GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 210–215, doi:10.1029/2012GL054286, 2013
The sign of the radiative forcing from marine cloud brightening
depends on both particle size and injection amount
K. Alterskjær1 and J. E. Kristjánsson1
Received 18 October 2012; revised 12 December 2012; accepted 22 December 2012; published 16 January 2013.
[1] Marine cloud brightening (MCB) is a proposed
technique to limit global warming through injections of sea
spray into the marine boundary layer. Using the
Norwegian Earth System Model, the sensitivity of MCB to
sea salt amount and particle size was studied by running a
set of simulations in which Aitken (re = 0.04 mm),
accumulation (re = 0.22 mm), or coarse (re = 2.46 mm) mode
sea salt emissions were increased uniformly by 1011 to
108 kg m2 s1. As desired, accumulation mode particles
had a negative radiative effect of down to 3.3 W m2.
Conversely, for Aitken mode particles, injections of
1010 kg m2 s1 or greater led to a positive forcing of up
to 8.4 W m2, caused by a strong competition effect
combined with the high critical supersaturation of Aitken
mode sea salt. The coarse mode particles gave a positive
forcing of up to 1.2 W m2 because of a decrease in
activation of background aerosols. Sensitivity experiments
show that the competition effect dominated our results. MCB
may have a cooling effect, but if the wrong size or injection
amount is used, our simulations show a warming effect on
the climate system. Citation: Alterskjær, K., and J. E.
Kristjánsson (2013), The sign of the radiative forcing from marine
cloud brightening depends on both particle size and injection amount,
Geophys. Res. Lett., 40, 210–215, doi: 10.1029/2012GL054286.
1. Introduction
[2] Deliberate engineered cooling of the global climate
has received increased scientific interest over the last decade
as mitigation strategies to limit global warming are yet to be
of significance. One climate engineering strategy involves
enhancing the albedo of marine clouds and thus increasing
the reflection of solar radiation from the Earth-atmosphere
system [Latham, 1990]. The idea is that spraying sea water
into the marine boundary layer will increase the number of
sea salt particles that ascend into overlying clouds and
increase their albedo through the aerosol indirect effect
[Twomey, 1974]. This may significantly affect the global
radiation budget because of the low albedo of the underlying
ocean surface in the subtropics, where extensive low clouds
are found.
All Supporting Information may be found in the online version of this
article.
1
Department of Geosciences, Meteorology and Oceanography Section,
University of Oslo, Oslo, Norway.
Corresponding author: K. Alterskjær, Department of Geosciences,
Meteorology and Oceanography Section, University of Oslo, PO Box 1022,
0315 Oslo, Norway. (karialt@geo.uio.no)
©2012. American Geophysical Union. All Rights Reserved.
0094-8276/13/2012GL054286
[3] Early estimates of the global radiative effect of marine
cloud brightening (MCB) assumed a certain change in cloud
droplet number concentration (CDNC) in seeded clouds and
found that MCB could wholly or partially cancel the positive
forcing associated with a doubling of CO2 from preindustrial
times [Latham et al., 2008; Jones et al., 2009; Rasch et al.,
2009]. Korhonen et al. [2010] used a global aerosol
transport model, while Pringle et al. [2012] used three
independent global aerosol models and a box model and
Wang et al. [2011] used a cloud-system-resolving model to
investigate what changes in CDNC were achievable from
sea salt injections, but these studies did not include estimates
of the radiative effect of the cloud seeding.
[4] To our knowledge only three global studies have so far
included estimates of the forcing from MCB using fully
prognostic treatments of sea salt. Jones and Haywood
[2012] studied the radiative impact and climate effects of
wind speed-dependent MCB while Alterskjær et al. [2012]
injected sea salt uniformly over the ocean to study the
geographical distribution of clouds susceptible to seeding.
Partanen et al. [2012] used wind speed-dependent
emissions of sea salt and estimated that injecting 20.6 Tg
yr1into the most sensitive stratocumulus regions led to a
forcing of 0.8 W m2. Decreasing the size of the particles
by 60% or multiplying the injections by five led to a forcing
of 2.1 and 2.2 W m2, respectively.
[5] In this study, we have investigated further the
importance of particle size and injection strength. This is
necessary to understand the outcome of a potential seeding
measure—What sea salt size category would be most
effective and therefore least costly? Is the same size category
most effective for all injection mass fluxes? And is there a
simple linear relation between injection strength and forcing?
