World Meteorological Organization
7bis avenue de la Paix
Case postale 2300
CH-1211 Geneva 2
Switzerland
United Nations Environment Programme
Ozone Secretariat
P.O. Box 30552
Nairobi, 00100
Kenya
US Department of Commerce
National Oceanic and Atmospheric Administration
14th Street and Constitution Avenue NWHerbert C. Hoover Building, Room 5128
Washington, D. C. 20230
National Aeronautics and Space Administration
Earth Science Division
NASA Headquarters
300 E. Street SW
Washington, D.C. 20546-0001
European Commission
Directorate-General for Research
B-1049 Bruxelles
Belgium

Published October 2022
ISBN: 978-9914-733-99-0
This document should be cited as:
World Meteorological Organization (WMO). Executive Summary. Scientific Assessment of Ozone Depletion: 2022,
GAW Report No. 278, 56 pp.; WMO: Geneva, 2022.
This report is available on the internet at the following locations:
https://ozone.unep.org/science/assessment/sap
https://www.csl.noaa.gov/assessments/ozone/2022
Note: Figures from this report are in the public domain and may be used without permission. Attribution to this document isencouraged.
The graphic illustrations on this cover are artistic renderings of Figure ES-1 (Panels a and d) reflecting the theme of thispublication and the underlying science.

World Meteorological Organization
Ozone Research and Monitoring – GAW Report No. 278

EXECUTIVE SUMMARY
Scientific Assessment of
Ozone Depletion: 2022

World Meteorological Organization
United Nations Environment Programme
National Oceanic and Atmospheric Administration
National Aeronautics and Space Administration
European Commission

HIGHLIGHTS
of theScientific Assessment of Ozone Depletion: 2022
Science has been one of the foundations of the Montreal Protocol’s success. This document highlights advances and updates in thescientific understanding of ozone depletion since the 2018 Scientific Assessment of Ozone Depletion and provides policy-relevant scientificinformation on current challenges and future policy choices.

A. Major achievements of the Montreal Protocol• Actions taken under the Montreal Protocol continued to decrease atmospheric abundances of controlled ozone-depleting substances (ODSs) and advance the recovery of the stratospheric ozone layer. The atmospheric abundances of both total tropospheric chlorine
and total tropospheric bromine from long-lived ODSs have continued to decline since the 2018 Assessment. New studies support
previous Assessments in that the decline in ODS emissions due to compliance with the Montreal Protocol avoids global warming ofapproximately 0.5–1 °C by mid-century compared to an extreme scenario with an uncontrolled increase in ODSs of 3–3.5% per year.
• Actions taken under the Montreal Protocol continue to contribute to ozone recovery. Recovery of ozone in the upper stratosphereis progressing. Total column ozone (TCO) in the Antarctic continues to recover, notwithstanding substantial interannual variability inthe size, strength, and longevity of the ozone hole. Outside of the Antarctic region (from 90°N to 60°S), the limited evidence of TCO
recovery since 1996 has low confidence. TCO is expected to return to 1980 values around 2066 in the Antarctic, around 2045 in the
Arctic, and around 2040 for the near-global average (60°N–60°S). The assessment of the depletion of TCO in regions around theglobe from 1980–1996 remains essentially unchanged since the 2018 Assessment.
• Compliance with the 2016 Kigali Amendment to the Montreal Protocol, which requires phase down of production and consumptionof some hydrofluorocarbons (HFCs), is estimated to avoid 0.3–0.5°C of warming by 2100. This estimate does not include contributions from HFC-23 emissions.

