ALBEDO ENHANCEMENT BY STRATOSPHERIC
SULFUR
INJECTIONS: A CONTRIBUTION TO RESOLVE A POLICY
DILEMMA?
"You take the blue pill and the story ends. You wake in your bed and you believe whatever you want to believe."
"You take the red pill and
you stay in Wonderland and I'll show you how deep the
rabbit-hole goes."
An Editorial Essay
Fossil fuel burning releases about 25 Pg of CO2 per year into the
atmosphere, which leads to global warming (Prentice et al., 2001).
However, it also emits 55 Tg S as SO2 per year (Stern, 2005), about
half of which is converted to submicrometer size sulfate particles,
the remainder being dry deposited. Recent research has shown that
the warming of earth by the increasing concentrations of CO2 and
other greenhouse gases is partially countered by some backscattering
to space of solar radiation by the sulfate particles, which act as
cloud condensation nuclei and thereby influence the micro-physical
and optical properties of clouds, affecting regional precipitation
patterns, and increasing cloud albedo (e.g., Rosenfeld, 2000;
Ramanathan et al., 2001; Ramaswamy et al., 2001). Anthropogenically
enhanced sulfate particle
concentrations thus cool the planet, offsetting an uncertain
fraction of the anthropogenic increase in greenhouse gas warming.
However, this fortunate coincidence is “bought” at a substantial
price. According to the World Health Organization, the pollution
particles affect health and lead to more than 500,000 premature
deaths per year worldwide (Nel, 2005). Through acid precipitation
and deposition, SO2 and sulfates also cause various kinds of
ecological damage. This creates a dilemma for environmental policy
makers, because the required emission reductions of SO2, and also
anthropogenic organics (except black carbon), as dictated by health
and ecological considerations, add to global warming and associated
negative consequences, such as sea level rise, caused by the
greenhouse gases. In fact, after earlier rises, global SO2 emissions
and thus sulfate loading have been declining at the rate of 2.7% per
year, potentially explaining the observed reverse from dimming to
brightening in surface solar radiation at many stations worldwide
(Wild et al., 2005). The corresponding increase in solar radiation
by 0.10% per year from 1983 to 2001 (Pinker et al., 2005)
contributed to the observed climate warming during the past decade.
According to model calculations by Brasseur and Roeckner (2005)
complete improvement in air quality could lead to a decadal global
average surface air temperature increase by 0.8 K on most continents
and 4 K in the Arctic.
Further studies by Andreae et al. (2005) and Stainforth et al.
(2005) indicate that global average climate warming during this
century may even surpass the highest values in the projected IPCC
global warming range of 1.4–5.8 ◦ C (Cubasch et al., 2001). Climatic
Change (2006) DOI: 10.1007/s10584-006-9101-y c Springer 2006
By far the preferred way to
resolve the policy makers’ dilemma is to lower the emissions of the
greenhouse gases. However, so far, attempts in that direction have
been grossly unsuccessful. While stabilization of CO2 would require
a
60–80% reduction in current anthropogenic CO2 emissions, worldwide
they actually increased by 2% from 2001 to 2002 (Marland et al.,
2005), a trend, which probably will not change at least for the
remaining 6-year term of the Kyoto pro-
tocol, further increasing the required emission restrictions.
Therefore, although by far not the best solution, the usefulness of
artificially enhancing earth’s albedo and thereby cooling climate by
adding sunlight reflecting aerosol in the stratosphere
(Budyko, 1977; NAS, 1992) might again be explored and debated as a
way to defuse the Catch-22 situation just presented and additionally
counteract the climate forcing of growing CO2 emissions. This can be
achieved by burning S2
or H2S, carried into the stratosphere on balloons and by artillery
guns to produce SO2.
To enhance the residence time of the material in the stratosphere
and minimize the required mass, the reactants might be released,
distributed over time, near the tropical upward branch of the
stratospheric circulation system. In the stratosphere,
chemical and micro-physical processes convert SO2 into
sub-micrometer sulfate particles. This has been observed in volcanic
eruptions e.g., Mount Pinatubo in June, 1991, which injected some 10
Tg S, initially as SO2, into the tropical stratosphere (Wilson et
al., 1993; Bluth et al., 1992). In this case enhanced reflection of
solar radiation to space by the particles cooled the earth’s surface
on average by 0.5 C in the year following the eruption (Lacis and
Mishchenko, 1995).
