Open Mind


November 16, 2008 · 15 Comments

The sequence “ABC” can stand for a lot of things, from the alphabet to a chain of liquor stores. But in the context of climate, it stands for Atmospheric Brown Clouds, which is the subject of a recent United Nations Environment Program report.

The report begins with a bleak outlook on the impact of ABCs:

… increasing amount of soot, sulphates and other aerosol components in atmospheric brown clouds (ABCs) are causing major threats to the water and food security of Asia and have resulted in surface dimming, atmospheric solar heating and soot deposition in the Hindu Kush-Himalayan-Tibetan (HKHT) glaciers and snow packs. These have given rise to major areas of concern, some of the most critical being observed decreases in the Indian summer monsoon rainfall, a north-south shift in rainfall patterns in eastern China, the accelerated retreat of the HKHT glaciers and decrease in snow packs, and the increase in surface ozone. All these have led to negative effects on water resources and crop yields. The emergence of the ABC problem is expected to further aggravate the recent dramatic escalation of food prices and the consequent challenge for survival among the world’s most vulnerable populations. Lastly, the human fatalities from indoor and outdoor exposures to ABC-relevant pollutants have also become a source of grave concern.

Atmospheric Brown Clouds are large concentrations of man-made aerosols emitted due to fossil fuel combustion, biofuel cooking, and biomass burning.. Some of them, particularly sulfates and nitrates, have a cooling effect because they primarily scatter incoming solar radiation back to space. Others, a large portion of which are referred to as “black carbon,” have a net warming effect because they absorb incoming solar radiation. But they have more impact than just absorbing sunlight; they re-distribute incoming energy, altering the balance between surface and atmosphere:

… these aerosols are referred to as black carbon. The heating aerosols absorb solar radiation. Furthermore, the ratio of absorption to scattering exceeds 10 per cent. These absorbing aerosols add solar energy to the atmosphere and alter the distribution of energy in two different ways. First, by absorbing direct solar radiation, which would have otherwise reached the surface, the absorbing aerosols lead to dimming at the surface. This effect is a redistribution of the solar energy between the surface and the atmosphere, and has a significant influence on the stability of the atmosphere by warming the air above and cooling the surface below, suppressing cumulus clouds and cumulus precipitation. Furthermore, dimming will lead to reduced evaporation of water vapour from the surface, thereby impacting precipitation. Second, by absorbing solar radiation reflected by the surface, atmosphere and clouds, the absorbing aerosols reduce the amount of solar radiation that is reflected to space. This results in a net heating of the surface-atmosphere system and therefore constitutes a positive radiative forcing of the climate system and contributes to global warming. Thus, black carbon aerosols are major agents of regional and global warming.

Furthermore, ABCs can have an impact on cloud properties:

Aerosols in ABCs nucleate cloud drops. The enhancement of the cloud drop population increases the reflection of solar radiation (making the clouds brighter) which leads to dimming and surface cooling. In regions with copious amounts of ABC aerosols, competition for water between nucleating aerosols causes cloud drop size to decrease, and this inhibits the formation of larger size drizzles and rain drops. The net effect is an extension of cloud lifetimes, that is, the polluted regions are cloudier with brighter clouds. This latter effect also leads to dimming and surface cooling. The radiative changes due to the two effects above are referred to as indirect radiative forcing.

Among the many impact of ABCs is to reduce greenhouse-gas warming at the surface while increasing warming in the atmosphere, to increase surface levels of ozone (threatening human health), and to alter the patterns of rainfall including the intensity of the Indian Monsoon. Also, the pollutants responsible for ABCs have a direct impact on food crops, reducing productivity quite apart from their effect on climate. Furthermore, they pose a threat to the Himalayan glaciers (and therefore to the water supply of Asia) because of the enhanced warming created at higher altitudes:

Most of the studies, if not all, attribute the retreat of the Himalayan glaciers to rising air temperatures. Warming is much more pronounced at elevated levels of the Himalayan-Tibetan region. The warming trend at elevated regions (>3 km) is as much as 0.25°C per decade since the 1950s.

