Stratospheric aerosol injection

Source: Wikipedia, the free encyclopedia.
Solar radiation
reduction due to volcanic eruptions, considered the best analogue for stratospheric aerosol injection.

Stratospheric aerosol injection is a proposed method of

global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter.[1] It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects.[2] The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] methodagreement that it could limit warming to below 1.5 °C (2.7 °F)."[3] However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible,[4] particularly if used in a suboptimal manner.[5]

Various forms of

Alumina, calcite and salt are also under consideration.[8][9] The leading proposed method of delivery is custom aircraft.[10]

Scientific basis

Natural and anthropogenic sulfates

There is a wide range of

bunker fuel),[12][13] biogenic from hydrosphere and biosphere, geological via volcanoes or weather-driven from wildfires and other natural combustion events.[14][15][13]

Inorganic aerosols are mainly produced when

photochemical decomposition of COS (carbonyl sulfide), or when solid sulfates in the sea salt spray can react with gypsum dust particles
).

This section is an excerpt from Sulfur dioxide § Air pollution.[edit]
Volcanic "injection"

Major

shield volcanoes doesn't result in plumes which reach the stratosphere.[20] However, before the Industrial Revolution, dimethyl sulfide pathway was the largest contributor to sulfate aerosol concentrations in a more average year with no major volcanic activity. According to the IPCC First Assessment Report, published in 1990, volcanic emissions usually amounted to around 10 million tons in 1980s, while dimethyl sulfide amounted to 40 million tons. Yet, by that point, the global human-caused emissions of sulfur into the atmosphere became "at least as large" as all natural emissions of sulfur-containing compounds combined: they were at less than 3 million tons per year in 1860, and then they increased to 15 million tons in 1900, 40 million tons in 1940 and about 80 millions in 1980. The same report noted that "in the industrialized regions of Europe and North America, anthropogenic emissions dominate over natural emissions by about a factor of ten or even more".[21] In the eastern United States, sulfate particles were estimated to account for 25% or more of all air pollution.[22] Exposure to sulfur dioxide emissions by coal power plants (coal PM2.5) in the US was associated with 2.1 times greater mortality risk than exposure to PM2.5 from all sources.[23]
Meanwhile, the Southern Hemisphere had much lower concentrations due to being much less densely populated, with an estimated 90% of the human population in the north. In the early 1990s, anthropogenic sulfur dominated in the Northern Hemisphere, where only 16% of annual sulfur emissions were natural, yet amounted for less than half of the emissions in the Southern Hemisphere.[24]

Acid rain-damaged forest in Europe's Black Triangle

Such an increase in sulfate aerosol emissions had a variety of effects. At the time, the most visible one was acid rain, caused by precipitation from clouds carrying high concentrations of sulfate aerosols in the troposphere.[25]

At its peak, acid rain has eliminated brook trout and some other fish species and insect life from lakes and streams in geographically sensitive areas, such as Adirondack Mountains in the United States.[26] Acid rain worsens soil function as some of its microbiota is lost and heavy metals like aluminium are mobilized (spread more easily) while essential nutrients and minerals such as magnesium can leach away because of the same. Ultimately, plants unable to tolerate lowered pH are killed, with montane forests being some of the worst-affected ecosystems due to their regular exposure to sulfate-carrying fog at high altitudes.[27][28][29][30][31] While acid rain was too dilute to affect human health directly, breathing smog or even any air with elevated sulfate concentrations is known to contribute to heart and lung conditions, including asthma and bronchitis.[22] Further, this form of pollution is linked to preterm birth and low birth weight, with a study of 74,671 pregnant women in Beijing finding that every additional 100 µg/m3 of SO2 in the air reduced infants' weight by 7.3 g, making it and other forms of air pollution the largest attributable risk factor for low birth weight ever observed.[32]

