The Big Picture on Aerosols

“We used to think of smoke mainly as a reflector, reflecting sunlight back to space,” Koren observed. “But here we show that, due to absorption, it chokes off cloud formation. This is one of aerosols’ most important contributions to the global radiant energy budget.”

But how could such a small-scale event—and one that only lasts a matter of weeks—possibly have a significant effect on Earth’s total energy budget?

Remer points out that human production of energy-absorbing aerosols is not unique to the Amazon Basin; the problem is much more widespread and happening year round. Ackerman observed a large, dense pall of the plumes over a large swath of the Indian subcontinent, exerting the same cloud-burning effect on cumulus clouds over the Bay of Bengal. And Remer says her team has also observed the semi-direct (warming) effect of aerosols over the Canadian boreal forest, during intense wildfires, as well as over parts of Africa during the burning season there.

When the separate contributions of changes in cloud reflectivity, cloud trapping of infrared radiation, and smoke reflectivity are combined, the total semi-direct effect of aerosols on the climate of the Amazon is warming. This graph shows the contributions of each component, and the total effect (black line) relative to smoke optical depth. Positive numbers indicate more energy at Earth’s surface (warming), and negative numbers indicate less energy at Earth’s surface (cooling). (Graph redrawn by Robert Simmon from Koren, 2004)

“We don’t know a lot about aerosol absorption on a global scale,” Remer admitted, “so it is impossible to say right now how significant the semi-direct effect is.”

When pressed to help put aerosols’ different influences into perspective, Remer said, “If you guess at the indirect effect and combine this with what we know about the aerosol direct effect, it is possible that the Earth should be cooling today, despite the warming influence of greenhouse gases.” In other words, it is possible that aerosols’ cooling influence should have overwhelmed greenhouse gases’ warming influence.

Remer said it is possible that the amount of solar energy aerosols reflect combined with the increased solar energy that aerosol-modified clouds reflect should produce a cooling influence that is greater than the warming influence of human-produced greenhouse gases. “Now, the semi-direct effect [of soot particles] could explain why we are witnessing a [global] warming instead of a [global] cooling,” she surmised.

• References
• Koren, Ilan, Y.J. Kaufman, L.A. Remer, and J.V. Martins, 2004: “Measurement of the effect of biomass burning aerosol on inhibition of cloud formation over the Amazon,” Science, 303, 1342-45.
• Hansen, James, M. Sato, A. Lacis, and R. Ruedy, 1997: “The missing climate forcing,” Phil. Trans. Royal Soc. London B, 352, 231-40.
• Ackerman, Andy S., O.B. Toon, D.E. Stevens, A.J. Heymsfield, V. Ramanathan, and E.J. Welton, 2000: “Reduction of Tropical Cloudiness by Soot,” Science, 288, 1042-47.

 When Smoke Kills Clouds

Aerosols supress clouds in locations around the globe, although the effects have not yet been thoroughly studied. This image shows smoke from fires in Alaska suppressing clouds along the border between the United States and Canada. The animation shows clouds disappearing as smoke moves into the region. These images were acquired by the NOAA GOES-West satellite on July 17, 2004.

• animations:
• small (1.2 MB QuickTime)
• large (5.2 MB QuickTime)

(Image and animations courtesy University of Wisconsin Space Science and Engineering Center)