Editing Sulfur dioxide (section)

==Environmental role==
===Air pollution===
[[File:Volcanic injection.svg|350px|thumb|The effect of major [[volcanic eruption]]s on sulfate aerosol concentrations and chemical reactions in the atmosphere]]
Major [[volcanic eruption]]s have an overwhelming effect on sulfate [[aerosol]] concentrations in the years when they occur: eruptions ranking 4 or greater on the [[Volcanic Explosivity Index]] inject {{chem2|SO2}} and water vapor directly into the [[stratosphere]], where they react to create sulfate aerosol plumes.<ref name="nasa-aerosols">{{cite web |url=http://volcanoes.usgs.gov/hazards/gas/s02aerosols.php |title=Volcanic Sulfur Aerosols Affect Climate and the Earth's Ozone Layer |access-date=February 17, 2009 |publisher=United States Geological Survey |archive-date=November 14, 2015 |archive-url=https://web.archive.org/web/20151114184944/https://volcanoes.usgs.gov/hazards/gas/s02aerosols.php |url-status=dead }}</ref> Volcanic emissions vary significantly in composition, and have complex chemistry due to the presence of ash particulates and a wide variety of other elements in the plume. Only [[stratovolcanoes]] containing primarily [[felsic]] magmas are responsible for these fluxes, as [[mafic]] magma erupted in [[shield volcanoes]] doesn't result in plumes which reach the stratosphere.<ref>{{cite journal |doi=10.1016/j.atmosenv.2004.06.017 |journal=Atmospheric Environment |volume=38 |issue=33 |year=2004 |pages=5637–5649 |title=Aerosol chemistry of emissions from three contrasting volcanoes in Italy |vauthors=Mathera TA, Oppenheimer AG, McGonigle A|bibcode=2004AtmEn..38.5637M }}</ref> 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&nbsp;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&nbsp;million tons per year in 1860, and then they increased to 15&nbsp;million tons in 1900, 40&nbsp;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".<ref name="IPCC_FAR">IPCC, 1990: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_chapter_01.pdf Chapter 1: Greenhouse Gases and Aerosols] [R.T. Watson, H. Rodhe, H. Oeschger and U. Siegenthaler]. In: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_full_report.pdf Climate Change: The IPCC Scientific Assessment] [J.T.Houghton, G.J.Jenkins and J.J.Ephraums (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 31–34,</ref> In the eastern United States in the early 2000s, sulfate particles were estimated to account for 25% or more of all [[air pollution]].<ref name="EPAHealth" /> Exposure to sulfur dioxide emissions by coal power plants (coal PM<sub>2.5</sub>) in the US was associated with 2.1 times greater mortality risk than exposure to PM<sub>2.5</sub> from all sources.<ref name="science2023mortality">{{cite journal |last1=Henneman |first1=Lucas |last2=Choirat |first2=Christine |last3=Dedoussi |first3=Irene |last4=Dominici |first4=Francesca |last5=Roberts |first5=Jessica|last6=Zigler |first6=Corwin |title=Mortality risk from United States coal electricity generation |journal=[[Science (journal)|Science]] |date=November 24, 2023 |volume=382 |issue=6673 |pages=941–946|doi=10.1126/science.adf4915 |pmid=37995235 |pmc=10870829 |bibcode=2023Sci...382..941H |language=en}}</ref> 
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.<ref>{{Cite journal|last1=Bates|first1=T. S.|last2=Lamb|first2=B. K.|last3=Guenther|first3=A.|last4=Dignon|first4=J.|last5=Stoiber|first5=R. E.|date=April 1992|title=Sulfur emissions to the atmosphere from natural sources|url=http://link.springer.com/10.1007/BF00115242|journal=Journal of Atmospheric Chemistry|language=en|volume=14|issue=1–4|pages=315–337|doi=10.1007/BF00115242 |bibcode=1992JAtC...14..315B |s2cid=55497518|issn=0167-7764}}</ref>
