marine cloud brightening

File:Ship Tracks off the California Coast - NASA Earth Observatory.jpg left this cluster of bright cloud trails lingering in the atmosphere in 2012. The narrow clouds, known as ship tracks, form when water vapor condenses around tiny particles of pollution that ships either emit directly as exhaust or that form as a result of gases within the exhaust.]]Marine cloud brightening is based on phenomena that are currently observed in the climate system. Today, emissions particles, such as soot, mix with clouds in the atmosphere and increase the amount of sunlight they reflect, reducing warming. This cooling effect is estimated at between 0.5 and 1.5 °C (0.9 and 2.7 °F), and is one of the most important unknowns in climate.{{Cite journal|author=Gunnar Myhre (Norway)|author2=Drew Shindell (US)|year=2013|title=Anthropogenic and Natural Radiative Forcing|url=https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf|journal=IPCC 5th Assessment Report|volume=Chapter 8|access-date=2017-01-22|archive-date=2018-10-22|archive-url=https://web.archive.org/web/20181022073632/https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf|url-status=live}} Marine cloud brightening proposes to generate a similar effect using benign material, such as sea salt. Marine stratocumulus clouds are thought to be the most suitable because of their prevalence, coverage, accessibility, and generally low cloud drop number concentration.{{Cite journal |last1=Chen |first1=C.-C. |last2=Richter |first2=J. H. |last3=Lee |first3=Walker R. |last4=Tye |first4=Mari |last5=MacMartin |first5=Douglas G. |last6=Kravitz |first6=Ben |date=2025 |title=Climate Impact of Marine Cloud Brightening Solar Climate Intervention Under a Susceptibility-Based Strategy Simulated by CESM2 |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JD041245?af=R |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=130 |issue=2 |pages=e2024JD041245 |doi=10.1029/2024JD041245 |bibcode=2025JGRD..13041245C |issn=2169-8996}} MCB also makes the clouds last longer.{{Cite web |last=Stein |first=Theo |date=2024-03-20 |title=Scientists detail research to assess viability and risks of marine cloud brightening |url=https://research.noaa.gov/scientists-detail-research-to-assess-viability-and-risks-of-marine-cloud-brightening/ |access-date=2025-03-07 |website=NOAA Research |language=en-US}} Although stratospheric aerosol injection would be much higher up, it could diffuse sunlight and so also brighten low-level marine clouds.{{Cite journal |last1=Gristey |first1=Jake J. |last2=Feingold |first2=Graham |date=2025 |title=Stratospheric Aerosol Injection Would Change Cloud Brightness |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL113914 |journal=Geophysical Research Letters |language=en |volume=52 |issue=6 |pages=e2024GL113914 |doi=10.1029/2024GL113914 |bibcode=2025GeoRL..5213914G |issn=1944-8007|doi-access=free }}

Most clouds are quite reflective, redirecting incoming solar radiation back into space. Increasing clouds' albedo would increase the portion of incoming solar radiation that is reflected, in turn cooling the planet. Clouds consist of water droplets, and clouds with smaller droplets are more reflective (because of the Twomey effect). Cloud condensation nuclei are necessary for water droplet formation. The central idea underlying marine cloud brightening is to add aerosols to atmospheric locations where clouds form. These would then act as cloud condensation nuclei, increasing the cloud albedo.

