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Cirrus cloud seeding has potential to cool climate
T. Storelvmo,
1
J. E. Kristjansson,
2
H. Muri,
2
M. Pfeffer,
2
D. Barahona,
3
and A. Nenes
4
Received 16 October 2012; revised 18 December 2012; accepted 24 December 2012; published 15 January 2013.
[1] Cirrus clouds, thin ice clouds in the upper troposphere,
have a net warming effect on Earth’s climate. Consequently,
a reduction in cirrus cloud amount or optical thickness
would cool the climate. Recent research indicates that by
seeding cirrus clouds with particles that promote ice
nucleation, their lifetimes and coverage could be reduced.
We have tested this hypothesis in a global climate model
with a state-of-the-art representation of cirrus clouds and find
that cirrus cloud seeding has the potential to cancel the entire
warming caused by human activity from pre-industrial times
to present day. However, the desired effect is only obtained
for seeding particle concentrations that lie within an optimal
range. With lower than optimal particle concentrations, a
seeding exercise would have no effect. Moreover, a higher
than optimal concentration results in an over-seeding that
could have the deleterious effect of prolonging cirrus lifetime
and contributing to global warming. Citation: Storelvmo T., J.
E. Kristjansson, H. Muri, M. Pfeffer, D. Barahona and A. Nenes
(2013), Cirrus cloud seeding has potential to cool climate, Geophys.
Res. Lett.,40,178–182, doi:10.1029/2012GL054201.
1. Introduction
[2] With the realization that Earth’s climate is changing at a
rapid pace, a number of mechanisms through which climate
could artificially be stabilized have been proposed in the
literature. Climate sensitivity, defined as the equilibrium
surface temperature response to a doubling of atmospheric
CO
2
, is poorly constrained, and very high climate sensitivities
cannot currently be ruled out [Roe and Baker,2007].This,
combined with what seems to be a difficult prospect of curbing
anthropogenic CO
2
emissions [Davis et al., 2010], the main
cause of modern climate change, has led many to propose
climate engineering as a cooling mechanism [Keith,2001;
Boyd, 2008]. Carbon capturing and sequestration is one
example of climate engineering that would directly target the
problem of rising atmospheric CO
2
concentrations [Metz
et al., 2005]. Another class of climate engineering proposals
is often termed solar radiation management (SRM), because
rather than reducing Earth’s greenhouse effect, their purpose
is to increase Earth’salbedo/reflectivity. SRM strategies
include stratospheric sulphur injection, mimicking volcanic
eruptions [Crutzen, 2006], and enhancement of marine
stratocumulus cloud albedo via sea salt injection [Latham,
1990]. Both mechanisms have been the focus of many recent
studies [Rasch et al., 2008; Wang et al., 2011], and several
complications have been identified. Examples are changes to
the local and regional hydrological cycles [Ricke et al.,
2010], as well as stratospheric ozone depletion in the case of
stratospheric sulphur injection [Tilmes et al., 2008]. Here we
address a climate engineering mechanism that has so far not
been tested: the perturbation of cirrus clouds to reduce their
lifetime and optical thickness, thereby cooling Earth’s climate.
[3] This idea was first put forth by Mitchell and Finnegan
[2009] and builds on the fact that spontaneous freezing of
liquid solution droplets requires high water vapor partial
pressures that well exceed that of saturation with respect to
a plane ice surface (i.e., supersaturation, S
i
). Spontaneous
freezing of droplets is a stochastic process that is referred
to as homogeneous nucleation [Koop et al., 2000].
[4] The homogeneous nucleation rate decreases with
increasing temperature (T), and for Thigher than about
35C, homogeneous ice nucleation does not occur in the
atmosphere [Pruppacher and Klett, 1997]. The presence of
a substrate to facilitate the formation of tiny ice crystals
can significantly lower the supersaturation required for ice
formation, a process known as heterogeneous ice nucleation.
Certain insoluble particles can provide such substrates in the
atmosphere and are termed ice nuclei (IN). Examples of
natural IN are mineral dust particles, as well as certain
primary biological particles [Pruppacher and Klett, 1997].