[[File:Acid rain woods1.JPG|thumb|left|Acid rain-damaged forest in Europe's [[Black Triangle (region)|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]].<ref>{{Cite journal|last1=Burns|first1= Douglas A.|last2=Aherne|first2=Julian|last3=Gay|first3=David A.|last4=Lehmann|first4=Christopher M.~B.|title =Acid rain and its environmental effects: Recent scientific advances|journal = Atmospheric Environment|language=en|year = 2016|volume=146|pages = 1–4|doi = 10.1016/j.atmosenv.2016.10.019|bibcode= 2016AtmEn.146....1B|doi-access=free}}</ref>
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.<ref name="EPASurface">{{Cite web|title=Effects of Acid Rain – Surface Waters and Aquatic Animals|url=http://www.epa.gov/acidrain/effects/surface_water.html|url-status=dead|archive-url=https://web.archive.org/web/20090514121649/http://www.epa.gov/acidrain/effects/surface_water.html|archive-date=May 14, 2009|website=US EPA}}</ref> 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 [[ecosystem]]s due to their regular exposure to sulfate-carrying fog at high altitudes.<ref>{{Cite journal|last1=Rodhe|first1=Henning|last2=Dentener|first2=Frank|last3=Schulz|first3=Michael|date=October 1, 2002|title=The Global Distribution of Acidifying Wet Deposition|url=https://doi.org/10.1021/es020057g|journal=Environmental Science & Technology|volume=36|issue=20|pages=4382–4388|doi=10.1021/es020057g|pmid=12387412|bibcode=2002EnST...36.4382R|issn=0013-936X}}</ref><ref name="EPA: Forests">US EPA: [http://www.epa.gov/acidrain/effects/forests.html Effects of Acid Rain – Forests] {{webarchive |url=https://web.archive.org/web/20080726034352/http://www.epa.gov/acidrain/effects/forests.html |date=July 26, 2008 }}</ref><ref>{{cite journal|doi=10.1126/science.272.5259.244|url=http://www.esf.edu/efb/mitchell/Class%20Readings/Sci.272.244.246.pdf|title=Long-Term Effects of Acid Rain: Response and Recovery of a Forest Ecosystem|year=1996|last1=Likens|first1=G. E.|last2=Driscoll|first2=C. T.|last3=Buso|first3=D. C.|journal=Science|volume=272|issue=5259|page=244|bibcode=1996Sci...272..244L|s2cid=178546205|access-date=February 9, 2013|archive-date=December 24, 2012|archive-url=https://web.archive.org/web/20121224203613/http://www.esf.edu/efb/mitchell/Class%20Readings/Sci.272.244.246.pdf|url-status=live}}</ref><ref>{{Cite journal|last1=Larssen|first1=T.|last2=Carmichael|first2=G. R.|date=October 1, 2000|title=Acid rain and acidification in China: the importance of base cation deposition|url=http://www.sciencedirect.com/science/article/pii/S0269749199002791|journal=Environmental Pollution|language=en|volume=110|issue=1|pages=89–102|doi=10.1016/S0269-7491(99)00279-1|pmid=15092859|issn=0269-7491|access-date=April 22, 2020|archive-date=March 30, 2015|archive-url=https://web.archive.org/web/20150330041614/http://www.sciencedirect.com/science/article/pii/S0269749199002791|url-status=live}}</ref><ref>{{Cite journal|last1=Johnson|first1=Dale W.|last2=Turner|first2=John|last3=Kelly|first3=J. M.|date=1982|title=The effects of acid rain on forest nutrient status|journal=Water Resources Research|language=en|volume=18|issue=3|pages=449–461|doi=10.1029/WR018i003p00449|bibcode=1982WRR....18..449J|issn=1944-7973}}</ref> 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]].<ref name="EPAHealth">[http://www.epa.gov/acidrain/effects/health.html Effects of Acid Rain – Human Health] {{Webarchive|url=https://web.archive.org/web/20080118120242/http://www.epa.gov/acidrain/effects/health.html |date=January 18, 2008 }}. Epa.gov (June 2, 2006). Retrieved on February 9, 2013.</ref> 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&nbsp;μg/m<sup>3</sup> 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.<ref>{{Cite journal |last1=Wang |first1=X. |last2=Ding |first2=H. |last3=Ryan |first3=L. |last4=Xu |first4=X. |s2cid=2707126 |date=May 1, 1997 |title=Association between air pollution and low birth weight: a community-based study |journal=Environmental Health Perspectives |volume=105 |issue=5 |pages=514–20 |issn=0091-6765 |pmc=1469882 |pmid=9222137 |doi=10.1289/ehp.97105514}}</ref>