Pollution controls and the discovery of radiative effects

Governmental action to combat the effects of acid rain

The discovery of these negative effects spurred the rush to reduce atmospheric sulfate pollution, typically through

developing countries.[39] While China and India have seen decades in rapid growth of sulfur emissions while they declined in the U.S. and Europe, they have also peaked in the recent years. In 2005, China was the largest polluter, with its estimated 25,490,000 short tons (23.1 Mt) emissions increasing by 27% since 2000 alone and roughly matching the U.S. emissions in 1980.[40] That year was also the peak, and a consistent decline was recorded since then.[41] Similarly, India's sulfur dioxide emissions appear to have been largely flat in the 2010s, as more coal-fired power plants were fitted with pollution controls even as the newer ones were still coming online.[42]

Sulfur dioxide in the world on April 15, 2017. Note that sulfur dioxide moves through the atmosphere with prevailing winds and thus local sulfur dioxide distributions vary day to day with weather patterns and seasonality.

Yet, around the time these treaties and technology improvements were taking place, evidence was coming in that sulfate aerosols were affecting both the

volcanic eruptions,[43] notably 1991 eruption of Mount Pinatubo in the Philippines,[44][45] had shown that the mass formation of sulfate aerosols by these eruptions formed a subtle whitish haze in the sky,[46] reducing the amount of Sun's radiation reaching the Earth's surface and rapidly losing the heat they absorb back to space, as well increasing clouds' albedo (i.e. making them more reflective) by changing their consistency to a larger amount of smaller droplets,[12] which was the principal reason for a clear drop in global temperatures for several years in their wake.[47] On the other hand, multiple studies have shown that between 1950s and 1980s, the amount of sunlight reaching the surface declined by around 4–5% per decade,[48][49][50] even though the changes in solar radiation at the top of the atmosphere were never more than 0.1-0.3%.[51] Yet, this trend (commonly described as global dimming) began to reverse in the 1990s, consistent with the reductions in anthropogenic sulfate pollution,[52][53][54] while at the same time, climate change accelerated.[55][56] Areas like eastern United States went from seeing cooling in contrast to the global trend to becoming global warming hotspots as their enormous levels of air pollution were reduced,[57] even as sulfate particles still accounted for around 25% of all particulates.[37][58][59]

Stratospheric sulfates from volcanic emissions cause transient cooling; the purple line showing sustained cooling is from tropospheric sulfate pollution.

As the real world had shown the importance of sulfate aerosol concentrations to the global climate, research into the subject accelerated. Formation of the aerosols and their effects on the atmosphere can be studied in the lab, with methods like

Paul Crutzen's detailed 2006 proposal.[1] Deploying in the stratosphere ensures that the aerosols are at their most effective, and that the progress of clean air measures would not be reversed: more recent research estimated that even under the highest-emission scenario RCP 8.5, the addition of stratospheric sulfur required to avoid 4 °C (7.2 °F) relative to now (and 5 °C (9.0 °F) relative to the preindustrial) would be effectively offset by the future controls on tropospheric sulfate pollution, and the amount required would be even less for less drastic warming scenarios.[67] This spurred a detailed look at its costs and benefits,[68] but even with hundreds of studies into the subject completed by the early 2020s, some notable uncertainties remain.[69]

Methods

Materials

Pinatubo eruption cloud. This volcano released huge quantities of stratospheric sulfur aerosols and contributed greatly to understanding of the subject.

Various forms of

photophoretic particles, metal oxides (as in Welsbach seeding, and titanium dioxide), and diamond are also under consideration.[18][73][74]

Delivery

Various techniques have been proposed for delivering the aerosol or precursor gases.[1] The required altitude to enter the stratosphere is the height of the tropopause, which varies from 11 kilometres (6.8 mi/36,000 ft) at the poles to 17 kilometers (11 mi/58,000 ft) at the equator.

aerosols
into the stratosphere

Injection system

The latitude and distribution of injection locations has been discussed by various authors. Whilst a near-equatorial injection regime will allow particles to enter the rising leg of the

Brewer-Dobson circulation, several studies have concluded that a broader, and higher-latitude, injection regime will reduce injection mass flow rates and/or yield climatic benefits.[79][80] Concentration of precursor injection in a single longitude appears to be beneficial, with condensation onto existing particles reduced, giving better control of the size distribution of aerosols resulting.[81] The long residence time of carbon dioxide in the atmosphere may require a millennium-timescale commitment to aerosol injection[82]
if aggressive emissions abatement is not pursued simultaneously.