Although climate cooling by sulfate aerosols also occurs in the troposphere (e.g., Ramaswamy et al., 2001), the great advantage of placing reflective particles in the stratosphere is their long residence time of about 1–2 years, compared to a week in the troposphere. Thus, much less sulfur, only a few percent, would be required in the stratosphere to achieve similar cooling as the tropospheric sulfate aerosol (e.g., Dickinson, 1996; Schneider, 1996; NAS, 1992; Stern, 2005). This would make it possible to reduce air pollution near the ground, improve ecological conditions and reduce the concomitant climate warming. The main issue with the albedo modification method is whether it is environmentally safe, without significant side effects.
We will next derive some useful metrics. First, a loading of 1 Tg S
in the stratosphere yields a global average vertical optical depth
of about 0.007 in the visible and corresponds to a global average
sulfur mixing ratio of ∼1 nmol/mole, about
six times more than the natural background (Albritton et al., 2001).
Second, to derive the radiative forcing caused by the presence of 1
Tg S in the stratosphere, we adopt a simple approach based on the
experience gained from the Mount Pinatubo volcanic eruption. For the
Mount Pinatubo eruption, Hansen et al. (1992) calculated a radiative
cooling of 4.5 W/m 2 caused by 6 Tg S, the amount of S that remained
in the stratosphere as sulfate six months after the eruption from
initially 10 Tg S (Bluth et al., 1992). Linear downscaling results
in a sulfate climate cooling efficiency of 0.75 W/m 2 per Tg S in
the stratosphere. The estimated annual cost to put 1 Tg S in the
stratosphere, based on information by the NAS (1992), at that time
would have been US $25 billion (NAS, 1992; Ron Nielsen, personal
communication). Thus, in order to compensate for enhanced climate
warming by the removal of anthropogenic aerosol (an uncertain mean
value of 1.4 W/m 2, according to Crutzen and Ramanathan (2003)), a
stratospheric sulfate loading of 1.9 Tg S would be required,
producing an optical depth of 1.3%. This can be achieved by a
continuous deployment of about 1–2 Tg S per year for a total price
of US $25–50 billion, or about $25–50 per capita in the affluent
world, for stratospheric residence times of 2 to 1 year,
respectively. The cost should be compared with resulting
environmental and societal benefits, such as reduced rates of sea
level rise. Also, in comparison, current annual global military
expenditures approach US$1000 billion, almost half in the U.S.A. The
amount of sulfur that is needed is only 2–4% of the current input of
55 Tg S/year (Stern, 2005). Although the particle sizes of the
artificial aerosols are smaller than those of the volcanic aerosol,
because of greater continuity of injections in the former, the
radiative forcings are rather similar for effective particle radii
ranging between 0.1 and 1 μm (see Table 2.4, page 27, Lacis and
Mishchenko, 1995). However the smaller particles have a longer
stratospheric residence time, so that less material needs to be
injected to cool climate, compared to the volcanic emission case. It
should be mentioned that Anderson et al. (2003a,b) state that the
radiative cooling by the aerosol could be much larger than the
figure of 1.4 W/m 2, derived by Crutzen and Ramanathan (2003), which
is based on the assumption of constant relative humidity in the
troposphere.
If Anderson et al. (2003a,b) are indeed correct, the result might be
a stronger climate heating from air pollution cleanup than derived
above (see also Andreae et al., 2005). To compensate for a doubling
of CO2, which causes a greenhouse warming of 4 W/m 2, the required
continuous stratospheric sulfate loading would be a sizeable 5.3 Tg
S, producing an optical depth of about 0.04. The Rayleigh scattering
optical depth at 0.5 μm is about 0.13, so that some whitening on the
sky, but also colorful sunsets and sunrises would occur. It should
be noted, however, that considerable whitening of the sky is already
occurring as a result of current air pollution in the continental
boundary layer.
