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REVIEW
An overview of geoengineering of climate
using stratospheric sulphate aerosols
BYPHILIP J. RASCH
1,
*,S
IMONE TILMES
1
,RICHARD P. TURCO
2
,
ALAN ROBOCK
3
,LUKE OMAN
4
,CHIH-CHIEH (JACK)CHEN
1
,GEORGIY
L. STENCHIKOV
3
AND ROLANDO R. GARCIA
1
1
National Center for Atmospheric Research, PO Box 3000-80307,
Boulder, CO 80307, USA
2
Department of Atmospheric and Oceanic Sciences, University of California,
Los Angeles, CA 90095-1565, USA
3
Department of Environmental Sciences, Rutgers University,
New Brunswick, NJ 08901, USA
4
Department of Earth and Planetary Sciences, Johns Hopkins University,
Baltimore, MD 21218, USA
We provide an overview of geoengineering by stratospheric sulphate aerosols. The state
of understanding about this topic as of early 2008 is reviewed, summarizing the past 30
years of work in the area, highlighting some very recent studies using climate models,
and discussing methods used to deliver sulphur species to the stratosphere. The studies
reviewed here suggest that sulphate aerosols can counteract the globally averaged
temperature increase associated with increasing greenhouse gases, and reduce changes to
some other components of the Earth system. There are likely to be remaining regional
climate changes after geoengineering, with some regions experiencing significant changes
in temperature or precipitation. The aerosols also serve as surfaces for heterogeneous
chemistry resulting in increased ozone depletion. The delivery of sulphur species to the
stratosphere in a way that will produce particles of the right size is shown to be a
complex and potentially very difficult task. Two simple delivery scenarios are explored,
but similar exercises will be needed for other suggested delivery mechanisms. While the
introduction of the geoengineering source of sulphate aerosol will perturb the sulphur
cycle of the stratosphere signicantly, it is a small perturbation to the total (stratosphere
and troposphere) sulphur cycle. The geoengineering source would thus be a small
contributor to the total global source of ‘acid rain’ that could be compensated for
through improved pollution control of anthropogenic tropospheric sources. Some areas of
research remain unexplored. Although ozone may be depleted, with a consequent
increase to solar ultraviolet-B (UVB) energy reaching the surface and a potential impact
on health and biological populations, the aerosols will also scatter and attenuate this part
of the energy spectrum, and this may compensate the UVB enhancement associated with
ozone depletion. The aerosol will also change the ratio of diffuse to direct energy reaching
Phil. Trans. R. Soc. A (2008) 366, 4007–4037
doi:10.1098/rsta.2008.0131
Published online 29 August 2008
One contribution of 12 to a Theme Issue ‘Geoscale engineering to avert dangerous climate change’.
* Author for correspondence (pjr@ucar.edu).
4007 This journal is q2008 The Royal Society
the surface, and this may influence ecosystems. The impact of geoengineering on these
components of the Earth system has not yet been studied. Representations for the
formation, evolution and removal of aerosol and distribution of particle size are still very
crude, and more work will be needed to gain confidence in our understanding of the
deliberate production of this class of aerosols and their role in the climate system.
Keywords: climate change; geoengineering; sulphate aerosols; global warming
1. Introduction
The concept of ‘geoengineering’ (the deliberate change of the Earth’s climate by
mankind; Keith 2000) has been considered at least as far back as the 1830s with
J. P. Espy’s suggestion (Fleming 1990) of lighting huge fires that would stimulate
convective updrafts and change rain intensity and frequency of occurrence.
Geoengineering has been considered for many reasons since then, ranging from
making polar latitudes habitable to changing precipitation patterns.
There is increasing concern by scientists and society in general that energy
system transformation is proceeding too slowly to avoid the risk of dangerous
climate change from humankind’s release of radiatively important atmospheric
constituents (particularly CO
2
). The assessment by the Intergovernmental Panel
on Climate Change (IPCC 2007a) shows that unambiguous indicators of human-
induced climate change are increasingly evident, and there has been little societal
response to the scientific consensus that reductions must take place soon to avoid
large and undesirable impacts.
To reduce carbon dioxide emissions soon enough to avoid large and
undesirable impacts requires a near-term revolutionary transformation of energy
and transportation systems throughout the world (Hoffert et al. 1998). The
size of the transformation, the lack of effective societal response and the inertia to
changing our energy infrastructure motivate the exploration of other strategies
to mitigate some of the planetary warming. For this reason, geoengineering
for the purpose of cooling the planet is receiving increasing attention. A
broad overview to geoengineering can be found in the reviews of Keith (2000),
WRMSR (2007), and the papers in this volume. The geoengineering paradigm
is not without its own perils ( Robock 2008). Some of the uncertainties
and consequences of geoengineering by stratospheric aerosols are discussed in
this paper.
This study describes an approach to cooling the planet, which goes back
to the mid-1970s, when Budyko (1974) suggested that, if global warming
ever became a serious threat, we could counter it with airplane flights in the
stratosphere, burning sulphur to make aerosols that would reflect sunlight away.
The aerosols would increase the planetary albedo and cool the planet,
ameliorating some (but as discussed below, not all) of the effects of increasing
CO
2
concentrations. The aerosols are chosen/designed to reside in the
stratosphere because it is remote, and they will have a much longer residence
time than tropospheric aerosols that are rapidly scavenged. The longer lifetime
means that a few aerosols need be delivered per unit time to achieve a given
aerosol burden, and that the aerosols will disperse and act to force the climate
system over a larger area.
P. J. Rasch et al.4008
Phil. Trans. R. Soc. A (2008)
Sulphate aerosols are always found in the stratosphere. Low background
concentrations arise due to transport from the troposphere of natural and
anthropogenic sulphur-bearing compounds. Occasionally much higher concen-
trations arise from volcanic eruptions, resulting in a temporary cooling of the
Earth system (Robock 2000), which disappears as the aerosol is flushed from
the atmosphere. The volcanic injection of sulphate aerosol thus serves as a
natural analogue to the geoengineering aerosol. The analogy is not perfect
because the volcanic aerosol is flushed within a few years, and the climate system
does not respond in the same way as it would if the particles were continually
replenished, as they would be in a geoengineering effort. Perturbations to
the system that might become evident with constant forcing disappear as the
forcing disappears.
This study reviews the state of understanding about geoengineering by
sulphate aerosols as of early 2008. We review the published literature, introduce
some new material and summarize some very recent results that are presented in
detail in the submitted articles at the time of the writing of this paper. In our
summary we also try to identify areas where more research is needed.
Since the paper by Budyko (1974), the ideas generated there have received
occasional attention in discussions about geoengineering (e.g. NAS92 1992;
Turco 1995; Govindasamy & Caldeira 2000,2003;Govindasamy et al. 2002;
Crutzen 2006;Wigley 2006;Matthews & Caldeira 2007).
There are also legal, moral, ethical, financial and international political issues
associated with a manipulation of our environment. Commentaries (Bengtsson
2006;Cicerone 2006;Kiehl 2006;Lawrence 2006;MacCracken 2006)toCrutzen
0°90°
evaporation
tropical
pipe
transport
sedimentation
tropopause
fold
aircraft sulfur
and soot
transport
of source
gases
cloud
scavenging
tropopause cloud
processes
nucleation
condensation
and coagulation
tropical
stratospheric
reservoir
polar
vortex
nucleation
PSCs
removal
~17 km
~8 km
Figure 1. A schematic of the processes that influence the life cycle of stratospheric aerosols
(adapted with permission from SPARC 2006).
4009Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
(2006) address some of these issues and remind us that this approach does not treat
all the consequences of higher CO
2
concentrations (such as ocean acidification;
others are discussed in Robock 2008). Recently, climate modellers have begun
efforts to provide more quantitative assessments of the complexities of
geoengineering by sulphate aerosols and the consequences to the climate system
(Rasch et al.2008;Tilmeset al.2008,submitted;Robock et al.2008).
2. An overview of stratospheric aerosols in the Earth system
(a)General considerations
Sulphate aerosols are an important component of the Earth system in the
troposphere and stratosphere. Because sulphate aerosols play a critical role in the
chemistry of the lower stratosphere and occasionally, following a volcanic
eruption, in the radiative budget of the Earth by reducing the incoming solar
energy reaching the Earth surface, they have been studied for many years. A
comprehensive discussion of the processes that govern the stratospheric sulphur
cycle can be found in the recent assessment of stratosphere aerosols (SPARC
2006). Figure 1, taken from that report, indicates some of the processes that are
important in that region.
Sulphate aerosols play additional roles in the troposphere (IPCC (2007a)and
references therein). As in the stratosphere they act to reflect incoming solar
energy (the ‘aerosol direct effect’), but also act as cloud condensation nuclei,
influencing the size of cloud droplets and the persistence or lifetime of clouds
(the ‘aerosol indirect effect’) and thus the reflectivity of clouds.
