IPCC:AR6/WGI/Chapter-6 - Revision history

Laura at 08:35, 22 May 2026

2026-05-22T08:35:25Z

← Older revision		Revision as of 08:35, 22 May 2026
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			\| prevurl = IPCC:AR6/WGI/Chapter-5
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Laura: Laura moved page IPCC:Wg1:Chapter:Chapter-6 to IPCC:AR6/WGI/Chapter-6 without leaving a redirect

2026-05-18T16:17:45Z

Laura moved page IPCC:Wg1:Chapter:Chapter-6 to IPCC:AR6/WGI/Chapter-6 without leaving a redirect

← Older revision	Revision as of 16:17, 18 May 2026
(No difference)

Laura: #119

2026-05-15T11:57:49Z

#119

← Older revision		Revision as of 11:57, 15 May 2026
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	In response to the outbreak of COVID-19 (officially the severe acute respiratory syndrome–coronavirus 2 or SARS-CoV-2), which was declared a pandemic on March 11 2020 by the World Health Organization (WHO), regulations were imposed by many countries to contain the spread of COVID-19. Restrictions were implemented on the movement of people, such as closing borders or requiring the majority of population to stay at home, for periods of several months. This Cross-Chapter Box assesses the influence of the COVID-19 containment on short-lived climate forcers (SLCFs) and long-lived greenhouse gases (LLGHGs), and related implications for the climate. Note that this assessment was developed late in the AR6 WGI process and is based on the available emerging literature.		In response to the outbreak of COVID-19 (officially the severe acute respiratory syndrome–coronavirus 2 or SARS-CoV-2), which was declared a pandemic on March 11 2020 by the World Health Organization (WHO), regulations were imposed by many countries to contain the spread of COVID-19. Restrictions were implemented on the movement of people, such as closing borders or requiring the majority of population to stay at home, for periods of several months. This Cross-Chapter Box assesses the influence of the COVID-19 containment on short-lived climate forcers (SLCFs) and long-lived greenhouse gases (LLGHGs), and related implications for the climate. Note that this assessment was developed late in the AR6 WGI process and is based on the available emerging literature.

	~~'''~~'''Emissions'''~~'''~~		'''Emissions'''

	Global fossil CO <sub>2</sub> emissions are estimated to have declined by 7% ( ''medium confidence'' ) in 2020 compared to 2019 emissions, with estimates ranging from 5.8% to 13.0% based on various combinations of data on energy production and consumption, economic activity and proxy activity data for emissions and their drivers (Forster et al. , 2020; Friedlingstein et al. , 2020; Le Quéré et al. , 2020; Liu et al. , 2020) . However, the concentration of atmospheric CO <sub>2</sub> continued to grow in 2020 compared to previous years ( [[#Dlugokencky--2021\|Dlugokencky and Tans, 2021]] ). Given the large natural interannual variability of CO <sub>2</sub> ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.1\|Section 5.2.1]] ), and the small expected impact of emissions in the CO <sub>2</sub> growth rate, there were no observed changes in CO <sub>2</sub> concentration that could be attributed to COVID-19 containment ( ''medium confidence'' ) ( [[#Chevallier--2020\|Chevallier et al., 2020]] ; [[#Tohjima--2020\|Tohjima et al., 2020]] ).		Global fossil CO <sub>2</sub> emissions are estimated to have declined by 7% ( ''medium confidence'' ) in 2020 compared to 2019 emissions, with estimates ranging from 5.8% to 13.0% based on various combinations of data on energy production and consumption, economic activity and proxy activity data for emissions and their drivers (Forster et al. , 2020; Friedlingstein et al. , 2020; Le Quéré et al. , 2020; Liu et al. , 2020) . However, the concentration of atmospheric CO <sub>2</sub> continued to grow in 2020 compared to previous years ( [[#Dlugokencky--2021\|Dlugokencky and Tans, 2021]] ). Given the large natural interannual variability of CO <sub>2</sub> ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.1\|Section 5.2.1]] ), and the small expected impact of emissions in the CO <sub>2</sub> growth rate, there were no observed changes in CO <sub>2</sub> concentration that could be attributed to COVID-19 containment ( ''medium confidence'' ) ( [[#Chevallier--2020\|Chevallier et al., 2020]] ; [[#Tohjima--2020\|Tohjima et al., 2020]] ).