We have used the Norwegian Earth System Model (NorESM)
[Bentsen et al., 2012] to investigate the global sensitivity to
particle size and injection strength. We have looked at how
injections of sea salt affect the cloud radiative properties and
how the increased competition effect resulting from the added
sea salt affects our global estimates. In section 2 we describe
the model used and the experimental design, while in section
3 we go through the results of our experiments, including
a presentation and discussion of sensitivity experiments
performed to test the robustness of our findings. We summarize and conclude in section 4.
2. Model and Methods
2.1. NorESM
[6] Simulations were performed using the NorESM, which
is based on the NCAR (National Center for Atmospheric
Research) CCSM4 (Community Climate System Model
version 4), but includes new treatments of clouds, aerosols,
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ALTERSKJÆR AND KRISTJÁNSSON: MARINE CLOUD BRIGHTENING
aerosol-radiation and aerosol-cloud interactions and chemistry
[Kirkevåg et al., 2012], along with a new ocean model
component. The aerosol module accounts for prognostic sea
salt, sulfate (SO4), particulate organic matter, black carbon,
and mineral dust as well as two gaseous aerosol precursors
producing sulfate (DMS and SO2). The model uses the
Mårtensson et al. [2003] scheme for wind speed and
temperature-dependent sea salt emissions [Struthers et al.,
2011]. It includes sea salt particles with dry number modal
radii of 0.022 mm (Aitken mode), 0.13 mm (accumulation
mode), and 0.74 mm (coarse mode) and geometric standard
deviations of 1.59, 1.59, and 2.0, respectively, corresponding
to dry effective radii of 0.04, 0.22, and 2.46 mm.
[7] The aerosol indirect effect is accounted for as described
in Hoose et al. [2009] and has a magnitude of 0.91 W m2 in
the current set up. The model uses the Abdul-Razzak and Ghan
[2000] cloud droplet nucleation scheme and parametrized
updraft velocities following Morrison and Gettelman [2008],
with annually averaged in-cloud velocities ranging between
about 10 and 100 cm s1 at a model hybrid level at ~945 hPa
over ocean (see Supporting Information). For an overview of
typically observed updrafts we refer the reader to Pringle
et al. [2012]. Model cloud properties were compared to
satellite retrievals in, e.g., Alterskjær et al. [2012] and Jiang
et al. [2012] indicating a general underestimation of marine
cloud CDNC and an overestimation of the simulated cloud
liquid water path (LWP). The low CDNC makes the model
more susceptible to MCB, while the high LWP has the
opposite effect. Alterskjær et al. [2012] also showed that the
simulated cloud fraction below 700 hPa is generally overestimated, except for a slight underestimation in the subtropical
stratocumulus regions.
2.2. Experimental Design
[8] Cloud brightening simulations were performed by
artificially increasing the emissions of each of the three fully
prognostic NorESM sea salt modes, meaning that the injected
sea salt was treated in a similar manner to natural sea salt and
that no explicit assumptions were made on their ability to act
as cloud condensation nuclei (CCN). The emissions were
uniform and confined between 30 S and 30 N based on the
findings of Alterskjær et al. [2012]. Most earlier studies have
used wind speed-dependent emissions due to the experimental
design proposed by Salter et al. [2008]. For simplicity, this
dependency is not included in our idealized study as we
believe that the emission technology may change prior to
possible implementation. Injection fluxes ranged from 1011
to 108 kg m2 s1 (Table 1), which corresponded to a global
sea salt emission increase of from 0.9% to 913%. A list of all
simulations performed is shown in Table 1.
[9] The model was run offline, meaning that the meteorological evolution remains unchanged between simulations, so that
the simulated change in radiative forcing is due to indirect
effects only. The control run uses year 2000 greenhouse gas
concentrations and year 2000 CMIP5 aerosol emissions and
the model resolution is 1.9 2.5 . It runs with 26 vertical
levels, with a top at about 2 hPa. All simulations include 1 year
of spin-up and results presented are averaged over the four
following years.
3. Results
3.1. Reference Simulations
[10] The radiative effect of sea salt injections was studied
by investigating the resulting change in shortwave cloud
forcing (SWCF) at the top of the atmosphere. Note that the
longwave effect was negligible. Figure 1a shows the resulting radiative effect, where reference model runs are marked
by Umin = 10 cm/s as this is the minimum sub-grid scale incloud vertical velocity used in the model parametrization
[Morrison and Gettelman, 2008]. The accumulation mode
sea salt is closest to the particle size suggested fit for cloud
brightening by Latham [2002] (0.13 mm dry radius), and all
simulations with injections of this mode (dark green) show
the negative radiative effect desired from climate engineering, with a maximum negative forcing of 3.3 W m2. By
comparison, a doubling of atmospheric CO2 yields a positive
forcing of about 3.7 W m2. Figures 1b and 1c show that the
negative forcing associated with MCB is directly linked to
an increase in the column-integrated CDNC, which in turn
leads to smaller droplets, a decrease in precipitation release,
and therefore to an increase in the LWP.
