clouds that can have either cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting.  In addition, many sources of black carbon also emit other particles whose effects counteract black carbon, providing a cooling effect.

Figure 9.1 Quantitative estimates of black carbon climate forcing.  This study indicates the direct effects due to black carbon are nearly twice the number reported in the 2007 IPCC Fourth Assessment [Bond et al., 2013].

The research team quantified all the complexities of black carbon and the impacts of co-emitted pollutants for different sources, taking into account uncertainties in measurements and calculations. The study suggests mitigation of black carbon emissions for climate benefits must consider all emissions from each source and their complex influences on climate.  Based on the analysis, black carbon emission reductions targeting diesel engines followed by some types of wood and coal burning in small household burners would have an immediate cooling impact.

In addition, the report finds black carbon is a significant cause of the rapid warming in the Northern Hemisphere at mid to high latitudes, including the northern United States, Canada, northern Europe and northern Asia. Its impacts can also be felt farther south, inducing changes in rainfall patterns from the Asian Monsoon.  This demonstrates that curbing black carbon emissions could have significant impact on reducing regional climate change while having a positive impact on human health.

“Policy makers, like the Climate and Clean Air Coalition, are talking about ways to slow global warming by reducing black carbon emissions. This study shows that this is a viable option for some black carbon sources and since black carbon is short lived, the impacts would be noticed immediately.  Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced,” says co-lead author Tami Bond from the University of Illinois at Urbana-Champaign.

International Geosphere-Biosphere Programme (IGBP) news release

13 Responses to Black Carbon Larger Cause Of Climate Change Than Previously Assessed

  1. Joan Savage says:

    The abstract says, “Sources that emit black carbon also emit other short-lived species…”

    We need to have an update on the half-life of black carbon retention in atmosphere. Perhaps that is found in the full article, which I have not yet read.

    Eliminating significant short-lived species might be the ‘low hanging fruit’ that can jump start more political mobilization.

  2. Jameson Quinn says:

    I live in Guatemala City, where I see clouds of black carbon emerging from busses every day. But it’s also, of course, a tropical latitude; there’s not a lot of snow around here for it to stain. How far does black carbon travel? Is tropical black carbon less of a problem than it is at higher latitudes?

  3. Steve Funk says:

    I would really like to see a followup study on how much wood heaters like mine are contributing. The paper appears to imply that the main contribution is from simple devices in underdeveloped areas. And how much of my wood heat emissions are pure black carbon and how much are aggregated particles that appear to have a slight net cooling effect, according to the graph.

  4. Faye Sinnott says:

    Eliminating black carbon would also have almost immediate health benefit, given the role of particulates on cardio and pulmonary physiology.

    So, in addition to reducing the rate of climate change….

  5. fj says:

    excellent advance in understanding the crisis

  6. Crispin in Waterloo says:

    As the great majority of BC emissions are from biomass burning, reducing BC only and (perhaps) retaining organic carbon (OC) emissions (which are cooling) will be difficult. Reduction of anthropogenic BC can be done by improving the combustion efficiency of anything that burns a carbonaceous fuel (like wood or oil). As combustion efficiency is in any case the proper goal of any fuel use, this is a good thing.

    It may be helpful to remember that BC is not an inherent property of a fuel, but how it is burned (i.e. the combustor is responsible for BC and co-emitted species). Thus a biofuel can be burned with virtually no BC at all when the combustion conditions are right (as has been demonstrated many times over).

  7. Joan Savage says:

    Black carbon (soot) in the atmosphere functions as a greenhouse gas, before it comes back down to settle on a surface. Most of the black carbon starts out with combustion activities in the lower and mid latitudes.

    In 2009 there was an NYT article on black carbon, with maps, that says it stays airborne for a few weeks.

    Given the history of volcanic ash, or smoke from forest fires, soot can cover many a mile in those few weeks aloft before it drops out.

    I’d like to find you a map for that, too. Maybe one will turn up.

  8. Dr.A.Jagadeesh says:

    Great post on a crucial issue. Well illustrated.
    Dr.A.Jagadeesh Nellore(AP),India

  9. Omega Centauri says:

    Or it might be the short term relief that lets us ignore the 800 pound gorilla. I think our best strategy is to oppose attacking the shortlived species until an earnest effort to reduce CO2 is underway. Otherwise we risk complacency.

  10. Alex Smith says:

    More resources on black carbon:

    Radio Ecoshock interview on BC with this study’s lead author Tami Bond, University of Illinois (20 min)

    Full Radio Ecoshock Show “Black Carbon = Fast Warming = Early Death (April 2010) here:

  11. Mulga Mumblebrain says:

    I agree. The black carbon problem cannot be addressed as some sort of substitute for action on the big dangers, which are anthropogenic emissions of greenhouse gases. Indeed when I peruse the ‘usual suspects’ who are pushing the black carbon bandwaggon, my native naivete and credulousness are sorely tested.

  12. Joan Savage says:

    To follow your metaphor and the news, black carbon/ soot is the 528 pound gorilla, along side the 800 pound gorilla.

    “.. to blame for 1.1 Watts per square meter (W/m²), approximately two thirds of the effect of the largest man made contributor to global warming, carbon dioxide.”

    Which should get the rest of us big apes jumping up and down.

    A factor in finding the solution is that a lot of the black carbon/soot is from Asia and tropical countries, while CO2 is more commonly from the temperate zone. This prompts me to think of an outrageous comparison to the old adage about hypocrisy, “the pot calling the kettle black.”

  13. David Lewis says:

    Hansen published Global Warming in the 21st Century: An alternative scenario in the year 2000, arguing “the danger of dramatic climate change” could be reduced, if the emissions of black carbon were addressed. “Such a focus on air pollution has practical benefits that unite the interests of developed and developing countries”.

    So we have Hansen writing more than a decade ago: “Conceivably, a reduction of climate forcing by 0.5 W/m2 or more could be obtained by reducing black carbon emissions from diesel fuel and coal”.

    But keep in mind Hansen’s year 2000 caveat: “But a quantitative understanding of the role of absorbing aerosol in climate change is required to formulate reliable policy recommendations“.

    And when you read his latest (January 2013) communication, i.e. Global Temperature Update Through 2012, it is apparent that little has changed:

    “The one major wild card in projections of future climate change is the unmeasured climate forcing due to aerosol changes and their effects on clouds. Anecdotal information indicates that particulate air pollution has increased in regions with increasing coal burning, but assessment of the climate forcing requires global measurement of detailed physical properties of the aerosols”.

    Eg: look at the error bar on “Human-Made Aerosols” in Fig 6 page 6 of Hansen’s January 2013 communication. The total effect of aerosols which includes black carbon, according to this, could be anywhere between -0.5 W/m2 and -2.5 W/m2.

    The latest assessment of black carbon described in this post, i.e. the one just published by Fahey, Forster fits into that range, i.e. what was already known.

    Hansen stood in front of the fall meeting of the AGU some years ago and displayed a slide with photos of his two grandchildren. He claimed that when his group had to come up with an estimate of the total effect of aerosols for model runs for the AR4, that the state of the science was so uncertain that what he did was ask his grandchildren. Conner could only count up to one, so that was his estimate. Sophie, he claimed, suggested that the Hansen GISS model should use the number 2. In talks given since then Hansen has said if you think this isn’t a good scientific method: “you should see what the other guys do”.