Evidence of Unrecognized Complexity in the Direct Aerosol Climate Forcing

A new paper has been communicated to the climate community by Meinrat (Andi) Andreae. The title of the paper is
“Black carbon or brown carbon? The nature of light-absorbing carbonaceousaerosols” with authors M. O. Andreae and A. Gelencsér and appears in Atmos. Chem. Phys. Discuss., 6, 3419–3463, 2006

The abstract reads,

” Although the definition and measurement methods of atmospheric “black carbonâ€? (“BCâ€?) have long been subjects of scientific controversy, the recent discovery of light absorbing carbon that is not black (“brown carbon, Cbrownâ€?) makes it imperative to re-assess and redefine the components that make up light-absorbing carbonaceous matter (LAC) in the atmosphere. Evidence for the atmospheric presence of Cbrown comes directly from aerosol absorption measurements near specific combustion sources, from observations of spectral properties of water extracts of continental aerosol, from laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that these species may severely bias measurements of “BCâ€? and “ECâ€? over vast parts of the troposphere, where mass concentration of Cbrown is high relative to that of combustion soot. We also imply that due to the strongly skewed absorption of Cbrown towards the UV, single-wavelength light absorption measurements may not be adequate for the assessment of absorption of solar radiation in the troposphere. The possible consequences of these effects on our understanding of tropospheric processes are discussed.”

The conclusion is particularly blunt. It reads,

“We conclude that, at the present state of the art, the uncertainties and potential biases in EC and BC measurements are large, and that there is a poor correlation between measured ECa or BCe values and atmospheric light absorption. If these errors are systematic, and unless there is some degree of fortuitous compensation between upward and downward biases, they could call into question even the sign of the direct forcing of anthropogenic aerosols on climate!”

(the specific definitions of “apparent ECâ€? (ECa) and “equivalent BCâ€? (BCe) are given in the paper).

This is a remarkable and important admission of how little we actually know concerning the human forcing of climate variability and change. If the global average direct radiative forcing from aerosols is so poorly understood, than the effect of the spatial variations of this forcing on planetary weather patterns, which respond to the spatial gradients of diabatic heating (see), are similarly very poorly understood.

This work further illustrates why a focus on the radiative effect of CO2 as the dominant human climate forcing is so inadequate.

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