Locally, the stratospheric albedo modification scheme, even when
conducted at remote tropical island sites or from ships, would be a
messy operation. An alternative may be to release a S-containing gas
at the earth’s surface, or better from
balloons, in the tropical stratosphere. A gas one might think of is
COS, which may be the main source of the stratospheric sulfate layer
during low activity volcanic periods (Crutzen, 1976), although this
is debated (Chin and Davis, 1993).
However, about 75% of the COS emitted will be taken up by plants,
with unknown long-term ecological consequences, 22% is removed by
reaction with OH, mostly in the troposphere, and only 5% reaches the
stratosphere to produce SO2
and sulfate particles (Chin and Davis, 1993). Consequently,
releasing COS at the ground is not recommended. However, it may be
possible to manufacture a special gas that is only processed
photochemically in the stratosphere to yield sulfate.
The compound should be non-toxic, insoluble in water, non-reactive with OH, it should have a relatively short lifetime of less than about 10 years, and should not significantly contribute to greenhouse warming, which for instance disqualifies SF6.
The albedo modification scheme presented here has been discussed
before, however, without linking opposite climate warming and
improved air quality considerations. Instead of sulfur, it ha also
been proposed to launch reflecting small
balloons or mirrors, or to add highly reflective nano-particles of
other material than sulfur (Teller et al., 1997; Keith, 2000)
An interesting alternative could be to release soot particles to create minor “nuclear winter” conditions. In this case earth’s albedo would actually decrease, but surface temperatures would, nevertheless, decline. Only 1.7% of the mass of sulfur would be needed to effect similar cooling at the earth’s surface, making the operations much cheaper and less messy. However, because soot particles absorb solar radiation very efficiently, differential solar heating of the stratosphere could change its dynamics. It would, however, also counteract stratospheric cooling by increasing CO2 and may even prevent the formation of polar stratospheric cloud particles, a necessary condition for ozone hole formation.
Since it is likely that the greenhouse warming is substantially
negated by the cooling effect of anthropogenic aerosol in the
troposphere, by 25–65% according to an estimate by Crutzen and
Ramanathan (2003), but possibly greater (Anderson
et al., 2003a,b), air pollution regulations, in combination with
continued growing emissions of CO2, may bring the world closer than
is realized to the danger described by Schneider (1996): “Supposing,
a currently envisioned low probability
but high consequence outcome really started to unfold in the decades
ahead (for example, 5 ◦C warming in this century) which I would
characterize as having potential catastrophic implications for
ecosystems... Under such a scenario, we would simply have to
practice geo-engineering...”
There are some worrying indications of potentially large climate
changes: for instance the locally drastic atmospheric warming by up
to 3 W/m 2 per decade in Alaska due to surface albedo decreases
through tree and shrub expansion (Chapin III et al., 2005), the
projected increase in surface temperatures by 2–3 K by the middle of
this century in Africa even with the Kyoto protocol in force (B.
Hewitson, University of Cape Town, quoted by Cherry, 2005) with
great impacts on biodiversity, and potentially also the 30% slowdown
in the north Atlantic overturning circulation during the past half
century (Bryden et al., 2005). Given the grossly disappointing
international political response to the required greenhouse gas
emissions, and further considering some drastic results of recent
studies (Andreae et al.,2005; Stainforth et al., 2005), research on
the feasibility and environmental consequences of climate
engineering of the kind presented in this paper, which might need to
be deployed in future, should not be tabooed. Actually, considering
the great importance of the lower stratosphere/upper troposphere
(LS/UT) for the radiation balance, chemistry, and dynamics of the
atmosphere, its research should anyhow be intensified. For instance,
it is not well known how much of the large quantities of
anthropogenic SO2 emitted at ground level reaches the LS/UT to
produce sulfate particles, what regulates temperatures, water vapour
concentrations and cirrus cloud formation in the LS/UT region, and
how these factors may change in response to growing CO2
concentrations, which are already 30–40% higher than ever
experienced during the past 650,000 years (Siegenthaler et al.,
2005). Progress in the understanding of the complicated earth
climate system is generally slow.
