Black carbon and climate change

Megan Pini in yellow

Al Gore’s latest book – Our Choice: A Plan to Solve the Climate Crisis – includes a fair bit of discussion of black carbon, a human pollutant that causes global warming, but not in the same way carbon dioxide (CO2) does. Greenhouse gasses like CO2 prevent long-wave infrared radiation from leaving the Earth into space. Black carbon, by contrast, warms the planet by absorbing a lot of short-wave radiation from the sun. In essence, it has a very low albedo.

Some other pertinent things to know about black carbon:

  • The largest source is biomass combustion – such as burning forests and grasslands to clear them for agriculture.
  • The areas where this is happening most are Brazil, Indonesia, and Central Africa.
  • Black carbon settling in the Arctic is a major cause of warming there: possibly responsible for 1°C of the 2.5°C of warming already observed there.
  • Black carbon is also a major threat to Himalayan glaciers, which in turn provide the source water for rivers of critical human importance, such as the Ganges.
  • Black carbon is washed out of the atmosphere by rain, and only has a lifetime of a few weeks. If we stopped emitting it, its contribution to climate change would cease quickly.

The last of those is very encouraging. Unlike CO2, which remains in the atmosphere for a very long span of time, black carbon is something we could tackle on a short timescale, by mandating things like filters on diesel engines and the cleaner burning of coal and biomass.

As mentioned before, recent research has also highlighted the importance of non-CO2 greenhouse gasses. Anything that allows us to take more rapid and effective action to halt climate change is welcome news. Also, it requires a lot less political will to install better filters on diesel engines than it does to curb activities that are critically linked to greenhouse gas emissions.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

11 thoughts on “Black carbon and climate change”

  1. It seems like incorporating black carbon into carbon pricing schemes could be problematic. A substance that only stays in the atmosphere for a few weeks is very different from one that lasts for many years.

    Perhaps the best approach is not to make people buy permits to emit black carbon, but rather to mandate the use of the best available technologies to prevent its release. For instance, filters on diesel engines and power plants.

  2. The same is true of HFC-23.

    Including emissions that are extremely cheap and easy to avoid in carbon pricing schemes certainly seems capable of causing disruption and strategic behaviour (‘accidentally’ producing HFC-23 when you can be paid to destroy it, etc).

  3. What is black carbon? The Wikipedia entry gives no description of its physical make up, only its effects, and that it is produced through incomplete combustion.

    This entry is an amazing example of how someone can know so much about a thing, without knowing what the thing “is”. I suppose this could be considered evidence for the unimportance of a things immediate physical substance compared to its indirect properties.

  4. Black carbon is soot.

    It consists of tiny solid particles of nearly pure carbon. In addition to causing warming, it often contains mutagens and probable human carcinogens.

  5. IPCC AR4
    Chapter 2: Changes in Atmospheric Constituents and in Radiative Forcing

    Aerosols are small particles present in the atmosphere with widely varying size, concentration and chemical composition. Some aerosols are emitted directly into the atmosphere while others are formed from emitted compounds. Aerosols contain both naturally occurring compounds and those emitted as a result of human activities. Fossil fuel and biomass burning have increased aerosols containing sulphur compounds, organic compounds and black carbon (soot). Human activities such as surface mining and industrial processes have increased dust in the atmosphere. Natural aerosols include mineral dust released from the surface, sea salt aerosols, biogenic emissions from the land and oceans and sulphate and dust aerosols produced by volcanic eruptions.

    Aerosol particles infl uence radiative forcing directly through refl ection and absorption of solar and infrared radiation in the atmosphere. Some aerosols cause a positive forcing while others cause a negative forcing. The direct radiative forcing summed over all aerosol types is negative. Aerosols also cause a negative radiative forcing indirectly through the changes they cause in cloud properties.

    The TAR categorised aerosol RFs into direct and indirect effects. The direct effect is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system. Sulphate, fossil fuel organic carbon, fossil fuel black carbon, biomass burning and mineral dust aerosols were all identified as having a signifi cant anthropogenic component and exerting a significant direct RF.