====Control measures====
[[File:Estimates of past and future SO2 global anthropogenic emissions.png|thumb|upright=1.25|Early 2010s estimates of past and future anthropogenic global sulfur dioxide emissions, including the [[Representative Concentration Pathway]]s. While no [[climate change scenario]] may reach Maximum Feasible Reductions (MFRs), all assume steep declines from today's levels. By 2019, sulfate emission reductions were confirmed to proceed at a very fast rate.<ref name=XuRamanathanVictor>{{Cite journal|last1=Xu|first1=Yangyang|last2=Ramanathan|first2=Veerabhadran|last3=Victor|first3=David G.|date=December 5, 2018|title=Global warming will happen faster than we think|journal=Nature|language=en|volume=564|issue=7734|pages=30–32 |url=https://www.researchgate.net/publication/329411074 |doi=10.1038/d41586-018-07586-5|pmid=30518902|bibcode=2018Natur.564...30X|doi-access=free}}</ref>]]
Due largely to the US EPA's [[Acid Rain Program]], the U.S. has had a 33% decrease in emissions between 1983 and 2002 (see table). This improvement resulted in part from [[flue-gas desulfurization]], a technology that enables SO<sub>2</sub> to be chemically bound in [[power plant]]s burning sulfur-containing coal or petroleum.
{| class="wikitable"
|-
! Year
! SO<sub>2</sub>
|-
| 1970
| {{convert|31161000|ST|Mt|sigfig=3}}
|-
| 1980
| {{convert|25905000|ST|Mt|sigfig=3}}
|-
| 1990
|{{convert|23678000|ST|Mt|sigfig=3}}
|-
| 1996
|{{convert|18859000|ST|Mt|sigfig=3}}
|-
| 1997
|{{convert|19363000|ST|Mt|sigfig=3}}
|-
| 1998
|{{convert|19491000|ST|Mt|sigfig=3}}
|-
| 1999
|{{convert|18867000|ST|Mt|sigfig=3}}
|}

In particular, [[calcium oxide|calcium oxide (lime)]] reacts with sulfur dioxide to form [[calcium sulfite]]:

: CaO + SO<sub>2</sub> →  CaSO<sub>3</sub>

Aerobic oxidation of the CaSO<sub>3</sub> gives CaSO<sub>4</sub>, [[anhydrite]]. Most gypsum sold in Europe comes from flue-gas desulfurization.

To control sulfur emissions, dozens of methods with relatively high efficiencies have been developed for fitting of coal-fired power plants.<ref>{{Cite journal|last1=Lin|first1=Cheng-Kuan|last2=Lin|first2=Ro-Ting|last3=Chen|first3=Pi-Cheng|last4=Wang|first4=Pu|last5=De Marcellis-Warin|first5=Nathalie|last6=Zigler|first6=Corwin|last7=Christiani|first7=David C.|date=February 8, 2018|title=A Global Perspective on Sulfur Oxide Controls in Coal-Fired Power Plants and Cardiovascular Disease|journal=Scientific Reports|language=en|volume=8|issue=1|pages=2611 |doi=10.1038/s41598-018-20404-2|pmid=29422539|issn=2045-2322|pmc=5805744|bibcode=2018NatSR...8.2611L}}</ref> Sulfur can be removed from coal during burning by using limestone as a bed material in [[fluidized bed combustion]].<ref>{{cite book |last=Lindeburg |first=Michael R. |title=Mechanical Engineering Reference Manual for the PE Exam |location=Belmont, C.A. |publisher=Professional Publications, Inc |year=2006 |pages=27–3 |isbn=978-1-59126-049-3}}</ref>

