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The Impact of Atmospheric Aerosols on North Atlantic Climate
Booth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N. 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature 484: 228-232.

The authors write that "a number of studies have provided evidence that aerosols can influence long-term changes in sea surface temperatures," citing Mann and Emanuel (2006) and Evan et al. (2009); but they say that "climate models have so far failed to reproduce these interactions," citing Knight (2009) and Ting et al. (2009). And they consequently note, as they phrase it, that "the role of aerosols in decadal variability remains unclear."

What was done
Hoping to bring some much needed clarity to the subject, Booth et al. used the Hadley Centre Global Environmental Model version 2 (HadGEM2-ES) - which is a next-generation Climate Model Intercomparison Project phase 5 (CMIP5) model - to determine whether older CMIP3 models "contained the complexity necessary to represent a forced Atlantic Multidecadal Oscillation."

What was learned
The five researchers were able to demonstrate, in their words, that "aerosol emissions and periods of volcanic activity explain 76% of the simulated multidecadal variance in detrended 1860-2005 North Atlantic sea surface temperatures," and that "after 1950, simulated variability is within observational estimates," while their estimates for 1910-1940 "capture twice the warming of previous generation models," although they still "do not explain the entire observed trend." Put another way, they state that "mechanistically, we find that inclusion of aerosol-cloud microphysical effects, which were included in few previous multimodel ensembles, dominates the magnitude (80%) and the spatial pattern of the total surface aerosol forcing in the North Atlantic."

What it means
Booth et al. conclude that "one of the reasons why the role of aerosols in driving multidecadal variability has not previously been identified" is due to the fact that "although all the CMIP3 models represented the direct effect of aerosols on shortwave radiation, most omitted or only partly represented the indirect aerosol effects," citing Chang et al. (2011). And as a result of their findings, they thus state that "we need to reassess the current attribution to natural ocean variability of a number of prominent past climate impacts linked to North Atlantic sea surface temperatures."

In similar fashion, we suggest that climatologists need to reassess the attribution of the post-Little Ice Age warming of the world to concomitant anthropogenic CO2 emissions, due to the possible warming effects of still other as-yet-unappreciated phenomena that are either "omitted or only partly represented" in current state-of-the-art climate models. Some of these phenomena that could cause warming to occur may be associated with things transpiring on (or within) the sun; while other phenomena that may thwart or significantly reduce the warming effect of rising atmospheric CO2 concentrations may be associated with a variety of biological responses of both marine and terrestrial vegetation to atmospheric CO2 enrichment, as well as to warming itself. For more on these important subjects, see Aerosols and Feedback Factors (Biophysical) in our Subject Index.

Chang, C.Y., Chiang, J.C.H., Wehner, M.F. Friedman, A. and Ruedy, R. 2011. Sulfate aerosol control of tropical Atlantic climate over the 20th century. Journal of Climate 24: 2540-2555.

Evan, A.T., Vimont, D.J., Heidinger, A.K., Kossin, J.P. and Bennartz, R. 2009. The role of aerosols in the evolution of tropical North Atlantic Ocean temperature anomalies. Science 324: 778-781.

Knight, J.R. 2009. The Atlantic Multidecadal Oscillation inferred from the forced climate response in coupled general circulation models. Journal of Climate 22: 1610-1625.

Mann, M.E. and Emanuel, K.A. 2006. Atlantic hurricane trends linked to climate change. EOS, Transactions, American Geophysical Union 87: 233-244.

Ting, M., Kushnir, Y., Seager, R. and Li, C. 2009. Forced and internal twentieth-century SST trends in the North Atlantic. Journal of Climate 22: 1469-1481.

Reviewed 17 October 2012