[11] Figure 1a also reveals a weaker negative forcing for
the largest compared to the second largest injections of the
Table 1. Sea Salt Injections Used to Simulate MCB and List of Simulations
Sea Salt Injections
Injections, 30 S to 30 N [kg m2 s1]
Total injections [Tg yr1]
Increase in global sea salt emissions
List of Simulationsa
Reference simulations (ref)
Aitken mode
Accumulation mode
Coarse mode
Fixed supersaturation
Aitken mode
Accumulation mode
Coarse mode
Enhanced vertical velocity (Umin30)
Aitken mode
Accumulation mode
Coarse mode
Control
1011
48.2
0.9%
1010
595.2
9.1%
3 1010
1785.5
27.4%
109
5935.6
91.3%
108
59436
913.1%
1011
1010
3 1010
109
108
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
This row lists the names used to identify injection amounts in kg m2 s1. Xs indicate simulations performed.
a
211
X
X
X
X
ALTERSKJÆR AND KRISTJÁNSSON: MARINE CLOUD BRIGHTENING
Figure 1. Dark colored bars present results from reference simulations (minimum updraft velocity of 10 cm s1) while light
colored bars present results from simulations with a minimum updraft velocity of 30 cm s1. Yellow bars represent control
run values for the reference run and the run of increased updraft, respectively. Figures 1a–1d show global annual averages,
while Figures 1e and 1f show annual results averaged over the oceanic injection region between 30 S and 30 N. (a) Change
in shortwave cloud forcing [W m2] with MCB. Stippled lines are results from simulations of fixed supersaturation. (b)
Column integrated cloud droplet number concentration [cm2]. (c) Change in liquid water path [g m2]. (d) Maximum
in-cloud supersaturation [%] at model hybrid layer ~945 hPa over ocean. Dashed lines indicate the critical supersaturation
of Aitken mode sea salt (upper) and accumulation mode SO4 (lower). (e) Aerosol number concentration [cm3]
at ~ 945 hPa. (f) In-cloud CDNC [cm3] at ~945 hPa.
accumulation mode sea salt. As described in Alterskjær et al.
[2012] this can be caused by an increased competition effect,
because the added sea salt particles swell, creating a moisture sink which lowers the maximum supersaturation (S)
and therefore increases the critical size that particles must
have to activate [Ghan et al., 1998; Korhonen et al.,
2010]. This competition effect may bring the S below that
necessary to activate background aerosols and in some cases
below that necessary to activate the added sea salt itself. Figure 1d shows that as the injection flux goes up, the S goes
down monotonically, as found in Korhonen et al. [2010]
and Wang et al. [2011], and for the accumulation mode 108
kg m2 s1 case, the globally and annually averaged S is
brought down to only 0.078%. Based on Köhler theory, this
is well above the critical S of 0.013% needed to activate
accumulation mode sea salt, but very close to the critical S
of the model’s accumulation mode droplets of pure H2SO4
of 0.07% (Figure 1d, lower purple dashed line). Our results
show that regionally and temporarily the S goes below the
value necessary for activation of accumulation mode sulfate,
and there is a reduction in activation of background aerosols
leading to a drop in both the CDNC and the LWP (Figures 1b
and 1c), so that the efficiency of the seeding goes down for
the largest injection mass.
[12] Figures 1e and 1f show the average aerosol number
concentration and CDNC obtained in the latitudinal band
where sea salt is injected for the model level where the
subtropical stratocumulus base is found most frequently
(945 hPa). The aerosol number concentration is on the order
of 103 to 106 cm3. For comparison, polluted urban areas
have particle concentrations of the order of 105 cm3
[Pruppacher and Klett, 1997]. Only one simulation gives
an averaged CDNC above 375 cm3, a value suggested to
offset the radiative forcing of a doubling of CO2 [Latham
et al., 2008].