Sulfur can also be removed from fuels before burning, preventing formation of SO<sub>2</sub> when the fuel is burnt. The [[Claus process]] is used in refineries to produce sulfur as a byproduct. The [[Stretford process]] has also been used to remove sulfur from fuel. [[Redox]] processes using iron oxides can also be used, for example, Lo-Cat<ref>[https://web.archive.org/web/20100304170107/http://www.gtp-merichem.com/support/index.php FAQ's About Sulfur Removal and Recovery using the LO-CAT® Hydrogen Sulfide Removal System]. gtp-merichem.com</ref> or Sulferox.<ref>[http://www.netl.doe.gov/technologies/coalpower/gasification/pubs/pdf/SFA%20Pacific_Process%20Screening%20Analysis_Dec%202002.pdf Process screening analysis of alternative gas treating and sulfur removal for gasification]. (December 2002) Report by SFA Pacific, Inc. prepared for U.S. Department of Energy (PDF) Retrieved on October 31, 2011.</ref>

Fuel additives such as [[calcium]] additives and magnesium carboxylate may be used in marine engines to lower the emission of sulfur dioxide gases into the atmosphere.<ref>May, Walter R.  [http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf Marine Emissions Abatement] {{Webarchive|url=https://web.archive.org/web/20150402130001/http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf |date=April 2, 2015 }}. SFA International, Inc., p. 6.</ref>

===Effects on ozone layer===
Sulfur dioxide aerosols in the stratosphere can contribute to [[ozone depletion]] in the presence of chlorofluorocarbons and other halogenated ozone-depleting substances.<ref name=klobas2017>{{cite journal|last1=Klobas|first1=J.E.|last2=Wilmouth|first2=D.M.|last3=Weisenstein|first3=D.K.|last4=Anderson|first4=J.G.|last5=Salawitch|first5=R.J.|title=Ozone depletion following future volcanic eruptions|journal=Geophysical Research Letters|volume=44|issue=14|pages=7490–7499|year=2017|doi=10.1002/2017GL073972|doi-access=free}}</ref>  The effects of volcanic eruptions containing sulfur dioxide aerosols on the ozone layer are complex, however.  In the absence of anthropogenic or biogenic halogenated compounds in the lower stratosphere, depletion of [[dinitrogen pentoxide]] in the middle stratosphere associated with its reactivity to the aerosols can promote ozone formation.<ref name=klobas2017/>  Injection of sulfur dioxide and large amounts of water vapor into the stratosphere following the [[2022 Hunga Tonga–Hunga Haʻapai eruption and tsunami|2022 eruption of Hunga Tonga-Hunga Haʻapai]] resulted in altered atmospheric circulation that promoted a decrease in ozone in the southern latitudes but an increase in the tropics.<ref>{{cite web|url=https://www.chemistry.harvard.edu/news/new-research-massive-2022-eruption-reduced-ozone-levels|title=New research: Massive 2022 eruption reduced ozone levels|website=Department of Chemistry and Chemical Biology|publisher=Harvard University|date=21 November 2023|access-date=7 January 2025}}</ref><ref>{{cite journal|last1=Wilmouth|first1=D.M.|last2=Østerstrøm|first2=F.F.|last3=Smith|first3=J.B.|last4=Anderson|first4=J.G.|last5=Salawitch|first5=R.J.|title=Impact of the Hunga Tonga volcanic eruption on stratospheric composition|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=120|issue=46|id=Art. No. e2301994120|doi=10.1073/pnas.2301994120|doi-access=free|year=2023|pages=e2301994120 |pmid=37903247 |pmc=10655571}}</ref>  The additional presence of hydrochloric acid in eruptions can result in net ozone depletion.<ref name=klobas2017/>

