Download GEOENGINEERING WITH SEA SPRAY: AEROSOL DIRECT AND

GEOENGINEERING WITH SEA SPRAY: AEROSOL DIRECT AND INDIRECT
EFFECTS
A.-I. PARTANEN1,2 , H. KOKKOLA2 ,S. ROMAKKANIEMI1 , S. DOBBIE3 , K. CARSLAW3 and
H. KORHONEN1,2
1
Department of Physics and Mathematics, University of Eastern Finland, Kuopio Campus, 70211
Kuopio, Finland.
2 Finnish
3
Meteorological Institute, Kuopio Unit, 70211, Kuopio, Finland.
School of Earth and Environment, University of Leeds, LS2 9JT, Leeds, United Kingdom.
Keywords: GEOENGINEERING, STRATOCUMULUS CLOUDS, GLOBAL MODELING,
AEROSOL FORCING.
INTRODUCTION
In recent years, altering the climate system deliberately has been a target of extensive research.
One of these geoengineering ideas is to seed low- level marine clouds with sea spray aerosol in order
to increase cloud droplet concentration (CDNC) (Latham, 1990). Increased CDNC would lead to
higher cloud albedo (first indirect effect, Twomey 1997), which in turn would affect Earth’s radiative
balance and cool the climate. Previous model studies have shown that that increased CDNC would
indeed have a major effect on Earth’s radiative balance (Latham et al., 2008; Jones et al., 2008;
Rasch et al., 2009). In these studies a fixed CDNC in the modified regions was assumed. Explicit
calculation of aerosol microphysics and cloud activation with a chemical transport model shows
that suffcient and homogeneous increase in CDNC is hard to achieve with previously suggested sea
spray fluxes (Korhonen et al., 2010).
Our goal is to get a better view on aerosol-cloud interactions in order to get better estimates on
radiative forcing caused by this geoengineering method. We consider also aerosol direct effect which
is prominent part of the forcing of the natural sea salt aerosol (Ayash et al., 2008). It has been
neglected by all previous studies on sea spray geoengineering.
METHODS
In the first stage of this study we conducted global simulations with aerosol-climate model ECHAM5HAM (Stier et al., 2005) to estimate the change in CDNC and radiative forcing caused by this
geoengineering method. Preliminary runs were done using horizontal resolution of T21 and Lin
and Leaitch (1997) parametrization for cloud droplet activation. Future runs will be done with
higher resolution and physically more realistic cloud droplet activation parametrization. In these
global simulations we modified four marine regions covering about 18 % of the ocean surface. They
are off the coasts of the United States, Namibia, Chile and Australia. These four regions are shown
in Figure 1. We performed five 10-year-long geoengineering runs with varying injection rate (baseline and its multiples) of sea salt particles. These wind-speed-dependent injection rates use the
source function by Korhonen et al.(2010) and fall roughly into the range of previous estimates of
the required sea salt flux (Latham, 2002).
RESULTS
Results from global simulations show that sea salt is not distributed homogeneously over modified
regions. This is due to both transport and spatially varying sea salt flux. Part of the injected sea salt
is transported thousands kilometers away from the source. To achieve homogeneous distribution
of sea salt over a region would require extremely careful planning when placing the vessels and it
migth prove to be impossible.
Change in CDNC is much lower than for example Latham et al. (2008) assumed. This is consistent
with the results by Korhonen et al. (2010). Increase in CDNC at cloud top over the modified
regions is only about 10 % on average in the baseline run. Doubling of the CDNC requires a
quadruple of the baseline flux. Our results show also that the change in CDNC is not spatially
homogeneous. Neither absolute or relative change of CDNC at cloud top follows the pattern of
the distribution of the additional sea salt mass. This shows that local meteorological conditions
play an important role in this study. There are also differences between the modified regions. For
example there is a weak response on the coast of Australia mainly due to the fact that clouds reside
on higher levels there and the majority of the particles do not reach the cloud top.
We calculate total radiative forcing as radiative flux perturbation (RFP) by comparing 10-year
mean values of the top of the atmosphere total radiation from geoengineered climate with the
control simulation. This allows for taking aerosol indirect effect into account. Results from the
run with double the baseline flux of sea salt are shown in Figure 1. The most negative RFPs are
situated over areas with large relative increase in CDNC at cloud top which is a very expected
result. Because direct effect from the sea salt aerosol also affects, RFPs cannot be predicted from
change in CDNC alone. Global mean of the RFP in the baseline run was -0.44 Wm−2 , which is
about 17 % of the radiative forcing caused by increased concentrations of long-lived greenhouse
gases since preindustrial times (Forster et al., 2007).
Figure 1: Total radiative flux perturbation in a simulation with double the baseline flux of sea salt.
Red lines indicate the geoengineered regions.
Sea salt aerosol direct effect is calculated comparing aerosol total direct effect between geoengineering and control simulation. We consider only the short-wave part of the direct effect. It was
significant part of the total forcing. In the baseline run it contributed more than 70 % to the global
mean total forcing.
CONCLUSIONS
These results show that aerosol microphysics and cloud droplet activation must be considered when
global simulations of marine stratocumulus geoengineering are carried out. Although original idea
by Latham (1990) was to exploit mainly the first aerosol indirect effect, direct effect of the sea
spray aerosol has an important role too in our results. We will improve the reliability of these
result by using a physically based cloud activation parametrization and higher model resolution.
To overcome the constraints of large grid-box size in a global model we will conduct simulations
with LEM. These future studies will give more insight about how this geoengineering scheme affects
individual clouds and their radiative properties.
ACKNOWLEDGEMENTS
This work was supported by the Nessling Foundation under grant 2009152.
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