Although our focus is on stratospheric aerosols, one cannot ignore the troposphere,
and so we include a brief discussion of some aspects of the tropospheric sulphur cycle
also. A very rough budget describing the sources, sinks and transformation
pathways
1
during volcanically quiescent times is displayed in figure 2. Sources, sinks
and burdens for sulphur species are much larger in the troposphere than in the
stratosphere. The sources of the aerosol precursors are natural and anthropogenic
sulphur-bearing reduced gases (DMS, dimethyl sulphide; SO
2
, sulphur dioxide; H
2
S,
hydrogen sulphide; OCS, carbonyl sulphide). These precursor gases are gradually
oxidized (through both gaseous and aqueous reactions) to end products involving
the sulphate anion (SO2K
4) in combination with various other cations. In the
troposphere where there is sufficient ammonia, most of the aerosols exist in the form
of mixtures of ammonium sulphate ((NH
4
)
2
SO
4
) and bisulphate ((NH
4
)HSO
4
).
The stratospheric sulphur-bearing gases oxidize (primarily via reactions with the
OH radical) to SO
2
, which is then further oxidized to gaseous H
2
SO
4
. Stratospheric
sulphate aerosols exist in the form of mixtures of condensed sulphuric acid (H
2
SO
4
),
water and, under some circumstances, hydrates with nitric acid (HNO
3
).
1
Sulphur emissions and burdens are frequently expressed in differing units. They are sometimes
specified with respect to their molecular weight. Elsewhere they are specified according to the
equivalent weight of sulphur. They may be readily converted by multiplying by the ratio of
molecular weights of the species of interest. We use only units of S in this paper, and have
converted all references in other papers to these units. Also, in the stratosphere, we have assumed
that the sulphate binds with water in a ratio of 75/25 H
2
SO
4
/water to form particles. Hence
3TgSO
2K
4Z2TgSO
2Z1TgSz4 Tg aerosol particles:
P. J. Rasch et al.4010
Phil. Trans. R. Soc. A (2008)
Although the OCS source is relatively small compared with other species, owing
to its relative stability, it is the dominant sulphur-bearing species in the atmosphere.
Oxidation of OCS is a relatively small contributor to the radiatively active sulphate
aerosol in the troposphere, but it plays a larger role in the stratosphere where it
contributes perhaps half the sulphur during volcanically quiescent conditions. Some
sulphur also enters the stratosphere as SO
2
and as sulphate aerosol particles. The
reduced sulphur species oxidize there and form sulphuric acid gas. The H
2
SO
4
vapour partial pressure in the stratosphere—almost always determined by
photochemical reactions—is generally supersaturated, and typically highly super-
saturated, over its binary H
2
O–H
2
SO
4
solution droplets. The particles form and
grow through vapour deposition, depending on the ambient temperature and
concentrations of H
2
OandH
2
SO
4
. These aerosol particles are then transported by
winds (as are their precursors). Above the lower stratosphere, the particles can
evaporate, and in the gaseous form the sulphuric acid can be photolysed to SO
2
,
where it can be transported as a gas, and may again oxidize and condense in some
other part of the stratosphere. Vapour deposition is the main growth mechanism in
the ambient stratosphere, and in volcanic clouds, over time.
Because sources and sinks of aerosols are so much stronger in the troposphere, the
lifetime of sulphate aerosol particles in the troposphere is a few days, while that of
stratospheric aerosol is a year or so. This explains the relatively smooth spatial
distribution of sulphate aerosol and resultant aerosol forcing in the stratosphere,
and much smaller spatial scales associated with tropospheric aerosol.
The net source
1
of sulphur to the stratosphere is believed to be of the order of
0.1 Tg S yr
K1
during volcanically quiescent conditions. A volcanic eruption
completely alters the balance of terms in the stratosphere. For example, the eruption
0.0001
(13 days)
0.06
(1.4 days)
0.3
(10 years)
0.01
(30 days)
0.2
(2 years)
2
(1 year)
0.6
(5 days)
0.4
(2 days)
CS2,H2S,DMS OCS SO2SO4=
surface
tropopause
15 2 65 1
0.004 0.03 0.03 0.06 0.1
(sed)
0.03 0.06
32
50
51
(scav)
2
15
Figure 2. A very rough budget (approx. 1 digit of accuracy) for most of the major atmospheric
sulphur species during volcanically quiescent situations, following Rasch et al. (2000),SPARC
(2006) and Montzka et al. (2007). Numbers inside boxes indicate species burden in units of Tg S,
and approximate lifetime against the strongest source or sink. Numbers beside arrows indicate net
source or sinks (transformation, transport, emissions, and deposition processes) in Tg S yr
K1
.
4011Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
of Mount Pinatubo is believed to have injected approximately 10 Tg S (in the form of
SO
2
) over a few days. This injection amount provides a source approximately 100
times that of all other sources over the year. The partial pressure of sulphuric acid gas
consequently reaches much higher levels than those during background conditions.
After an eruption, new particles are nucleated only in the densest parts of eruption
clouds. These rapidly coagulate and disperse to concentration levels that do not
aggregate significantly. Particle aggregation is controlled by Brownian coagulation
(except perhaps under very high sulphur loadings). Coagulation mainly limits the
number of particles, rather than the overall size of the particles, which depends more
on the sulphur source strength (although considering the overall sulphur mass
balance, the two processes both contribute). The particles’ growth is thus influenced
by both vapour deposition and proximity to other particles.
The primary loss mechanism for sulphur species from the stratosphere is believed
to be the sedimentation of the aerosol particles. Particle sedimentation is governed
by Stokes’ equation for drag corrected to compensate for the fact that in the
stratosphere at higher altitudes the mean free path between air molecules can far
exceed the particle size, and particles fall more rapidly than they would otherwise.
The aerosol particles settle out (larger particles settle faster), gradually entering the
troposphere, where they are lost via wet and dry deposition processes.
Examples of the nonlinear relationships between SO
2
mass injection, particle
size and visible optical depth as a function of time assuming idealized dispersion
can be found in Pinto et al. (1998). These are detailed microphysical simulations,
although in a one-dimensional model with specified dispersion. The rate of
dilution of injected SO
2
is critical owing to the highly nonlinear response of
particle growth and sedimentation rates within expanding plumes; particles have
to be only 10 mm or less to fall rapidly, which greatly restricts the total suspended
mass, optical depth and infrared effect. The mass limitation indicates that
10 times the mass injection (of say Pinatubo) might result in only a modestly
larger visible optical depth after some months.
The life cycle of these particles is thus controlled by a complex interplay
between meteorological fields (like wind, humidity and temperature), the local
concentrations of the gaseous sulphur species, the concentration of the particles
themselves and the size distribution of the particles.
In the volcanically quiescent conditions (often called background conditions),
partial pressures of sulphur gases remain relatively low, and the particles are
found to be quite small (Bauman et al. 2003), with a typical size distribution that
can be described with a lognormal distribution with a dry mode radius, standard
deviation and effective radius of 0.05/2.03/0.17 mm, respectively. After volcanic
eruptions when sulphur species concentrations get much higher, the particles
grow much larger (Stenchikov et al. 1998). Rasch et al. (2008) used numbers for a
size distribution 6–12 months after an eruption for the large volcanic-like
distribution of 0.376/1.25/0.43 mm following Stenchikov et al. (1998) and Collins
et al. (2004). There is uncertainty in the estimates of these size distributions, and
volcanic aerosol standard distribution s
LN
was estimated to range from 1.3 to
greater than 2 in Steele & Turco (1997).
When the particles are small, they primarily scatter in the solar part of the
energy spectrum, and play no role in influencing the infrared (long-wave) part of
the energy spectrum. Larger particles seen after an eruption scatter and absorb
in the solar wavelengths, but also absorb in the infrared (Stenchikov et al. 1998).
P. J. Rasch et al.4012
Phil. Trans. R. Soc. A (2008)
Thus small particles tend to scatter solar energy back to space. Large particles
scatter less efficiently, and also trap some of the outgoing energy in the infrared.
The size of the aerosol thus has a strong influence on the climate.
(b)Geoengineering considerations
To increase the mass and number of sulphate aerosols in the stratosphere, a
new source must be introduced. Using Pinatubo as an analogue, Crutzen (2006)
estimated a source of 5 Tg S yr
K1
would be sufficient to balance the warming
associated with a doubling of CO
2
.Wigley (2006) used an energy balance model
to conclude that approximately 5 Tg S yr
K1
in combination with emission
mitigation would suffice. These studies assumed that the long-term response of
the climate system to a more gradual injection would be similar to the transient
response to a Pinatubo-like transient injection. A more realistic exploration can
be made in a climate system model (see §2d).
Rasch et al. (2008) used a coupled climate system model to show that the
amount of aerosol required to balance the warming is sensitive to particle size,
and that nonlinearities in the climate system mattered. Their model suggested
that 1.5 Tg S yr
K1
might suffice to balance the GHG warming, if the particles
looked like those during background conditions (unlikely, as will be seen in §2c),
and perhaps twice that would be required if the particles looked more like
volcanic aerosols. Robock et al. (2008) used 1.5–5 Tg S yr
K1
in a similar
study, assuming larger particle sizes (which, as will be seen in §2c, is probably
more realistic). They explored the consequences of injections in polar regions
(where the aerosol would be more rapidly flushed from the stratosphere) and
tropical injections.