Marine cloud brightening on a small scale already occurs unintentionally due to the aerosols in ships' exhaust, leaving ship tracks.{{Cite journal| last1=Hobbs| first1=Peter V.| last2=Garrett| first2=Timothy J.| last3=Ferek| first3=Ronald J.| last4=Strader| first4=Scott R.| last5=Hegg| first5=Dean A.| last6=Frick| first6=Glendon M.| last7=Hoppel| first7=William A.| last8=Gasparovic| first8=Richard F.| last9=Russell| first9=Lynn M.| date=2000-08-01| title=Emissions from Ships with respect to Their Effects on Clouds| journal=Journal of the Atmospheric Sciences| volume=57| issue=16| pages=2570–2590| doi=10.1175/1520-0469(2000)057<2570:efswrt>2.0.co;2| bibcode=2000JAtS...57.2570H| issn=0022-4928| url=https://authors.library.caltech.edu/28361/1/HOBjas00.pdf| access-date=2018-11-21| archive-date=2017-08-15| archive-url=https://web.archive.org/web/20170815102705/http://authors.library.caltech.edu/28361/1/HOBjas00.pdf| url-status=live}} Changes to shipping regulations enacted by the United Nations’ International Maritime Organization to reduce certain aerosols are hypothesized to be leading to reduced cloud cover and increased oceanic warming, providing additional support to the potential effectiveness of marine cloud brightening at modifying ocean temperature.{{Cite news |last=Voosen |first=Paul |date=August 2, 2023 |title='We're changing the clouds.' An unforeseen test of geoengineering is fueling record ocean warmth |work=Science |url=https://www.science.org/content/article/changing-clouds-unforeseen-test-geoengineering-fueling-record-ocean-warmth |access-date=August 4, 2023}} Different cloud regimes are likely to have differing susceptibility to brightening strategies, with marine stratocumulus clouds (low, layered clouds over ocean regions) most sensitive to aerosol changes.{{Cite journal| last1=Salter| first1=Stephen| last2=Sortino| first2=Graham| last3=Latham| first3=John| date=2008-11-13| title=Sea-going hardware for the cloud albedo method of reversing global warming| journal=Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|language=en|volume=366|issue=1882|pages=3989–4006|doi=10.1098/rsta.2008.0136|issn=1364-503X|pmid=18757273|bibcode=2008RSPTA.366.3989S| doi-access=}}{{Cite journal| last1=Oreopoulos|first1=Lazaros| last2=Platnick| first2=Steven| date=2008-07-27|title=Radiative susceptibility of cloudy atmospheres to droplet number perturbations: 2. Global analysis from MODIS|journal=Journal of Geophysical Research: Atmospheres| language=en |volume=113|issue=D14|pages=D14S21|doi=10.1029/2007JD009655|issn=2156-2202| bibcode=2008JGRD..11314S21O}} These marine stratocumulus clouds are thus typically proposed as the target. They are common over the cooler regions of subtropical and midlatitude oceans, where their coverage can average over 50% over a year.{{Cite journal| last=Wood| first=Robert| date=2012-02-09| title=Stratocumulus Clouds| journal=Monthly Weather Review| volume=140| issue=8 |pages=2373–2423| doi=10.1175/MWR-D-11-00121.1| issn=0027-0644| bibcode=2012MWRv..140.2373W| doi-access=free}} The leading possible source of additional cloud condensation nuclei is salt from seawater, although there are others.{{Cite journal|last1=Wingenter|first1=Oliver W.|last2=Haase|first2=Karl B.|last3=Zeigler|first3=Max|last4=Blake|first4=Donald R.|last5=Rowland|first5=F. Sherwood|last6=Sive|first6=Barkley C.|last7=Paulino|first7=Ana|last8=Thyrhaug|first8=Runar|last9=Larsen|first9=Aud|date=2007-03-01|title=Unexpected consequences of increasing CO2 and ocean acidity on marine production of DMS and CH2ClI: Potential climate impacts|journal=Geophysical Research Letters|language=en|volume=34|issue=5|pages=L05710|doi=10.1029/2006GL028139|issn=1944-8007|bibcode=2007GeoRL..34.5710W|url=http://oceanrep.geomar.de/5437/1/2006GL028139.pdf|doi-access=free|access-date=2019-12-11|archive-date=2017-08-26|archive-url=https://web.archive.org/web/20170826225043/http://oceanrep.geomar.de/5437/1/2006GL028139.pdf|url-status=live}}

Even though the importance of aerosols for the formation of clouds is, in general, well understood, many uncertainties remain. The IPCC Fifth Assessment Report considers aerosol-cloud interactions as one of the current major challenges in climate modeling in general.{{Cite book|url=http://ebooks.cambridge.org/ref/id/CBO9781107415324|title=Climate Change 2013 – The Physical Science Basis by Intergovernmental Panel on Climate Change|doi=10.1017/cbo9781107415324|date=March 2014|publisher=Cambridge University Press|isbn=9781107415324|editor1-last=Intergovernmental Panel On Climate Change}} In particular, the number of droplets does not increase proportionally when more aerosols are present, and can even decrease.{{Cite journal|last1=Leaitch|first1=W. R.|last2=Lohmann|first2=U.|last3=Russell|first3=L. M.|last4=Garrett|first4=T.|last5=Shantz|first5=N. C.|last6=Toom-Sauntry|first6=D.|last7=Strapp|first7=J. W.|last8=Hayden|first8=K. L.|last9=Marshall|first9=J.|date=2010-08-18|title=Cloud albedo increase from carbonaceous aerosol|journal=Atmos. Chem. Phys.|volume=10|issue=16|pages=7669–7684|doi=10.5194/acp-10-7669-2010|bibcode=2010ACP....10.7669L|issn=1680-7324|url=http://www.atmos-chem-phys.net/10/7669/2010/acp-10-7669-2010.html|doi-access=free|access-date=2019-09-04|archive-date=2019-09-04|archive-url=https://web.archive.org/web/20190904020619/https://www.atmos-chem-phys.net/10/7669/2010/acp-10-7669-2010.html|url-status=live|hdl=20.500.11850/22281|hdl-access=free}}{{Cite journal|last1=Chen|first1=Y.-C.| last2=Christensen| first2=M. W.| last3=Xue|first3=L.| last4=Sorooshian| first4=A.| last5=Stephens| first5=G. L.|last6=Rasmussen|first6=R. M.|last7=Seinfeld|first7=J. H.| date=2012-09-12| title=Occurrence of lower cloud albedo in ship tracks|journal=Atmos. Chem. Phys.| volume=12|issue=17| pages=8223–8235| doi=10.5194/acp-12-8223-2012|issn=1680-7324|bibcode=2012ACP....12.8223C| doi-access=free}} Extrapolating the effects of particles on clouds observed on the microphysical scale to the regional, climatically relevant, scale is not straightforward.{{Cite journal|last1=Martin|first1=G. M.|last2=Johnson|first2=D. W.| last3=Spice| first3=A.| date=1994-07-01| title=The Measurement and Parameterization of Effective Radius of Droplets in Warm Stratocumulus Clouds|journal=Journal of the Atmospheric Sciences|volume=51|issue=13|pages=1823–1842|doi= 10.1175/1520-0469(1994)051<1823:tmapoe>2.0.co;2|bibcode=1994JAtS...51.1823M|issn=0022-4928|doi-access=free}} For example deployment in the South Pacific or South Atlantic could increase rainfall in western and central Africa but reduce it in southern Africa.{{Cite report |url=https://meetingorganizer.copernicus.org/EGU24/EGU24-6419.html |title=Africa's Climate Response to Marine Cloud Brightening |last1=Odoulami |first1=Romaric C. |last2=Hirasawa |first2=Haruki |last3=Kouadio |first3=Kouakou |last4=Patel |first4=Trisha D. |last5=Quagraine |first5=Kwesi A. |last6=Pinto |first6=Izidine |last7=Egbebiyi |first7=Temitope S. |last8=Abiodun |first8=Babatunde J. |last9=Lennard |first9=Christopher |date=2024-03-07 |publisher=Copernicus Meetings |issue=EGU24-6419 |language=en}}