[13] The change in SWCF resulting from injections of
Aitken mode sea salt is shown as dark blue columns in
Figure 1a. For the smallest injection amount the cooling effect of Aitken mode sea salt is larger than for accumulation
mode sea salt. This is because we add a larger number of
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ALTERSKJÆR AND KRISTJÁNSSON: MARINE CLOUD BRIGHTENING
particles per injection mass for the Aitken mode, and more
particles are activated to form cloud droplets (Figure 1b).
A large increase in CDNC leads to an increase in LWP that
is larger for Aitken mode than for accumulation mode
injections for this mass flux.
[14] Figure 1d shows that the suppression of S is greater
for the Aitken mode particles than for accumulation or
coarse mode particles, which is due to the larger surface area
for water vapor to condense on. Also, as the particles
themselves are small in the Aitken mode injections, their
critical S (0.16%, Figure 1d, upper purple dashed line) is
higher than for accumulation mode sea salt. Figure 1d shows
that the S is above this value only for the smallest injection
mass flux of Aitken mode sea salt. For injection strengths
of 1010 kg m2 s1 or greater, the water vapor condensing
on the added sea salt brings the globally averaged S below
that necessary to activate the Aitken mode sea salt itself.
Thus the added particles may not contribute as CCN and
the lowered S instead suppresses activation of background
aerosols. As a result, the simulated change in SWCF is
positive and increasingly more so as the injection mass flux
increases, reaching a maximum forcing in our simulations of
+8.4 W m2. This is the opposite effect of what one would
seek to achieve if performing climate engineering.
[15] The radiative effect of injecting coarse mode sea salt
is positive in our simulations, albeit much less so than for
the Aitken mode particles (Figure 1a, dark red bars). The
reason for this warming effect is twofold. First, the number
of particles per mass is small for the coarse mode case, so
that while the sea salt particles are always large enough to
activate, in number they do not contribute much to the
CDNC. Second, while the added surface area is low for this
mode, water vapor does condense on the sea salt and the S is
decreased (Figure 1d). This keeps some of the background
aerosols from activating, and as long as this number is
greater than the number of new droplets created on the
coarse mode sea salt, there will be a warming effect. Figure 1
shows that both the CDNC and the LWP decrease with
increasing injection strength. The radiative effect of this
sea salt mode increases substantially for the largest injection
flux, for which a positive forcing of 1.2 W m2 is reached
(Figure 1a). In this case, the globally averaged S is brought
below 0.16% (Figure 1d), the critical S of Aitken mode sea
salt. This indicates that injecting enough coarse mode sea
salt to shut off the activation of natural Aitken mode sea salt
may lead to a warming of the climate system.
[16] In agreement with our findings, Pringle et al. [2012]
found in their box model investigation that a decrease in
CDNC was possible under certain conditions. However,
these conditions were not met in their global study, possibly
due to their moderate injection amounts relative to our
values.
3.2. Sensitivity Tests
[17] The maximum simulated negative forcing of 3.3 W
m2 was achieved when injecting 109 kg m2 s1 of sea
salt between 30 S and 30 N, corresponding to injecting
5936 Tg of sea salt per year. By comparison Partanen
et al. [2012] simulated a forcing of 5.1 W m2 when
injecting only 443.9 Tg yr1 of sea salt of a comparable size
distributed over all ocean regions. A direct comparison of
these two studies is difficult due to the differences in
experimental design, but the findings indicate that NorESM
may be less sensitive to sea salt seeding than the
ECHAM5.5-HAM2 used by Partanen et al. [2012]. First,
the forcing estimate from Partanen et al. [2012] was based
on a simulation without “ultrafine” sea salt (dry diameter
100 nm), which was included in our study. Including these
particles in their model weakened their forcing estimate to
4.5 W m2. Second, the updraft vertical velocity in the
Partanen et al. [2012] study was very high, ranging between
1.0 and 1.4 m s1, which according to the authors eliminated
the competition effect from their study. Our results indicate
that the competition effect significantly reduces the
maximum in-cloud supersaturation. If overestimated, this
competition effect will influence our results substantially
and may lead to an underestimation of the effectiveness
of MCB.
[18] To investigate this closer we conducted two sets of
sensitivity simulations. In the first set, the maximum in-cloud
supersaturation was set to a fixed value of 0.2% (annually averaged control run S is 0.18% globally around 945 hPa) and the
model was run for an injection strength of 1010 kg m2 s1
for the Aitken and accumulation modes and for a strength of
108 kg m2 s1 for the coarse mode. The second set involved
increasing the minimum sub-grid scale in-cloud vertical
velocity in the Morrison and Gettelman [2008] parametrization
from 10 to 30 cm s1. This increased the average updraft
velocity over ocean between 30 S and 30 N from 30.4 to
41.6 cm s1 around 945 hPa (see Supporting Information) and
increased the maximum in-cloud supersaturation. This setup
was run for all injection strengths for the Aitken and the accumulation mode injections and for the 1010 kg m2 s1 injections for the coarse mode sea salt.