All of these studies suggest that a source 15–30 times that of the current non-
volcanic sources of sulphur to the stratosphere would be needed to balance warming
associated with a doubling of CO
2
. It is important to note that in spite of this very
large perturbation to the stratospheric sulphur budget, it is a rather small
perturbation to the total sulphur budget of the atmosphere. This suggests that the
enhanced surface deposition (as for example ‘acid rain’) from a stratospheric
geoengineering aerosol would be small compared with that arising from tropospheric
sources globally, although it could be important if it occurred in a region that
normally experienced little deposition from other sources.
There are competing issues in identifying the optimal way to produce a
geoengineering aerosol. Since ambient aerosol can be a primary scavenger of new
particles and vapours, their very presence limits new particle formation. When
the stratosphere is relatively clean, the H
2
SO
4
supersaturation can build up, and
nucleation of new particles over time occurs more easily, with less scavenging of
the new particles. Thus, the engineered layer itself becomes a limiting factor in
the ongoing production of optically efficient aerosols.
Many of the earlier papers on geoengineering with stratospheric aerosols have
listed delivery systems that release sulphur in very concentrated regions, using
artillery shells, high flying jets, balloons, etc. These will release the sulphur in
relatively small volumes of air. Partial pressures of sulphuric acid gas will get
quite high, with consequences to particle growth and lifetime of the aerosols
(see §2cfor more detail).
4013
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
An alternative would be to use a precursor gas that is quite long-lived in the
troposphere but oxidizes in the stratosphere and then allow the Earth’s natural
transport mechanisms to deliver that gas to the stratosphere, and diffuse it prior
to oxidation. OCS might serve as a natural analogue to such a gas (Turco et al.
1980), although it is carcinogenic and a greenhouse gas.
Current sources of OCS are (1–2 Tg S yr
K1
(Montzka et al. 2007). Perhaps
15 per cent of that is estimated to be of anthropogenic origin. Only
approximately 0.03–0.05 Tg S yr
K1
is estimated to reach the tropopause and
enter the stratosphere (figure 2 and SPARC 2006). Residence times in the
troposphere are estimated to be approximately 1–3 years, and much longer (3–10
years) in the stratosphere. Turco et al. (1980) speculated that if anthropogenic
sources of OCS were to be increased by a factor of 10, then a substantial increase
in sulphate aerosols would result. If we assume that lifetimes do not change (and
this would require careful research in itself ), then OCS concentrations would in
fact need to be enhanced by a factor of 50 to produce a 1 Tg S yr
K1
source.
It might also be possible to create a custom molecule that breaks down in the
stratosphere that is not a carcinogen, but using less reactive species would
produce a reservoir species that would require years to remove if society needs to
stop production. Problems with this approach would be reminiscent of the
climate impacts from the long-lived chlorofluorocarbons (CFCs), although
lifetimes are shorter.
(c)Aerosol injection scenarios
An issue that has been largely neglected in geoengineering proposals to modify
the stratospheric aerosol is the methodology for injecting aerosols or their
precursors to create the desired reflective shield.
As exemplified in §2d, climate simulations to date have employed specified
aerosol parameters, including size, composition and distribution, often with
these parameters static in space and time. In this section, we consider transient
effects associated with possible injection schemes that use aircraft platforms, and
estimate the microphysical and dynamical processes that are likely to occur close
to the injection point in the highly concentrated injection stream. There are
many interesting physical limitations to such injection schemes for vapours
and aerosols, including a very high sensitivity to the induced nucleation rates
(e.g. homogeneous nucleation) that would be very difficult to quantify within
injection plumes.
Two rather conservative injection scenarios are evaluated, both assume
baseline emission equivalent to approximately 2.5 Tg S yr
K1
(which ultimately
forms approx. 10 Tg of particles) as follows: (i) insertion of a primary aerosol,
such as fine sulphate particles, using an injector mounted aboard an aircraft
platform cruising in the lower stratosphere and (ii) sulphur-enhanced fuel
additives employed to emit aerosol precursors in a jet engine exhaust stream. In
each case injection is assumed to occur uniformly between 15 and 25 km, with
the initial plumes distributed throughout this region to avoid hot spots.
Attempts to concentrate the particles at lower altitudes, within thinner layers, or
regionally—at high latitudes, for example—would tend to exacerbate problems
in maintaining the engineered layer, by increasing the particle number density
and thus increasing coagulation.
P. J. Rasch et al.4014
Phil. Trans. R. Soc. A (2008)
Our generic platform is a jet-fighter-sized aircraft carrying a payload of
10 metric tons of finely divided aerosol, or an equivalent precursor mass, to be
distributed evenly over a 2500 km flight path during an 4-hour flight (while few
aircraft are currently capable of sustained flight at stratospheric heights,
platform design issues are neglected at this point). The initial plume cross section
is taken to be 1 m
2
, which is consistent with the dimensions of the platform. Note
that, with these specifications, a total aerosol mass injection of 10 Tg of particles
per year would call for 1 million flights, and would require several thousand
aircraft operating continuously in the foreseeable future. To evaluate other
scenarios or specifications, the results described below may be scaled to a
proposed fleet or system.
(i) Particle properties
The most optically efficient aerosol for climate modification would have sizes,
R
p
, of the order of 0.1 mm or somewhat less (here we use radius rather than
diameter as the measure of particle size, and assume spherical, homogeneous
particles at all times). Particles of this size have close to the maximum
backscattering cross section per unit mass; they are small enough to remain
suspended in the rarefied stratospheric air for at least a year and yet are large
enough and thus could be injected at low enough abundances to maintain the
desired concentration of dispersed aerosol against coagulation for perhaps
months (although long-term coagulation and growth ultimately degrade the
optical efficiency at the concentrations required—see below). As the size of the
particles increases, the aerosol mass needed to maintain a fixed optical depth
increases roughly as wR
p
, the local mass sedimentation flux increases as wR4
p,
and the particle infrared absorptivity increases as wR3
p(e.g. Seinfeld & Pandis
1997). Accordingly, to achieve, and then stabilize, a specific net radiative forcing,
similar to those discussed in §2d, larger particle sizes imply increasingly greater
mass injections, which in turn accelerate particle growth, further complicating
the maintenance of the engineered layer.
This discussion assumes a monodispersed aerosol. However, an evolving
aerosol, or one maintained in a steady state, exhibits significant size dispersion.
Upper-tropospheric and stratospheric aerosols typically have a lognormal-like
size distribution with dispersion s
LN
w1.6–2.0 (ln s
LN
w0.47–0.69). Such
distributions require a greater total particle mass per target optical depth than
a nearly monodispersed aerosol of the same mean particle size and number
concentration. Accordingly, the mass injections estimated here should be
increased by a factor of approximately 2, other things remaining equal (i.e. for
s
LN
w1.6–2.0, the mass multiplier is in the range of 1.6–2.6).
(ii) Aerosol microphysics
A bottleneck in producing an optically efficient uniformly dispersed aerosol—
assuming perfect disaggregation in the injector nozzles—results from coagulation
during early plume evolution. For a delivery system with the specifications
given above, for example, the initial concentration of plume particles of radius
R
po
Z0.08 mm would be approximately 1!10
9
cm
K3
, assuming sulphate-like
particles with a density of 2 g cm
K3
. This initial concentration scales inversely
with the plume cross-sectional area, flight distance, particle specific density and
4015
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
cube of the particle radius, and also scales directly with the mass payload.
For example, if R
po
were 0.04 or 0.16 mm, the initial concentration would
be approximately 1!10
10
or 1!10
8
cm
K3
, respectively, other conditions
remaining constant.
For an injected aerosol plume, the initial coagulation time constant is
tco Z2
npoKco
;ð2:1Þ
where n
po
is the initial particle concentration (#/cm
3
)andK
co
is the self-
coagulation kernel (cm
3
s
K1
) corresponding to the initial aerosol size. For
R
po
w0.1 mm, K
N
w3!10
K9
cm
3
s
K1
(e.g. Turco et al.1979;Yu & Turco 2001).
Hence, in the baseline injection scenario, t
co
w0.07–7 s, for R
po
w0.04–0.16 mm,
respectively. To assess the role of self-coagulation, these time scales must be
compared with typical small-scale mixing rates in a stably stratified environment,
as well as the forced mixing rates in a jet exhaust wake.
Turco & Yu (1997,1998,1999) derived analytical solutions of the aerosol
continuity equation which describe the particle microphysics in an evolving
plume. The solutions account for simultaneous particle coagulation and
condensational growth under the influence of turbulent mixing, and address
the scavenging of plume vapours and particles by the entrained background
aerosol. A key factor—in addition to the previous specifications—is the growth,
or dilution, rate of a plume volume element (or, equivalently, the plume cross-
sectional area). The analytical approach incorporates arbitrary mixing rates
through a unique dimensionless parameter that represents the maximum total
number of particles that can be maintained in an expanding, coagulating volume
element at any time. Turco & Yu (1998,1999) show that these solutions can be
generalized to yield time-dependent particle size distributions, and accurately
reproduce numerical simulations from a comprehensive microphysical code.