    Black carbon (BC) is a primary aerosol emitted directly at the source from incomplete combustion processes such as fossil fuel and biomass burning and therefore much atmospheric BC is of anthropogenic origin. Global, present-day fossil fuel emission estimates range from 5.8 to 8.0 TgC yr–1 (Haywood and Boucher, 2000 and references therein). Bond et al. (2004) estimated the total current global emission of BC to be approximately 8 TgC yr–1, with contributions of 4.6 TgC yr–1 from fossil fuel and biofuel combustion and 3.3 TgC yr–1 from open biomass burning, and estimated an uncertainty of about a factor of two. Ito and Penner (2005) suggested fossil fuel BC emissions for 2000 of around 2.8 TgC yr–1. The trends in emission of fossil fuel BC have been investigated in industrial areas by Novakov et al. (2003) and Ito and Penner (2005). Novakov et al. (2003) reported that significant decreases were recorded in the UK, Germany, the former Soviet Union and the USA over the period 1950 to 2000, while significant increases were reported in India and China. Globally, Novakov et al. (2003) suggested that emissions of fossil fuel BC increased by a factor of three between 1950 and 1990 (2.2 to 6.7 TgC yr–1) owing to the rapid expansion of the USA, European and Asian economies (e.g., Streets et al., 2001, 2003), and have since fallen to around 5.6 TgC yr–1 owing to further emission controls. Ito and Penner (2005) determined a similar trend in emissions over the period 1950 to 2000 of approximately a factor of three, but the absolute emissions are smaller than in Novakov et al. (2003) by approximately a factor of 1.7.

    Black carbon aerosol strongly absorbs solar radiation. Electron microscope images of BC particles show that they are emitted as complex chain structures (e.g., Posfai et al., 2003), which tend to collapse as the particles age, thereby modifying the optical properties (e.g., Abel et al., 2003). The Indian Ocean Experiment (Ramanathan et al., 2001b and references therein) focussed on emissions of aerosol from the Indian sub-continent, and showed the importance of absorption by aerosol in the atmospheric column. These observations showed that the local surface forcing (–23 W m–2) was signifi cantly stronger than the local RF at the TOA (–7 W m–2). Additionally, the presence of BC in the atmosphere above highly refl ective surfaces such as snow and ice, or clouds, may cause a signifi cant positive RF (Ramaswamy et al., 2001). The vertical profile is therefore important, as BC aerosols or mixtures of aerosols containing a relatively large fraction of BC will exert a positive RF when located above clouds. Both microphysical (e.g., hydrophilic-to hydrophobic nature of emissions into the atmosphere, aging of the aerosols, wet deposition) and meteorological aspects govern the horizontal and vertical distribution patterns of BC aerosols, and the residence time of these aerosols is thus sensitive to these factors (Cooke et al., 2002; Stier et al., 2006b).

    The TAR assessed the RF due to fossil fuel BC as being +0.2 W m–2 with an uncertainty of a factor of two. Those models since the TAR that explicitly model and separate out the RF due to BC from fossil fuels include those from Takemura et al. (2000), Reddy et al. (2005a) and Hansen et al. (2005) as summarised in Table 2.5. The results from a number of studies that continue to group the RF from fossil fuel with that from biomass burning are also shown. Recently published results (A to K) and AeroCom studies (L to T) suggest a combined RF from both sources of +0.44 ± 0.13 W m–2 and +0.29 ± 0.15 W m–2 respectively. The stronger RF estimates from the models A to K appear to be primarily due to stronger sources and column loadings as the direct RF/column loading is similar at approximately 1.2 to 1.3 W mg–1 (Table 2.5). Carbonaceous aerosol emission inventories suggest that approximately 34 to 38% of emissions come from biomass burning sources and the remainder from fossil fuel burning sources. Models that separate fossil fuel from biomass burning suggest an equal split in RF. This is applied to those estimates where the BC emissions are not explicitly separated into emission sources to provide an estimate of the RF due to fossil fuel BC. For the AeroCom results, the fossil fuel BC RF ranges from +0.08 to +0.18 W m–2 with a mean of +0.13 W m–2 and a standard deviation of 0.03 W m–2. For model results A to K, the RFs range from +0.15 W m–2 to approximately +0.27 W m–2, with a mean of +0.25 W m–2 and a standard deviation of 0.11 W m–2. The mean and median of the direct RF for fossil fuel BC from grouping all these studies together are +0.19 and +0.16 W m–2, respectively, with a standard deviation of nearly 0.10 W m–2. The standard deviation is multiplied by 1.645 to approximate the 90% confi dence interval and the best estimate is rounded upwards slightly for simplicity, leading to a direct RF estimate of +0.20 ± 0.15 W m–2. This estimate does not include the semi-direct effect or the BC impact on snow and ice surface albedo (see Sections 2.5.4 and