3.2.1. Fixed Supersaturation
[19] Setting the in-cloud supersaturation to a fixed value
increased the magnitude of the negative forcing of the
accumulation mode sea salt from 1.27 to 1.43 W m2
for an injection strength of 1010 kg m2 s1 (Figure 1a,
green dashed line). More strikingly, for the Aitken mode,
removing the competition effect led to a change in the sign
of the resulting forcing; going from +1.27 to 5.91 W m2.
The high number to mass ratio combined with an S that is
above the critical S of the Aitken mode sea salt (0.16%),
leads to a very large cooling effect. These results confirm
that the competition effect dramatically limits the simulated
cooling effect achieved from cloud seeding in the reference
simulations.
[20] Latham et al. [2008] suggested that seeding clouds
with coarse mode sea salt particles might lead to a warming
because the “giant salt nuclei” may lead to an early onset of
precipitation and a decrease in LWP. We investigated this by
running the 108 kg m2 s1 coarse mode case with a
constant S of 0.2%. This brought the change in SWCF from
1.2 (Figure 1a) to 0 W m2, indicating that there is no contribution to the simulated warming in our coarse mode results
from other sources than the competition effect.
3.2.2. Enhanced Minimum Vertical Velocity
[21] Increasing the minimum in-cloud vertical velocity in the
second set of sensitivity simulations led to an increase in the
globally and annually averaged control run S from 0.18% to
0.26% around 945 hPa (Figure 1d, yellow bars). This led to activation of smaller background particles in the increased updraft
simulations (Umin30) than in the reference simulations (ref)
and therefore increased the CDNC (Figure 1b, yellow bars)
and decreased the effective radius around 945 hPa from
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ALTERSKJÆR AND KRISTJÁNSSON: MARINE CLOUD BRIGHTENING
8.7 mm in ref to 8.2 mm in Umin30. The LWP increased from
126 g m2 in ref to 137 g m2 in the Umin30 simulation due
to suppression of precipitation. Combined, the influence on
the effective radius and the LWP led to a control run that had
a higher cloud albedo in Umin30 than in the ref.
[22] For accumulation mode sea salt injections, the radiative forcing achieved was smaller in magnitude than in the
ref simulations, except for the simulation of maximum sea
salt seeding mass (Figure 1a, green bars). The small magnitude was caused by (i) reduced precipitation suppression due
to high control run CDNC [Rasch and Kristjánsson, 1998,
equation 21] and (ii) a reduced sensitivity for albedo to
change with LWP due to high control run LWP. Assuming
an asymmetry parameter of 0.85 and an optical depth t ¼
3LWP
2rL re gives a change in cloud albedo, A [Hobbs, 1993], with
LWP of
dA
4:67rL re
¼
dLWP ðLWP þ 4:67rL re Þ2
estimates because of the nonlinear relation between added
mass and forcing caused by, e.g., differences in the regional
meteorological conditions—Are there low clouds above?
Are there updrafts that can carry the injected sea salt aloft?
Is the particle number in the region already high rendering
the clouds less sensitive to the injected sea salt?
[27] This study does not account for kinetic limitations on
the activation of giant CCN [Chuang et al., 1997]. In regions
of high S this may lead to an overestimated CDNC, while the
opposite may be true in regions of low S where an
overestimated droplet activation may lead to an exaggerated
competition effect. Not accounting for this kinetic limitation
may be especially important for the coarse mode sea salt
injections, for which we expect the simulated positive
radiative forcing to be an upper estimate.
4. Conclusions
(1)
where re is the cloud droplet effective radius and rL the density
of water. Combined, (i) and (ii) led to a smaller radiative effect
of MCB for Umin30 than for ref accumulation mode seeding.
[23] For the maximum injections of accumulation mode
sea salt, the high Umin30 control run S needed a large
reduction to bring it below that necessary to activate
background sulfate. Contrary to ref simulations, the Umin30
simulations resulted in an S that was well above the critical
limit (Figure 1d, lower purple dashed line). The reduced
competition effect resulted in an MCB that still served to
increase the CDNC and LWP and therefore to increase the
magnitude of the SW cloud forcing.