Although aerosol properties (concentration, size) normally vary across the plume
cross section (e.g. Brown et al. 1996;Du
¨rbeck & Gerz 1996), uniform mixing is
assumed, and only the mean behaviour is considered.
(iii) Quiescent injection plumes
An otherwise passive (non-exhaust) injection system generally has limited
turbulent energy, and mixing is controlled more decisively by local environ-
mental conditions. If the quiescent plume is embedded within an aircraft wake,
however, the turbulence created by the exhaust, and wing vortices created at the
wingtips, can have a major impact on near-field mixing rates (e.g. Schumann
et al. 1998). For a quiescent plume, we adopt a linear cross-sectional growth
model that represents a small-scale turbulent mixing perpendicular to the plume
axis (e.g. Justus & Mani 1979). Observations and theory lead to the following
empirical representation for the plume volume:
VðtÞ=V0Zð1Ct=tmixÞ;ð2:2Þ
where Vis the plume volume element of interest (equivalent to the cross-
sectional area in the near-field), V
0
is its initial volume and t
mix
is the mixing
time scale. For the situations of interest, we estimate 0.1%t
mix
%10 s.
P. J. Rasch et al.4016
Phil. Trans. R. Soc. A (2008)
Following Turco & Yu (1999, eqn (73)), we find for a self-coagulating primary
plume aerosol
NpðtÞ=Npo Z1
1Cfmlnð1Cfc=fmÞ;ð2:3Þ
where N
p
is the total number of particles in the evolving plume volume element
at time t, and N
po
is the initial number. We also define the scaled time, f
c
Zt/t
co
,
and scaled mixing rate, f
m
Zt
mix
/t
co
. The local particle concentration is
npðtÞZNpðtÞ=VðtÞ.
In figure 3, predicted changes in particle number and size are illustrated as a
function of the scaled time for a range of scaled mixing rates. The ranges of
parameters introduced earlier result in an approximate range of 0.014%f
m
%140.
At the lower end, prompt coagulation causes only a small reduction in the
number of particles injected, while at the upper end reductions can exceed 90 per
cent in the first few minutes. Particle self-coagulation in the plume extending
over longer time scales further decreases the initial population—by a factor of a
1000 after one month in the most stable situation assumed here, but by only
some 10s of per cent for highly energetic and turbulent initial plumes.
The dashed line in figure 3 shows the effect of coagulation at the ‘unit mixing
time’, at which the plume volume has effectively doubled. Clearly, prompt
coagulation significantly limits the number of particles that can be injected into
the ambient stratosphere when stable stratification constrains early mixing.
Initial particle concentrations in the range of approximately 10
10
–10
11
cm
K3
would be rapidly depleted, as seen by moving down the unit mixing time line in
figure 3 (further, 10
11
cm
K3
of 0.08 mm sulphate particles exceed the density of
1.0
2.1
4.6
1011010
210310410 510 6
10–3
10–2
10–1
1
10
unit mixing time
100
1000
10 000
Figure 3. Evolution of the total concentration of particles N
p
and the mass-mean particle radius R
p
in an expanding injection plume. Both variables are scaled against their initial values in the
starting plume. The time axis (f
c
Zt/t
co
) is scaled in units of the coagulation time constant t
co
.
Each solid line, corresponding to a fixed value of f
m
, gives the changes in N
p
and R
p
for a specific
mixing time scale t
mix
measured relative to the coagulation time scale t
co
or f
m
Zt
mix
/t
co
. The
heavy dashed line shows the changes at the unit mixing time, for which f
c
Zf
m
when the plume
cross-sectional area has roughly doubled; the longer the mixing time scale, the greater the
reduction in particle abundance and particle radius.
4017Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
stratospheric air). A consequence of prompt coagulation is that it is increasingly
difficult to compensate for plume coagulation (at a fixed mass injection rate) by
reducing the starting particle size. Initial particle concentrations could
simultaneously be reduced to offset coagulation, but the necessary additional
flight activity would affect payload and/or infrastructure. It is also apparent that
rapid mass injections in the forms of liquids or powders for the purpose of
reducing flight times would lead to mass concentrations greatly exceeding those
assumed above (generally !1!10
K4
gcm
K3
), causing large particle or droplet
formation and rapid fallout.
(iv) Aerosol injection in aircraft jet exhaust
The effects of high-altitude aircraft on the upper troposphere and lower
stratosphere have been extensively studied, beginning with the supersonic
transport programmes of the 1970s and extending to recent subsonic aircraft
impact assessments (under various names) in the USA and Europe (e.g.
NASA-AEAP 1997). These projects have characterized aircraft emissions and jet
plume dynamics, and developed corresponding models to treat the various
chemical, microphysical and dynamical processes.
Enhancing aircraft fuel with added sulphur compounds (H
2
S, S
n
) could
increase the particle mass in a jet wake. It is well established that ultrafine
sulphate particles are generated copiously in jet exhaust streams during flight
(e.g. Fahey et al. 1995). The particles appear to be nucleated by sulphuric acid on
ions (hereafter chemiions, e.g. Yu & Turco (1997,1998b)) formed in the
combustion process of jet engines by radical reactions. Sulphuric acid is a
by-product of sulphur residues in the fuel (typically less than 1% sulphur by
weight); most of this fuel sulphur is emitted as SO
2
. The fraction emitted as
H
2
SO
4
decreases as the fuel sulphur content increases, and accounts for roughly 2
per cent of the total sulphur as the fuel sulphur content approaches
approximately 1 per cent.
The concentrations of chemiions in jet emissions are strongly limited by ion–
ion recombination along the engine train to approximately 1!10
9
cm
K3
at the
exit plane (e.g. Arnold et al. 2000). Considering a variety of direct measurements
of particles in jet wakes, Ka
¨rcher et al. (2000) showed that chemiion nucleation is
consistent with the observed relative constancy of the ultrafine volatile (non-
soot) particle emission factor, E
p
w1–2!10
17
kg
K1
fuel (where it should be noted
that the concentrations of soot particles are typically less than 1 per cent of the
total number of particles emitted). E
p
is quite insensitive to the fuel sulphur
content, a fact that is also consistent with a chemiion nucleation source. While
vapour trails formed in jet wakes can significantly modify the injected particle
properties (e.g. Yu & Turco 1998a), condensation trails are extremely rare under
normally dry stratospheric conditions.
If we imagine enhanced jet fuel sulphur contents of 5 per cent by weight
(10–100 times current amounts) for geoengineering purposes, then the annual
consumption of approximately 50 Tg of such fuel during stratospheric flight
(approx. half the amount used by current commercial aviation) could emit up to
2.5 Tg of sulphur that would eventually generate roughly 10 Tg of sulphate
aerosol. The total number of particles emitted—for E
p
w1!10
17
kg
K1
fuel—
would amount to approximately 5!10
27
. This number, uniformly dispersed over
P. J. Rasch et al.4018
Phil. Trans. R. Soc. A (2008)
a 10-km thick layer from 15 to 25 km, yields an average concentration of
approximately 1!10
3
cm
K3
with a particle radius of roughly 0.06 mm; in other
words, an ideal geoengineered solar shield. These estimates (i) assume no
unexpected chemistry or microphysics in the early wake that would alter the
emission factor significantly, (ii) allow for an ideal distribution of sulphate mass
among the particles, and (iii) ignore coagulation following emission.
The mixing rates in a jet wake are very rapid. Schumann et al. (1998) fit a
wide range of exhaust plume observations in the upper troposphere and lower
stratosphere with a ‘universal’ mixing curve. We use their result in the form,
V=V0Z100t0:8;tR0:0032 s:ð2:4Þ
Equation (2.4) describes, roughly, plume dilution starting at the exhaust exit
prior to mixing with turbine bypass air, through the jet zone, vortex region and
into the ambient mixing regime. Schumann et al. (1998) state that the fit is best
between 1 and 50 s. For the approximately 1!10
9
cm
K3
incipient particles in the
initial exhaust stream, the extent of self-coagulation can be projected using the
more general analytical approach discussed earlier ( Turco & Yu 1999). Thus,
even at 10
5
s, approximately three-quarters of the initial particles remain
(compared with an estimated 0.0007% if mixing were completely suppressed).
Clearly, prompt coagulation is not an issue in a jet exhaust plume.
(v) Longer term plume processing
The extended microphysical processing of an injection plume can be critical
owing to the long induction time before the plume becomes widely dispersed as a
part of the background aerosol. Yu & Turco (1999) studied the far-wake regime
of jet exhaust for upper tropospheric conditions to estimate the yield of cloud
condensation nuclei from volatile aircraft particulate emissions. In their
simulations, the background aerosol surface area density (SAD) ranged from
12.7 to 18.5 mm
2
cm
K3
for summer conditions. The resulting scavenging of fresh
plume particles amounted to approximately 95 per cent after 10 days (that is, the
effective emission index was decreased by a factor of 20). Moreover, only
approximately 1 in 10 000 of the original particles had grown to 0.08 mmat
that time, corresponding to a fuel sulphur content of 0.27 per cent by weight,
with 2 per cent emitted as H
2
SO
4
. For a geoengineering scheme with 5 per cent
fuel sulphur, although the primary exhaust sulphuric acid fraction would
probably be less than 1 per cent, the initial growth rate of the chemiions would
probably be accelerated.