ABC solar heating (by black carbon) of the atmosphere is suggested to be as important as GHG warming in accounting for the anomalously large warming trend observed in the elevated regions.

Decreased reflection of solar radiation by snow and ice due to black carbon deposition is emerging as another major contributor to the melting of snow packs and glaciers. Recent ice core observations reveal large depositions of sulphates and black carbon, with a large increasing trend during the past few decades. Furthermore, new atmospheric observations by Project ABC in elevated regions of the Himalayas (1 - 5 km) within 100 km of the Mt Everest region, suggest large black carbon concentrations ranging from a few hundred to a few thousand ng m-3.

The report states changes induced by ABCs with high confidence:

It is certain that ABCs have caused dimming at the surface.

It is certain that soot in ABCs has increased solar heating of the atmosphere.

It is virtually certain that India and China are dimmer (at the surface) today by at least 6 per cent, compared with the pre-industrial values. Absorbed solar radiation at the surface in China and India are lower today by 15 W m-2 or more, compared with the pre-industrial values.

It is highly likely that black carbon (BC) in ABCs has increased the vertically averaged annual mean solar absorption in the troposphere (from the surface up to 14 km in altitude) by about 15 per cent (about 14 W m-2) and the solar heating at elevated levels (1 - 4 km) over India and China by as much as 20 - 50 per cent (6 - 20 W m-2).

Overall, the report concludes that ABCs represent a serious threat to human health and food security, and that the problem is already becoming serious in large regions of Asia. But in a disturbing damned-if-you-do-and-damned-if-you-don’t, eliminating ABCs will also remove the mitigating effect on greehouse-gas warming:

… aerosols in ABCs have masked 20 - 80 per cent of GHG forcing in the past century.

There’s much more in the report, including a detailed look at the impact of ABCs on human health and on the food supply. It’s a serious alarm-bell about the effect human activity is having on the environment. I strongly recommend you download the report and read it.

Categories: Global Warming

15 responses so far ↓

  • Ray Ladbury // November 16, 2008 at 8:27 pm

    Tamino, Nice summary on a topic about which there’s a lot of confusion. As to the mitigating effect of aerosols, I think that given the different timescales on which these forcings operate, the most we can hope for would be a respite of a couple of decades from greenhouse warming. Indeed, by masking the effects of warming, sulfate aerosols may reinforce the sanguine attitude of the denialist crowd. One of the best things we could do to diminish future warming would be to help India, China and Brazil to resolve their energy problems without burning coal. It might buy time and develop technology while we, the legacy industrial economies, catch up.

  • Gareth // November 16, 2008 at 8:40 pm

    A rather extreme example an be seen at the Earth Observatory.

  • Nexus 6 // November 16, 2008 at 9:32 pm

    But what about the benefits? These reports are so one-sided. As anyone who spends a bit of time in SE Asia would know, the ABCs result in some of the most spectacular sunsets you’ll see anywhere. Surely they’re worth a bit of climate change and loss of food security ;)

  • S2 // November 16, 2008 at 10:09 pm


    Thanks for bringing this to our attention. It makes sobering reading.

    The report concentrates on Asia, but mentions that similar effects are being seen in the Amazon Basin and in Africa (two of their 13 “hotspots” are Cairo and Lagos). I know that Cape Town also has it’s own “brown haze” now.

  • cce // November 16, 2008 at 10:29 pm

    The best course of action is to focus on reducing the pollution of the third world that has the most effect on public health. Improving the efficiency of cook stoves and adopting newer diesel engines would clean up the air substantially, and reduce the localized climatic effects.
    (”Say no to filth”)

  • Bob North // November 17, 2008 at 8:49 pm

    Any info on to what extent it is believed that such ABC’s were present over the US and western europe prior to the implementation of air pollution regulations? I agree with CCE that health effects are a primary concern for cleaning up air pollution.

