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Cooling the Earth's Surface by Stratospheric Sulphur Injections*

Paul J. Crutzen

* To be published in German in JAHRBUCH ÖKOLOGIE 2011

The warming of earth's climate by increasing CO2 and other greenhouse gases is partially countered by some backscattering to space of solar radiation by sulfate particles. This effect is enhanced as the particles also act as cloud condensation nuclei. Thereby they influence the micro-physical and optical properties of clouds, affecting cloud brightness (albedo) and precipitation. Anthropogenically enhanced sulfate particle concentrations thus cool the planet, offsetting a fraction of the anthropogenic increase in greenhouse gas warming. The cooling effect is most efficient if the sulfate particles are produced in the stratosphere.

1. An environmental policy dilemma

Near the ground the cooling effect of sulfur particles is "bought" at a substantial price. According to the World Health Organization (WHO), 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 emission reductions of SO2, and also anthropogenic organics (except black carbon), for health and ecological considerations, add to global warming and associated negative consequences, such as sea level rise. According to model calculations by Brasseur and Roeckner (2005), complete improvement in air quality could lead to a global average surface air temperature increase by 0.8 °C on most continents and 4 °C in the Arctic.

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 in the atmosphere would require a 60-80 % reduction in current anthropogenic CO2 emissions, worldwide they actually are increasing. Therefore, the usefulness of artificially enhancing earth's albedo and thereby cooling climate by adding sunlight-reflecting aerosol in the stratosphere (Budyko 1977) is here again debated as a way to defuse the Catch-22 situation just presented and counteract climate heating by growing CO2 emissions.

This can be achieved by burning S2 or H2S, carried up into the stratosphere on balloons, by rockets or artillery guns to produce SO2. The reactants might be released, distributed over time, near the tropical upward branch of the stratospheric circulation system at about 25 km altitude. In the stratosphere, chemical and micro-physical processes convert SO2 into sub-micrometer size sulfate particles. This has been observed in volcanic eruptions. For instance Mount Pinatubo in June, 1991, injected about 10 million tons of S, initially as SO2, into the tropical stratosphere (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 (Robock 2000).

2. Why inject sulfur in the stratosphere?

Although climate cooling by sulfate aerosols also occurs in the troposphere, the great advantage of placing reflective particles in the stratosphere is their long residence time of about 1-2 years, compared to a week or so in the troposphere due to uptake in precipitation. Thus, much less sulfur, only a few percent, would be required in the stratosphere to achieve similar cooling as the tropospheric sulfate aerosol (Stern 2005). This would make it possible to cut air pollution near the ground, improve ecological and health conditions and still reduce climate warming.

The main issue with the albedo modification method is whether there are side effects. According to scenarios of the Intergovernmental Panel on Climate Change (IPCC 2007), by the end of this century there will be a greenhouse warming in the range of 1.1-6.4 °C. The required stationary stratospheric sulfate loading for a doubling of CO2 would be 5.3 million tons, requiring a stratospheric input of 2.7-5.3 million tons of S per year (Crutzen 2006). This causes some whitening of the sky, but also more colorful sunsets and sunrises. It should be noted, however, that considerable whitening of the sky is already occurring in many regions of the world because of air pollution.

3. Research on the environmental impact of the S injection scheme

Given the grossly disappointing international political response to the required greenhouse gas emissions (as exemplified by the Copenhagen climate conference in December 2009), research on the feasibility and environmental consequences of climate engineering of the kind presented in this paper, might need to be deployed and should not be tabooed. Research has already started with model investigations and, dependent on their outcome, possibly to be followed, step by step, by atmospheric tests.

As natural sulfur injections occur occasionally in the form of explosive volcanic eruptions at low latitudes, they have already provided excellent opportunities for model development and testing (e.g. Robock 2000). Besides a temperature drop, global precipitation effects have been observed. Similar effects are likely following sulfur seeding in the stratosphere.

Needless to say, the possibility of adverse environmental side effects must be fully researched before countermeasures to greenhouse warming are attempted. Among the possible negative side effects, those on stratospheric ozone first spring to mind. Recent model calculations by Tilmes et al. (2009) indicate a delay by several decades in the recovery of the ozone hole.