B. Current Scientific and Policy Challenges• The recent identification of unexpected CFC-11 emissions led to scientific investigations and policy responses. Observations andanalyses revealed the source region for at least half of these emissions and substantial emissions reductions followed. Regional datasuggest some CFC-12 emissions may have been associated with the unreported CFC-11 production. Uncertainties in emissions frombanks and gaps in the observing network are too large to determine whether all unexpected emissions have ceased.
• Unexplained emissions have been identified for other ODSs (CFCs-13, 112a, 113a, 114a, 115, and CCl4), as well as HFC-23. Some ofthese unexplained emissions are likely occurring as leaks of feedstocks or by-products, and the remainder is not understood.
• Outside of the polar regions, observations and models are in agreement that ozone in the upper stratosphere continues to recover.
In contrast, ozone in the lower stratosphere has not shown signs of recovery. Models simulate a small recovery in mid-latitude lower-stratospheric ozone in both hemispheres that is not seen in observations. Reconciling this discrepancy is key to ensuring a fullunderstanding of ozone recovery.
• The existing network of atmospheric monitoring stations provides measurements of global surface concentrations of long-lived ODSs
and HFCs resulting from anthropogenic emissions. However, gaps in regional atmospheric monitoring limit the scientific community’s
ability to identify and quantify emissions of controlled substances from many source regions.
• Several space-borne instruments providing vertically resolved, global measurements of ozone-related atmospheric constituents (e.g.,reactive chlorine, water vapor, and long-lived transport tracers) are due to be retired within a few years. Without replacements of these
instruments, the ability to monitor and explain changes in the stratospheric ozone layer in the future will be impeded.
• The impact on the ozone layer of stratospheric aerosol injection (SAI), which has been proposed as a possible option to offset global
warming, has been assessed following the terms of reference for the 2022 SAP Assessment Report. Important potential consequences, such as deepening of the Antarctic ozone hole and delay in ozone recovery, were identified. Many knowledge gaps and uncertainties prevent a more robust evaluation at this time.
• Heightened concerns about influences on 21st century ozone include impacts of: further increases in nitrous oxide (N2O), methane(CH4), and CO2 concentrations; rapidly expanding ODS and HFC feedstock use and emissions; climate change on TCO in the tropics;
extraordinary wildfires and volcanic eruptions; increased frequency of civilian rocket launches and the emissions of a proposed new
fleet of supersonic commercial aircraft.

C. Future Policy Considerations• If ODS feedstock emissions as currently estimated were to be eliminated in future years, the return of mid-latitude Equivalent Effective
Stratospheric Chlorine (EESC) to 1980 abundances could be advanced by almost 4 years, largely due to reductions in CCl4, and thereby reduce total climate forcing from ODSs.
• Eliminating future emissions of methyl bromide (CH3Br) from quarantine and pre-shipment applications currently allowed by
the Montreal Protocol would accelerate the return of mid-latitude EESC to 1980 abundances by two years (as noted in previous
Assessments).
• Emissions of anthropogenic very short-lived chlorine substances, dominated by dichloromethane (CH2Cl2), continue to grow andcontribute to ozone depletion. If CH2Cl2 emissions continue at their current level, they will continue to deplete approximately 1 DU ofannually averaged global TCO. Elimination of these emissions would rapidly reverse this depletion.
• A 3% reduction in anthropogenic N2O emissions, averaged over 2023–2070, would lead to an increase in annually averaged global
TCO of about 0.5 DU over the same period, and a decrease of about 0.04 Wm–2 in radiative forcing, averaged over 2023–2100.
• Global emissions of long-lived HFC-23, which are largely a by-product of HCFC-22 production, are as much as eight times larger than
expected and are likely to grow unless abatement increases during HCFC-22 production or feedstock use of HCFC-22 decreases.
• The current combined GWP-weighted emissions of CFCs plus HCFCs are comparable to those of HFCs. Reductions in the future
emissions of CFCs and HCFCs requires addressing releases from banks and continuing production and use in allowed manufacturingof feedstocks, in by-products, or in unknown uses, depending upon the compound.

CONTENTS
Preface1

Introduction

5

1. Abundances and Trends in Ozone-Depleting Substances (ODSs)

9

2. Hydrofluorocarbons (HFCs)

14

3. Stratospheric Ozone

16

4. Stratospheric Ozone Change and Its Influence on Climate

19

5. Stratospheric Aerosol Injection and Potential Impacts on Ozone

21

6. Policy-Relevant Scenarios and Information

23

Scientific Summaries of the Chapters

25

Authors, Contributors, and Reviewers

44

PREFACE
This document contains information upon which the Parties to the Montreal Protocol on Substances that Deplete the
Ozone Layer (“The Parties”) will base their future decisions regarding protection of the stratospheric ozone layer and climate from the production and consumption of ozone-depleting substances (ODSs) and their replacements.