Advantages of the technique

The advantages of this approach in comparison to other possible means of solar geoengineering are:

This graph shows baseline radiative forcing under three different Representative Concentration Pathway scenarios, and how stratospheric aerosol injection, first deployed in 2034, can be tuned to either halve the speed of warming by 2100, to halt the warming, or to reverse it entirely.[83]

Uncertainties

It is uncertain how effective any solar geoengineering technique would be, due to the difficulties modeling their impacts and the complex nature of the global climate system. Certain efficacy issues are specific to stratospheric aerosols.

  • Lifespan of aerosols: Tropospheric sulfur aerosols are short-lived.
    Brewer-Dobson circulation above the tropical tropopause. Further, sizing of particles is crucial to their endurance.[89]
  • Aerosol delivery: There are two proposals for how to create a stratospheric sulfate aerosol cloud, either through the release of a precursor gas (SO
    2    ) or the direct release of sulfuric acid (H
    2    SO
    4    ) and these face different challenges.    [90] If SO
    2     gas is released it will oxidize to form H
    2    SO
    4     and then condense to form droplets far from the injection site.[91] Releasing SO
    2     would not allow control over the size of the particles that are formed but would not require a sophisticated release mechanism. Simulations suggest that as the SO
    2     release rate is increased there would be diminishing returns on the cooling effect, as larger particles would be formed which have a shorter lifetime and are less effective scatterers of light.[92] If H
    2    SO
    4     is released directly then the aerosol particles would form very quickly and in principle the particle size could be controlled although the engineering requirements for this are uncertain. Assuming a technology for direct H
    2    SO
    4     release could be conceived and developed, it would allow control over the particle size to possibly alleviate some of the inefficiencies associated with SO
    2     release.[90]
  • Strength of cooling: The magnitude of the effect of forcing from aerosols by decreasing
    clouds which can alter this estimate of aerosol cooling, and consequently, our knowledge of how many millions of tons must be deployed annually to achieve the desired effect.[95][96][97][98][99][100][101]
Anthropogenic sulfate aerosols have decreased precipitation over most of Asia (red), but increased it over some parts of Central Asia (blue).[102]
  • Hydrological cycle: Since the historical
    vector-borne diseases. Considering the already-extensive present-day mosquito habitat, it is currently unclear whether those changes are likely to be positive or negative.[69]

Cost

Early studies suggest that stratospheric aerosol injection might have a relatively low direct cost. The annual cost of delivering 5 million tons of an albedo enhancing aerosol (sufficient to offset the expected warming over the next century) to an altitude of 20 to 30 km is estimated at US$2 billion to 8 billion.[110] In comparison, the annual cost estimates for climate damage or emission mitigation range from US$200 billion to 2 trillion.[110]

A 2016 study finds the cost per 1 W/m2 of cooling to be between 5–50 billion USD/yr.[111] Because larger particles are less efficient at cooling and drop out of the sky faster, the unit-cooling cost is expected to increase over time as increased dose leads to larger, but less efficient, particles by mechanism such as coalescence and Ostwald ripening.[112] Assume RCP8.5, -5.5 W/m2 of cooling would be required by 2100 to maintain 2020 climate. At the dose level required to provide this cooling, the net efficiency per mass of injected aerosols would reduce to below 50% compared to low-level deployment (below 1W/m2).[113] At a total dose of -5.5 W/m2, the cost would be between 55-550 billion USD/yr when efficiency reduction is also taken into account, bringing annual expenditure to levels comparable to other mitigation alternatives.