It has been suggested that MCB should be used to preserve Arctic sea ice.{{Cite web |last=Hunt |first=Professor Hugh |date=2024-08-04 |title=The Imperative for Marine Cloud Brightening: A Call for $100 Million to Preserve Arctic Sea Ice |url=https://genn.cc/marine-cloud-brightening/ |access-date=2025-03-07 |website=ClimateGenn |language=en-US}}

The modeling evidence of the global climatic effects of marine cloud brightening remains limited. Current modeling research indicates that marine cloud brightening could substantially cool the planet. A 2020 study found a substantial increase in cloud reflectivity from shipping in the southeast Atlantic basin, suggesting that a regional-scale test of MCB in stratocumulus‐dominated regions could be successful.{{cite journal |last1=Diamond |first1=Michael S. |last2=Director |first2=Hannah M. |last3=Eastman |first3=Ryan |last4=Possner |first4=Anna |first5=Robert |last5=Wood |date=2020 |title=Substantial Cloud Brightening From Shipping in Subtropical Low Clouds |journal=AGU Advances |volume=1 |issue=1 |doi=10.1029/2019av000111 |doi-access=free|bibcode=2020AGUA....100111D }}

Studies in the late 2010s estimated that this technique could produce up to 2 W/m² of negative radiative forcing,{{efn|1=there may be a mistake in AR6 which says up to 5 W/m² of negative radiative forcing. {{Cite book |last=Intergovernmental Panel on Climate Change (IPCC) |url=https://www.ipcc.ch/report/ar6/wg1/ |title=Climate Change 2021: The Physical Science Basis – Working Group I Contribution to the Sixth Assessment Report of the IPCC |date=2021 |publisher=Cambridge University Press |pages=}}{{rp|624}} - The [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-4/#4.6.3.3 IPCC AR6 stated] "1–5 W m–2, depending on the scale and amount of sea salt injection; heterogeneous radiative forcing" for "Global Mean Negative Radiative Forcing Potential and Characteristics". But looking at the underlying sources, it appears that the IPCC report is not quite correct. Both of the newer IPCC-cited articles https://acp.copernicus.org/articles/18/621/2018/ 1], [https://acp.copernicus.org/articles/17/13071/2017/ 2 gave maximum global negative RF as 2.0 W/m2, and the older two give no values. However, one of the newer cited articles does refer back to [https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011JD016428 this one from 2012] which concludes "we predicted a [global] radiative flux perturbation (RFP) of −5.1 W m−2, which is enough to counteract warming from doubled CO2 concentration." This seems like an outlier.}}{{Cite journal |last1=Stjern |first1=Camilla W. |last2=Muri |first2=Helene |last3=Ahlm |first3=Lars |last4=Boucher |first4=Olivier |last5=Cole |first5=Jason N. S. |last6=Ji |first6=Duoying |last7=Jones |first7=Andy |last8=Haywood |first8=Jim |last9=Kravitz |first9=Ben |last10=Lenton |first10=Andrew |last11=Moore |first11=John C. |last12=Niemeier |first12=Ulrike |last13=Phipps |first13=Steven J. |last14=Schmidt |first14=Hauke |last15=Watanabe |first15=Shingo |date=2018-01-19 |title=Response to marine cloud brightening in a multi-model ensemble |url=https://acp.copernicus.org/articles/18/621/2018/ |journal=Atmospheric Chemistry and Physics |language=English |volume=18 |issue=2 |pages=621–634 |doi=10.5194/acp-18-621-2018 |doi-access=free |bibcode=2018ACP....18..621S |issn=1680-7316|hdl=10871/39951 |hdl-access=free }}{{Cite journal |last1=Ahlm |first1=Lars |last2=Jones |first2=Andy |last3=Stjern |first3=Camilla W. |last4=Muri |first4=Helene |last5=Kravitz |first5=Ben |last6=Kristjánsson |first6=Jón Egill |date=2017-11-06 |title=Marine cloud brightening – as effective without clouds |url=https://acp.copernicus.org/articles/17/13071/2017/ |journal=Atmospheric Chemistry and Physics |language=English |volume=17 |issue=21 |pages=13071–13087 |doi=10.5194/acp-17-13071-2017 |doi-access=free |bibcode=2017ACP....1713071A |issn=1680-7316|hdl=10852/59113 |hdl-access=free }} which is less than human-caused radiative forcing of almost 3 W/m².