At typical mixing rates, background aerosol concentrations would be present
in an injection plume within a minute or less. The natural stratosphere has an
ambient aerosol concentration of 1–10 cm
K3
, with an effective surface area of less
than 1 mm
2
cm
K3
. However, in a geoengineered stratosphere, at the desired
baseline optical depth, a SAD greater than 10 mm
2
cm
K3
would prevail. Further,
any attempt to concentrate the engineered layer regionally or vertically, or both,
would greatly exacerbate both self-coagulation and local scavenging.
The coagulation kernel for collisions of the background engineered particles
(assuming a minimum radius of approx. 0.1–0.2 mm following ageing) with jet
exhaust nanoparticles of approximately 10–80 nm is approximately 1!10
K7
to
4!10
K9
cm
3
s
K1
, respectively (Turco et al. 1979). Using a mean scavenging
4019
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
kernel for growing jet particles of approximately 2!10
K8
cm
3
s
K1
, and a
background concentration of 120 cm
K3
(determined for a doubling of the mass
injection rate to maintain the optical depth, see below), the estimated scavenging
factor is exp(K2.5!10
K6
t). After 1 day, the reduction in number is a factor of
approximately 0.80, and over 10 days, approximately 0.1, consistent with the
result of Yu & Turco (1999). Keeping in mind that the optical requirements of
the engineered layer are roughly based on total cross section (ignoring infrared
effects), while the scavenging collision kernel is also approximately proportional
to the total background particle surface area (for the particle sizes relevant to
this analysis), larger particles imply a lower concentration (and greater injection
mass loading) but about the same overall scavenging efficiency.
The background aerosol will also affect the partitioning of any injected
vapours between new and pre-existing particles. Considering the injection of SO
2
in jet exhaust as an example, it should be noted that SO
2
oxidation in the
stabilized plume should occur over roughly a day, unless oxidants are purposely
added to the plume. By this time the SO
2
would be so dilute and relative
humidity so low that additional nucleation would be unlikely.
At approximately 1 day, the residual plume exhaust particles may have achieved
sizes approaching 0.05 mm(Yu & Turco 1999). Then, considering the considerably
larger surface area of the background aerosol, only a fraction of the available
precursor vapours would migrate to new particles, with the rest absorbed on pre-
existing aerosol. Using an approach similar to that in Turco & Yu (1999), we infer
that the jet-fuel sulphur injection scenario partitions roughly 20 per cent of the
injected sulphur into new particles, with the rest adding to the background mass.
Considering the higher fuel sulphur content, and reduced number of condensation
sites, the residual injected plume particles could grow on average to approximately
0.08 mm. While this is a desirable size, the effective emission index is an order of
magnitude below that needed to maintain the desired layer under the conditions
studied. Either the fuel sulphur content or fuel consumption could be doubled to
regain the overall target reflectivity. Nevertheless, as the expanding injection
plumes merge and intermix following the early phase of coagulation scavenging, the
aerosol system undergoes continuing self-coagulation as the layer approaches, and
then maintains, a steady state. The consequences of this latter phase are not
included in these estimates.
(vi) Summary
A primary conclusion of the present analysis is that the properties of aerosols
injected directly into the stratosphere from a moving (or stationary) platform, or
in the exhaust stream of a jet aircraft, can be severely affected by prompt and
extended microphysical processing as the injection plume disperses, especially
owing to self-coagulation and coagulation scavenging by the background aerosol.
Early coagulation can increase mass requirements by a factor of 2 or more
primarily because increased particle size leads to reduced optical efficiency. In
addition, the resulting dispersion in particle sizes implies even greater mass
injections by up to a factor of approximately 2. Thus, consideration of particle
aggregation and size dispersion increases, at least by several fold, the estimated
engineering and infrastructure development effort needed to produce a required
net solar forcing. We wish to emphasize that these calculations are merely one
P. J. Rasch et al.4020
Phil. Trans. R. Soc. A (2008)
exploration of an idealized set of delivery scenarios. Many others are possible,
and would require similar sets of calculations, and, if deemed promising, far more
elaborate studies.
(d)Global modelling
Most of the studies mentioned in the previous sections calibrated their
estimates of the climate response to geoengineering aerosol (Crutzen 2006;
Wigley 2006) based upon historical observations of the aerosol produced by
volcanic eruptions. Crutzen and Wigley focused primarily upon the surface
temperature cooling, resulting from the aerosol’s shielding effect. Trenberth &
Dai (2007) analysed historical data to estimate the role of the shielding on the
–90 0 90
10
20
30
40(a)
(b)
(c)
(km)
(latitude)
0246810
6 7 8 9 101112
–1 –2 –3 –4 –5
Figure 4. (a–c) Examples of distribution of the geoengineering aerosol for June, July and August
from a 20-year simulation for a 2 Tg S yr
K1
emission. The white contour in (a) shows the region
where temperatures fall below 194.5 K, and indicates approximately where ozone depletion may be
important (see §2d(i)).
4021Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
hydrological cycle, and concluded that there would be a substantial reduction in
precipitation over land, with a consequent decrease in runoff and river discharge
to the ocean.
The analogy between a volcanic eruption and geoengineering via a sulphate
aerosol strategy is imperfect. The aerosol forcing from an eruption lasts a few
years at most, and eruptions occur only occasionally. There are many timescales
within the Earth system, and their transient response to the eruption is not likely
to be the same as the response to the continuous forcing required to counter the
warming associated with greenhouse gases. Furthermore, we have no precise
information on the role the eruptions might have on a world warmer than today.
For example, the response of the biosphere to a volcanic eruption might be
somewhat different in a warmer world than it is today. It is thus of interest to
explore the consequences of geoengineering using a climate model—a tool (albeit
imperfect) that can simulate some of the complexities of the Earth system—and
ask how the Earth’s climate might change if one could successfully introduce
particles into the stratosphere.
Govindasamy & Caldeira (2000,2003), Govindasamy et al. (2002) and
Matthews & Caldeira (2007) introduced this line of exploration, mimicking the
impact of stratospheric aerosols by reducing the solar constant to diminish the
energy entering the atmosphere (by 1.8%). These studies are discussed in more
detail elsewhere in this volume, so we will not review them further here.
Rasch et al. (2008) used a relatively simple representation of the stratospheric
sulphur cycle to study this problem. The aerosol and precursor distributions’
evolution is controlled by production, transport and loss processes as the model
atmosphere evolves. The aerosols are sensitive to changes in model climate and
this allows some feedbacks to be explored (for example changes in temperature
of the tropical tropopause, and lower stratosphere, and changes to cross tropo-
pause transport). Their model used a ‘bulk’ aerosol formulation carrying only the
aerosol mass (the particle size distribution was prescribed). They used an
atmosphere ocean general circulation model, a coupled variant of the NCAR
community atmosphere model (CAM3; Collins et al. 2006), coupled to a slab
ocean model (SOM). The model was designed to produce a reasonable climate
for the troposphere and middle atmosphere. The use of a SOM with a
thermodynamic sea ice model precluded a dynamic response from the ocean
and sea ice, which requires a more complex model such as that of Robock et al.
(2008) discussed below.
The model was used to explore the evolution of the sulphate aerosol and the
climate response to different amounts of precursor injection, and the size of the
aerosol. SO
2
was injected uniformly and continuously in a 2 km thick region at
25 km between 108N and 108S. Owing to the difficulties of modelling the particle
size evolution discussed in §2c, the study assumed the distribution to be either
‘small’ such as that seen during volcanically quiescent situations or ‘large’ such
as particles seen following an eruption. Figure 4 shows the aerosol distribution
and radiative forcing for an example simulation (assuming a 2 Tg S yr
K1
source
and particle size similar to a volcanic aerosol). We have chosen to focus on the
June, July, August season to highlight some features that disappear when
displaying annual averages. The aerosol is not distributed uniformly in space and
time. The mass of aerosol is concentrated in equatorial regions near the precursor
injection source region, and in polar regions in areas where air densities are
P. J. Rasch et al.4022
Phil. Trans. R. Soc. A (2008)
higher, and mixing into the troposphere is less than the mid-latitudes and
sub-tropics, where relatively rapid exchange with the troposphere takes place.
Aerosol burdens are the highest in the winter hemisphere, but because
solar insolation is lower there, radiative forcing is also lower than that in
the summer hemisphere. Maximum radiative forcing occurs in the high latitudes
of the summer hemisphere, acting to effectively shield the high latitudes
resulting in a substantial recovery of sea ice compared with the 2!CO
2
scenario
(Rasch et al. 2008).
While the largest forcing in the annually averaged sense occurs in equatorial
regions, the seasonal forcing is the largest in the summer hemisphere. The most
sensitivity in the response occurs at the poles, consistent with the general
behaviour of climate models to uniform radiative forcing from greenhouse gases
(IPCC 2007a), and also to the response to volcanic eruptions (Robock 2000), and
to simpler explorations of geoengineering (Govindasamy & Caldeira 2000).
