A New Paper On The Role Of Biomass Burning On The Climate System – Tosca Et Al 2009

Papers which document human climate forcings other than CO2 continue to be published.

Today I was alerted to a paper that demonstrates the major role of biomass burning on regional climate. The paper is

M. G. Tosca, J. T. Randerson, C. S. Zender, M. G. Flanner, and P. J. Rasch, 2009: Do biomass burning aerosols intensify drought in equatorial Asia during El Nino? Manuscript prepared for Atmos. Chem. Phys. with version 3.0 of the LATEX class copernicus.cls. Date: 6 August 2009

The abstract reads

“During El Nino years, fires in tropical forests and peatlands in equatorial Asia create large regional smoke clouds. We characterized the sensitivity of these clouds to regional drought, and we investigated their effects on climate by using an atmospheric general circulation model. Satellite observations during 2000-2006 indicated that El Ni˜no-induced regional drought led to increases in fire emissions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investigate how climate responded to this forcing. We conducted two 30 year simulations in which monthly fire emissions were prescribed for either a high (El Ni˜no; 1997) or low (La Ni˜na; 2000) fire year using a satellite derived time series of fire emissions. Our simulations included the direct and semi-direct effects of aerosols on the radiation budget within the model. Fire aerosols reduced net shortwave radiation at the surface during August–October by 19.1 ± 12.9 Wm−2 (10%) in a region that encompassed most of Sumatra and Borneo (90E–120E, 5S–5N). The reductions in net radiation cooled sea surface temperatures (SSTs) and land surface temperatures by 0.5 ± 0.3 and 0.4 ± 0.2C during these months. Tropospheric heating from black carbon (BC) absorption averaged 20.5 ± 9.3 Wm−2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and increased atmospheric heating reduced regional precipitation by 0.9 ± 0.6 mmd−1 (10%). The vulnerability of ecosystems to fire was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satellite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensifies drought stress during El Nino.”

The finding that the tropospheric heating from black carbon absorption averaged 20.5 ± 9.3 Wm−2  is on the same order of the heating rates that we found 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.

As we wrote in our paper

“….. 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 new Tosca et al 2009 further confirms our conclusion on the major role of human aerosols in altering regional climate.

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