Editing Contrail (section)

==Impacts on climate==
[[Image:Sfc.contrail.1.26.01.JPG|thumb|[[NASA]] photograph showing aircraft contrails and natural clouds]]
It is considered that the largest contribution of aviation to climate change comes from contrails.<ref>{{cite web |author=KATIE CAMERO |title=Aviation's dirty secret: Airplane contrails are a surprisingly potent cause of global warming Warming effect of thin, white clouds will triple by 2050 |url=https://www.science.org/content/article/aviation-s-dirty-secret-airplane-contrails-are-surprisingly-potent-cause-global-warming |website=www.science.org |access-date=10 May 2024 |date=28 June 2019}}</ref>  
In general, aircraft contrails trap [[outgoing longwave radiation]] emitted by the Earth and atmosphere more than they reflect incoming [[solar radiation]], resulting in a net increase in [[radiative forcing]]. In 1992, this warming effect was estimated between 3.5&nbsp;mW/m<sup>2</sup> and 17&nbsp;mW/m<sup>2</sup>.<ref>{{cite journal|last=Ponater|first=M.|display-authors= etal |year=2005|title=On contrail climate sensitivity|journal=[[Geophysical Research Letters]]|volume=32|issue=10|pages=L10706|doi=10.1029/2005GL022580|bibcode=2005GeoRL..3210706P|doi-access=free}}</ref>
In 2009, its 2005 value was estimated at 12&nbsp;mW/m<sup>2</sup>, based on the [[Atmospheric reanalysis|reanalysis]] data, [[climate model]]s, and [[Atmospheric radiative transfer codes|radiative transfer codes]]; with an uncertainty range of 5 to 26&nbsp;mW/m<sup>2</sup>, and with a low level of scientific understanding.<ref>{{cite journal|last=Lee|first=D. S.|display-authors= etal|year=2009 |title=Aviation and global climate change in the 21st century|journal=[[Atmos. Environ.]]|volume=43|issue=22|pages=3520–3537|doi=10.1016/j.atmosenv.2009.04.024|pmid=32362760|pmc=7185790|bibcode=2009AtmEn..43.3520L|url=http://elib.dlr.de/59761/1/lee.pdf |archive-url=https://web.archive.org/web/20160716195614/http://elib.dlr.de/59761/1/lee.pdf |archive-date=2016-07-16 |url-status=live}}</ref> 
[[File:Bomber stream.jpg|thumb|left|USAAF 8th Air Force B-17s and their contrails]]
Contrail cirrus may be air traffic's largest radiative forcing component, larger than all {{CO2}} accumulated from aviation, and could triple from a 2006 baseline to 160–180&nbsp;mW/m<sup>2</sup> by 2050 without intervention.<ref>{{cite news |url=https://www.newscientist.com/article/2207886-it-turns-out-planes-are-even-worse-for-the-climate-than-we-thought/ |website=New Scientist |title=It turns out planes are even worse for the climate than we thought |date= 27 June 2019 |author=Michael Le Page |access-date=13 October 2021}}</ref><ref>{{cite journal |journal=Atmospheric Chemistry and Physics |title=Contrail cirrus radiative forcing for future air traffic |year=2019 |url=https://acp.copernicus.org/articles/19/8163/2019/ |last1=Bock |first1=Lisa |last2=Burkhardt |first2=Ulrike |volume=19 |issue=12 |page=8163 |doi=10.5194/acp-19-8163-2019 |bibcode=2019ACP....19.8163B |doi-access=free}}</ref> For comparison, the total radiative forcing from human activities amounted to 2.72 W/m<sup>2</sup> (with a range between 1.96 and 3.48W/m<sup>2</sup>) in 2019, and the increase from 2011 to 2019 alone amounted to 0.34W/m<sup>2</sup>.<ref name="IPCC_WGI_SPM">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3–32, {{doi|10.1017/9781009157896.001}}.</ref> Contrail effects differ a lot depending on when they are formed, as they decrease the daytime temperature and increase the nighttime temperature, reducing their difference.<ref>{{citation |author1= Bernhardt, J. |author2= Carleton, A. M. |date= 14 March 2015 |title= The impacts of long-lived jet contrail 'outbreaks' on surface station diurnal temperature range |journal= Journal of International Climatology |volume= 35 |issue= 15 |pages= 4529–4538 |doi= 10.1002/joc.4303 |bibcode= 2015IJCli..35.4529B |s2cid= 128789946 |url= https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/joc.4303}}</ref> In 2006, it was estimated that [[Red-eye flight|night flights]] contribute 60 to 80% of contrail radiative forcing while accounting for 25% of daily air traffic, and winter flights contribute half of the annual mean radiative forcing while accounting for 22% of annual air traffic.<ref>{{cite journal |last=Stuber|first=Nicola | display-authors= etal |date= 15 June 2006 |title=The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7095 |pages=864–7 |doi=10.1038/nature04877 |pmid=16778887 |bibcode=2006Natur.441..864S |s2cid=4348401 |url=https://www.nature.com/articles/nature04877}}</ref>