The climatic impacts of marine cloud brightening would be rapidly responsive and reversible. If the brightening activity were to change in intensity, or stop altogether, then the clouds' brightness would respond within a few days to weeks, as the cloud condensation nuclei particles precipitate naturally.

Again unlike stratospheric aerosol injection, marine cloud brightening might be able to be used regionally, albeit in a limited manner.{{Cite journal|last1=Latham|first1=John|last2=Gadian|first2=Alan|last3=Fournier|first3=Jim|last4=Parkes|first4=Ben|last5=Wadhams|first5=Peter|last6=Chen|first6=Jack|date=2014-12-28|title=Marine cloud brightening: regional applications|journal=Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|language=en|volume=372|issue=2031|pages=20140053|doi=10.1098/rsta.2014.0053|issn=1364-503X|pmc=4240952|pmid=25404682|bibcode=2014RSPTA.37240053L}} Marine stratocumulus clouds are common in particular regions, specifically the eastern Pacific Ocean and the eastern South Atlantic Ocean. A typical finding among simulation studies was a persistent cooling of the Pacific, similar to the La Niña phenomenon, and, despite the localized nature of the albedo change, an increase in polar sea ice.{{Cite journal |last1=Jones |first1=Andy |last2=Haywood |first2=Jim |last3=Boucher |first3=Olivier |date=2009-05-27 |title=Climate impacts of geoengineering marine stratocumulus clouds |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=114 |issue=D10 |pages=D10106 |bibcode=2009JGRD..11410106J |doi=10.1029/2008JD011450 |issn=2156-2202 |hdl-access=free |hdl=10871/9161}}{{Cite journal|last1=Latham|first1=John|last2=Rasch|first2=Philip|last3=Chen|first3=Chih-Chieh|last4=Kettles|first4=Laura|last5=Gadian|first5=Alan|last6=Gettelman|first6=Andrew|last7=Morrison|first7=Hugh|last8=Bower|first8=Keith|last9=Choularton|first9=Tom|date=2008-11-13|title=Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds|journal=Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|language=en|volume=366|issue=1882|pages=3969–3987|doi=10.1098/rsta.2008.0137|issn=1364-503X|pmid=18757272|bibcode=2008RSPTA.366.3969L|s2cid=6985875}}{{Cite journal|last1=Rasch|first1=Philip J.|last2=Latham|first2=John|last3=Chen|first3=Chih-Chieh (Jack)|date=2009-01-01|title=Geoengineering by cloud seeding: influence on sea ice and climate system|journal=Environmental Research Letters|language=en|volume=4|issue=4|pages=045112|doi=10.1088/1748-9326/4/4/045112|issn=1748-9326|bibcode=2009ERL.....4d5112R|doi-access=free}}{{Cite journal|last1=Hill|first1=Spencer|last2=Ming|first2=Yi|date=2012-08-16|title=Nonlinear climate response to regional brightening of tropical marine stratocumulus|journal=Geophysical Research Letters|language=en|volume=39|issue=15|pages=L15707|doi=10.1029/2012GL052064|issn=1944-8007|bibcode=2012GeoRL..3915707H|doi-access=}}{{Cite journal|last1=Baughman|first1=E.|last2=Gnanadesikan|first2=A.|last3=Degaetano|first3=A.|last4=Adcroft|first4=A.|date=2012-05-18|title=Investigation of the Surface and Circulation Impacts of Cloud-Brightening Geoengineering|journal=Journal of Climate|volume=25|issue=21|pages=7527–7543|doi=10.1175/JCLI-D-11-00282.1|issn=0894-8755|bibcode=2012JCli...25.7527B|s2cid=130550105|doi-access=free}} Studies aim at making simulation findings derived from different models comparable.{{update-inline|date=February 2025}}{{Cite journal|last1=Alterskjær|first1=K.|last2=Kristjánsson|first2=J. E.|date=2013-01-16|title=The sign of the radiative forcing from marine cloud brightening depends on both particle size and injection amount|journal=Geophysical Research Letters|language=en|volume=40|issue=1|pages=210–215|doi=10.1029/2012GL054286|issn=1944-8007|bibcode=2013GeoRL..40..210A|doi-access=free}}{{Cite journal|last1=Kravitz|first1=Ben|last2=Caldeira|first2=Ken|last3=Boucher|first3=Olivier|last4=Robock|first4=Alan|last5=Rasch|first5=Philip J.|last6=Alterskjær|first6=Kari|last7=Karam|first7=Diana Bou|last8=Cole|first8=Jason N. S.|last9=Curry|first9=Charles L.|date=2013-08-16|title=Climate model response from the Geoengineering Model Intercomparison Project (GeoMIP)|journal=Journal of Geophysical Research: Atmospheres|language=en|volume=118|issue=15|pages=8320–8332|doi=10.1002/jgrd.50646|issn=2169-8996|bibcode=2013JGRD..118.8320K|url=http://publications.iass-potsdam.de/pubman/item/escidoc:134078|hdl=10871/21039|s2cid=53139265|hdl-access=free}}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}