Bismuth tri-iodide (BiI3) is an example of an artificial IN
and has been suggested as cirrus seeding material [Mitchell
and Finnegan, 2009]. It has been suggested that BiI3 can
initiate freezing at a supersaturation as low as 5%, while
homogeneous ice nucleation requires a supersaturation of
the order of 50% at typical cirrus temperatures. Due to the
low concentration of IN in the upper troposphere (UT),
homogeneous freezing is thought to dominate cirrus cloud
formation [Karcher and Lohmann, 2003; Mitchell et al.,
2011]. Hence, the addition of very efficient IN in the right
concentration may result in fewer, larger ice crystals. The
heterogeneously formed ice crystals would deplete water
vapor as they grow and prevent the supersaturations required
for the onset of homogeneous freezing. Larger ice crystals
would reduce cirrus optical thickness and shorten cloud
lifetimes through increased ice crystal sedimentation velocities.
Both mechanisms would yield a smaller greenhouse effect
(Figure 1). Mitchell and Finnegan [2009] proposed that
the seeding material could be injected at cirrus levels by
commercial aircraft. A background concentration of seeding
material would build up, and cirrus clouds would form in an
environment sufficiently enriched in IN for homogeneous
freezing to be suppressed.
1
Department of Geology and Geophysics, Yale University, New Haven,
Connecticut, USA.
2
Department of Geoscience, University of Oslo, Oslo, Norway.
3
Global Modeling and Assimilation Office, NASA Goddard Space
Flight Center, Greenbelt, Maryland, USA.
4
Schools of Chemical and Biomolecular Engineering and Earth
and Atmospheric Sciences, Georgia Institute of Technology, Atlanta,
Georgia, USA.
Correspondence author: T. Storelvmo, Department of Geology and
Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511,
USA. (trude.storelvmo@yale.edu)
©2012. American Geophysical Union. All Rights Reserved.
0094-8276/13/2012GL054201
178
GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 178–182, doi:10.1029/2012GL054201, 2013
[5] Here we have tested the effect of background concen-
trations of seeding IN spanning several orders of magnitude
in numerical simulations using a modified version of the
National Center for Atmospheric Research Community
Atmosphere Model (CAM, version 5).
2. Modeling Tool and Experimental Setup
[6] The CAM5 was, for the purpose of this study, run at a
horizontal resolution of 1.9latitude and 2.5longitude, with
30 vertical levels, a finite volume dynamical core and a
timestep of 20 min. All simulations were conducted with
climatological sea surface temperatures corresponding to the
year 2000. CAM5’s predecessor is described in Gent et al.
[2011], but its cloud microphysics has since been updated
[Gettelman et al., 2008, 2010] and it also has a recently
developed modal aerosol treatment [Liu et al., 2012a]. The
aerosol size distribution can now be represented by either three
or seven lognormal modes (MAM3 and MAM7, respec-
tively). The treatment of cirrus cloud microphysics in CAM5
has also been significantly improved relative to earlier ver-
sions [Liu et al., 2007], but was in this study partly replaced
by an alternative and more flexible cirrus scheme, developed
by Barahona and Nenes [2008, 2009]. The scheme is based
on an analytical solution of the governing equations of a cool-
ing air parcel. It explicitly accounts for the effect of cloud
formation conditions and aerosol properties on the cirrus ice
crystal concentration. Competition between homogeneous
and heterogeneous ice nucleation, hence the influence of ice
nuclei on ice crystal concentration, is also accounted for. Het-
erogeneous ice nucleation is described through a generalized
ice nucleation spectrum, which can have any functional form,
providing flexibility in describing ice nucleation on different
IN. While the CAM5 cirrus scheme has already been carefully
validated, particularly in terms of global cloud and radiation
fields [Liu et al., 2012b; Gettelman et al., 2008, 2010], we
show in Table 1 global averages of some key cloud and radi-
ation fields for the standard CAM5, the modified CAM5 used
in this study, as well as from observations. Both model ver-
sions were run with homogeneous nucleation only, for tem-
peratures below 38C. The introduction of the new cirrus
scheme does not dramatically change the cloud and radiation
fields. However, it does produce more ice crystals at cirrus
levels, which leads to optically thicker and longer-lived cirrus
clouds, hence the slightly larger ice and liquid water paths (the
latter due to reduced accretion of liquid by falling ice crystals).
