IPCC Expert Meeting on Geoengineering

Annex 4: Poster Abstracts
Sea spray geoengineering: global model simulations with explicit aerosol
microphysics
Hannele Korhonen 1 *, Antti-Ilari Partanen 1 , Harri Kokkola 1 , Sami Romakkaniemi 2 , Veli-Matti Kerminen 3 , Ari
Laaksonen 2,3 and Kari E. J. Lehtinen 1,2
1
Finnish Meteorological Institute, Kuopio Unit, Finland
2
University of Eastern Finland, Department of Applied Physics, Finland
3
Finnish Meteorological Institute, Climate Change, Finland
One much discussed technique for solar radiation management (SRM) is to use artificial sea spray emissions from wind
powered vessels to increase the concentration of submicron sea salt particles in the marine boundary layer (Latham, 1990).
It has been hypothesized that these artificially emitted aerosol particles could act as cloud condensation nuclei (CCN) and
thus increase the cloud droplet number concentration (CDNC) in the marine stratocumulus clouds. This in turn would lead,
at least in theory, to a higher cloud albedo and thus planetary cooling.
Using a detailed aerosol microphysics model (GLOMAP) within a global chemical transport model, we have recently shown
that artificial sea spray emissions are unlikely to lead to homogeneous CDNC fields over vast oceanic areas (Korhonen et
al., 2010). Our results also indicated that the relative enhancement of CDNC in low-level clouds is likely to be lower than
assumed in previous climate model studies.
We have now repeated a similar set of simulations in ECHAM-HAM aerosol-climate model in order to quantify the direct
and indirect effects from sea spray injections as well as to study the effect of the particle injection size on the resulting
planetary cooling (Partanen et al., 2011). ECHAM-HAM includes an explicit and prognostic calculation of cloud
microphysics and interaction of aerosol particles with clouds. We assume a wind-speed dependent number flux for the
injected particle population and follow its transport and transformation due to microphysical processes in the atmosphere.
This additional flux is assumed either over all the oceans or in three optimized regions covering 3.3% of the planet’s
surface area.
Applying the baseline scenario (i.e. injected particle size 250 nm) in the optimized regions, we predict a global mean
radiative flux perturbation (RFP) of -0.8 Wm -2 . This compares well with an earlier published estimate of -0.97 Wm -2 which
assumed a fixed prescribed CDNC of 375 cm -3 in the geoengineered regions (Jones et al., 2009). In our simulations the
mean regional CDNC varies between 194 and 286 cm -3 but cloud cover increases by 2-5 percentage points. It is noteworthy
that both the absolute CDNC values as well as their relative changes (74-80%) are clearly higher than predicted in an
earlier study using similar emission fluxes. In the ECHAM simulations, multiples of the baseline sea spray flux cause almost
a linear increase in CDNC but the RFP is clearly sublinear (global mean RFP with 5 times the baseline flux is -2.2 Wm -2 ).
Since the three optimal geoengineering regions are characterized with persistent stratocumulus decks, inside them
practically all of the radiative effect originates from aerosol indirect effects. However, the direct effect can be significant
outside these regions: when all oceanic regions are seeded, the direct effect is about 65% of the aerosol indirect effects.
For a constant volume emission flux of sea spray, the size at which the individual particles are injected becomes very
important. Reducing the injection size from 250 nm to 100 nm, which is typically still large enough for cloud activation in
marine boundary layer, increases the global mean RFP in the run of optimized regions to -2.1 Wm -2 . On the other hand,
injection at 500 nm has only very minor effects on CDNC due to the low number flux (13% of the baseline flux) and
produces roughly the same direct forcing as the baseline simulation.
The presented work is part of an interdisciplinary project “Aerosol intervention technologies to cool the climate: costs,
benefits, side effects and governance” funded by the Academy of Finland. In addition to cloud seeding with artificial sea
spray emissions, the project focuses on the climate and other environmental effects stratospheric particle injections and
reduction of black carbon (BC) emissions. We investigate also how the introduction of geoengineering alternatives may
lead to reframing of global and national climate policies as well as how the existing legal and political frameworks will
affect the pace and direction of the package of admissible alternatives.
<strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 50