The climatic response of the albedo enhancement experiment would take effect within about half a year, as demonstrated by the Mount Pinatubo eruption (Robock 2000). As an escape route against strongly increasing temperatures, the albedo adjustment scheme can become effective at rather short notice, at the latest when climate is heating up by more than 2 °C since pre-industrial times, the so-called "tolerable (?) window" for climate warming (Bruckner & Schellnhuber 1999), as accepted by the delegates at the recent Copenhagen climate conference. If sizeable reductions in greenhouse gas emissions will not happen and temperatures rise rapidly, then climatic engineering, such as presented here, is an option to rapidly reduce temperature rises and counteract other climatic effects. Such a modification can be stopped on short notice, if undesirable and unforeseen side effects become apparent.

There is, therefore, a strong need to estimate the negative, as well as positive, side effects of the proposed stratospheric modification scheme.

4. Additional considerations

The chances of unexpected global environmental effects should not be underrated, as 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. Climate heating is known to be particularly strong in arctic regions (Chapin et al. 2005), which may trigger accelerated biospheric CO2 and CH4 emissions.

Reductions in CO2 and other greenhouse gas emissions ought clearly be the main priority. However, this is a decades-long process and so far there is little reason to be optimistic.

There is in fact a serious additional problem. Should measures to limit CO2 emissions prove unsuccessful, growing uptake of CO2 will lead to acidification of the upper ocean waters, leading to dissolution of calcifying organisms (Royal Society 2005). CO2 sequestration in deep geological strata, if available, will then be needed to bring down atmospheric CO2 concentrations.

5. In conclusion

The arguments presented in this paper call for active scientific research on geoengineering to prevent climate to heat beyond the global average temperature "tolerable (?) window" value of 2 °C. Scientific, legal, ethical, and societal issues, regarding the climate modification scheme are many. Building trust between international politicians, scientists and the general public would be needed to make such a large-scale climate modification acceptable, even if it would appear scientifically 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. That is why I wrote this article.

P.S.: This is a revised version of an earlier paper (Crutzen 2006). Since publication by the original version a global debate on geo-engineering "erupted" and several reviews by international groups of scientists have been conducted.

In the following we will reproduce the main conclusions of the studies by the American Meteorological Society (2009) and the British Royal Society (2009).

The troposphere is the lowest region of the atmosphere, extending to 17 km in the tropics and 8 km near the poles. Extending to about 50 km above the troposphere is the stratosphere. The stratosphere is a stable layer of the atmosphere.


Bluth, G. J. S.: 1992, Geophys. Res. Lett. 19, 151-154.
Brasseur, G. P. & Roeckner, E.: 2005, Geophys. Res. Letters 32, L23704, doi:10.1029/2005GL023902.
Bruckner, T. & Schellnhuber, H. J.: 1999, 'IPTS Report 34, May, 6.
Budyko, M. I.: 1977, 'Climatic Changes, American Geophysical Society', Washington, D.C., 244 pp. Chapin III, F. S. et al.: 2005, Science 310, 657-660.
Crutzen P. J.: 2006, Climatic Change 77, 211-219.
Intergovernmental Panel for Climate Change (IPCC, 2007, Solomon et al., Climate Change 2007, The physical science base, Cambridge. Nel, A.: 2005, 'Air pollution-related illness: Effects of particles', Science 308, 804.
Royal Society: June 2005, Ocean acidification due to increasing atmospheric carbon dioxide, 57 pp.
Robock, A.: 2000, Rev. Geophys. 38, 191-219.
Stern, D. I.: 2005, 'Global sulfur emissions from 1850 to 2000', Chemosphere 58, 163-175.
Tilmes, S. et al.: 2009, Geophys. Res. 114, D12305


Geoengineering the Climate System

A Policy Statement of the American Meteorological Society (AMS)

(Adopted by the AMS Council on 20 July 2009)

Human responsibility for most of the well-documented increase in global average temperatures over the last half century is well established. Further greenhouse gas emissions, particularly of carbon dioxide from the burning of fossil fuels, will almost certainly contribute to additional widespread climate changes that can be expected to cause major negative consequences for most nations 1)

Three proactive strategies could reduce the risks of climate change:

1) mitigation: reducing emissions;

2) adaptation: moderating climate impacts by increasing our capacity to cope with them; and

3) geoengineering: deliberately manipulating physical, chemical, or biological aspects of the Earth system. 2)


This policy statement focuses on large-scale efforts to geoengineer the climate system to counteract the consequences of increasing greenhouse gas emissions.

Geoengineering could lower greenhouse gas concentrations, provide options for reducing specific climate impacts, or offer strategies of last resort if abrupt, catastrophic, or otherwise unacceptable climate-change impacts become unavoidable by other means. However, research to date has not determined whether there are large-scale geoengineering approaches that would produce significant benefits, or whether those benefits would substantially outweigh the detriments. Indeed, geoengineering must be viewed with caution because manipulating the Earth system has considerable potential to trigger adverse and unpredictable consequences.