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	Using similar methodologies, [[#Forster--2020\|Forster et al. (2020)]] assembled activity data and emissions estimates for other greenhouse gases and aerosols and their precursors. Anthropogenic NO <sub>x</sub> emissions, which are largely from the transport sector, are estimated to have decreased by a maximum of 35% in April ( ''medium confidence'' ). Species whose emissions are dominated by other sectors, such as methane and NH <sub>3</sub> from agriculture, saw smaller reductions.		Using similar methodologies, [[#Forster--2020\|Forster et al. (2020)]] assembled activity data and emissions estimates for other greenhouse gases and aerosols and their precursors. Anthropogenic NO <sub>x</sub> emissions, which are largely from the transport sector, are estimated to have decreased by a maximum of 35% in April ( ''medium confidence'' ). Species whose emissions are dominated by other sectors, such as methane and NH <sub>3</sub> from agriculture, saw smaller reductions.

	~~'''~~'''Abundances and air quality'''~~'''~~		'''Abundances and air quality'''

	Owing to the short atmospheric lifetimes of SLCFs relevant to air quality, changes in their concentrations were detected within a few days after lockdowns had been implemented (e.g., Bauwens et al. , 2020; Venter et al. , 2020; Gkatzelis et al. , 2021; Shi et al. , 2021) . The COVID-19-driven economic slowdown has illustrated how complex the relationship is between emissions and air pollutant concentrations due to non-linearity in the atmospheric chemistry leading to secondary compound formation (Section 6.1, Box 6.1; [[#Kroll--2020\|Kroll et al., 2020]] ).		Owing to the short atmospheric lifetimes of SLCFs relevant to air quality, changes in their concentrations were detected within a few days after lockdowns had been implemented (e.g., Bauwens et al. , 2020; Venter et al. , 2020; Gkatzelis et al. , 2021; Shi et al. , 2021) . The COVID-19-driven economic slowdown has illustrated how complex the relationship is between emissions and air pollutant concentrations due to non-linearity in the atmospheric chemistry leading to secondary compound formation (Section 6.1, Box 6.1; [[#Kroll--2020\|Kroll et al., 2020]] ).

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	Except for ozone, temporary improvement of air quality during lockdown periods was observed in most regions of the world ( ''high confidence'' ), resulting from a combination of interannual meteorological variability and the impact of COVID-19 containment measures ( ''high confidence'' ). Estimated air pollution reductions associated with lockdown periods are lower than what can be expected from integrated mitigation policy leading to lasting reductions ( ''medium confidence'' ).		Except for ozone, temporary improvement of air quality during lockdown periods was observed in most regions of the world ( ''high confidence'' ), resulting from a combination of interannual meteorological variability and the impact of COVID-19 containment measures ( ''high confidence'' ). Estimated air pollution reductions associated with lockdown periods are lower than what can be expected from integrated mitigation policy leading to lasting reductions ( ''medium confidence'' ).

	~~'''~~'''Radiative forcings'''~~'''~~		'''Radiative forcings'''

	COVID-19-related emissions changes primarily exerted effective radiative forcing (ERF) through reduced emissions rates of CO <sub>2</sub> and methane, altered abundance of SLCFs, notably ozone, NO <sub>2</sub> and aerosols, and through other changes in anthropogenic activities, notably a reduction in the formation of aviation-induced cirrus clouds.		COVID-19-related emissions changes primarily exerted effective radiative forcing (ERF) through reduced emissions rates of CO <sub>2</sub> and methane, altered abundance of SLCFs, notably ozone, NO <sub>2</sub> and aerosols, and through other changes in anthropogenic activities, notably a reduction in the formation of aviation-induced cirrus clouds.

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	The available studies are in broad agreement on the sign and magnitude of contributions to ERF from COVID-19-related emissions changes during 2020. The range in peak global mean ERF in spring 2020 was [0.025 to 0.2] W m <sup>–2</sup> ( ''medium confidence'' ), composed of a positive forcing from aerosol–climate interactions of [0.1 to 0.3] W m <sup>–2</sup> , and negative forcings from CO <sub>2</sub> (–0.01 W m <sup>–2</sup> ), NO <sub>x</sub> (–0.04 W m <sup>–2</sup> ) and contrail cirrus (–0.04 W m <sup>–2</sup> ) ( ''limited evidence'' , ''medium agreement'' ). By the end of 2020, the ERF was at half the peak value ( ''medium confidence'' ).		The available studies are in broad agreement on the sign and magnitude of contributions to ERF from COVID-19-related emissions changes during 2020. The range in peak global mean ERF in spring 2020 was [0.025 to 0.2] W m <sup>–2</sup> ( ''medium confidence'' ), composed of a positive forcing from aerosol–climate interactions of [0.1 to 0.3] W m <sup>–2</sup> , and negative forcings from CO <sub>2</sub> (–0.01 W m <sup>–2</sup> ), NO <sub>x</sub> (–0.04 W m <sup>–2</sup> ) and contrail cirrus (–0.04 W m <sup>–2</sup> ) ( ''limited evidence'' , ''medium agreement'' ). By the end of 2020, the ERF was at half the peak value ( ''medium confidence'' ).