Other possible side effects

Turner was inspired by dramatic sunsets caused by volcanic aerosols[114]

Solar geoengineering in general poses various problems and risks. However, certain problems are specific to or more pronounced with stratospheric sulfide injection.[115]

  • polar stratospheric clouds before the levels of CFCs and other ozone destroying gases fall naturally to safe levels because stratospheric aerosols, together with the ozone destroying gases, are responsible for ozone depletion.[119][120] The injection of other aerosols that may be safer such as calcite has therefore been proposed.[8] The injection of non-sulfide aerosols like calcite (limestone) would also have a cooling effect while counteracting ozone depletion and would be expected to reduce other side effects.[8]
  • Whitening of the sky: Volcanic eruptions are known to affect the appearance of sunsets significantly,[121] and a change in sky appearance after the eruption of Mount Tambora in 1816 "The Year Without A Summer" was the inspiration for the paintings of J. M. W. Turner.[122] Since stratospheric aerosol injection would involve smaller quantities of aerosols, it is expected to cause a subtler change to sunsets and a slight hazing of blue skies.[123][124] How stratospheric aerosol injection may affect clouds remains uncertain.[125]
  • Stratospheric temperature change: Aerosols can also absorb some radiation from the Sun, the Earth, and the surrounding atmosphere. This changes the surrounding air temperature and could potentially impact the stratospheric circulation, which in turn may impact the surface circulation.[126]
  • Deposition and acid rain: The surface deposition of sulfate injected into the stratosphere may also have an impact on ecosystems. However, the amount and wide dispersal of injected aerosols means that their impact on particulate concentrations and acidity of precipitation would be very small.[67]
  • Ecological consequences: The consequences of stratospheric aerosol injection on ecological systems are unknown and potentially vary by ecosystem with differing impacts on marine versus terrestrial biomes.[127][128][129]
  • Mixed effects on agriculture: A historical study in 2018 found that stratospheric sulfate aerosols injected by the volcanic eruptions of Chicón (1982) and Mount Pinatubo (1991) had mixed effects on global crop yields of certain major crops.[130] Based on several studies, the IPCC Sixth Assessment Report suggests that crop yields and carbon sinks would be largely unaffected or may even increase slightly, because reduced photosynthesis due to lower sunlight would be offset by CO2 fertilization effect and the reduction in thermal stress, but there's less confidence about how the specific ecosystems may be affected.[69]
  • Inhibition of Solar Energy Technologies: Uniformly reduced net shortwave radiation would hurt solar photovoltaics by the same 2-5% as for plants.[131] the increased scattering of collimated incoming sunlight would more drastically reduce the efficiencies (by 11% for RCP8.5) of concentrating solar thermal power for both electricity production [132][131] and chemical reactions, such as solar cement production.[133]

Outdoors research

In 2009, a Russian team tested aerosol formation in the lower troposphere using helicopters.[134] In 2015, David Keith and Gernot Wagner described a potential field experiment, the Stratospheric Controlled Perturbation Experiment (SCoPEx), using stratospheric calcium carbonate[135] injection,[136] but as of October 2020 the time and place had not yet been determined.[137][138] SCoPEx is in part funded by Bill Gates.[139][140] Sir David King, a former chief scientific adviser to the government of the United Kingdom, stated that SCoPEX and Gates' plans to dim the sun with calcium carbonate could have disastrous effects.[141]

In 2012, the

STFC to the tune of £2.1 million[142] and was one of the first UK projects aimed at providing evidence-based knowledge about solar radiation management.[142] Although the field testing was cancelled, the project panel decided to continue the lab-based elements of the project.[143]
Furthermore, a consultation exercise was undertaken with members of the public in a parallel project by Cardiff University, with specific exploration of attitudes to the SPICE test.
ETC Group drafted an open letter calling for the project to be suspended until international agreement is reached,[145] specifically pointing to the upcoming convention of parties to the Convention on Biological Diversity in 2012.[146]

Governance

Most of the existing governance of stratospheric sulfate aerosols is from that which is applicable to solar radiation management more broadly. However, some existing legal instruments would be relevant to stratospheric sulfate aerosols specifically. At the international level, the Convention on Long-Range Transboundary Air Pollution (CLRTAP Convention) obligates those countries which have ratified it to reduce their emissions of particular transboundary air pollutants. Notably, both solar radiation management and climate change (as well as greenhouse gases) could satisfy the definition of "air pollution" which the signatories commit to reduce, depending on their actual negative effects.[147] Commitments to specific values of the pollutants, including sulfates, are made through protocols to the CLRTAP Convention. Full implementation or large scale climate response field tests of stratospheric sulfate aerosols could cause countries to exceed their limits. However, because stratospheric injections would be spread across the globe instead of concentrated in a few nearby countries, and could lead to net reductions in the "air pollution" which the CLRTAP Convention is to reduce so they may be allowed.

