Black Carbon Soot's Climate Warming Effect May Be Canceled By Its Increasing Cloud Production

clouds photo

photo: Jose Roberto V Moraes via flickr

In the past eighteen months or so there's been a greater acknowledgment that black carbon soot, from burning fossil fuels and biomass, plays a bigger role in increasing global warming and glacier melting than we had thought. Well, a new paper in Atmospheric Chemistry and Physics (hat tip: Climate Progress) says that calculating the net warming effect is actually more complicated than sometimes presented--in fact in someplaces increases in cloud cover may cancel out warming. From the NASA science brief on the paper:

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.

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.

The report says that in some places cloud cover increases, while in others in decreases, and "most of the global models 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." (my emphasis)

The caveat in that: "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."

So, NASA's Goddard Institute for Space Studies is basically saying that reducing black carbon soot likely isn't the silver bullet for stopping temperature rise, as has sometimes been presented--including by this author, though I don't think I ever characterized it as a 'silver bullet', only that the benefits would take place more quickly than with other components of global warming, which remain in the atmosphere longer.

In short, as Climate Progress points out, there's no substitute for rapid reductions in carbon emissions if we want to reduce the impact of climate change.

That said though, the studies on the impact of black carbon on Himalayan glacier melting note that the reason the pollution in speeding melting is because when the soot falls on the ice and snow it changes reflectivity and increases melting. So, it's possible that reducing black carbon pollution could still be of benefit here. As would it in decreasing particulate air pollution and the myriad negative health effects caused by that.

Read the original study: Black carbon semi-direct effects on cloud cover: review and synthesis [PDF]

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More on Black Carbon:
Black Carbon Identified as a Key Element in Himalayan Glacier Melting
Black Carbon Pollution From Fossil Fuels Causes Twice the Warming As Burning Biomass
Black Carbon May Be Planet's Second Most Potent GHG
Black Soot Coating Himalayan Glaciers is Accelerating Melting

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