	~~'''~~'''Climate responses'''~~'''~~		'''Climate responses'''

	Changes in atmospheric composition due to COVID-19 emissions reductions are not thought to have caused a detectable change in global temperature or rainfall in 2020 ( ''high confidence'' ). A large ensemble of Earth system model (ESM) simulations show an ensemble average reduction in Aerosol Optical Depth (AOD) in some regions, notably Eastern and Southern Asia ( [[#Fyfe--2021\|Fyfe et al., 2021]] ). This result is supported by observational studies finding decreases in optical depth in 2020 ( [[#Gkatzelis--2021\|Gkatzelis et al., 2021]] ; [[#Ming--2021\|Ming et al., 2021]] ; [[#van%20Heerwaarden--2021\|van Heerwaarden et al., 2021]] ), which may have contributed to observed increases in solar irradiance ( [[#van%20Heerwaarden--2021\|van Heerwaarden et al., 2021]] ) or solar clear-sky reflection ( [[#Ming--2021\|Ming et al., 2021]] ).		Changes in atmospheric composition due to COVID-19 emissions reductions are not thought to have caused a detectable change in global temperature or rainfall in 2020 ( ''high confidence'' ). A large ensemble of Earth system model (ESM) simulations show an ensemble average reduction in Aerosol Optical Depth (AOD) in some regions, notably Eastern and Southern Asia ( [[#Fyfe--2021\|Fyfe et al., 2021]] ). This result is supported by observational studies finding decreases in optical depth in 2020 ( [[#Gkatzelis--2021\|Gkatzelis et al., 2021]] ; [[#Ming--2021\|Ming et al., 2021]] ; [[#van%20Heerwaarden--2021\|van Heerwaarden et al., 2021]] ), which may have contributed to observed increases in solar irradiance ( [[#van%20Heerwaarden--2021\|van Heerwaarden et al., 2021]] ) or solar clear-sky reflection ( [[#Ming--2021\|Ming et al., 2021]] ).

Laura: /* Chapter 6: Short-lived Climate Forcers */

2026-05-13T14:04:36Z

Chapter 6: Short-lived Climate Forcers

← Older revision		Revision as of 14:04, 13 May 2026
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	<div id="chapter-authors"></div>		<div id="chapter-authors"></div>

	Coordinating Lead Authors:		'''Coordinating Lead Authors:'''

	Sophie Szopa (France), Vaishali Naik (United States of America)		Sophie Szopa (France), Vaishali Naik (United States of America)

	Lead Authors:		'''Lead Authors:'''

	Bhupesh Adhikary (Nepal), Paulo Artaxo (Brazil), Terje Berntsen (Norway), William D. Collins (United States of America), Sandro Fuzzi (Italy), Laura Gallardo (Chile), Astrid Kiendler-Scharr (Germany/Austria), Zbigniew Klimont (Austria/Poland), Hong Liao (China), Nadine Unger (United Kingdom/United States of America), Prodromos Zanis (Greece)		Bhupesh Adhikary (Nepal), Paulo Artaxo (Brazil), Terje Berntsen (Norway), William D. Collins (United States of America), Sandro Fuzzi (Italy), Laura Gallardo (Chile), Astrid Kiendler-Scharr (Germany/Austria), Zbigniew Klimont (Austria/Poland), Hong Liao (China), Nadine Unger (United Kingdom/United States of America), Prodromos Zanis (Greece)

	Contributing Authors:		'''Contributing Authors:'''