  • ed // November 17, 2008 at 10:39 pm

    In light of Ray’s comment “One of the best things we could do to diminish future warming would be to help India, China and Brazil to resolve their energy problems without burning coal. It might buy time and develop technology while we, the legacy industrial economies, catch up” . What exactly does “resolve their energy problems” mean? These economies are transitioning from no electricity or burning dung to finally have electricity. For a China that is commissioning a new coal plant every other week what’s the alternative? Wind farms? The only realistic alternative would be massive construction of nuke plants with the possibility of them coming on line in 5 years? It’s an impossible sales job to convince these countries to forgo utilizing the cheap coal available to them within their borders. Unfortunately this kicks the CO2 problem down the road 20-40 years until those plants shut down or their coal runs out and they transition to Fusion or whatever we’ve developed between now and then. I don’t think you can make the same case for USA created ABC’s given the scrubbers and technology in use in the USA. The net effect is that these countries will not allow the “first world” to dictate their energy policies.

  • Dave A // November 17, 2008 at 10:53 pm

    Interesting that the lead author of this report also had a letter in Nature last year that said the opposite

    “We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent. Our general circulation model simulations, which take into account the recently observed widespread occurrence of vertically extended atmospheric brown clouds over the Indian Ocean and Asia, suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends. We propose that the combined warming trend of 0.25 K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers”

  • cce // November 18, 2008 at 1:04 am

    In what way is Ramanathan saying “the opposite”? He is restating the findings of this, and earlier reports, which have been his research interest for years. ABCs cause dimming of the surface, warming of the atmosphere, and soot on snow, all of which cause significant regional climate change (eg Himalayas, Indian monsoon, soot in the Arctic).

  • Philippe Chantreau // November 18, 2008 at 1:19 am

    Dave A, the opposite of what? I find that abstract in excellent agreement with the report, although the report is more up to date.

    Ramanathan says exactly the same thing in that abstract as in the report, i.e. that ABCs induce surface cooling AND lower atmospheric warming.

    The major finding from Ramanathan unmanned aerial flights (letter) was that the net effect to the surface+atmosphere system is warming and it contributes significantly to regional atmospheric warming trends, even though it still masks the GHG warming. That is also clearly stated in the report.

    The additional info that is in the report and may not have been known at the time of the letter seems to concern the concentrations and effect of black carbon deposits on Himalayan snow and ice. These glaciers take a double whammy due to the atmospheric warming happening at their altitude because of the aerosols and the reduced albedo from BC deposits.

    I don’t see where the contradiction is.

  • Greg Simpson // November 18, 2008 at 8:24 pm

    I had just started seeing ABC used on climate pages and I was wondering what it stood for, thanks.

    Ed, China is building a number of nuclear power plants, and dabbling in wind energy too. I wish they (and we) were doing more, but I think they deserve some credit.

  • chriscolose // November 19, 2008 at 3:46 am

    The TOA RF is the surface+atmsophere forcing, and the net effect of Black Carbon is to increase the radiative heating of the atmosphere and decrease the radiative heating of the surface. All aerosols lead to reduction of solar radiation. So, absorbing aerosols that heat the lower atmosphere may still have an overall cooling effect at the top-of-atmosphere where forcing is defined. For single scattering albedo greater than about 0.89 the aerosol particles will cool at top-of-atmosphere and below 0.89, warm. But any single scattering below 1 means that they warm at the level they reside. Aerosol always cool the surface, warm (if they absorb) at the altitude where they reside, and either cool or warm at the TOA.

  • Douglas Watts // November 21, 2008 at 7:16 am

    ABC = gross, unchecked pollution.

    It is an amazing part of the human species that we can argue about everything, forever. I’m sure if Tamino put up a post about not eating out of a public toilet, someone would argue the risks are unproven and overblown.

  • Ray Ladbury // November 21, 2008 at 1:11 pm

    Douglas Watts, If they were arguing that the rest of us could eat out of a public toilet, I’d say you’re right, especially if they could make a profit off of us.

  • naught101 // November 28, 2008 at 9:19 pm

    It would be interesting to see an estimate of the breakdown of warming effects, lifetime, and sources for each component of the clouds.

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