There is some potential for changes to precipitation patterns and amplitude,{{Cite journal|last1=Bala|first1=G.|last2=Caldeira|first2=Ken|last3=Nemani|first3=Rama|last4=Cao|first4=Long|last5=Ban-Weiss|first5=George|last6=Shin|first6=Ho-Jeong|date=2010-06-24|title=Albedo enhancement of marine clouds to counteract global warming: impacts on the hydrological cycle|journal=Climate Dynamics|language=en|volume=37|issue=5–6|pages=915–931|doi=10.1007/s00382-010-0868-1|issn=0930-7575|bibcode=2011ClDy...37..915B|s2cid=129530032}}{{Cite journal|last1=Jones|first1=Andy|last2=Haywood|first2=Jim|last3=Boucher|first3=Olivier|date=2011-04-01|title=A comparison of the climate impacts of geoengineering by stratospheric SO2 injection and by brightening of marine stratocumulus cloud|journal=Atmospheric Science Letters|language=en|volume=12|issue=2|pages=176–183|doi=10.1002/asl.291|bibcode=2011AtScL..12..176J|s2cid=121136324 |issn=1530-261X}} although modeling suggests that the changes are likely less than those for stratospheric aerosol injection and considerably smaller than for unabated anthropogenic global warming. The effects may be like La Niña.{{Cite journal |last1=Chen |first1=C.-C. |last2=Richter |first2=J. H. |last3=Lee |first3=Walker R. |last4=Tye |first4=Mari |last5=MacMartin |first5=Douglas G. |last6=Kravitz |first6=Ben |date=2025 |title=Climate Impact of Marine Cloud Brightening Solar Climate Intervention Under a Susceptibility-Based Strategy Simulated by CESM2 |url=https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2024JD041245 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=130 |issue=2 |pages=e2024JD041245 |doi=10.1029/2024JD041245 |bibcode=2025JGRD..13041245C |issn=2169-8996}}

Regional implementations of MCB would need care to avoid causing possibly adverse consequences in areas far away from the region they are aiming to help. For example, a potential Marine Cloud Brightening aimed at cooling the Western United States could risk causing increasing heat in Europe, due to climate teleconnections such as unintended perturbation of the Atlantic meridional overturning circulation.

Marine cloud brightening was originally suggested by John Latham in 1990.{{Cite journal|last=Latham|first=John|date=1990|title=Control of Global Warming?|journal=Nature|volume=347|issue=6291|pages=339–340|doi=10.1038/347339b0|bibcode=1990Natur.347..339L|s2cid=4340327}} Because clouds remain a major source of uncertainty in climate change, some research projects into cloud reflectivity in the general climate change context have provided insight into marine cloud brightening specifically. For example, one project released smoke behind ships in the Pacific Ocean and monitored the particulates' impact on clouds.{{Cite journal|last1=Russell|first1=Lynn M.|last2=Sorooshian|first2=Armin|last3=Seinfeld|first3=John H.|last4=Albrecht|first4=Bruce A.|last5=Nenes|first5=Athanasios|last6=Ahlm|first6=Lars|last7=Chen|first7=Yi-Chun|last8=Coggon|first8=Matthew|last9=Craven|first9=Jill S.|date=2013-05-01|title=Eastern Pacific Emitted Aerosol Cloud Experiment|journal=Bulletin of the American Meteorological Society|volume=94|issue=5|pages=709–729|doi=10.1175/BAMS-D-12-00015.1|issn=0003-0007|bibcode=2013BAMS...94..709R|url=https://authors.library.caltech.edu/39251/1/bams-d-12-00015.1.pdf|hdl=10945/46393|s2cid=122917010 |access-date=2018-11-21|archive-date=2018-07-23|archive-url=https://web.archive.org/web/20180723124051/https://authors.library.caltech.edu/39251/1/bams-d-12-00015.1.pdf|url-status=live}} Although this was done in order to better understand clouds and climate change, the research has implications for marine cloud brightening.