Cirrus ice crystals concentrations lie in the range 10–
1000 L
1
, with a global annual average at 200 hPa of
~400 L
1
. This is somewhat higher than the concentrations
reported for example from the recent SPARTICUS campaign
[Mitchell et al., 2011], but values are very sensitive to the
treatment of subgrid-scale vertical velocity (see section 3).
Here, we made the assumption that under unseeded condi-
tions, cirrus clouds form solely through homogeneous ice
nucleation. The concentration of solution droplets that could
potentially nucleate homogeneously corresponds to the pre-
dicted number concentration of particles in the Aitken mode
of the MAM3 aerosol module.
[7] Based on the number concentration and size of the
solution droplets, as well as temperature and vertical velocity,
the homogeneous nucleation rate were calculated [Barahona
and Nenes, 2008]. The seeding IN were all conservatively
assumed to activate and nucleate ice at a supersaturation of
10% [Mitchell et al., 2011]. We have carried out 20 model
simulations, each 10 years of length after a spin-up of
3 months, in which the concentration of seeding IN in
the UT (IN
s
) was varied from 0 to 1500 L
1
. Note that in the
following, cirrus clouds refer to all clouds forming in the
UT, which will here correspond to the part of the atmosphere
with temperatures lower than 38C.
3. Results
[8] The delivery method, dispersion, and atmospheric fate
of the seeding IN are beyond the scope of the present study.
Here, we focus on the effect of seeding on cirrus cloud
properties and Earth’s energy budget, under the assumption
that there exist some means to build up uniform background
concentrations of seeding IN in the UT. Figure 2a shows
simulated global annual mean vertically integrated ice amount
(ice water path, IWP) and high cloud coverage (CC
HGH
), as a
function of IN
s
. For low IN
s
concentrations (<5L
1
), IWP
and CC
HGH
remain very similar to their values under pure
a) unseeded b) seeded
Figure 1. Conceptual schematic of changes in cirrus cloud properties in response to seeding. Red arrows represent longwave
(LW) radiation and blue arrows represent shortwave (SW) radiation. The seeded cirrus clouds on average reflect slightly less SW
radiation back to space, but also allow more LW radiation to escape to space, and the latter effect dominates.
Table 1. Simulated Global and Annual Mean Cloud Cover (CC),
Ice Water Path (IWP), Liquid Water Path (LWP), and Net Cloud
Forcing (NCF) From the Standard and Modified CAM5.1
(CAM5.1-HOM and CAM5.1-BN09, Respectively) as well as From
Satellite Observations (OBS)K
a
CC (%) IWP (gm
2
) LWP (gm
2
) NCF (Wm
2
)
CAM5.1-HOM 64.3 17.8 44.2 27.6
CAM5.1-BN09 68.8 21.9 47.1 26.3
OBS 71 20–70 30–50 17.2 to 23.8
a
Observations are taken from a combination of CloudSat and CALIPSO
retrievals (CC, IWP, and LWP), and from ERBE and CERES (NCF).
STORELVMO ET AL.: CIRRUS CLOUD SEEDING CAN COOL CLIMATE
179
homogeneous freezing, i.e., IN
s
=0L
1
, our reference case
(REF). However, for IN
s
in the range 5–100 L
1
,bothare
suppressed and ice crystals are 10–20% larger than in the case
of pure homogeneous freezing (Figure 2b). In this IN
s
range,
we also observed a small reduction in liquid water path, due
to increased accretion of liquid by falling ice crystals. Finally,
for IN
s
>100 L
1
, seeding leads to the opposite effect; smaller
ice crystals and the consequent increase in IWP and CC
HGH
.