Therefore it is recommended to intensify research in order to
challenge the climate modification idea here presented, starting
with model investigations and, dependent on their outcome, followed
step by step by small scale atmospheric tests. Also, as natural
sulfur injection experiments occur intermittently in the form of
explosive volcanic eruptions, often at low latitudes, they provide
excellent opportunities for model development and testing (e.g.,
Robock, 2000). Researchers at the Lawrence Livermore Laboratory are
so far the only ones who have modelled the stratospheric albedo
modification scheme. In a first study, Govindasamy and Caldeira
(2000) simulated this by reducing the solar luminosity by 1.8%, to
balance future climate warming by a doubling of CO2. Although solar
radiative forcing has a different physics and spatial distribution
than the infrared effects caused by CO2, the model results indicated
that the global temperature response by both perturbations at the
Earth’ surface and atmosphere largely cancelled out. Although these
preliminary model results would be in favor a stratospheric sulfur
injection operation, the required annual S inputs are large, so that
the possibility of adverse environmental side effects needs to be
fully researched before the countermeasure to greenhouse warming is
attempted. What has to be done first, is to explore whether using a
sulfur injection scheme with advanced micro-physical and radiation
process descriptions will show similar model results as the
simple solar luminosity adjustment scheme of Govindasamy and
Caldeira (2000). Further studies, following those conducted by
Govindasamy (2003), should
address the biological effects of the albedo modification scheme. As
already mentioned, injection of soot may be an alternative, but in
need of critical analysis.
Such studies by themselves, even when the experiment is never done,
will be very informative. Among possible negative side effects,
those on stratospheric ozone first spring to mind. Fortunately, in
this case one can build on the experience with past volcanic
eruptions, such as El Chichon in 1982 and Mount Pinatubo in 1991,
which injected 3–5 Tg S (Hofmann and Solomon, 1989) and 10 Tg S (Bluth
et al., 1992), respectively, in the stratosphere. Local ozone
destruction in the El Chichon case was about 16% at 20 km altitude
at mid-latitudes (Hofmann and Solomon, 1989). For Mount Pinatubo,
global column ozone loss was about 2.5% (Kinnison et al., 1994). For
the climate engineering experiment, in which the cooling effect of
all tropospheric anthropogenic aerosol is removed, yielding a
radiative heating of 1.4 W/m 2 (Crutzen and Ramanathan, 2003), a
stratospheric loading of almost 2 Tg S, and an input of 1–2 Tg S/yr
is required, depending on stratospheric residence times. In this
case, stratospheric sulfate injections would be 5 times less than
after the Mount Pinatubo eruption, leading to much smaller
production of ozone-destroying Cl and ClO radicals, whose formation
depends on particle surface-catalyzed heterogeneous reactions
(Wilson, 1993). Compensating for a CO2 doubling would
lead to larger ozone loss but not as large as after Mount Pinatubo.
Furthermore, the amounts of stratospheric chlorine radicals, coming
from past production of the chloro-fluoro-carbon gases, are now
declining by international regulation, so that ozone will
significantly recover by the middle of this century. If instead of
SO2, elemental carbon would be injected in the stratosphere, higher
temperatures might prevent the formation of polar stratospheric ice
particles and thereby hinder the formation of ozone holes. This and
the consequences of soot deposition on polar glaciers should be
checked by model calculations.
In contrast to the slowly developing effects of greenhouse warming
associated with anthropogenic CO2 emissions, the climatic response
of the albedo enhancement experiment would start taking effect
within about half a year, as demonstrated by the Mount Pinatubo
eruption (Hansen et al., 1992). Thus, provided the technology to
carry out the stratospheric injection experiment is in place, as an
escape route against strongly increasing temperatures, the albedo
adjustment scheme can become effective at rather short notice, for
instance if climate heats up by more than 2 ◦ C globally or when the
rates of temperatures increase by more than 0.2 ◦ C/decade), i.e.