A research coalition called the Marine Cloud Brightening Project was formed in order to coordinate research activities. Its proposed program includes modeling, field experiments, technology development and policy research to study cloud-aerosol effects and marine cloud brightening. The proposed program currently serves as a model for process-level (environmentally benign) experimental programs in the atmosphere.{{Cite journal|last1=Keith|first1=David W.|last2=Duren|first2=Riley|last3=MacMartin|first3=Douglas G.|date=2014-12-28|title=Field experiments on solar geoengineering: report of a workshop exploring a representative research portfolio|journal=Phil. Trans. R. Soc. A|language=en|volume=372|issue=2031|pages=20140175|doi=10.1098/rsta.2014.0175|issn=1364-503X|pmc=4240958|pmid=25404684|bibcode=2014RSPTA.37240175K}}{{Better cite|reason=The sentence says “currently” but the cite is not current|date=February 2025}}Formed in 2009 by Kelly Wanser with support from Ken Caldeira,{{Cite book|title=The Planet Remade: How Geoengineering Could Change the World|last=Morton|first=Oliver|publisher=Princeton Press|year=2015|isbn=9781400874453}} the project is now housed at the University of Washington.{{Cite web |title=Marine Cloud Brightening Program |url=https://atmos.uw.edu/faculty-and-research/marine-cloud-brightening-program/ |access-date=2025-03-08 |website=Department of Atmospheric and Climate Science |language=en-US}}

The shipping industry may have been carrying out an unintentional experiment in marine cloud brightening due to the emissions of ships and causing a global temperature reduction of as much as 0.25 ˚C lower than they would otherwise have been.{{cite news|last1=Temple|first1=James|title=We're about to kill a massive, accidental experiment in reducing global warming|url=https://www.technologyreview.com/s/610007/were-about-to-kill-a-massive-accidental-experiment-in-halting-global-warming/|access-date=22 January 2018|work=MIT Technology Review|date=22 January 2018|archive-date=22 January 2018|archive-url=https://web.archive.org/web/20180122173807/https://www.technologyreview.com/s/610007/were-about-to-kill-a-massive-accidental-experiment-in-halting-global-warming/|url-status=live}} A 2020 study found a substantial increase in cloud reflectivity from shipping in the southeast Atlantic, suggesting that a regional-scale test of MCB in stratocumulus‐dominated regions could be successful.

Marine cloud brightening is being field tested as a way to shade and cool the Great Barrier Reef in Australia, as part of the Reef Restoration and Adaptation Program.{{Cite web |last=Peterson |first=Whitney |date=2024-07-22 |title=On the ground with Holly Scoble: Reflections a year after joining the Marine Cloud Brightening fieldwork |url=https://sgdeliberation.org/reflections-a-year-after-joining-the-marine-cloud-brightening-fieldwork/ |access-date=2025-02-21 |website=DSG |language=en-US}} As of 2024 it is thought that the salt spray can deliver particles into low clouds.{{Cite web |last=Collins |first=Sarah |date=2024-10-16 |title=Buying time: can science save the Great Barrier Reef? |url=https://www.cam.ac.uk/stories/saving-great-barrier-reef |access-date=2025-02-21 |website=www.cam.ac.uk |language=en}} Although research is not yet complete, experts on the project say that if deployed it would not effect any other countries.{{Cite web |last=Mowbray |first=Sean |date=2024-08-12 |title=Cloud brightening over oceans may stave off climate change, but with risk |url=https://news.mongabay.com/2024/08/cloud-brightening-over-oceans-may-stave-off-climate-change-but-with-risk/ |access-date=2025-02-21 |website=Mongabay Environmental News |language=en-US |quote=Any eventual MCB deployment over the Great Reef would not have transboundary effects according to research, say experts affiliated with the project.}} Unlike experiments in some other places, this research is supported locally and by most Australians.{{Cite web |title=Geoengineering could fight climate change—if the public can be convinced |url=https://www.science.org/content/article/geoengineering-fight-climate-change-if-public-can-convinced?utm_campaign=Science&utm_medium=ownedSocial&utm_source=facebook&fbclid=IwY2xjawJd6aZleHRuA2FlbQIxMAABHjy-VOB0BKorNN8XM515MuVc-JVTGOEY1WzyDbiyreI3S4Weiz3LSy-rD2KW_aem_iPAGy5X85BH7uz8xnvnVow |access-date=2025-04-05 |website=www.science.org |language=en}}

The leading proposed method for marine cloud brightening is to generate a fine mist of salt from seawater, and to deliver into targeted banks of marine stratocumulus clouds from ships traversing the ocean. This requires technology that can generate optimally-sized (~200 nm) sea-salt particles and deliver them at sufficient force and scale to penetrate low-lying marine clouds. The resulting spray mist must then be delivered continuously into target clouds over the ocean.{{Rp|pages=39-43}}