From Figure 2, three distinct regimes can be identified:
(1)the sub-optimal seeding regime—IN
s
is insufficient for
suppression of homogeneous nucleation, and the cirrus clouds
remain unaffected by the seeding; (2) the optimal seeding re-
gime—homogeneous nucleation is suppressed, and IN
s
is
low enough to reduce ice crystal concentration and increase
crystal size, with associated reductions in cirrus cloud
amount and coverage; and (3) the over-seeding regime—
homogeneous nucleation is suppressed, but more ice crystals
nucleate on seeds than would otherwise have nucleated
homogeneously in the unseeded case. Table 2 gives approxi-
mate IN
s
intervals for these three regimes in our control model
setup (CTL). As a consequence of the increase in ice crystal
sizes and decrease in cirrus cloud amount in the optimal
seeding regime, cirrus clouds become optically thinner, as
illustrated by the reduction in longwave cloud forcing
(LWCF), shown in Figure 2c. The reduced LWCF allows
for more outgoing longwave radiation at the top of the
atmosphere (TOA), corresponding to a negative radiative
forcing (i.e., cooling) of about 7 Wm
2
. This cooling is partly
compensated for by a reduction in cirrus cloud albedo and
hence the shortwave cloud forcing (SWCF), such that the
maximum reduction in the net cloud forcing (NCF) amounts
to 2.0 Wm
2
. While changes in the net shortwave flux at the
TOA are very similar to the changes in SWCF, the reduction
in UT water vapor in response to the seeding increases the
outgoing longwave radiation further by up to 0.5Wm
2
and
hence amplifies the cooling.
[9] Hence, cirrus cloud seeding could potentially eliminate
a forcing equivalent to that which has been causing climate
a)
0.01 0.10 1.0 10.0 100.0 1000.0
16
18
20
22
24
IWP (g/m2)
38
39
40
41
42
43
44
45
46
47
High Cloud Cover (%)
Column Ice Amount and High Cloud Cover
vs. Seeding IN Concentration
b)
0.1 1.0 10.0 100.0
Seeding IN concentration (l-1)
Seeding IN concentration (l-1)
Seeding IN concentration (l-1)
30
32
34
36
38
40
Ice Crystal Effective
Radius (μm), 200hPa
70
72.5
75
77.5
Ice Crystal Effective
Radius (μm), 300hPa
Ice Crystal Effective Radius
vs. Seeding IN Concentration
0.01 0.1 1.0 10.0 100.0
-8
-6
-4
-2
0
2
4
6
Δ Cloud Forcing (W/m2)
ΔNCF
ΔLWCF
ΔSWCF
Cloud Radiative Forcings Anomalies
vs. Seeding IN Concentration
c)
Figure 2. CAM5 simulations of macro-physical and radiative properties of high clouds as a function of IN
s
. Each circle
corresponds to an individual 10 year CAM5 simulation. (a) High cloud amount (i.e., cloud cover integrated from 400 to
50 hPa) and vertically integrated ice amount (ice water path, IWP), (b) ice crystal effective radius at 300 hPa (red solid line)
and 200 hPa (blue solid line), and (c) changes in longwave, shortwave and net cloud forcing (SWCF, LWCF, and NCF, re-
spectively) at the top of the atmosphere (TOA), relative to REF. Solid lines represent moving averages. Error bars represent
one standard deviation, calculated based on annual averages.
Table 2. Approximate Sub-optimal, Optimal, and Over-seeding
INs Concentrations for the CTL, WW
c,LOW
and W
c,HGH
Sets
of Simulations
Case
Sub-optimal
IN
s
(L
1
)
Optimal IN
s
(L
1
)
Over-seeding
IN
s
(L
1
)
CTL <55–100 >100
W
c,LOW
<11–25 >25
W
c,HGH
<20 20–200 >200
STORELVMO ET AL.: CIRRUS CLOUD SEEDING CAN COOL CLIMATE
180
change to date. However, this would require seeding IN
concentrations finely tuned to lie exactly in the optimal IN
s
win-
dow. While the main perceived risk of under-seeding is a costly,
wasted effort, over-seeding could actually lead to the opposite of
the desired effect. This is illustrated in Figure 2; IN
s
concentra-
tions larger than 100 L
1
would lead to an increase in IWP and
a decrease in ice crystal sizes relative to the unseeded atmosphere,
and hence a warming rather than a cooling. Based on Figure 2,
we have approximated the optimal IN
s
,IN
s,o
,to15L
1
and have
displayed anomalies in several cirrus cloud properties relative to
REF for IN
s,o
in Figure 3. Evident is the strong reduction in ice
crystal number concentrations in the UT (Figure 3a), which
allows individual ice crystals to grow larger via vapor deposition
(Figure 3b). The larger ice crystals in turn lead to reduced cloud
ice (Figure 3c) and cloud coverage (Figure 3d), as a result of
the faster sedimentation of the larger ice crystals. As expected,
the strongest perturbations are found at mid-latitudes, where cir-
rus clouds form in situ, rather than in the tropics, where anvil cir-
rus are produced by convective outflow.
