IPCC Expert Meeting on Geoengineering

Annex 4: Poster Abstracts
Tropospheric sulfate burdens as a consequence of stratospheric sulfate
geoengineering
Jason M. English 1,2 , Brian Toon 1,2 *, Michael J. Mills 3
1
Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA
2
Laboratory for Atmospheric and Space Physics, Boulder, CO, USA
3
NCAR Earth System Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
Stratospheric sulfur injection is a leading geoengineering idea to counteract climate change. Sulfate aerosols can cool the
lower atmosphere by scattering and absorbing incoming shortwave radiation. The stratosphere is a more efficient location
than the troposphere due to longer aerosol lifetime, but has unintended consequences such as changes to heterogeneous
chemistry and ozone destruction.
Volcanoes act as natural tests to this idea; after major volcanic eruptions, reduced surface temperatures and reductions in
the stratospheric ozone column are observed. However, little is known about the efficacy and consequences of continuous
stratospheric sulfur injection. Simulations with prescribed size distributions have found a generally linear association
between injection rate, sulfate burden, and temperature reduction (e.g. Rasch et al., 2008; Robock et al., 2008). The first
microphysical aerosol simulations were conducted by Heckendorn et al. (2009) using a 2d aerosol model, and found
geoengineered aerosols to grow much larger than previously assumed, resulting in a significantly lower lifetime and
temperature change efficacy. All previous work has focused on the stratosphere and the earth’s surface, while impacts in
the middle and upper troposphere have not been well studied. We present geoengineering simulations using a 3d
microphysical aerosol model fully coupled with a 3d general circulation model and full sulfur chemistry. We compare our
stratospheric burden results to Heckendorn et al.(2009), and conduct a detailed analysis of the upper tropospheric sulfate
burden.
We use the WACCM3/CARMA model that includes three-dimensional treatment of SO 2 and OCS surface emissions, 63-
species chemistry, nucleation, growth, coagulation, and wet deposition (English et al., 2011). The model is run using 4 x 5
degrees horizontal resolution, 66 vertical levels, and 42 dry sulfuric acid aerosol bins. We use a numerically-efficient form of
binary homogeneous nucleation (Zhao and Turco, 1995), and enhanced coagulation due to Van der Waal’s forces (Chan
and Mozurkewich, 2005). We specify four geoengineering scenarios (1, 2, 5, and 10 Tg sulfur annual injection) of SO 2 at
the 50 mb level, between 4 degrees S and 4 degrees N, across all longitudes (similar to Heckendorn et al., 2009).
Simulations are run for 5 years, with the 5th year analyzed.
We find the relationship between SO 2 injection rate and stratospheric mass burden is highly non-linear, with reduced
efficacy at higher injection rates (Figure A.4.5). To achieve 6 Tg S burden, 10 Tg S/yr injection rate is required, which is five
times more than simulations that assumed prescribed size distributions (Rasch et al., 2008), and very close to that
predicted by the only other microphysical study done (Heckendorn et al., 2009). The burden discrepancy is attributed to the
particles growing to larger sizes (1 micrometer radius) than previously assumed, which have faster fall velocities and shorter
stratospheric lifetimes. (Additionally, larger particles are less effective at scattering shortwave radiation). Additionally, we
find a significantly increased sulfate burden across large regions of the troposphere. Fully one-third of the sulfate burden is
in the troposphere, with tropospheric enhancements observed in many regions, particularly the high latitude upper
troposphere. For the 10 Tg simulation, we find the sulfate mass burden between the tropopause and 100 hPa below the
tropopause is 30 times higher and the next 100 hPa region down is 10 times higher than the unperturbed scenario. The
entire tropospheric burden is five times higher than normal. This enhancement of sulfate aerosols can have implications for
tropospheric cloud properties, radiative forcing, and upper tropospheric chemistry. Additionally, increased acid deposition
occurs in mid- and high- latitudes, as has been previously noted (e.g. Kravitz et al., 2009).
These results highlight numerous limitations and consequences of stratospheric sulfate geoengineering. In addition to
limited efficacy, ozone destruction, and surface acid deposition that have been published previously, we find a significantly
enhanced upper tropospheric sulfate burden which may alter tropospheric clouds, chemistry, and radiative forcing. We
recommend geoengineering ideas to be studied in more detail before they are seriously considered as climate intervention
options.
<strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 62