    The presence of soot particles in snow could cause a decrease in the albedo of snow and affect snowmelt. Initial estimates by Hansen et al. (2000) suggested that BC could thereby exert a positive RF of +0.2 W m–2. This estimate was refined by Hansen and Nazarenko (2004), who used measured BC concentrations within snow and ice at a wide range of geographic locations to deduce the perturbation to the surface and planetary albedo, deriving an RF of +0.15 W m–2. The uncertainty in this estimate is substantial due to uncertainties in whether BC and snow
    particles are internally or externally mixed, in BC and snow particle shapes and sizes, in voids within BC particles, and in the BC imaginary refractive index. Jacobson (2004) developed a global model that allows the BC aerosol to enter snow via precipitation and dry deposition, thereby modifying the snow albedo and emissivity. They found modelled concentrations of BC within snow that were in reasonable agreement with those from many observations. The model study found that BC on snow and sea ice caused a decrease in the surface albedo of 0.4% globally and 1% in the NH, although RFs were not reported. Hansen et al. (2005) allowed the albedo change to be proportional to local BC deposition according to Koch (2001) and presented a further revised estimate of 0.08 W m–2. They also suggested that this RF mechanism produces a greater temperature response by a factor of 1.7 than an equivalent CO2 RF, that is, the ‘efficacy’ may be higher for this RF mechanism (see Section This report adopts a best estimate for the BC on snow RF of +0.10 ± 0.10 W m–2, with a low level of scientific understanding (Section 2.9, Table 2.11).

  6. Climate science is bloody complicated!

    Not only is the climate system internally complex, but all the stuff we’re throwing up into the atmosphere interacts with all the other stuff we’re emitting.

    Al Gore, at least, has the dual advantages of being very smart and being able to call on world class experts to explain things to him.

  7. f you “just want to get away” from Earth for a while by indulging your new hobby of space exploration, you may find the planet pretty steamed when you get back. According to researchers, the budding space tourism industry may be mooning efforts to slow climate change by blasting extra soot, or black carbon, from rocket engines high into the atmosphere. Ten years of soot from commercial space flight might do as much harm as soot from all current commercial air travel. (Soot particles are “dark and light-absorbing and therefore warm the climate.” They come from burning stuff, like coal and wood.)

    More out-of-this-world soot is astronaut the kind of action on climate we’ve been hoping for.

    It’s particularly bad news because these tourist spaceships thrust up, up, and away from the part of the atmosphere where weather occurs — plus, new hybrid rocket engines are expected to be even sootier than a dance sequence from Mary Poppins. Without rain clouds to wash the sky clean, particles of soot won’t get flushed out of the stratosphere, instead likely hanging around to heat up the planet for a while.

  8. Global shipping firms are not only taking advantage of melting ice in the Arctic Ocean — they’re actually helping to drive the meltdown that continues to unlock sea routes across the top of the world.

    And as a rapidly warming Arctic encourages more ship traffic through Canada’s Northwest Passage and along other polar routes, the sooty emissions from passing freighters will significantly accelerate climate change in the region, according to a new Canadian-American study that, for the first time, predicts the potential impact of engine exhaust particles on the Arctic environment.

    “One of the most potent ‘short-lived climate forcers’ in diesel emissions is black carbon, or soot,” says the study’s lead author, James Corbett, a University of Delaware marine scientist. “Ships operating in or near the Arctic use advanced diesel engines that release black carbon into one of the most sensitive regions for climate change.”

    The study, published in the latest issue of the journal Atmospheric Chemistry and Physics, includes findings from three other U.S. scientists, as well as B.C.-based Transport Canada researcher Susie Harder and Ottawa-based researcher Maya Gold of the Canadian Coast Guard.

    The researchers estimate that about two per cent of current global ship traffic will be diverted to the Arctic by 2030 and that the figure could rise to five per cent by 2050.

    A major concern among scientists has been the environmental effect of pumping pollutants directly into the Arctic region, where exhaust impurities released by the burning of ships’ bunker fuel are known to elevate ambient air and ice temperatures.

  9. During a 10-year investigation detailed in the latest issue of the Journal of Geophysical Research, Stanford University scientist Mark Jacobson isolated the widespread warming effects from all sources of soot — the visible residue of burned wood, crops, oil, biomass and other fuels — from the climate impacts caused by greenhouse gases such as carbon dioxide and methane.

    He concluded that soot is currently the No. 2 driver of climate change — behind CO2 but ahead of methane — and that curbing emissions of black carbon would produce the fastest, most effective and most affordable international response to climate change and the shrinking of the Arctic sea ice.

    “Controlling soot may be the only method of significantly slowing Arctic warming within the next two decades,” Jacobson said at the time.

Leave a Reply

Your email address will not be published. Required fields are marked *