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:
- In must-see AGU video, Richard Alley explains “The Biggest Control Knob: Carbon Dioxide in Earth’s Climate History”
- Why solar energy trumps coal power: Exclusive new Caldeira analysis explains “the burning of organic carbon warms the Earth about 100,000 times more from climate effects than it does through the release of chemical energy in combustion.”