In the earliest published studies, John Latham and Stephen Salter proposed a fleet of around 1500 unmanned Rotor ships, or Flettner ships, that would spray mist created from seawater into the air.{{cite journal|last=Latham |first=J. |year=2002 |title=Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds |journal=Atmos. Sci. Lett. |pages=52–58 |doi=10.1006/asle.2002.0099 |url=http://www.mmm.ucar.edu/people/latham/files/cloud_albedo_atmos_sci_lett_2002.pdf |volume=3 |issue=2–4 |bibcode=2002AtScL...3...52L |s2cid=209933251 |url-status=dead |archive-url=https://web.archive.org/web/20110716111854/http://www.mmm.ucar.edu/people/latham/files/cloud_albedo_atmos_sci_lett_2002.pdf |archive-date=2011-07-16 }}

Subsequent researchers determined that transport efficiency was only relevant for use at scale, and that for research requirements, standard ships could be used for transport. (Some researchers considered aircraft as an option, but concluded that it would be too costly.) Droplet generation and delivery technology is critical to progress, and technology research has been focused on solving this challenging problem.{{Cn|date=April 2025}}

As of 2025 how far the plume would travel and how much would reach the cloud layer is not known.{{Cite journal |last1=Hernandez-Jaramillo |first1=Diana C. |last2=Kelaher |first2=Brendan |last3=Harrison |first3=Daniel P. |date=2025-02-26 |title=A review of plume dispersion and measurement techniques applicable to marine cloud brightening |journal=Frontiers in Marine Science |language=English |volume=12 |doi=10.3389/fmars.2025.1450175 |doi-access=free |bibcode=2025FrMaS..1250175H |issn=2296-7745}}

Other methods were proposed and discounted, including:

• Using small droplets of seawater into the air through ocean foams. When bubbles in the foams burst, they loft small droplets of seawater.{{Cite journal| doi = 10.3354/cr00885 | last1 = Evans | first1 = J. | last2 = Stride | first2 = E. | last3 = Edirisinghe | first3 = M. | last4 = Andrews | first4 = D. | last5 = Simons | first5 = R. | title = Can oceanic foams limit global warming? | journal = Climate Research | volume = 42 | issue = 2 | pages = 155–160 | year = 2010 | bibcode = 2010ClRes..42..155E | doi-access = free }}
• Using piezoelectric transducers. This would create Faraday waves at a free surface. If the waves are steep enough, droplets of sea water will be thrown from the crests and the resulting salt particles can enter into the clouds. However, a significant amount of energy is required.{{cite journal |last1=Barreras |first1=F. |last2=Amaveda |first2=H. |last3=Lozano |first3=A. |date=2002 |title=Transient high-frequency ultrasonic water atomization |journal=Experiments in Fluids |volume=33 |issue=3 |pages=405–413 |doi=10.1007/s00348-002-0456-1|bibcode=2002ExFl...33..405B }}
• Using engine or smoke emissions as a source for CCN.{{clarify|date=March 2025}} Paraffin oil particles have also been proposed, though their viability has been discounted.

The costs of marine cloud brightening remain largely unknown. A report of the US National Academies suggested roughly five billion US dollars annually for a large deployment program.

{{See also|Solar radiation modification#Governance and policy risks}}

Marine cloud brightening would be governed primarily by international law because it would likely take place outside of countries' territorial waters, and because it would affect the environment of other countries and of the oceans. For the most part, the international law governing solar radiation management in general would apply. For example, according to customary international law, if a country were to conduct or approve a marine cloud brightening activity that would pose significant risk of harm to the environments of other countries or of the oceans, then that country would be obligated to minimize this risk pursuant to a due diligence standard. In this, the country would need to require authorization for the activity (if it were to be conducted by a private actor), perform a prior environmental impact assessment, notify and cooperate with potentially affected countries, and inform the public.{{Citation |last=Hubert |first=Anna-Maria |title=Marine Scientific Research |date=2018 |work=Handbook on Marine Environment Protection |pages=933–951 |editor-last=Salomon |editor-first=Markus |url=http://link.springer.com/10.1007/978-3-319-60156-4_50 |access-date=2025-04-06 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-60156-4_50 |isbn=978-3-319-60154-0 |editor2-last=Markus |editor2-first=Till}}