Geoengineering proposals fall into at least three broad categories:

1) reducing the levels of atmospheric greenhouse gases through large-scale manipulations (e.g., ocean fertilization or afforestation using non-native species);

2) exerting a cooling influence on Earth by reflecting sunlight (e.g., putting reflective particles into the atmosphere, putting mirrors in space, increasing surface reflectivity, or altering the amount or characteristics of clouds); and

3) other large-scale manipulations designed to diminish climate change or its impacts (e.g., constructing vertical pipes in the ocean that would increase downward heat transport).

Geoengineering proposals differ widely in their potential to reduce impacts, create new risks, and redistribute risk among nations. Techniques that remove CO2 directly from the air would confer global benefits but could also create adverse local impacts. Reflecting sunlight would likely reduce Earth's average temperature but could also change global circulation patterns with potentially serious consequences such as changing storm tracks and precipitation patterns. As with inadvertent human-induced climate change, the consequences of reflecting sunlight would almost certainly not be the same for all nations and peoples, thus raising legal, ethical, diplomatic, and national security concerns.

Exploration of geoengineering strategies also creates potential risks. The possibility of quick and seemingly inexpensive geoengineering fixes could distract the public and policy makers from critically needed efforts to reduce greenhouse gas emissions and build society's capacity to deal with unavoidable climate impacts. Developing any new capacity, including geoengineering, requires resources that will possibly be drawn from more productive uses. Geoengineering technologies, once developed, may enable short-sighted and unwise deployment decisions, with potentially serious unforeseen consequences.

Even if reasonably effective and beneficial overall, geoengineering is unlikely to alleviate all of the serious impacts from increasing greenhouse gas emissions. For example, enhancing solar reflection would not diminish the direct effects of elevated CO2 concentrations such as ocean acidification or changes to the structure and function of biological systems.

Still, the threat of climate change is serious. Mitigation efforts so far have been limited in magnitude, tentative in implementation, and insufficient for slowing climate change enough to avoid potentially serious impacts. Even aggressive mitigation of future emissions cannot avoid dangerous climate changes resulting from past emissions, because elevated atmospheric CO2 concentrations persist in the atmosphere for a long time. Furthermore, it is unlikely that all of the expected climate-change impacts can be managed through adaptation. Thus, it is prudent to consider geoengineering's potential benefits, to understand its limitations, and to avoid ill-considered deployment.

Therefore, the American Meteorological Society (AMS) recommends:

1) Enhanced research on the scientific and technological potential for geoengineering the climate system, including research on intended and unintended environmental responses.

2) Coordinated study of historical, ethical, legal, and social implications of geoengineering that integrates international, interdisciplinary, and intergenerational issues and perspectives and includes lessons from past efforts to modify weather and climate.

3) Development and analysis of policy options to promote transparency and international cooperation in exploring geoengineering options along with restrictions on reckless efforts to manipulate the climate system.

Geoengineering will not substitute for either aggressive mitigation or proactive adaptation, but it could contribute to a comprehensive risk management strategy to slow climate change and alleviate some of its negative impacts. The potential to help society cope with climate change and the risks of adverse consequences imply a need for adequate research, appropriate regulation, and transparent deliberation.

[This statement is considered in force until July 2012 unless superseded by a new statement issued by the AMS Council before this date.]
© American Meteorological Society, 45 Beacon Street, Boston, MA 02108-3693

1) For example, impacts are expected to include further global warming, continued sea level rise, greater rainfall intensity, more serious and pervasive droughts, enhanced heat stress episodes, ocean acidification, and the disruption of many biological systems. These impacts will likely lead to the inundation of coastal areas, severe weather, and the loss of ecosystem services, among other major negative consequences.

2) These risk management strategies sometimes overlap and some specific actions are difficult to classify uniquely. To the extent that a geoengineering approach improves society's capacity to cope with changes in the climate system, it could reasonably be considered adaptation. Similarly, geological carbon sequestration is considered by many to be mitigation even though it requires manipulation of the Earth system.


Stop emitting CO2 - or geoengineering could be our only hope

The future of the Earth could rest on potentially dangerous and unproven geoengineering technologies unless emissions of carbon dioxide can be greatly reduced, the latest Royal Society report has found.

