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Stratospheric sulfur aerosols (geoengineering)
Encyclopedia
The ability of stratospheric sulfate aerosols to create a global dimming
effect has made them a possible candidate for use in geoengineering
projects to limit the effect and impact of climate change
due to rising levels of greenhouse gases. Delivery of precursor sulfide gases such as hydrogen sulfide
(H2S) or sulfur dioxide
(SO2) by artillery
, aircraft
and balloons has been proposed.
Tom Wigley calculated the impact of injecting sulfate particles, or aerosols, every one to four years into the stratosphere in amounts equal to those lofted by the volcanic eruption of Mount Pinatubo
in 1991, but did not address the many technical and political challenges involved in potential geoengineering efforts. If found to be economically, environmentally and technologically viable, such injections could provide a "grace period" of up to 20 years before major cutbacks in greenhouse gas emissions would be required, he concludes.
Direct delivery of precursors is proposed by Paul Crutzen. This would typically be achieved using sulfide gases such as dimethyl sulfide
, sulfur dioxide
(SO2), carbonyl sulfide
, or hydrogen sulfide
(H2S). These compounds would be delivered using artillery, aircraft (such as the high-flying F15C) or balloons, and result in the formation of compounds with the sulfate anion SO42-.
According to estimates by the Council on Foreign Relations
, "one kilogram of well placed sulfur in the stratosphere would roughly offset the warming effect of several hundred thousand kilograms of carbon dioxide."
(H2SO4). This acidic liquid solution is in the form of a vapor
and condenses onto particles of solid matter, either meteor
itic in origin or from dust carried from the surface to the stratosphere. Secondary or heterogeneous aerosol
formation occurs when H2SO4 vapor condenses onto existing aerosol particles. Existing aerosol particles or droplets also run into each other, creating larger particles or droplets in a process known as coagulation
. Warmer atmospheric temperatures also lead to larger particles. These larger particles would be less effective at scattering sunlight
because the peak light scattering is achieved by particles with a diameter of 0.3 μm.
in general is a controversial technique, and carries problems and risks, such as weaponisation. However, certain problems are specific to, or more pronounced with this particular technique.
Further, the delivery methods may cause significant problems, notably climate change
and possible ozone depletion
in the case of aircraft
, and litter
in the case of untethered balloons.
, which varies from 11 km (6.8 miles/36,000 feet) at the poles to 17 km (11 miles/58,000 feet) at the equator.
Global dimming
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...
effect has made them a possible candidate for use in geoengineering
Geoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...
projects to limit the effect and impact of climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
due to rising levels of greenhouse gases. Delivery of precursor sulfide gases such as hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
(H2S) or sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
(SO2) by artillery
Artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...
, aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
and balloons has been proposed.
Tom Wigley calculated the impact of injecting sulfate particles, or aerosols, every one to four years into the stratosphere in amounts equal to those lofted by the volcanic eruption of Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...
in 1991, but did not address the many technical and political challenges involved in potential geoengineering efforts. If found to be economically, environmentally and technologically viable, such injections could provide a "grace period" of up to 20 years before major cutbacks in greenhouse gas emissions would be required, he concludes.
Direct delivery of precursors is proposed by Paul Crutzen. This would typically be achieved using sulfide gases such as dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
, sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
(SO2), carbonyl sulfide
Carbonyl sulfide
Carbonyl sulfide is the chemical compound with the formula OCS. Commonly written as COS, it is a colourless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl group double bonded to a sulfur atom...
, or hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
(H2S). These compounds would be delivered using artillery, aircraft (such as the high-flying F15C) or balloons, and result in the formation of compounds with the sulfate anion SO42-.
According to estimates by the Council on Foreign Relations
Council on Foreign Relations
The Council on Foreign Relations is an American nonprofit nonpartisan membership organization, publisher, and think tank specializing in U.S. foreign policy and international affairs...
, "one kilogram of well placed sulfur in the stratosphere would roughly offset the warming effect of several hundred thousand kilograms of carbon dioxide."
Aerosol formation
Primary aerosol formation, also known as homogeneous aerosol formation results when gaseous SO2 combines with water to form aqueous sulfuric acidSulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
(H2SO4). This acidic liquid solution is in the form of a vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
and condenses onto particles of solid matter, either meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
itic in origin or from dust carried from the surface to the stratosphere. Secondary or heterogeneous aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
formation occurs when H2SO4 vapor condenses onto existing aerosol particles. Existing aerosol particles or droplets also run into each other, creating larger particles or droplets in a process known as coagulation
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
. Warmer atmospheric temperatures also lead to larger particles. These larger particles would be less effective at scattering sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
because the peak light scattering is achieved by particles with a diameter of 0.3 μm.