===Impact on climate change===
{{excerpt|Global dimming#History|paragraph=2}}
{{excerpt|Global dimming#Causes|paragraph=1|hat=no|files=no}}

====Projected impacts====
[[File:Physical Drivers of climate change.svg|thumb|upright=1.35|The extent to which physical factors in the atmosphere or on land affect [[climate change]], including the cooling provided by sulfate aerosols and the dimming they cause. The large [[error bar]] shows that there are still substantial unresolved uncertainties.]]
{{excerpt|Global dimming#Future|paragraphs=1,3|hat=no|files=no}}

====Solar geoengineering====
[[File:SPICE SRM overview.jpg|thumb|upright=1.5|alt=refer to caption and image description|Proposed tethered balloon to inject [[aerosols]] into the stratosphere]]
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 [[Ion chromatography|ion-chromatography]] and [[mass spectrometry]]<ref>{{Cite journal |last1=Kobayashi |first1=Yuya |last2=Ide |first2=Yu |last3=Takegawa |first3=Nobuyuki |date=3 April 2021 |title=Development of a novel particle mass spectrometer for online measurements of refractory sulfate aerosols |url=https://doi.org/10.1080/02786826.2020.1852168 |journal=Aerosol Science and Technology |volume=55 |issue=4 |pages=371–386 |doi=10.1080/02786826.2020.1852168 |bibcode=2021AerST..55..371K |s2cid=229506768 |issn=0278-6826}}</ref> Samples of actual particles can be recovered from the [[stratosphere]] using balloons or aircraft,<ref>{{cite journal |url=https://www.researchgate.net/publication/234296252_DUSTER_Aerosol_collection_in_the_stratosphere |journal=Societa Astronomica Italiana |title=The DUSTER experiment: collection and analysis of aerosol in the high stratosphere |author1=Palumbo, P. |author2= A. Rotundi |author3=V. Della Corte |author4=A. Ciucci |author5=L. Colangeli |author6=F. Esposito |author7=E. Mazzotta Epifani |author8=V. Mennella |author9=J.R. Brucato |author10=F.J.M. Rietmeijer |author11=G. J. Flynn |author12=J.-B. Renard   |author13=J.R. Stephens |author14=E. Zona |access-date=19 February 2009 }}</ref> and remote [[satellite]]s were also used for observation.<ref name=":32">{{Cite journal |last1=Myhre |first1=Gunnar |last2=Stordal |first2=Frode |last3=Berglen |first3=Tore F. |last4=Sundet |first4=Jostein K. |last5=Isaksen |first5=Ivar S. A. |date=1 March 2004 |title=Uncertainties in the Radiative Forcing Due to Sulfate Aerosols |journal=Journal of the Atmospheric Sciences |language=EN |volume=61 |issue=5 |pages=485–498 |doi=10.1175/1520-0469(2004)061<0485:UITRFD>2.0.CO;2 |bibcode=2004JAtS...61..485M |s2cid=55623817 |issn=0022-4928|doi-access=free }}</ref> This data is fed into the [[climate model]]s,<ref>{{Cite journal |last1=Zhang |first1=Jie |last2=Furtado |first2=Kalli |last3=Turnock |first3=Steven T. |last4=Mulcahy |first4=Jane P. |last5=Wilcox |first5=Laura J. |last6=Booth |first6=Ben B. |last7=Sexton |first7=David |last8=Wu |first8=Tongwen |last9=Zhang |first9=Fang |last10=Liu |first10=Qianxia |date=22 December 2021 |title=The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth system models |url=https://acp.copernicus.org/articles/21/18609/2021/ |journal=Atmospheric Chemistry and Physics |volume=21 |issue=4 |pages=18609–18627 |language=en |doi=10.5194/acp-21-18609-2021 |bibcode=2021ACP....2118609Z |doi-access=free }}</ref> as the necessity of accounting for aerosol cooling to truly understand the rate and evolution of warming had long been apparent, with the [[IPCC Second Assessment Report]] being the first to include an estimate of their impact on climate, and every major model able to simulate them by the time [[IPCC Fourth Assessment Report]] was published in 2007.