The Charge to the Assessment Panels
Specifically, Article 6 of the Montreal Protocol on Substancesthat Deplete the Ozone Layer states:
Beginning in 1990, and at least every four years thereafter,
the Parties shall assess the control measures provided for in
Article 2 and Articles 2A to 2I on the basis of available scientific, environmental, technical and economic information.
To provide the mechanisms whereby these assessments are
conducted, the Montreal Protocol further states:“. . . the Parties shall convene appropriate panels of experts”
and “the panels will report their conclusions . . . to the Parties.”
To meet this request, the Scientific Assessment Panel (SAP),
the Environmental Effects Assessment Panel, and the Technology
and Economic Assessment Panel each prepare, every 4 years,
major assessments that update the state of understanding in theirpurviews. These assessments are made available to the Parties inadvance of their annual meetings at which they consider amendments and adjustments to the provisions of the Montreal Protocol.

Sequence of Scientific Assessments
The 2022 Assessment is the latest in a series of assessments
prepared by the world’s leading experts in the atmospheric sciences and under the auspices of the Montreal Protocol in coordination with the World Meteorological Organization (WMO) andthe United Nations Environment Programme (UN Environment).
The 2022 Assessment is the tenth in the series of major assessments that have been prepared by the Scientific Assessment Panelas direct input to the Montreal Protocol process. The chronologyof the ten scientific assessments of ozone depletion, along withother relevant reports and international policy decisions, are summarized in Table ES-1.

2022 Assessment Terms of Reference
The terms of reference of the 2022 Assessment for the SAP
were decided at the 31st Meeting of the Parties to the Montreal
Protocol in Rome, Lazio, Italy (4–8 November 2019) in theirDecision XXXI/21 (items 1–3 and 5):
1. To request the Scientific Assessment Panel, the Environmental
Effects Assessment Panel and the Technology and Economic
Assessment Panel to prepare quadrennial assessment reports
and submit them to the Secretariat by 31 December 2022
for consideration by the Open-ended Working Group andthe Meeting of the Parties in 2023, and to present a synthesis report by 30 April 2023, noting that the panels should

continue to exchange information, during the process of developing their respective reports in order to avoid duplicationand provide comprehensive information to the parties to theMontreal Protocol;
2. To request the assessment panels to bring to the notice of theparties any significant developments which, in their opinion,
deserve such notice, in accordance with Decision IV/13;
3. To encourage the assessment panels to closely involve relevant scientists from Article 5 parties with a view to promoting
gender and regional balance, to the best of their ability, in
producing the reports;
5. That the 2022 report of the Scientific Assessment Panelshould include:a) An assessment of the state of the ozone layer and its future evolution;b) An evaluation of global and polar stratospheric ozone,
including the Antarctic ozone hole and Arctic winter/spring ozone depletion and the predicted changes inthose phenomena;c) An evaluation of trends in the top-down derived
emissions, abundances and fate in the atmosphereof trace gases of relevance to the Montreal Protocol
on Substances that Deplete the Ozone Layer, in particular controlled substances and other substances ofimportance to the ozone layer, which should include a
comparison of bottom-up and top-down estimations ofsuch emissions with a view to addressing unidentified
emission sources and discrepancies between reported
emissions and observed atmospheric concentrations;d) An evaluation of consistency with reported production
and consumption of those substances and the likely implications for the state of the ozone layer, including its
interaction with the climate system;e) An assessment of the interaction between changes instratospheric ozone and the climate system, including
possible future policy scenarios relating to ozone depletion and climate impacts;f) Early identification and quantification, where possible,of any other issues of importance to the ozone layer andthe climate system consistent with the objectives of theVienna Convention for the Protection of the Ozone Layer
and the Montreal Protocol;g) An assessment of information and research related tosolar radiation management and its potential effect on
the stratospheric ozone layer;h) Relevant information on any newly detected substancesthat are relevant for the Montreal Protocol.

