New Paper “A Model Investigation Of Aerosol-Induced Changes In Tropical Circulation” By Ming and Ramaswamy 2011

In our paper

Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974

we reported that the heterogeneous character of aerosol forcing (in terms of the horizontal pressure gradient force) of atmospheric circulations was significantly larger than from the more homogeneous forcing from added greenhouse gases.

There is an important new model process study paper which confirms our finding, and examines this effect in more detail. It is

Yi Ming and V. Ramaswamy, 2011. A Model Investigation of Aerosol-induced Changes in Tropical Circulation. J of Climate (in press)

with the abstract (highlight added)

We study how anthropogenic aerosols, alone or in conjunction with radiatively active gases, affect the tropical circulation with an atmosphere/mixed layer ocean general circulation model. Aerosol-induced cooling gives rise to a substantial increase in the overall strength of the tropical circulation, a robust outcome consistent with a thermodynamical scaling argument. Owing to the interhemispheric asymmetry in aerosol forcing, the zonal-mean and zonally asymmetrical components of the tropical circulation respond differently. The Hadley circulation weakens in the Northern Hemisphere, but strengthens in the Southern Hemisphere. The resulting northward cross-equatorial moist static energy flux compensates partly for the aerosol radiative cooling in the Northern Hemisphere. In contrast, the less restricted zonally asymmetrical circulation does not show sensitivity to the spatial structure of aerosols, and strengthens in both hemispheres. Our results also point to the possible role of aerosols in driving the observed reduction in the equatorial sea level pressure gradient. These circulation changes have profound implications for the hydrological cycle. We find that aerosols alone make the subtropical dry zones in both hemispheres wetter, as the local hydrological response is controlled thermodynamically by atmospheric moisture content. The deep tropical rainfall undergoes a dynamically induced southward shift, a robust pattern consistent with the adjustments in the zonal-mean circulation and in the meridional moist static energy transport. Less certain is the magnitude of the shift. The nonlinearity exhibited by the combined hydrological response to aerosols and radiatively active gases is dynamical in nature.”

The paper starts with the text

“Although much remains to be done to gain a more definitive understanding of the climate effects of aerosols (radiative and microphysical alike) (e.g., Forster et al. 2007), it has been widely accepted that aerosol cooling “masked”, on the global scale, a considerable fraction of greenhouse gas warming since the preindustrial times (e.g., Hegerl et al. 2007). Unlike well mixed greenhouse gases, the spatial distributions of aerosols are highly non-uniform owing to inhomogeneous emission sources and short lifetimes (on the order of days).This basic recognition leads one to speculate that aerosols may be more capable of altering atmosphericand oceanic circulation, especially on the regional scale, than greenhouse gases.”

The excellent Ming and Ramaswamy paper also support our conclusion in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union

that the IPCC hypothesis

“Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and are dominated by the emissions into the atmosphere of greenhouse gases, the most important of which is CO2. The adverse impact of these gases on regional and global climate constitutes the primary climate issue for the coming decades”

should be rejected. The only hypothesis that is scientifically robust is

Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first-order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades.

As we wrote in our paper

“In addition to greenhouse gas emissions, other first-order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008]…..”

With the new Ming and Ramaswamy paper, we are even more confident of our findings reported in the Pielke et al 2009 EOS paper.

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