The report (published 1st September 2009, by the Royal Society (1), the UK's national academy of science) found that unless future efforts to reduce greenhouse gas emissions are much more successful than they have been so far, additional action in the form of geoengineering will be necessary if we are to cool the planet.

Geoengineering technologies were found to be very likely to be technically possible and some were considered to be potentially useful to augment the continuing efforts to mitigate climate change by reducing emissions. However, the report identified major uncertainties regarding their effectiveness, costs and environmental impacts.

Professor John Shepherd, who chaired the Royal Society's geoengineering study (2), said: "It is an unpalatable truth that unless we can succeed in greatly reducing CO2 emissions we are headed for a very uncomfortable and challenging climate future, and geoengineering will be the only option left to limit further temperature increases. Our research found that some geoengineering techniques could have serious unintended and detrimental effects on many people and ecosystems - yet we are still failing to take the only action that will prevent us from having to rely on them. Geoengineering and its consequences are the price we may have to pay for failure to act on climate change."

The report assesses the two main kinds of geoengineering techniques - Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM). CDR techniques address the root of the problem - rising CO2 - and so have fewer uncertainties and risks, as they work to return the Earth to a more normal state. They are therefore considered preferable to SRM techniques, but none has yet been demonstrated to be effective at an affordable cost, with acceptable environmental impacts, and they only work to reduce temperatures over very long timescales.

SRM techniques act by reflecting the sun's energy away from Earth, meaning they lower temperatures rapidly, but do not affect CO2 levels. They therefore fail to address the wider effects of rising CO2, such as ocean acidification, and would need to be deployed for a very long time. Although they are relatively cheap to deploy, there are considerable uncertainties about their regional consequences, and they only reduce some, but not all, of the effects of climate change, while possibly creating other problems.

The report concludes that SRM techniques could be useful if a threshold is reached where action to reduce temperatures must be taken rapidly, but that they are not an alternative to emissions reductions or CDR techniques.

Professor Shepherd added: "None of the geoengineering technologies so far suggested is a 'magic bullet', and all have risks and uncertainties associated with them. It is essential that we strive to cut emissions now, but we must also face the very real possibility that we will fail. If 'Plan B' is to be an option in the future, considerable research and development of the different methods, their environmental impacts and governance issues must be undertaken now. Used irresponsibly or without regard for possible side effects, geoengineering could have catastrophic consequences similar to those of climate change itself. We must ensure that a governance framework is in place to prevent this."

Of the CDR techniques assessed, the following were considered to have most useful potential:

CO2 capture from ambient air - this would be the preferred method of geoengineering, as it effectively reverses the cause of climate change. At this stage, no cost-effective methods have yet been demonstrated and much more research and development is needed.

Enhanced weathering - this technique, which utilises naturally occurring reactions of CO2 from the air with rocks and minerals, was identified as a prospective longer-term option. However, more research is needed to find cost-effective methods and to understand the wider environmental implications.

Land use and afforestation - the report found that land use management could and should play a small but significant role in reducing the growth of atmospheric CO2 concentrations. However, the scope for applying this technique would be limited by land use conflicts, and all the competing demands for land must be considered when assessing the potential for afforestation and reforestation.

Should temperatures rise to such a level where more rapid action needs to be taken, the following SRM techniques were considered to have most potential:

Stratospheric aerosols - these were found to be feasible, and previous volcanic eruptions have effectively provided short-term preliminary case studies of the potential effectiveness of this method. The cost was assessed as likely to be relatively low and the timescale of action short. However, there are some serious questions over adverse effects, particularly the depletion of stratospheric ozone.

Space-based methods - these were considered to be a potential SRM technique for long-term use, if the major problems of implementation and maintenance could be solved. At present, the techniques remain prohibitively expensive, complex and would be slow to implement.

Cloud albedo approaches - the effects would be localised and the impacts on regional weather patterns and ocean currents are of considerable concern but are not well understood. The feasibility and effectiveness of the technique is uncertain. A great deal more research would be needed before this technique could be seriously considered.

The following techniques were considered to have lower potential:

Biochar (CDR technique) - the report identified significant doubts relating to the potential scope, effectiveness and safety of this technique and recommends that substantial research would be required before it could be considered for eligibility for UN carbon credits.

Ocean fertilisation (CDR technique) - the report found that this technique had not been proved to be effective and had high potential for unintended and undesirable ecological side effects.

Surface albedo approaches (SRM technique, including white roof methods, reflective crops and desert reflectors) - these were found to be ineffective, expensive and, in some cases, likely to have serious impacts on local and regional weather patterns.

Catherine de Lange | Source: alphagalileo