NASA: Does heating from black carbon increase cooling from clouds?

Black carbon particles, commonly called soot, are dark and light-absorbing and therefore warm the climate. Soot comes from combustion of fossil and biofuels, especially burning of diesel, coal and wood. Due to its warming effects, reduction of soot could help cool climate. However, soot absorption also affects cloud distributions and the verdict on how the clouds change is unclear. Because clouds mostly cool the climate, the possibility that soot absorption could increase cloud cover needs to be considered.

Turns out the net warming effect from absorbing aerosols (AAs) such as black carbon (BC) or dust is more complicated than previously realized.  That’s the conclusion of a major review of the literature on the “semi-direct effects of absorbing aerosols,” in Atmospheric Chemistry and Physics, “Black carbon absorption effects on cloud cover: Review and synthesis,” by two NASA Goddard Institute for Space Studies.

And that means reducing BC may not be the silver bullet solution many thought.  The study concludes that in some climate model studies, “the cooling effect of BC due to cloud changes is strong enough to essentially cancel the warming direct effects.”

The news release, quoted above, explains why:

In a recent review of past studies of the effect of soot and other absorbing aerosols on clouds, we found ten processes described that either increase or decrease cloud cover. The cloud response depends on conditions such as relative altitude of smoke and cloud, cloud type, and meteorological conditions.

On one hand, soot embedded within clouds promotes cloud evaporation. On the other hand smoke or soot pollution blown from land up and over stratocumulus cloud decks over the oceans has been observed to stabilize and promote cloud persistence. But lofted smoke over dry land environments appears to inhibit formation of convective cumulus clouds. The studies indicate that more strongly absorbing aerosols, as would be expected from sources with lots of black carbon (like diesel or wood-smoke), have largest cloud response.

Global model studies of soot effects on clouds do indeed find a variety of cloud responses, with increased clouds in some regions and decreased clouds in others. Most of the global model studies indicate that the net cloud response to absorbing particles is cooling. This suggests the need for caution when pursuing mitigation of soot in order to cool climate. At the same time, relatively few global model studies have been conducted, and the global model cloud responses should be better tested against cloud scale models and field studies.

More research is clearly needed.  For now, we need to keep our eyes on the prize — sharp carbon dioxide reductions:


11 Responses to NASA: Does heating from black carbon increase cooling from clouds?

  1. catman306 says:

    Reducing soot would have the added benefit of reducing human lung ailments. But, of course, those saved lungs will need to dwell on a livable planet, which is iffy without significant CO2 reduction.

  2. Anonymous says:

    Would the US do something like this?

  3. Scott says:

    …but black carbon has more important negative impacts that make BC reduction an immediate high priority, and makes this observation a bit moot. Namely, that black carbon and particulates have dramatically increased the melt of the world’s tropic and sub tropic glaciers. eg:

    [JR: No question that BC accelerates ice melt. Probably on net a bad thing for climate.]

  4. richard pauli says:

    Yet another aspect for the perfect storm.

    Aerosols can reduce sun’s energy reaching the surface, but once deposited on the surface of snow it speeds melt (remember soot on snow?)

    With less snow-cover this is moot. So the suggested conclusion is that – if other factors were controlled – and say we managed to remove coal soot and other particles from the air – then more sunlight hits the ground and the now open sea. This would make things radically warmer.

    Oh joy – new cascading tipping points feeding my climate anxiety. Warming without CO2! Soot helps cool us! “Cats and Dogs living together!” …. Perfect storm.

    Who is controlling this movie?

  5. richard pauli says:

    Just be careful Joe – because “More research needed” can be code for “Lets just keep polluting while we distract you with delay and distraction”

    The words “More research is needed” should always be paired with “We have to act now to change things”

    The risk is that it may become “Let’s not do anything rash – like halt CO2, without doing more research”

    Just like tobacco – “more research” could become an obstructionist bumpersticker.

    [JR: Uhh, did you read the post to the end?]

  6. catman306 says:

    There’s another wonderful satellite shot, this time of THREE hurricanes at Dr. Jeff Masters:

    Who’s directing traffic out there?

  7. Lou Grinzo says:


    “Who’s directing traffic out there?”


  8. climate undergrad says:

    ‘The words “More research is needed” should always be paired with “We have to act now to change things”’


    “More research is clearly needed. For now, we need to keep our eyes on the prize — sharp carbon dioxide reductions.:

  9. EnviroWhacko says:

    Let’s just get back to basics here: Warmer lakes and oceans mean more evaporation, which means more condensation (clouds) which means more rainfall. Now that rainfall may not necessarily occur exactly in the same area as the evaporation took place, but will happen nonetheless. That will be determined mainly by winds and cold air masses. Carbon soot may have an effect on where the rain falls, but that rain will fall somewhere on the globe; hopefully it will fall on land somewhere, not ocean.

  10. Omega Centauri says:

    And to amplify Scott at 3, BC promotes ice melt at medium and high latitudes as well. Aerial BC has a greater effect on total shortwave energy reflected when it overlies a highly reflective surface, such as seaice seasonal snow, of glacier. So reducing BC could probably slow sea ice an icecap retreat, and also slow high latitude warming somewhat. Obviously if pursued in isolation any benefits will be swamped by CO2 induced warming. But it is probably cost effective as far as it goes.

  11. Also relevant:

    A glaciation indirect aerosol effect caused by soot aerosols
    U. Lohmann

    Anthropogenic aerosols can influence the climate indirectly by changing the optical properties and precipitation formation of water clouds. An indirect effect that has not been considered involves the subset of anthropogenic aerosols that act as ice nuclei and thereby determines the lifetime of ice and mixed-phase clouds. If, in addition to mineral dust, a fraction of the hydrophilic soot aerosol particles is assumed to act as contact ice nuclei as evident from recent laboratory studies, then increases in aerosol concentration from pre-industrial times to present-day pose a new indirect effect, a “glaciation indirect effect”, on clouds. Here increases in contact ice nuclei in the present-day climate result in more frequent glaciation of clouds and increase the amount of precipitation via the ice phase. This effect can at least partly offset the solar indirect aerosol effect on water clouds.

    I.e. this is an indirect as opposed to a semi-direct effect, but it also has the direction of BC *perhaps* causing more precipitation (by acting as ice nuclei), therby off setting some of its warming effects.

    Overall, I would still think that reducing soot is a good idea, not only for health, but also for climate reasons.