The stratospheric injection of sulfate aerosols would cause the Vienna Convention for the Protection of the Ozone Layer to be applicable due to their possible deleterious effects on stratospheric ozone. That treaty generally obligates its Parties to enact policies to control activities which "have or are likely to have adverse effects resulting from modification or likely modification of the ozone layer."[148] The Montreal Protocol to the Vienna Convention prohibits the production of certain ozone depleting substances, via phase outs. Sulfates are presently not among the prohibited substances.

In the United States, the Clean Air Act might give the United States Environmental Protection Agency authority to regulate stratospheric sulfate aerosols.[149]

Welsbach seeding

Welsbach seeding is a patented

micron) metal oxide particles (thorium dioxide, aluminium oxide). The purpose of the Welsbach seeding would be to "(reduce) atmospheric warming due to the greenhouse effect resulting from a greenhouse gases layer," by converting radiative energy at near-infrared
wavelengths into radiation at far-infrared wavelengths, permitting some of the converted radiation to escape into space, thus cooling the atmosphere. The seeding as described would be performed by airplanes at altitudes between 7 and 13 kilometres.

Patent

The method was patented by Hughes Aircraft Company in 1991, US patent 5003186.[150] Quote from the patent:

"Global warming has been a great concern of many environmental scientists. Scientists believe that the greenhouse effect is responsible for global warming. Greatly increased amounts of heat-trapping gases have been generated since the Industrial Revolution. These gases, such as CO2, CFC, and methane, accumulate in the atmosphere and allow sunlight to stream in freely but block heat from escaping (greenhouse effect). These gases are relatively transparent to sunshine but absorb strongly the long-wavelength infrared radiation released by the earth."

"This invention relates to a method for the reduction of global warming resulting from the greenhouse effect, and in particular to a method which involves the seeding of the earth's stratosphere with Welsbach-like materials."

Feasibility]This is not considered to be a viable option by current geoengineering experts; in fact the proposed mechanism is considered to violate the second law of thermodynamics.[151] Currently proposed atmospheric geoengineering methods would instead use other aerosols, at considerably higher altitudes.[152]

History

sulfate aerosols if global warming ever became a pressing issue.[153] Such controversial climate engineering proposals for global dimming have sometimes been called a "Budyko Blanket".[154][155][156]

In popular culture

In the film Snowpiercer, as well as in the television spin-off, an apocalyptic global ice-age is caused by the introduction of a fictional substance, dubbed, CW-7 into the atmosphere, with the intention of preventing global-warming by blocking out the light of the sun.[157][158]

In the novel

climate mitigation measure following a catastrophic and deadly heatwave.[159]

The bestselling novel Termination Shock by Neal Stephenson revolves around a private initiative by a billionaire, with covert support or opposition from some national governments, to inject sulfur into the stratosphere using recoverable gliders launched with a railgun. ;[160]

See also

  • Carl Auer von Welsbach – Austrian scientist and inventor (1858–1929)
  • Chemtrail conspiracy theory – Conspiracy theory about contrails
  • Climate change – Current rise in Earth's average temperature and its effects
  • Climate change mitigation – Actions to reduce net greenhouse gas emissions to limit climate change
  • Climate engineering – Deliberate and large-scale intervention in the Earth's climate system
  • Cloud seeding – Method that condenses clouds to cause rainfall
  • Global dimming – Reduction in the amount of sunlight reaching Earth's surface
  • Solar geoengineering
     – Reflection of sunlight to reduce global warming
  • Termination Shock – Science fiction novel by Neal Stephenson
  • Weather Modification Operations and Research Board
     – Act of intentionally altering or manipulating the weather

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External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Stratospheric_aerosol_injection&oldid=1216883063"