Arguments for the technique
The arguments in favour of this approach are:- Natural process — Stratospheric sulfur aerosols are created by existing atmospheric processes (especially volcanoes), the behaviour of which has been studied observationally. Other, more speculative geoengineering schemes, do not have natural analogs (e.g. space sunshade).
- Speed of action — Solar radiation managementSolar radiation managementSolar radiation management projects are a type of geoengineering which seek to reflect sunlight and thus reduce global warming. Examples include the creation of stratospheric sulfur aerosols. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems...
works quickly, in contrast to carbon sequestration projects such as carbon dioxide air captureCarbon dioxide air captureCarbon dioxide removal methods refers to a number of technologies which reduce the levels of carbon dioxide in the atmosphere. Among such technologies are bio-energy with carbon capture and storage, biochar, direct air capture, ocean fertilization and enhanced weathering.CDR is a different...
which would take longer to have an effect, as the latter relies on removing large amounts of carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
before they become effective; however, gaps in understanding of these processes exist (e.g. the effect on stratospheric climate and on rainfall patterns) and further research is needed. - Technological feasibility — In contrast to other geoengineering schemes, such as space sunshade, the technology required is pre-existing: chemical manufacturing, artillery shells, fighter aircraftFighter aircraftA fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
, weather balloons, etc. - Cost — The low-tech nature of this approach has led commentators to suggest it will cost less than many other interventions. Costs cannot be derived in a wholly objective fashion, as pricing can only be roughly estimated at an early stage. However, an assessment reported in Newscientist suggests it would be cheap relative to cutting emissions. According to Paul Crutzen annual cost of enough stratospheric sulfur injections to counteract effects of doubling CO2 concentrations would be $25–50 billion a year. This is over 100 times cheaper than producing the same temperature change by reducing CO2 emissions.
- Efficacy — Most geoengineering schemes can only provide a limited intervention in the climate - one cannot reduce the temperature by more than a certain amount with each technique. New research by Lenton and Vaughan suggests that this technique may have a high radiative 'forcing potential'.
- Tipping points — Application of this technique may prevent climate tipping elementsTipping point (climatology)A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...
, such as the loss of the Greenland ice sheetGreenland ice sheetThe Greenland ice sheet is a vast body of ice covering , roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost long in a north-south direction, and its greatest width is at a latitude of 77°N, near its...
Efficacy problems
All geoengineering schemes have potential efficacy problems, due to the difficulty of modelling their impact and the inherently complex nature of the global climate system. Nevertheless, certain efficacy issues are specific to the use of this particular technique.- Lifespan of aerosols — Tropospheric sulfur aerosols are short lived. Delivery of particles into the lower stratosphere will typically ensure that they remain aloft only for a few weeks or months. To ensure endurance, high-level delivery is needed, ensuring a typical endurance of several years. Further, sizing of particles is crucial to their endurance.
- Aerosol delivery — Even discounting the challenges of lifting, there are still significant challenges in designing a delivery system that is capable of delivering the precursor gases in the right manner to encourage effective aerosol formation. For example, it is unclear whether aerial shells should be designed to leak slowly or burst suddenly. The size of aerosol particles is also crucial, and efforts must be made to ensure optimal delivery.
- Distribution — It is logistically difficult to deliver aerosols evenly around the globe. Challenges therefore exist in creating a network of delivery points sufficient to allow viable geoengineering from a limited number of launching sites.
Possible side effects
GeoengineeringGeoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...
in general is a controversial technique, and carries problems and risks, such as weaponisation. However, certain problems are specific to, or more pronounced with this particular technique.
- DroughtDroughtA drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
, particularly monsoonMonsoonMonsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
failure in AsiaAsiaAsia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
and AfricaAfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
is a major risk. - Ozone depletionOzone depletionOzone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...
is a potential side effect of sulfur aerosols; and these concerns have been supported by modelling. - Tarnishing of the sky: Aerosols will noticeably affect the appearance of the sky, resulting in a potential "whitening" effect, and altered sunsetSunsetSunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...
s. - TropopauseTropopauseThe tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
warming and the humidification of the stratosphere. - Effect on clouds: Cloud formation may be affected, notably cirrus cloudCirrus cloudCirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...
s and polar stratospheric clouds. - Effect on ecosystemEcosystemAn ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s: The diffusion of sunlight may affect plant growth.but more importantly increase the rate of ocean acidification by the deposition of hydrogen ions from the acidic rain - Effect on solar energy: Incident sunlight will be lower, which may affect solar power systems both directly and disproportionately, especially in the case that such systems rely on direct radiation.