Stratosphere–troposphere exchange (STE) processes respond to greenhouse gas
forcing and interact with geoengineering. Nonlinear feedbacks modulate STE
processes and influence the amount of aerosol precursor required to counteract
CO
2
warming. Rasch et al. (2008) found that approximately 50 per cent more
aerosol precursor must be injected than would be estimated if STE processes
did not change in response to greenhouse gases or aerosols. Aerosol particle size
was also found to play a role. Roughly double the aerosol mass is required
to counteract greenhouse warming if the aerosol particles were as large as
those seen during volcanic eruptions because larger particles are less effective
at scattering incoming energy and trapping some of the outgoing energy. An
estimate of 2 Tg S yr
K1
was considered to be more than enough to balance the
warming in global-mean terms from a doubling of CO
2
if particles were small
(probably unlikely), but insufficient if the particles were large. Small particles
were optimal for geoengineering through radiative effects, though they also
provided more surface area for chemical reactions to occur. The reduced single
scattering albedo of the larger particles and increased absorption in the infrared
regime lessen the impact of the geoengineering, making large particle sulphate
less effective in cooling the planet. That study also indicated the potential for
ozone depletion. Ozone depletion issues are discussed in more detail in §2d(i).
A typical surface temperature change from present day to a doubling of
current carbon dioxide levels (denoted 2!CO
2
) scenario is shown in figure 5,
along with the result of geoengineering at 2 Tg S yr
K1
(assuming a volcanic-sized
particle). The familiar CO
2
warming signal, particularly at high latitude,
is evident, with a substantial reduction resulting from geoengineering. The
simulation uses an emission rate that is not sufficient to completely counter-
balance the warming. Geoengineering at this amplitude leaves the planet
0.25–0.5 K warmer than present over most of the globe, with the largest warming
remaining at the winter pole. It is also straightforward to produce an emission
that is sufficient to over-cool the model (e.g. Rasch et al. 2008). The polar
regions and continents show the most sensitivity to the amplitude of
the geoengineering.
Robock et al. (2008; hereafter referred to as the ‘Rutgers’ study) moved to the
next level of sophistication in modelling geoengineering on the climate system.
They used the GISS atmospheric model (Schmidt et al. 2006) and included a
similar formulation for sulphate aerosols (Oman et al.2005,2006a,b) with a
4023
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
substantially lower horizontal (4!58) and vertical (23 layers to 80 km) spatial
resolution than Rasch et al. (2008). Instead of using a slab ocean and sea ice
model, they included a full ocean and sea ice representation. While Rasch et al.
(2008) examined the steady-state response of the system for present and doubled
CO
2
concentrations, Robock et al. (2008) explored solutions with transient CO
2
forcings using an IPCC A1B scenario with transient greenhouse gas forcing. They
examined the consequences of injections of aerosol precursors at various altitudes
and latitudes to a 20-year burst of geoengineering, between 2010 and 2030. We
focus on two of their injection scenarios: (i) an injection of 2.5 Tg S yr
K1
in the
tropics at altitudes between 16 and 23 km; and (ii) an injection of 1.5 Tg S yr
K1
at latitude 688N between 10 and 15 km. They chose a dry mode radius of
0.25 mm, intermediate to the ranges explored in the Rasch et al. (2008) study.
The mid-latitude injection produces a shorter lifetime for the aerosol, and
concentrates its impact on the Arctic, although, as they show (and as seen
below), it has global consequences. This type of geoengineering scenario shares
some commonalities with scenarios described by Caldeira elsewhere in this
volume. Robock et al. (2008) also showed that geoengineering is able to return
sea ice, surface temperature and precipitation patterns to values closer to the
present day values in a climate system model.
As an example, we show changes in precipitation for a few scenarios from
Rasch et al. (2008) and Robock et al. (2008) in figure 6, again for a JJA season.
Because the signals are somewhat weaker than evident in the surface
temperature changes shown above, we have hatched areas where changes exceed
2 s.d. of an ensemble of control simulations to indicate differences that are likely
to be statistically important. Figure 6a,bshows results from the NCAR model
from Rasch et al. (2008), and figure 6c,d(labelled Rutgers) shows results from
the GISS model as described in Robock et al. (2008).
As noted in IPCC (2007b), projections of changes from forcing agents to the
hydrologic cycle through climate models are difficult. Uncertainties are larger
than in projections of temperature, and important deficiencies remain in the
simulation of clouds, and tropical precipitation in all climate models, both
–4–3–2–1–0.5 0.5 1 2 3 4
(a)(b)
Figure 5. (a,b) The surface temperature difference from present day during June, July and August
with the 2!CO
2
simulation and the geoengineering simulation using 2 Tg S yr
K1
emission (which
is not sufficient to entirely balance the greenhouse warming).
P. J. Rasch et al.4024
Phil. Trans. R. Soc. A (2008)
regionally and globally, so results from models must be interpreted carefully and
viewed cautiously. Nevertheless, climate models do provide information about
the fundamental driving forces of the hydrologic cycle and its response to changes
in radiative forcing (e.g. Annamalai et al. 2007).
The NCAR results (figure 6a), consistent with IPCC (2007c)and the 20C
models summarized there, suggest a general intensification in the hydrologic
cycle in a doubled CO
2
world with substantial increases in regional maxima (such
as monsoon areas) and over the tropical Pacific, and decreases in the subtropics.
Geoengineering (figure 6b, in this case not designed to completely compensate for
the CO
2
warming) reduces the impact of the warming substantially. There are
many fewer hatched areas, and the white regions indicating differences of less
than 0.25 mm d
K1
are much more extensive.
The Rutgers simulations show a somewhat different spatial pattern, but,
again, the perturbations are much smaller than those evident in an
‘ungeoengineered world’ with CO
2
warming. Figure 6cshows the precipitation
–3–2–1
–0.50 –0.25 0.25 0.50
123
(a)(b)
(c)(d)
Figure 6. Change in precipitation associated with perturbations to greenhouse gases and
geoengineering for two models during the June, July and August months: (a,b) shows differences
between present day and doubling of CO
2
in the NCAR model CCSM using a SOM. (a) The
changes induced by 2!CO
2
.(b) The additional effect of geoengineering (with a 2 Tg S yr
K1
source). (c,d) The precipitation changes for the GISS model using an A1B transient forcing
scenario and full ocean model (between 2020 and 2030) with geoengineering. (c) The changed
distribution using 1.5 Tg S yr
K1
injection at 688N. (d) The change introduced by a 2.5 Tg S yr
K1
injection in the tropics. Hatching shows areas where difference exceeds 2 s.d. of an ensemble of
samples from a control simulation.
4025Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
distributions for the polar injection; figure 6dshows the distributions for the
equatorial injection. Both models show changes in the Indian and SE Asian
monsoon regions, and common signals in the equatorial Atlantic. There are
few common signals between the NCAR and Rutgers estimates. Robock et al.
(2008) have emphasized that the perturbations that remain in the monsoon
regions after geoengineering are considerable and expressed concern that these
perturbations would influence the lives of billions of people. This would certainly
be true. However, it is important to keep in mind that: (i) the perturbations after
geoengineering are smaller than those without geoengineering; (ii) the remaining
perturbations are less than or equal to 0. 5 mm d
K1
in an area where seasonal
precipitation rates reach 6–15 mm d
K1
; (iii) the signals differ between the NCAR
and Rutgers simulations in these regions; and (iv) monsoons are a notoriously
difficult phenomenon to model (Annamalai et al. 2007). These caveats only serve
to remind the reader about the importance of a careful assessment of the
consequences of geoengineering, and the general uncertainties of modelling
precipitation distributions in the context of climate change.
(i) Impact on chemistry and the middle atmosphere
Historically, most attention has focused on the surface chemistry responsible
for chlorine activation and ozone depletion taking place on Polar Stratospheric
Clouds, but ozone loss also occurs on sulphate aerosols, and this is evident
following volcanic eruptions (Solomon 1999;Stenchikov et al. 2002). Ozone
depletion depends upon a complex interaction between meteorological effects (for
example temperature of the polar vortex, frequency and occurrence of sudden
warmings), stratospheric photochemistry and, critically, halogen concentrations
connected with the release of CFCs in the last few decades. Reductions in the
ozone column following Pinatubo of 2 per cent in the tropics and 5 per cent in
higher latitudes were observed when particle SAD exceeded 10 (mm)
2
cm
K3
(e.g.
Solomon 1999). Rasch et al. (2008) noted that regions with high aerosol SAD
associated with geoengineering sulphate aerosol were coincident with cold
temperatures (figure 4) and indicated concern that ozone depletion might be
possible, at least until most active chlorine has been flushed from the
stratosphere (thought to occur after approx. 2050). Recently, Tilmes and
colleagues have begun to explore some aspects of ozone depletion associated with
geoengineering, and we summarize some of that work here.