Starting from the 1990s, it was suggested that contrails during daytime have a strong cooling effect, and when combined with the warming from night-time flights, this would lead to a substantial [[diurnal temperature variation]] (the difference in the day's highs and lows at a fixed station).<ref>{{citation |author= Perkins, Sid. |title= September's Science: Shutdown of airlines aided contrail studies |journal= Science News |date= 11 May 2002 |publisher= Science News Online |url= https://www.sciencenews.org/article/septembers-science-shutdown-airlines-aided-contrail-studies  |access-date=13 October 2021}}</ref> When [[Closings and cancellations following the September 11 attacks|no commercial aircraft flew]] across the USA following the [[September 11 attacks]], the [[diurnal temperature variation]] was widened by {{cvt|1.1|C-change|F-change}}.<ref name=Travis2002Aug>{{cite journal |author= Travis, D. J. |author2= A. Carleton |author3= R. G. Lauritsen |date= August 2002 |title=Contrails reduce daily temperature range |journal= Nature |volume= 418 |issue= 6898 |page= 601 |doi= 10.1038/418601a |pmid= 12167846 |bibcode= 2002Natur.418..601T |s2cid= 4425866 |doi-access= free }}</ref> Measured across 4,000 [[weather station]]s in the continental United States, this increase was the largest recorded in 30 years.<ref name=Travis2002Aug/> Without contrails, the local diurnal temperature range was {{convert|1|°C-change|°F-change|abbr=on}} higher than immediately before.<ref>{{cite journal|last=Travis|first=D. J.|author2=A. M. Carleton|author3=R. G. Lauritsen|date=March 2004|title=Regional Variations in U.S. Diurnal Temperature Range for the 11–14 September 2001 Aircraft Groundings: Evidence of Jet Contrail Influence on Climate|journal=J. Clim.|volume=17|issue=5|page=1123|doi=10.1175/1520-0442(2004)017<1123:RVIUDT>2.0.CO;2|bibcode=2004JCli...17.1123T|url= https://journals.ametsoc.org/view/journals/clim/17/5/1520-0442_2004_017_1123_rviudt_2.0.co_2.xml}}</ref> In the southern US, the difference was diminished by about {{cvt|6|F-change|C-change|order=flip}}, and by {{cvt|5|F-change|C-change|order=flip}} in the US midwest.<ref>{{citation |url= https://www.sciencedaily.com/releases/2015/06/150618122236.htm |title= Jet contrails affect surface temperatures |work= [[Science Daily]] |date= 18 June 2015 |access-date=13 October 2021}}</ref><ref name=contrails>{{cite journal|title=Contrails reduce daily temperature range|first1=David J.|last1=Travis|last2=Carleton|first2=Andrew M.|last3=Lauritsen|first3=Ryan G.|journal=[[Nature (journal)|Nature]]|page=601|volume=418|year=2002|url=http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf|doi=10.1038/418601a|pmid=12167846|issue=6898|bibcode=2002Natur.418..601T|s2cid=4425866|url-status = dead|archive-url=https://web.archive.org/web/20060503192714/http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf|archive-date=3 May 2006}}</ref> However, follow-up studies found that a natural change in cloud cover can more than explain these findings.<ref>{{cite journal|last1=Kalkstein|last2=Balling Jr.|year=2004|title=Impact of unusually clear weather on United States daily temperature range following 9/11/2001|journal=Climate Research|volume=26|page=1|doi=10.3354/cr026001|bibcode=2004ClRes..26....1K|url=http://www.int-res.com/abstracts/cr/v26/n1/p1-4/|doi-access=free}}</ref> The authors of a 2008 study wrote, "The variations in high cloud cover, including contrails and contrail-induced cirrus clouds, contribute weakly to the changes in the diurnal temperature range, which is governed primarily by lower altitude clouds, winds, and humidity."<ref>{{cite journal|year=2008|doi=10.1029/2008GL036108|title=Do contrails significantly reduce daily temperature range?|journal=Geophysical Research Letters|volume=35|issue=23|pages=L23815|bibcode=2008GeoRL..3523815H|last1=Hong|first1=Gang|last2=Yang|first2=Ping|last3=Minnis|first3=Patrick|last4=Hu|first4=Yong X.|last5=North|first5=Gerald|doi-access=free}}</ref> 
[[File:Ashcloud.png|thumb|The sky above [[Würzburg]] without contrails after [[Air travel disruption after the 2010 Eyjafjallajökull eruption|air travel disruption in 2010]] (left) and with regular air traffic and the right conditions (right)]]