- Deposition effects: Although predicted to be insignificant, there is nevertheless a risk of direct environmental damage from falling particles.
- Uneven effects: Aerosols are reflective, making them more effective during the day. Greenhouse gases block outbound radiation at all times of day.
Further, the delivery methods may cause significant problems, notably climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
and possible ozone depletion
Ozone depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...
in the case of aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
, and litter
Litter
Litter consists of waste products such as containers, papers, wrappers or faeces which have been disposed of without consent. Litter can also be used as a verb...
in the case of untethered balloons.
Delivery methods
Various techniques have been proposed for delivering the aerosol precursor gases (H2S and SO2). The required altitude to enter the stratosphere is the height of the tropopauseTropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
, which varies from 11 km (6.8 miles/36,000 feet) at the poles to 17 km (11 miles/58,000 feet) at the equator.
- Aircraft such as the F15-C variant of the F-15 EagleF-15 EagleThe McDonnell Douglas F-15 Eagle is a twin-engine, all-weather tactical fighter designed by McDonnell Douglas to gain and maintain air superiority in aerial combat. It is considered among the most successful modern fighters with over 100 aerial combat victories with no losses in dogfights...
have the necessary flight ceiling, but limited payload. Military tanker aircraft such as the KC-135 StratotankerKC-135 StratotankerThe Boeing KC-135 Stratotanker is an aerial refueling military aircraft. It and the Boeing 707 airliner were developed from the Boeing 367-80 prototype. The KC-135 was the US Air Force's first jet-powered refueling tanker and replaced the KC-97 Stratotanker...
and KC-10 ExtenderKC-10 ExtenderThe McDonnell Douglas KC-10 Extender is the military adaptation of the three-engined DC-10 airliner for the United States Air Force . The KC-10 incorporates military-specific equipment for its primary roles of transport and aerial refueling. It was developed to supplement the KC-135 Stratotanker...
also have the necessary ceiling and have greater payload. - Modified ArtilleryArtilleryOriginally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...
might have the necessary capability, but requires a polluting and expensive gunpowderSmokeless powderSmokeless powder is the name given to a number of propellants used in firearms and artillery which produce negligible smoke when fired, unlike the older gunpowder which they replaced...
charge to loft the payload. - High-altitude balloons can be used to lift precursor gases, in tanks, bladders or in the balloons' envelope. Balloons can also be used to lift pipes and hoses, but no moored balloonMoored balloonA moored balloon is an inflated fabric structure, often shaped like an airship and usually filled with helium, that is restrained by a cable attached to the ground or a vehicle. Moored balloons differ from airships and free balloons in that it is not free-flying.Moored balloons are sometimes...
has ever been deployed to the necessary altitude.
External links
- Tropospheric Aerosol Program, United States Department of Energy Atmospheric Science Program (ASP)
- What can we do about climate change?, Oceanography magazine
- Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change, Lawrence Livermore National Laboratory
- Climate Change: A geoengineering fix?, Aerospace America
- The Geoengineering Option:A Last Resort Against Global Warming?, Council on Foreign RelationsCouncil on Foreign RelationsThe Council on Foreign Relations is an American nonprofit nonpartisan membership organization, publisher, and think tank specializing in U.S. foreign policy and international affairs...
- Geo-Engineering Climate Change with Sulfate Aerosols, Pacific Northwest National LaboratoryPacific Northwest National LaboratoryPacific Northwest National Laboratory is one of the United States Department of Energy National Laboratories, managed by the Department of Energy's Office of Science. The main campus of the laboratory is in Richland, Washington....
- Geo-Engineering Research, Parliamentary Office of Science and TechnologyParliamentary Office of Science and TechnologyThe Parliamentary Office of Science and Technology, is the Parliament of the United Kingdom's in-house source of independent, balanced and accessible analysis of public policy issues related to science and technology...
- Geo-engineering Options for Mitigating Climate Change, Department of Energy and Climate ChangeDepartment of Energy and Climate ChangeThe Department of Energy and Climate Change is a British government department created on 3 October 2008 by Prime Minister Gordon Brown to take over some of the functions of the Department for Business, Enterprise and Regulatory Reform and Department for Environment, Food and Rural Affairs...
- Unilateral Geoengineering, Council on Foreign RelationsCouncil on Foreign RelationsThe Council on Foreign Relations is an American nonprofit nonpartisan membership organization, publisher, and think tank specializing in U.S. foreign policy and international affairs...
- An overview of geoengineering of climate using stratospheric sulphate aerosols, The Royal Society