Importance of tropospheric volcanic aerosol for indirect radiative forcing of climate 1School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
16 Aug 2012
2National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
Received: 12 Mar 2012 – Published in Atmos. Chem. Phys. Discuss.: 21 Mar 2012 Abstract. Observations and models have shown that continuously degassing volcanoes have
a potentially large effect on the natural background aerosol loading and the
radiative state of the atmosphere. We use a global aerosol microphysics model
to quantify the impact of these volcanic emissions on the cloud albedo
radiative forcing under pre-industrial (PI) and present-day (PD) conditions.
We find that volcanic degassing increases global annual mean cloud droplet
number concentrations by 40% under PI conditions, but by only 10%
under PD conditions. Consequently, volcanic degassing causes a global annual
mean cloud albedo effect of −1.06 W m−2 in the PI era but only
−0.56 W m−2 in the PD era. This non-equal effect is explained partly
by the lower background aerosol concentrations in the PI era, but also
because more aerosol particles are produced per unit of volcanic sulphur
emission in the PI atmosphere. The higher sensitivity of the PI atmosphere to
volcanic emissions has an important consequence for the anthropogenic cloud
radiative forcing because the large uncertainty in volcanic emissions
translates into an uncertainty in the PI baseline cloud radiative state.
Assuming a −50/+100% uncertainty range in the volcanic sulphur flux,
we estimate the annual mean anthropogenic cloud albedo forcing to lie between
−1.16 W m−2 and −0.86 W m−2. Therefore, the volcanically
induced uncertainty in the PI baseline cloud radiative state substantially
adds to the already large uncertainty in the magnitude of the indirect
radiative forcing of climate.
Revised: 26 Jul 2012 – Accepted: 08 Aug 2012 – Published: 16 Aug 2012
Citation: Schmidt, A., Carslaw, K. S., Mann, G. W., Rap, A., Pringle, K. J., Spracklen, D. V., Wilson, M., and Forster, P. M.: Importance of tropospheric volcanic aerosol for indirect radiative forcing of climate, Atmos. Chem. Phys., 12, 7321-7339, doi:10.5194/acp-12-7321-2012, 2012.