Tilmes et al. (2007) estimated Arctic ozone depletion for the 1991–1992 winter
following the eruption of Mt Pinatubo based on satellite observations and
aircraft and balloon data, and found enhanced ozone loss in connection with
enhanced SAD. They used an empirical relationship connecting meteorological
conditions and ozone depletion to estimate 20–70 Dobson units (DU, a unit of
mass of ozone in a column) of extra ozone depletion from the volcanic aerosols in
the Arctic for the two winters following the eruption.
Tilmes et al.(2008)estimated the impact of geoengineered aerosols for future
halogen conditions using a similar empirical relationship, but this time including
aerosol loading and changing halogen content in the stratosphere. They based their
estimates of ozone depletion on an extrapolation of present meteorological condition
into the future, and assumptions about the amount and location of the
geoengineering aerosol. They predicted a substantial increase of chemical ozone
P. J. Rasch et al.4026
Phil. Trans. R. Soc. A (2008)
depletion in the Arctic polar regions, especially for very cold winters, and a delay of
30–70 years in the recovery of the Antarctic ozone hole. This estimate of ozone
depletion might be considered high, because they chose to impose a forcing (through
the geoengineering aerosol) that was sufficient to counter the warming from a
doubling of CO
2
even during the early twenty-first century, at a time when the
halogen content is high and the CO
2
concentrations are relatively low, so a strong
geoengineering is not required. However, even after 2050, accounting for the
projected decline in halogens and the increase in CO
2
concentrations, they found a
substantial depletion of ozone in polar regions, especially for very cold winters.
Tilmes et al. (submitted) extended their earlier calculation by using one of the
aerosol distributions calculated in Rasch et al. (2008) to explore the impact of
geoengineered sulphate aerosols. Rather than estimating ozone depletion using
the empirical relationships, the study used the interactive chemistry climate
model Whole Atmosphere Chemistry Climate Model (WACCM). The configu-
ration included an explicit representation of the photochemistry relevant to the
middle atmosphere (Kinnison et al. 2007), and a SOM. This model allows a first-
order response of the troposphere to greenhouse warming, to changes to the
middle atmosphere chemical composition and circulation structures, and exposes
the interaction between the chemistry and dynamics. As in Tilmes et al. (2008),
sulphur loading appropriate to substantially counteract the warming associated
with a doubling of CO
2
was assumed, resulting in a possible overestimation of the
impact of geoengineering before 2050.
Two simulations of the time period 2010–2050 were performed as follows: (i) a
baseline run without geoengineering aerosols; and (ii) a simulation containing
geoengineering aerosols. For the baseline run, monthly mean background values of
aerosols were assumed to match background SAGEII estimates (SPARC 2006). For
the geoengineering run, a repeating annual cycle of aerosols derived from the run
scenario labelled as ‘volc2’ from Rasch et al. (2008) was employed. That scenario
assumed aerosols with a particle size distribution similar to that following a volcanic
eruption, and an aerosol burden produced from a 2 Tg S yr
K1
injection of SO
2
.Both
model simulations used the IPCC A1B greenhouse gas scenario and changing
halogen conditions for the stratosphere. In the model simulations, the halogen
content in the stratosphere was assumed to decrease to 1980 values by approximately
2060 (Newman et al. 2006). The study thus explored the impact of geoengineering
during a period with significant amount of halogens in the stratosphere so that ozone
depletion through surface chemistry is important.
In addition to the desired cooling of the surface and tropospheric
temperatures, the enhanced sulphate aerosols in the stratosphere directly
influence middle atmosphere temperatures, chemistry and wind fields. The
temperature-dependent heterogeneous reaction rates in the stratosphere affect
the amount of ozone. Ozone plays an important role in the energy budget of the
stratosphere, absorbing incoming solar energy and outgoing energy in the
infrared. It therefore influences temperatures (and indirectly the wind field),
especially in polar regions. Additional aerosol heating also results in warmer
temperatures in the tropical lower stratosphere (between 18 and 30 km). This
results in an increase of the temperature gradient between tropics and polar
regions (as mentioned in Robock 2000). As a consequence, the polar vortex
becomes stronger and colder, and the Arctic polar vortex exists longer with
geoengineering than without, which influences polar ozone depletion.
4027
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
In the tropics and mid-latitudes, enhanced heterogeneous reactions cause a
slight increase of ozone owing to the shift of the NO
x
/NO
y
equilibrium towards
NO
y
in the region of high-aerosol loading with an increase of ozone loss rates
above and below this layer owing to the higher temperatures in the
geoengineering run. On average, the column ozone increases by 2–3 per cent
maximum at 20–308north and south. In polar regions, an increase in
heterogeneous reaction rates has a more severe impact on the ozone layer.
Chemical ozone loss in the polar vortex between early winter and spring can be
derived for both model simulations. These results can be compared with
estimates derived from observations between 1991 and 1992 and 2004 and 2005
for both hemispheres (Tilmes et al.2006,2007), as displayed in figure 7.
Estimates for present day depletion are indicated in black triangles. Estimates
for the control simulations and geoengineered atmosphere are shown in black and
red diamonds, respectively.
The WACCM model does a relatively good job of reproducing the ozone
depletion for the Antarctic vortex (figure 7b). Ozone loss decreases linearly with
time (black diamonds), and year to year variability in the model is similar to that
of the observations. The WACCM model suggests a 40–50 DU increase in ozone
depletion in the Antarctic vortex owing to geoengineering.
1980 2000 2020
y
ear
2040 2060
0
20
40
60
80
100
120
140
(a)
(b)
ozone loss (DU)
0
50
100
150
ozone loss (DU)
Figure 7. Partial chemical ozone depletion between 350 and 550 K in (a) the Arctic vortex core in
April and (b) the Antarctic vortex core in October. The depletion is estimated using the baseline
model run (black diamonds), the geoengineering run (red diamonds) and observations (Tilmes
et al. 2006; black triangles).
P. J. Rasch et al.4028
Phil. Trans. R. Soc. A (2008)
The model reproduces the depletion and variability much less realistically in the
Arctic (figure 7a). Averaged temperatures in the simulated vortex are similar to
observations, but the model does not reproduce the observed chemical response in
ozone. The simulated polar vortex is 2–58(in latitude) too small and the vortex
boundary is not as sharp as that seen in the observations. The ozone depletion starts
later in the winter owing to warmer temperatures in the beginning of the winter and
there is less illumination at the edge of the smaller vortex (necessary to produce the
depletion). Chemical ozone depletion for the WACCM3 baseline run in the Arctic is
less than half of that derived from observations. Underestimates of bromine
concentrations may also contribute to the underestimation of chemical ozone loss.
Examples of spatial changes in ozone depletion are shown in figure 8, which
displays the difference between the baseline and geoengineering runs for
Antarctica (figure 8a,b) for two winters with similar temperature structure and
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
45–45
90–90
–135 135
180
463
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
O3 (DU)
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
45–45
90–90
–135 135
180
180
135–135
90–90
–45 45
52
56
60
64
68
72
76
80
84
88
0
180
135–135
90–90
–45 45
52
56
60
64
68
72
76
80
84
88
0
512
450
425
400
375
350
325
300
275
250
225
200
175
150
O3 (DU)
(a)(b)
(c)(d)
Figure 8. Column ozone for (a,c) baseline run and (b,d) geoengineering run for two
meteorologically similar Antarctic winters in mid-October 2025 (a,b) and the coldest simulated
Arctic winters (c,d) in the beginning of April (DU, Dobson units).
4029Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
for the coldest winter of each simulation in the Arctic (figure 8c,d). Antarctic
winters show approximately 30 DU smaller column ozone values for the
geoengineering simulation. Larger ozone losses occur over a wider area of the
vortex for the geoengineering model run.
The geoengineering simulation suggests that ozone depletion will be somewhat
larger in the Arctic. The amplitude of the variability in ozone depletion is
increased in the geoengineering simulation, and colder vortex temperatures occur
during winter and spring. The coldest three winters of the geoengineering run are
1–2.58C colder than the coldest winter in the baseline run (between 20 and 25 km
in altitude) between mid-December and March. The warm Arctic winter in the
baseline case shows little ozone depletion (figure 8c). The colder temperatures and
larger vortex in the geoengineering run result in increased depletion compared
with the control. The Arctic ozone column falls below 250 DU in the vortex core
and reaches latitudes of 708N. Note that the inability of the model to reproduce
the chemical signal of observed ozone depletion in the unperturbed calculation
means one must be cautious in interpreting the model estimates for the Arctic.
These studies (Tilmes et al.2008,submitted) thus indicate that geoengineering
may have a significant impact on the ozone layer, with a possible decrease in ozone by
an additional 40–50 DU when geoengineering is employed, and a possible delay of a
few decades before the ozone recovery would begin again. More precise quantification
of these effects will require a better specification of the aerosol evolution, and more
realistic representations for the model dynamics and chemistry.
3. Summary, discussion and conclusions
Geoengineering by stratospheric aerosols as a possible means of mitigating the
climate change associated with increased greenhouse gases has been reviewed.
Sulphate aerosols in the stratosphere will increase the reflectivity of the planet
and counteract some of the effects of CO
2
warming. Part of the attraction of
using stratospheric aerosols arises because volcanic eruptions form a natural but
imperfect analogue to geoengineering. Observations following major volcanic
eruptions have demonstrated that sulphate aerosol, in sufficient amounts, will
cool the planet, and that the Earth system can survive this kind of perturbation.