Marine cloud brightening activities would be further governed by the international law of the sea, and particularly by the United Nations Convention on the Law of the Sea (UNCLOS). Parties to the UNCLOS are obligated to "protect and preserve the marine environment," including by preventing, reducing, and controlling pollution of the marine environment from any source.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 192 |page=477 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}}{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 194 |pages=478–479 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} The "marine environment" is not defined but is widely interpreted as including the ocean's water, lifeforms, and the air above.{{Cite journal|last1=Valencia|first1=Mark J.|last2=Akimoto|first2=Kazumine|date=2006-11-01|title=Guidelines for navigation and overflight in the exclusive economic zone|journal=Marine Policy|volume=30|issue=6|pages=704–711|doi=10.1016/j.marpol.2005.11.002|bibcode=2006MarPo..30..704V }} "Pollution of the marine environment" is defined in a way that includes global warming and greenhouse gases.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 1.1.4 |page=399 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}}{{Cite journal|last=Boyle|first=Alan|date=2012-01-01|title=Law of the Sea Perspectives on Climate Change|journal=The International Journal of Marine and Coastal Law|volume=27|issue=4|pages=831–838|doi=10.1163/15718085-12341244|issn=1571-8085}} The UNCLOS could thus be interpreted as obligating the involved parties to use methods such as marine cloud brightening if these were found to be effective and environmentally benign. Whether marine cloud brightening itself could be such pollution of the marine environment is unclear. At the same time, in combating pollution, Parties are "not to transfer, directly or indirectly, damage or hazards from one area to another or transform one type of pollution into another."{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 195 |page=479 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} If marine cloud brightening were found to cause damage or hazards, the UNCLOS could prohibit it. If marine cloud brightening activities were to be "marine scientific research"—also an undefined term—then UNCLOS Parties have a right to conduct the research, subject to some qualifications.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 239 |page=495 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}}{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Articles 242–244 |page=496 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} Like all other ships, those that would conduct marine cloud brightening must bear the flag of the country that has given them permission to do so and to which the ship has a genuine link, even if the ship is unmanned or automated.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Articles 91–92 |page=433 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} The flagged state must exercise its jurisdiction over those ships.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 94 |pages=434–435 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} The legal implications would depend on, among other things, whether the activity were to occur in territorial waters, an exclusive economic zone (EEZ), or the high seas; and whether the activity was scientific research or not. Coastal states would need to approve any marine cloud brightening activities in their territorial waters. In the EEZ, the ship must comply with the coastal state's laws and regulations.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 58.3 |page=419 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} It appears that the state conducting marine cloud brightening activities in another state's EEZ would not need the latter's permission, unless the activity were marine scientific research. In that case, the coastal state should grant permission in normal circumstances.{{cite book |date=1994 |title=United Nations Convention on the Law of the Sea |series=Treaty Series |publisher=United Nations |volume=1833 |chapter=Article 246 |pages=497–498 |url=https://treaties.un.org/doc/Publication/UNTS/Volume%201833/v1833.pdf}} States would be generally free to conduct marine cloud brightening activities on the high seas, provided that this is done with "due regard" for other states' interests. There is some legal unclarity regarding unmanned or automated ships.{{Cite journal|last=Van Hooydonk|first=Eric|date=2014|title=The Law of Unmanned Merchant Shipping: An Exploration|url=http://www.ericvanhooydonk.be/media/54f3185ce9304.pdf|journal=The Journal of International Maritime Law|volume=20|access-date=2016-10-17|archive-date=2017-04-21|archive-url=https://web.archive.org/web/20170421014647/http://www.ericvanhooydonk.be/media/54f3185ce9304.pdf|url-status=dead}}

As of 2025 MCB is being considered for addition to the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, which might mean that parties to the convention would have to assess projects under Annex V of the convention’s London Protocol.{{Cite journal |last1=Foster |first1=Rose |last2=Shumway |first2=Nicole |last3=Harrison |first3=Daniel |last4=Fidelman |first4=Pedro |date=2025-01-01 |title=Governing marine cloud brightening for ecosystem conservation under a warming climate |journal=Earth System Governance |volume=23 |pages=100240 |doi=10.1016/j.esg.2025.100240 |issn=2589-8116|doi-access=free }}

Marine cloud brightening appears to have most of the advantages and disadvantages of solar radiation management in general. For example, it presently appears to be inexpensive relative to suffering climate change damages and greenhouse gas emissions abatement, fast acting, and reversible in its direct climatic effects. Some advantages and disadvantages are specific to it, relative to other proposed solar radiation management techniques.

Compared with other proposed solar radiation management methods, such as stratospheric aerosols injection, marine cloud brightening may be able to be partially localized in its effects. This could, for example, be used to stabilize the West Antarctic Ice Sheet. Furthermore, marine cloud brightening, as it is currently envisioned, would use only natural substances sea water and wind, instead of introducing human-made substances into the environment.

Potential disadvantages include that specific MCB implementations could have a varying effect across time; the same intervention might even become a net contributor to global warming some years after being first launched, though this could be avoided with careful planning. {{Cite journal|last1=Wan|first1=Jessica S.|last2=Chen|first2=Chih-Chieh Jack|last3=Tilmes|first3=Simone|last4=Luongo|first4=Matthew T.|last5=Richter|first5=Jadwiga H.|last6=Ricke|first6=Katharine|date=2024|title= Diminished efficacy of regional marine cloud brightening in a warmer world|journal=Nature Climate Change|volume=14 |issue=8 |pages=808–814 |doi=10.1038/s41558-024-02046-7}}

• Climate engineering
• Cirrus cloud thinning

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