New Paper “Recent Northern Hemisphere Tropical Expansion Primarily Driven By Black Carbon And Tropospheric Ozone” By Allen Et Al 2012

source of image Allen et al 2012 

I was alerted to a new paper [h/t Jos de Laat] that provides information on the effect of non-CO2 climate forcings. This paper reinforces the conclusion in our paper

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.

where we wrote [highlight added]

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], the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer.

The new paper is

Allen, Robert J., Steven C. Sherwood, Joel R. Norris and Charles S. Zender: 2012: Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone. Nature 485,350–354(17 May 2012 )doi:10.1038/nature11097Received 16 December 2011 Accepted 29 March 2012 Published online 16 May 2012

with the abstract

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents—including black carbon aerosols and tropospheric ozone—are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.

Extracts from the Allen et al paper read

Our analysis strongly suggests that recent Northern Hemisphere tropical expansion is driven mainly by black carbon and tropospheric ozone, with greenhouse gases playing a smaller part. Compared to observations, the magnitude of the simulated change is underestimated. This could be related to the aforementioned caveats with the observations, model deficiencies, or deficient black carbon aerosol forcing.

The upper end of the observed range of black carbon would reconcile the 2.5 factor shortfall in all-forcings expansion relative to observations, if responses varied linearly with forcing. However, our results show that responses are not always linear, and it still seems likely that models are insufficiently sensitive to these forcings. As long as this insensitivity applies equally to different forcings, our results point to anthropogenic pollutants other than CO2 rather than global warming as the culprit in recent Northern Hemisphere tropical expansion.

The Allen et al paper also provides further support for 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

where we reported that

This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate.

The Allen et al explanations are based on modeling. However, it is refreshing to finally

  •  see a broadening of the assessment of first order climate forcings beyond CO2

and

  • that a global average surface temperature is an inadequate metric to assess the role of humans on the climate system. Heterogenous climate forcings are particularly important in affecting the climate.

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