outside the so-called “tolerable window” for climate warming (e.g.,
Bruckner and Schellnhuber, 1999). Taking into account the warming of
climate by up to 1 ◦ C by air pollution reduction (Brasseur and
Roeckner, 2005), the tolerable window for greenhouse gas emissions
might be as low as 1◦C, noteven counting positive biological
feedbacks. As mentioned before, regionally more rapid climate
changes are already happening in the Arctic (Chapin et al., 2005) or
are in petto for Africa (Cherry, 2005). Already major species
extinctions by current climate warming have been reported by Pounds
et al. (2005) and Root et al. (2003). If sizeable reductions in
greenhouse gas emissions will not happen and temperatures rise
rapidly, then climatic engineering, such as presented here, is the
only option available to rapidly reduce temperature rises and
counteract other climatic effects. Such a modification could also be
stopped on short notice, if undesirable and unforeseen side effects
become apparent, which would allow the atmosphere to return to its
prior state within a few years. There is, therefore,
a strong need to estimate negative, as well as positive, side
effects of the proposed stratospheric modification schemes. If
positive effects are greater than the negative effects, serious
consideration should be given to the albedo modification
scheme.
Nevertheless, again I must stress here that the albedo enhancement
scheme should only be deployed when there are proven net advantages
and in particular when rapid climate warming is developing,
paradoxically, in part due to improve-
ments in worldwide air quality. Importantly, its possibility should
not be used to justify inadequate climate policies, but merely to
create a possibility to combat potentially drastic climate heating
(e.g. Andreae et al., 2005; Stainforth et al., 2005; Crutzen and
Ramanathan, 2003; Anderson et al., 2003a,b). The chances of
unexpected climate effects should not be underrated, as clearly
shown by the sudden and unpredicted development of the antarctic
ozone hole. Current CO2
concentrations are already 30–40% larger than at any time during the
past 650,000 years (Siegenthaler et al., 2005). Climate heating is
known to be particularly strong in arctic regions (Chapin et
al., 2005), which may trigger accelerated CO2 and CH4 emissions in a
positive feedback mode. Earth system is increasingly in the
non-analogue condition of the Anthropocene.
Reductions in CO2 and other greenhouse gas emissions are clearly the
main priorities (Socolow et al., 2004; Lovins, 2005). However, this
is a decades-long process and so far there is little reason to be
optimistic. There is in fact a serious additional issue. Should the
proposed solutions to limit CO2 emissions prove unsuccessful and
should CO2 concentrations rise to high levels with risk of
acidification of the upper ocean waters, leading to dissolution of
calcifying organisms
(Royal Society, 2005; Orr et al., 2005), underground CO2
sequestration (Lackner, 2003, if proven globally significant , will
be needed to bring down atmospheric CO2 concentrations. However,
that kind of sequestration does not allow for rapid remedial
response. Reforestation could do so, but has its own problems. A
combination of efforts may thus be called for, including the
stratospheric albedo enhancement scheme.
In conclusion: The first modelling results and the arguments
presented in this paper call for active scientific research of the
kind of geo-engineering, discussed in this paper. The issue has come
to the forefront, because of the dilemma facing inter-
national policy makers, who are confronted with the task to clean up
air pollution, while simultaneously keeping global climate warming
under control. Scientific, legal, ethical, and societal issues,
regarding the climate modification scheme are
many (Jamieson, 1996; Bodansky, 1996). Building trust between
scientists and the general public would be needed to make such a
large-scale climate modification acceptable, even if it would be
judged to be advantageous. Finally, I repeat: the very best would be
if emissions of the greenhouse gases could be reduced so much that
the stratospheric sulfur release experiment would not need to take
place. Currently, this looks like a pious wish.
Acknowledgements
Thanks go to many colleagues, in particular Ron Nielsen for advice
on cost estimates, and to him, and colleagues V. Ramanathan, Jos
Lelieveld, Carl Brenninkmeijer, Mark Lawrence, Yoya Joseph, and
Henning Rodhe for advice and criticism on this paper. Part of this
study was conducted during a stay at the International Institute of
Advanced Systems Analysis (IIASA) in Laxenburg, Austria and
discussed with Bob Ayres and Arnulf Grubler.
.
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CRUTZEN
Max-Planck-Institute for Chemistry
PAUL J
.
CRUTZEN
Department of Atmospheric Chemistry
P.O. Box 3060, 55020 Mainz, Germany
E-mail: crutzen@mpch-mainz.mpg.de
and
Scripps Institution of Oceanography
University of California, San Diego
9500 Gilman Drive MC 0239-0221
La Jolla, CA 92093-0239, USA
E-mail: crutzen@fiji.ucsd.edu
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