<ref>{{cite web|url=https://earthobservatory.nasa.gov/features/Aerosols/page3.php|title=Aerosols and Incoming Sunlight (Direct Effects)|publisher=[[NASA]]|date=2 November 2010}}</ref> Many scientists also see the other side of this research, which is learning how to cause the same effect artificially.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2006/09/060914182715.htm |title=Stratospheric Injections Could Help Cool Earth, Computer Model Shows | access-date=19 February 2009 |publisher=ScienceDaily |date=15 September 2006 }}</ref> While discussed around the 1990s, if not earlier,<ref>{{cite journal |journal=Phil. Trans. R. Soc. A |year=1996 |volume=366 |pages=4039–56 |title=Global and Arctic climate engineering: numerical model studies |doi=10.1098/rsta.2008.0132 |author1=Launder B. |author2=J.M.T. Thompson |pmid=18757275 |issue=1882 |bibcode=2008RSPTA.366.4039C|doi-access=free }}</ref> stratospheric aerosol injection as a [[solar geoengineering]] method is best associated with [[Paul Crutzen]]'s detailed 2006 proposal.<ref name="Crutzen062">{{Cite journal|last1=Crutzen|first1=P. J.|year=2006|title=Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?|journal=Climatic Change|volume=77|issue=3–4|pages=211–220|bibcode=2006ClCh...77..211C|doi=10.1007/s10584-006-9101-y|doi-access=free}}</ref> 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 [[Representative Concentration Pathway|RCP 8.5]],  the addition of stratospheric sulfur required to avoid {{convert|4|C-change|F-change}} relative to now (and {{convert|5|C-change|F-change}} 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.<ref name="Visioni2020">{{Cite journal|last1=Visioni|first1=Daniele|last2=Slessarev|first2=Eric |last3=MacMartin|first3=Douglas G|last4=Mahowald|first4=Natalie M|last5=Goodale|first5=Christine L|last6=Xia|first6=Lili|date=1 September 2020|title=What goes up must come down: impacts of deposition in a sulfate geoengineering scenario|journal=Environmental Research Letters|volume=15|issue=9|pages=094063|doi=10.1088/1748-9326/ab94eb|bibcode=2020ERL....15i4063V|issn=1748-9326|doi-access=free}}</ref> This spurred a detailed look at its costs and benefits,<ref>{{cite web |url=http://www.met.reading.ac.uk/pg-research/downloads/2009/pgr-charlton.pdf |title=Costs and benefits of geo-engineering in the Stratosphere |author1=Andrew Charlton-Perez |author2=Eleanor Highwood |access-date=17 February 2009 |archive-date=14 January 2017 |archive-url=https://web.archive.org/web/20170114032949/http://www.met.reading.ac.uk/pg-research/downloads/2009/pgr-charlton.pdf |url-status=dead }}</ref> but even with hundreds of studies into the subject completed by the early 2020s, some notable uncertainties remain.<ref name="IPCC_WGI_SRM" >{{Cite journal |last1=Trisos |first1=Christopher H. |last2=Geden |first2=Oliver |last3=Seneviratne |first3=Sonia I. |last4=Sugiyama |first4=Masahiro |last5=van Aalst |first5=Maarten |last6=Bala |first6=Govindasamy |last7=Mach |first7=Katharine J. |last8=Ginzburg |first8=Veronika |last9=de Coninck |first9=Heleen |last10=Patt |first10=Anthony |title=Cross-Working Group Box SRM: Solar Radiation Modification |url=https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter16.pdf |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |year=2021 |volume=2021 |pages=1238 |doi=10.1017/9781009157896.007|bibcode=2021AGUFM.U13B..05K }}</ref>