[24] For the Aitken mode, Figure 1d (light blue columns)
shows that both 1011 and 1010 kg m2 s1 injections have
Umin30 S around 945 hPa that are above the critical limit to
activate the added Aitken mode sea salt. The two cases
therefore led to negative forcing, which for the smallest
injections was stronger than the forcing produced in the ref
simulation (Figure 1a). The increased updraft led to
increased activation of the added sea salt particles
themselves and therefore to a more effective MCB, while
for the corresponding accumulation mode case the increased
updraft mainly increased activation of background particles,
thus leading to a decrease the efficiency of MCB. The high
Umin30 control run S leads to a “delay” in the competition
effect, reducing the positive forcing for high Aitken mode
injections because more sea salt is needed to remove enough
water vapor to bring the S below that necessary for
activation of the Aitken mode particles.
[25] For the coarse mode, we saw no significant change
between the ref and the Umin30 simulations.
[26] The sensitivity experiments greatly suppress the
competition effect, but nevertheless do not lead to negative
radiative flux perturbations of the same magnitude as that
found by Partanen et al. [2012]. One possible reason is that
the Umin30 simulations have average updrafts of 41.6 cm
s1 around 945 hPa (see Supporting Information), which is
still well below that of Partanen et al. [2012]. The NorESM
may also be less sensitive to the added sea salt than the
ECHAM5.5-HAM2 used by Partanen et al. [2012] due to
the large model LWP (equation (1)). Additionally, Partanen
et al. [2012] seeded all ocean areas, while we only seeded
between 30 S and 30 N. This may influence the forcing
[28] In this study we have investigated how deliberate
injections of sea salt into the marine boundary layer affect
the global radiative budget as a function of both particle size
category and injection mass flux. Using the NorESM we find
that injecting accumulation mode sea salt between 30 S and
30 N leads to a desired negative radiative effect of down to
3.3 W m2 which would almost cancel the positive forcing
of a doubling of atmospheric CO2. On the other hand, for
Aitken mode injections greater or equal to 1010 kg m2 s1,
the simulated net radiative effect is positive, reaching a
maximum of 8.4 W m2. This is because the competition effect reduces the maximum supersaturation and suppresses
activation of both the added Aitken mode sea salt and
background particles, leading to a decrease in both CDNC
and LWP. When coarse mode sea salt is injected, the
simulated net radiative effect is always positive. While the
coarse mode sea salt itself is large enough to activate, its
small number to mass ratio leads to a lower increase in
CDNC due to the added sea salt than the decrease in
activation of background aerosols due to the competition
effect.
[29] We also performed sensitivity tests which show that
the size of the competition effect is crucial for the simulated
forcing achieved. Omitting this effect led to a negative
forcing of 5.9 W m2 when injecting 1010 kg m2 s1 of
Aitken mode sea salt, whereas the same injections gave a
positive forcing of 1.3 W m2 in the reference simulation.
Increasing the minimum in-cloud updraft velocity increased
the S and led to a “delay” in the competition effect.
However, for accumulation mode injections, the increased
updraft mainly led to increased activation of background
aerosols, leading to a reduced efficiency of the MCB.
[30] The results presented in this study clearly show that
the effectiveness of MCB is very sensitive to the injection
mass flux and particle size. While a cooling effect is
simulated for certain sea salt injections, emitting the wrong
particle size or the wrong amount leads to a simulated
warming of the climate system, which is opposite to what
one seeks to achieve by climate engineering.
[31] This study aims to be a first step in investigating the
global effect of MCB of different sized particles and injection amounts. Not all sea salt particle sizes are represented
and the model resolution is coarse. Wood [2007] showed
that the cloud lifetime effect may change sign depending
on the vertical placement of the cloud base and Wang
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ALTERSKJÆR AND KRISTJÁNSSON: MARINE CLOUD BRIGHTENING
et al. [2011] showed that the cloud albedo effect depends
strongly on the fine scale atmospheric state, neither of which
are well represented in climate models. However, coarse
models such as the NorESM are currently the only tool
available to study the global effects of MCB.
[32] Acknowledgments. This study was partly funded by the
European Commission’s 7th Framework Program through the IMPLICC
project (FP7-ENV-2008-1-226567), and by the Norwegian Research
Council through the EarthClim project (207711/E10) and its programme
for supercomputing (NOTUR) through a grant for computing time. The
authors are thankful to Philip Rasch and to the IMPLICC consortium for
constructive discussions.
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