	Wenche Aas (Norway), Dimitris Akritidis (Greece), Robert J. Allen (United States of America), Nicolas Bellouin (United Kingdom/France), Sophie Berger (France/Belgium), Sara M. Blichner (Norway), Josep G. Canadell (Australia), William Collins (United Kingdom), Owen R. Cooper (United States of America), Frank J. Dentener (EU/The Netherlands), Sarah Doherty (United States of America), Jean-Louis Dufresne (France), Sergio Henrique Faria (Spain/Brazil), Piers Forster (United Kingdom), Tzung-May Fu (China), Jan S. Fuglestvedt (Norway), John C. Fyfe (Canada), Aristeidis K. Georgoulias (Greece), Matthew J. Gidden (Austria/United States of America), Nathan P. Gillett (Canada), Paul Ginoux (United States of America), Paul T. Griffiths (United Kingdom), Jian He (United States of America/China), Christopher Jones (United Kingdom), Svitlana Krakovska (Ukraine), Chaincy Kuo (United States of America), David S. Lee (United Kingdom), Maurice Levasseur (Canada), Martine Lizotte (Canada), Thomas K. Maycock (United States of America), Jean-François Müller (Belgium), Helène Muri (Norway), Lee T. Murray (United States of America), Zebedee R. J. Nicholls (Australia), Jurgita Ovadnevaite (Ireland/Lithuania), Prabir K. Patra (Japan/India), Fabien Paulot (United States of America/France, United States of America), Pallav Purohit (Austria/India), Johannes Quaas (Germany), Joeri Rogelj (United Kingdom/Belgium), Bjørn H. Samset (Norway), Chris Smith (United Kingdom), Izuru Takayabu (Japan), Marianne Tronstad Lund (Norway), Alexandra P. Tsimpidi (Germany/Greece), Steven Turnock (United Kingdom), Rita Van Dingenen (Italy/Belgium), Hua Zhang (China), Alcide Zhao (United Kingdom/China)		Wenche Aas (Norway), Dimitris Akritidis (Greece), Robert J. Allen (United States of America), Nicolas Bellouin (United Kingdom/France), Sophie Berger (France/Belgium), Sara M. Blichner (Norway), Josep G. Canadell (Australia), William Collins (United Kingdom), Owen R. Cooper (United States of America), Frank J. Dentener (EU/The Netherlands), Sarah Doherty (United States of America), Jean-Louis Dufresne (France), Sergio Henrique Faria (Spain/Brazil), Piers Forster (United Kingdom), Tzung-May Fu (China), Jan S. Fuglestvedt (Norway), John C. Fyfe (Canada), Aristeidis K. Georgoulias (Greece), Matthew J. Gidden (Austria/United States of America), Nathan P. Gillett (Canada), Paul Ginoux (United States of America), Paul T. Griffiths (United Kingdom), Jian He (United States of America/China), Christopher Jones (United Kingdom), Svitlana Krakovska (Ukraine), Chaincy Kuo (United States of America), David S. Lee (United Kingdom), Maurice Levasseur (Canada), Martine Lizotte (Canada), Thomas K. Maycock (United States of America), Jean-François Müller (Belgium), Helène Muri (Norway), Lee T. Murray (United States of America), Zebedee R. J. Nicholls (Australia), Jurgita Ovadnevaite (Ireland/Lithuania), Prabir K. Patra (Japan/India), Fabien Paulot (United States of America/France, United States of America), Pallav Purohit (Austria/India), Johannes Quaas (Germany), Joeri Rogelj (United Kingdom/Belgium), Bjørn H. Samset (Norway), Chris Smith (United Kingdom), Izuru Takayabu (Japan), Marianne Tronstad Lund (Norway), Alexandra P. Tsimpidi (Germany/Greece), Steven Turnock (United Kingdom), Rita Van Dingenen (Italy/Belgium), Hua Zhang (China), Alcide Zhao (United Kingdom/China)

	Review Editors:		'''Review Editors:'''

	Yugo Kanaya (Japan), Michael J. Prather (United States of America), Noureddine Yassaa (Algeria)		Yugo Kanaya (Japan), Michael J. Prather (United States of America), Noureddine Yassaa (Algeria)

	Chapter Scientist:		'''Chapter Scientist:'''

	Chaincy Kuo (United States of America)		Chaincy Kuo (United States of America)
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	<div id="chapter-citation"></div>		<div id="chapter-citation"></div>

	This chapter should be cited as:		'''This chapter should be cited as:'''

	Szopa, S., V. Naik, B. Adhikary, P. Artaxo, T. Berntsen, W.D. Collins, S. Fuzzi, L. Gallardo, A. Kiendler-Scharr, Z. Klimont, H. Liao, N. Unger, and P. Zanis, 2021: Short-Lived Climate Forcers. In ''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. 817–922, doi: [https://doi.org/10.1017/9781009157896.008 10.1017/9781009157896.008] .		Szopa, S., V. Naik, B. Adhikary, P. Artaxo, T. Berntsen, W.D. Collins, S. Fuzzi, L. Gallardo, A. Kiendler-Scharr, Z. Klimont, H. Liao, N. Unger, and P. Zanis, 2021: Short-Lived Climate Forcers. In ''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. 817–922, doi: [https://doi.org/10.1017/9781009157896.008 10.1017/9781009157896.008] .

imported>Design: /* References */

2026-05-11T08:43:21Z

References

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