A Climatologically Significant Aerosol Longwave Indirect Effect In The Arctic

A paper early in 2006 in Nature by Dan Lubin and Andrew M. Vogelmann entitled

“A climatologically significant aerosol longwave indirect effect in the Arctic” ; Vol 439 26 January 2006 doi:10.1038/nature04449 [pages 453-456],

presents further evidence on the complexity of climate forcings in the Arctic (thank you to Marcia Wyatt for alerting me to this paper!).

The abstract reads,

“The warming of Arctic climate and decreases in sea ice thickness and extent observed over recent decades are believed to result from increased direct greenhouse gas forcing, changes in atmospheric dynamics having anthropogenic origin, and important positive reinforcements including ice–albedo and cloud–radiation feedbacks. The importance of cloud–radiation interactions is being investigated through advanced instrumentation deployed in the high Arctic since 1997. These studies have established that clouds, via the dominance of longwave radiation, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer9. The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes. Here we use multisensor radiometric data to show that enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the ‘first indirect’ effect. Under frequently occurring cloud types we find that this leads to an increase of an average 3.4 watts per square metre in the surface longwave fluxes. This is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant.”

The states at the end,

“In conclusion, we provide observational evidence that the first aerosol indirect effect operates in low, optically thin, single-layered Arctic clouds with a concomitant increase in the downwelling longwave flux. The cloud amount during the Arctic spring generally exceeds 80%, which implies that the observed longwave enhancement has climatological significance.”

The conclusion that this aerosol cloud effect “…is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant”,

along with the finding from

Hansen, J., and L. Nazarenko 2004. Soot climate forcing via snow and ice albedos. Proc. Natl. Acad. Sci. 101, 423-428, doi:10.1073/pnas.2237157100,

that the albedo effect of soot on snow and ice can result in a radiative forcing in the Northern Hemisphere of +0.3 W per meter squared, provides yet another example of why a focus on the radiative forcing of CO2 alone is inaccurately narrow.

In the IPCC report that is being released this Friday, whether or not this peer reviewed recognition of the diversity of human climate forcings in the Arctic this will be one benchmark to assess whether or not the IPCC assessment is an honest balanced presentation of climate science, or, as it has been in the past, a document to be used for political advocacy.