Although the topic has been discussed over the last 30 years, only very recently
have attempts been made to understand the interactions between various
components of the climate system using modern tools for understanding climate
consequences. These tools provide opportunities to quantify the interactions and
consequences, and to explore those consequences on time scales that are much
longer than the influence of a single volcanic eruption.
We have shown that state-of-the-art climate models used to simulate the
Earth system produce the intended physical response to geoengineering, i.e. the
Earth does cool, and many components of the system return to a state more like
an unperturbed Earth. However,
— Our studies have shown that the delivery of aerosols or their precursors, at
least using our hypothetical aircraft, is a formidable task. For the
conservative scenarios we have explored, it would take of the order of a
million flights of 4-hour duration (2500 km) per year to deliver the nominal
P. J. Rasch et al.4030
Phil. Trans. R. Soc. A (2008)
amount of aerosol (10 Tg particles yr
K1
Z2.5 Tg S yr
K1
)neededtobalance
the warming associated with increasing greenhouse gas emissions. These
numbers are still quite rough, and it is possible that up to four times as much
sulphur might be required. We have not investigated the entire spectrum
of delivery systems. The issues and methodology we have suggested may
be relevant to other proposed delivery systems (artillery shells, balloons,
hoses, other aircraft), although details will certainly be different. It may be
possible to design more efficient methods for delivery, but all will require
careful attention to detail and the difficulty of designing a system that
produces particles of the right size over broad regions of the stratosphere
should not be underestimated.
— Although it is possible to cool the Earth to approximately the same globally
averaged surface temperature, it is not likely that all aspects of the physical system
will return to a state such as that prior to human-induced CO
2
increases. It is
important to emphasize the uncertainties in our characterization of these issues.
We have made initial exploratory forays into understanding the consequences of
geoengineering, but much work remains to be done. The high sensitivity of polar
regions to processes regulating energy in and out of the system would make it
difficult to reproduce precisely the seasonal cycle of the polar climate for a pre-
industrial (or even present day) world with geoengineering.
A recent study by Stenchikov et al. (2006) showed that models have difficulty in
capturing the regional response of the climate system to volcanic eruptions. They
argued that volcanoes’ influence on the Arctic Annular Oscillation is associated
with the extra heating in the equatorial lower stratosphere, changing the
temperature gradient in the lower stratosphere vortex and producing stronger
westerlies and a winter warming over northern Eurasia and North America.
Models identified in that paper (which were reviewed in the IPCC report) tended
to underestimate, and misplace the Northern Hemisphere winter surface
temperature warming seen over Siberia in the observations following an eruption.
This suggests that while the zeroth-order response of a surface cooling is likely to be
robust, the first-order response of other components of the climate systems is a
difficult problem and that model regional responses to stratospheric forcing
changes must be viewed with caution.
As discussed in §2d, there are also hints that rainfall patterns would be
different from an undisturbed Earth, although it is likely that they would be
much closer to that distribution than in a world with 2!CO
2
and no
geoengineering.
— An increase in aerosol burden is likely to increase ozone depletion. We have
shown that current chemistry climate models have difficulty in reproducing
quantitatively the dynamics and chemistry of the arctic middle atmosphere.
Better coupled chemistry climate models would allow an improved estimate of
ozone, sulphate aerosol and dynamical interactions. The first step is to improve
the models’ capability in reproducing present day ozone representation,
particularly for the Northern Hemisphere.
— Reductions in ozone will lead to increases in solar ultraviolet-B radiation reaching
the Earth’s surface with a potential impact on human health (Ambach &
Blumthaler 1993;Madronich & de Gruijl 1993) and biological populations
(Blaustein et al.1994). The increase in UV associated with ozone depletion could
be compensated for by increased light extinction and attenuation by the aerosol
4031
Review. Geoengineering by sulphate aerosols
Phil. Trans. R. Soc. A (2008)
cloud itself; Vogelmann et al.(1992)and Wetzel et al.(2003)explored the
compensation between these effects. Vogelmann et al. (1992) studied the effect for
volcanic eruptions and concluded that, for stratospheric aerosol with an optical
depth of 0.1–0.2 (approx. the value required for geoengineering), ozone and aerosol
effects approximately compensated. At higher aerosol amounts, the aerosol
attenuation did not balance the enhancement from ozone, and UV was enhanced
at the surface. This kind of calculation should be repeated with a focus upon
geoengineering and global warming, since ozone distributions and aerosol spatial
and particle size distributions might differ significantly for geoengineering
scenarios compared with their volcanic eruption counterpart.
—Gu et al. (2003) showed that volcanic aerosols from the Pinatubo eruption
substantially increase diffuse radiation worldwide, with a resulting enhance-
ment to photosynthesis and uptake of CO
2
. The same effect is to be
anticipated with the geoengineering shield. Govindasamy et al. (2002)
explored some aspects of interactions between the physical Earth system
and the biosphere. They showed that stabilizing the temperature but not CO
2
induced a change in Net Primary Productivity. Their study had a number
of limitations as follows: (i) they used a prescribed CO
2
concentration,
eliminating important feedbacks, (ii) they did not use a biospheric model that
included nutrient limitation, (iii) they did not include an ocean biosphere,
and (iv) their model was not sensitive to changes in the ratio of direct to
diffuse radiation.
While ecosystems can survive occasional volcanic eruptions, it is not clear
whether the consequences to ecosystems would be from long-term changes in
direct/diffuse energy, or increases in UV radiation. These issues argue for
more attention on the consequences of stratospheric aerosols to ecosystems.
The change in ratio of direct to diffuse radiation will also have an effect on solar
energy production with technologies that make use of solar concentrators.
Advances in solar energy production which operate efficiently in the presence
of diffuse radiation are also possible, but a different technology is needed.
Characterizing the consequences of geoengineering for these technologies
is worthwhile.
— As mentioned in §§1and2b,d, larger aerosol particles exhibit significant
absorption in the infrared part of the energy spectrum. The cooling resulting from
the scattering of incoming solar energy is thus partly compensated for by the
absorption in the infrared. The proclivity of this geoengineering method to form
large particles makes it a less efficient solution than it would be if small particles
were easily generated and maintained.
— There are also occasional concerns voiced about increases to acid rain from
this type of geoengineering. We have shown that, although the perturbations
to the stratospheric sulphur cycle are quite large (increasing the background
sources there by a factor of 15–30), they are perhaps 2 per cent of the total
(troposphereCstratosphere) sulphur sources. Therefore it is unlikely that
geoengineering will have a significant impact on acid deposition and the
global increment could easily be balanced by a small reduction in tropo-
spheric emissions. On the other hand, it is possible that the deposition of
the geoengineering aerosol could influence a region that normally sees little
sulphate deposition from tropospheric sources if it occurs there. This should be
looked into.
P. J. Rasch et al.4032
Phil. Trans. R. Soc. A (2008)
— It is obvious that current models of the sulphur cycle could be substantially
improved. It would be desirable to move beyond the bulk aerosol formulations
used here to models that included the evolution of the particle size distribution,
accounting explicitly for aerosol growth and coagulation. This would include a
mechanism to move from the source as determined by the delivery system, to
evolution within the plume and finally to scales resolved by a global model.
— It is clear that this geoengineering method will not alleviate the problems
engendered by absorption of CO
2
in the oceans, with a resulting decrease in
ocean pH.
Substantial reductions in greenhouse gas emissions must take place soon to
avoid large and undesirable climate impacts. This study has reviewed one
technique that might be used in a planetary emergency to mitigate some of the
effects of a projected global warming. We emphasize that, while the studies
highlighted here are a step along the way, we believe no proposal (including the
ideas explored here) has yet completed the series of steps required for a
comprehensive and thoroughly studied geoengineering mitigation strategy
occurring in the peer reviewed literature (Cicerone 2006). Our review of studies
of geoengineering by sulphate aerosols suggests it will ameliorate some
consequences of global warming. The study highlights some positive aspects of
the strategy. However, many uncertainties remain in understanding the influence
of geoengineering on the climate system (particularly on aspects related to
likely impacts on the biosphere). More work is required to understand the
costs, benefits and risks involved, and to reconcile the legal, political and ethical
issues of geoengineering.
We thank Alvia Gaskill, Ken Caldeira and three anonymous referees for their constructive
suggestions. Dennis Shea kindly provided computer scripts facilitating some of the model analysis.
Work by R.P.T. was supported by NSF grant ATM-07-30761. A.R., L.O. and G.L.S. were
supported by NSF grant ATM-0730452. P.J.R. and S.T. were supported by NCAR and the NSF.
C.-C.C. was supported by a grant from the U. Calgary. NCAR simulations were performed at
NCAR. Rutgers simulations were carried out at NASA GISS. Model development and computer
time at GISS were supported by NASA climate modelling grants. The National Center for
Atmospheric Research is sponsored by the National Science Foundation. P.J.R. thanks Diane
Pendlebury for her help in redrafting figure 1.
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