[10] Several studies have indicated that the relative impor-
tance of homogeneous versus heterogeneous ice nucleation is
very sensitive to the vertical velocity at the cloud-scale
[Karcher and Lohmann, 2003; DeMott et al., 1997]. CAM5
parameterizes this subgrid-scale updraft velocity as a single
value for each model grid box, proportional to the square root
of the turbulent kinetic energy (TKE), Wc¼ffiffiffiffiffiffiffiffiffiffiffi
2
3TKE
p.
[11] We have tested the robustness of our results to increased/
decreased vertical velocities, by repeating the set of IN
s
perturbation simulations, but with Wc¼Wc;HGH ¼ffiffiffiffiffiffiffiffiffiffiffi
8
3TKE
p
and Wc¼Wc;LOW ¼ffiffiffiffiffiffiffiffiffiffiffi
1
6TKE
p, respectively. Figure 4 shows
a) b)
c) d)
Figure 3. Simulated changes in zonal and annual mean cloud properties induced by a seeding IN concentration of 15 l
1
(relative to REF): (a) in-cloud ice crystal number concentration, (b) ice crystal effective radius, (c) ice mass mixing ratio, and
(d)cloud coverage. All plots are based on 10 year model simulations.
0.01 0.1 1.0 10 100
Seeding IN concentration (l-1)
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Cloud Forcing (W/m2)
ΔNCF, Wc,LOW
ΔNCF, Wc,HGH
ΔNCF, Wc
Cloud Radiative Forcings Anomalies
vs. Seeding IN Concentration
Figure 4. Change in the net cloud forcing (NCF) as a func-
tion of IN
s
at the TOA relative to REF for default, doubled
and halved subgrid-scale vertical velocity (W
c
,W
c,HGH
,
W
c,LOW
, respectively).
STORELVMO ET AL.: CIRRUS CLOUD SEEDING CAN COOL CLIMATE
181
the change in NCF (relative to REF) as a function of IN
s
for
simulations with W
c
,W
c,HGH
,andW
c,LOW
. Evident from Table 1
is a shift in the optimal IN
s
interval toward lower (higher)
values when W
c
is decreased (increased). The magnitude of
the cooling is also affected, and becomes smaller (larger)
when W
c
is decreased (increased). Higher vertical velocities
lead to higher homogeneous nucleation rates, and hence a
stronger perturbation when homogeneous nucleation is
suppressed. Higher vertical velocities also require higher
IN
s
concentrations in order for homogeneous nucleation to
be suppressed. While previous studies of the effect of
anthropogenic IN on cirrus have reported a sensitivity to the
concentration of solution droplets available for homogeneous
nucleation [Penner et al., 2009], we found minor changes in
a simulation reducing the concentration of solution droplets
available by 50%.
4. Discussion and Outlook
[12] Further investigations of the viability of cirrus seeding
as a means of stabilizing Earth’s climate will require simula-
tions of the atmospheric lifetimes of seeding IN, fromthe point
of emission, through potential ice nucleation, and subsequent
sedimentation and deposition on Earth’s surface. Laboratory
investigations of ice nucleation on BiI3 are also required to
shed further light on the geoengineering process investigated
here. The present study has demonstrated that successful cirrus
cloud seeding requires seeding IN concentrations that lie in a
relatively narrow optimal interval. The bounds of this interval
are set by the vertical velocities in the UT, for which only
sparse and sporadic measurements exist. A premature
implementation of cirrus seeding before knowledge of vertical
velocities at cirrus levels is improved could accelerate global
warming as opposed to prevent it.
[13]Acknowledgments. The work presented in this paper was sup-
ported in part by the facilities and staff of the Yale University Faculty of Arts
and Sciences High Performance Computing Center. The Research Council of
Norway, through grant number 216763/F11, made this collaboration possible,
and H. M. and M. P. were supported through the grants 184714/S30 and
207711/E10. T.S. is thankful to B. Dobbins (Yale University) for technical
support and to J. Wettlaufer (Yale University) for helpful comments.
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