In 2011, a study of British meteorological records taken during [[World War II]] identified one event where the temperature was {{convert|0.8|°C-change|°F-change|abbr=on}} higher than the day's average near [[airbase]]s used by [[USAAF]] [[strategic bomber]]s after they flew in a formation. However, its authors cautioned that this was a single event, making it difficult to draw firm conclusions from it.<ref>{{cite web|url=http://www.scientificamerican.com/article/contrails-aviation-affects-climate/|title=World War II Bomber Contrails Show How Aviation Affects Climate|first=Umair |last=Irfan |work=scientificamerican.com (ClimateWire) |date=7 July 2011 |access-date=13 October 2021}}</ref><ref>{{cite web|url=http://www.livescience.com/14944-wwii-bombing-raids-contrails-weather-climate.html|title=WWII Bombing Raids Altered English Weather |work=livescience.com |last=Parry|first=Wynne |date=7 July 2011 |access-date=13 October 2021}}</ref><ref>{{Cite journal|last1=Ryan|first1=A. C.|display-authors= etal |title=World War II contrails: A case study of aviation-induced cloudiness|journal=International Journal of Climatology|year=2012|volume=32|issue=11|pages=1745–1753|doi=10.1002/joc.2392|bibcode=2012IJCli..32.1745R|s2cid=129296874 |doi-access=free}}</ref> Then, the global response to the [[2020 coronavirus pandemic]] led to a reduction in global air traffic of nearly 70% relative to 2019. Thus, it provided an extended opportunity to study the impact of contrails on regional and global temperature. Multiple studies found "no significant response of diurnal surface air temperature range" as the result of contrail changes, and either "no net significant global ERF" (effective [[radiative forcing]]) or a very small warming effect.<ref>{{cite journal|date=29 September 2021|title=An Observational Constraint on Aviation-Induced Cirrus From the COVID-19-Induced Flight Disruption|journal=Geophysical Research Letters|volume=48|issue=20|pages=e2021GL095882|last1=Digby|first1=Ruth A. R.|last2=Gillett|first2=Nathan P.|last3=Monahan|first3=Adam H.|last4=Cole|first4=Jason N. S. |doi=10.1029/2021GL095882 |pmid=34924638 |pmc=8667656 |doi-access=free}}</ref><ref>{{cite journal|date=18 June 2021|url=https://acp.copernicus.org/articles/21/9405/2021/ |doi=10.5194/acp-21-9405-2021|title=The climate impact of COVID-19-induced contrail changes|journal=Atmospheric Chemistry and Physics|volume=21|pages=9405–9416|last1=Gettelman|first1=Andrew|last2=Chen|first2=Chieh-Chieh|last3=Bardeen|first3=Charles G.|issue=12 |doi-access=free}}</ref><ref>{{cite journal |last1=Zhu |first1=Jialei |last2=Penner |first2=Joyce E. |last3=Garnier |first3=Anne |last4=Boucher |first4=Olivier |last5=Gao |first5=Meng |last6=Song |first6=Lei |last7=Deng |first7=Junjun |last8=Liu |first8=Cong-qiang |last9=Fu |first9=Pingqing |date=18 March 2022 | title=Decreased Aviation Leads to Increased Ice Crystal Number and a Positive Radiative Effect in Cirrus Clouds |journal=AGU Advances | volume=3 |issue=2 |page=ee2020GL089788 |doi=10.1029/2021AV000546 |doi-access=free |hdl=2027.42/172020 |hdl-access=free }}</ref> 

An EU project launched in 2020 aims to assess the feasibility of minimising contrail effects by the operational choices in making flight plans.<ref>{{cite web |title=A unique opportunity to accelerate development {{!}} EUROCONTROL |url=https://www.eurocontrol.int/article/unique-opportunity-accelerate-development |website=www.eurocontrol.int |access-date=10 May 2024 |language=en |date=16 November 2020}}</ref> Other similar projects include ContrailNet from Eurocontrol,<ref>{{cite web |title=EUROCONTROL launches ContrailNet - the new network to create a common repository of contrail observation data {{!}} EUROCONTROL |url=https://www.eurocontrol.int/news/eurocontrol-launches-contrailnet-new-network-create-common-repository-contrail-observation |website=www.eurocontrol.int |access-date=12 May 2024 |language=en |date=7 November 2023}}</ref>    Reviate,<ref>{{cite web |title=Reviate - Contrail avoidance for the climate |url=https://contrails.org/ |website=contrails.org |access-date=12 May 2024}}</ref> and the Ciconia project,<ref>{{cite web |last1=Andrews |first1=Siân |title=Leading the Way in Contrail Avoidance |url=https://nats.aero/blog/2023/12/leading-the-way-in-contrail-avoidance/ |website=NATS Blog |access-date=12 May 2024 |date=13 December 2023}}</ref> as well as Google's 'project contrails'.<ref>{{cite web |title=Project Contrails: Preventing Contrails with AI - Google Research |url=https://sites.research.google/contrails/ |website=Project Contrails: Preventing Contrails with AI - Google Research |access-date=12 May 2024 |language=en}}</ref>