Decision XXXI/2: Potential areas of focus for the 2022 quadrennial reports of the Scientific Assessment Panel, the Environmental Effects Assessment Panel and the Technology
and Economic Assessment Panel
1

1

The Final Author Meeting was held in Geneva, Switzerland on 25–29 July 2022. Shown is the iconic Jet d’Eau on Lake Geneva.

The Assessment Process
The process of writing the current Assessment started earlyin 2020. The co-chairs of the Scientific Assessment Panel (SAP) of
the Montreal Protocol (David W. Fahey, Paul A. Newman, John A.
Pyle, and Bonfils Safari) considered suggestions from the Parties
regarding experts from their countries who could participate in
the process. A Scientific Steering Committee (SSC), comprising
the co-chairs and an ad-hoc international scientific advisory group,
was formed to suggest authors and reviewers from the world scientific community and to help craft the Assessment outline. As inprevious Assessments, the participants represented experts fromthe developed and developing world who bring a special perspective to the process and whose involvement in the Assessment
contributes to capacity building. The Authors, Contributors, andReviewers section at the end of this document provides a listing of
the approximately 230 scientists from 30 countries who contributed to the preparation and review of the Assessment.
An initial letter was sent to a large number of scientists andpolicymakers in November 2020 soliciting comments and inputs
on a draft outline along with suggestions for authors for the 2022
Assessment. This was followed by revisions to the outline and recruitment of lead authors and co-authors. Revised chapter outlines
were developed between February and April 2021 through a series of online meetings of the SSC and lead authors. The chapter
writing process produced four drafts between August 2021 andDavid W. Fahey
Paul A. Newman
John A. Pyle
Bonfils Safari
Co-chairs of the Scientific Assessment
Panel of the Montreal Protocol

2

September 2022 aided by a virtual meeting of the author teamand SSC in March 2022 and an in-person meeting in July 2022 at
WMO Headquarters in Geneva, Switzerland. The first drafts of thechapters were formally peer-reviewed by over 100 expert reviewers. The chapters were revised by the author teams based on theextensive review comments (numbering over 3500). Review editors for each chapter provided oversight of the revision process toensure that all comments were addressed appropriately.
At a meeting in Geneva, Switzerland, held on 25–29 July
2022, the Executive Summary contained herein was prepared andcompleted by the 74 attendees of the meeting. These attendees
included the steering committee, chapter lead authors, review
editors, some chapter co-authors (selected by the chapter leads),
reviewers (selected by the review editors), and some leading experts invited by the steering committee. The Executive Summary,initially drafted by the Assessment SSC, was reviewed, revised,and approved line-by-line. The section of Assessment highlights
was drafted during the meeting.
The success of the 2022 Assessment depended on the combined efforts and commitment of a large international team ofscientific researchers who volunteered their time as lead authors,
contributors, reviewers, and review editors and on the skills anddedication of the assessment coordinator and the editorial andproduction staff, who are listed at the end of this report.

Table ES-1. Chronology of scientific reports and policy decisions related to ozone depletion.
Year

Policy Decisions

1981

Scientific Reports
The Stratosphere 1981: Theory and Measurements. WMO No. 11.

1985

Vienna Convention

1987

Montreal Protocol

Atmospheric Ozone 1985. Three volumes. WMO No. 16.

1988

International Ozone Trends Panel Report 1988.
Two volumes. WMO No. 18.

1989

Scientific Assessment of Stratospheric Ozone: 1989.
Two volumes. WMO No. 20.

1990

London Adjustment
and Amendment

1991

Scientific Assessment of Ozone Depletion: 1991. WMO No. 25.

1992

Methyl Bromide: Its Atmospheric Science, Technology, and Economics(Montreal Protocol Assessment Supplement). UNEP (1992)

1992

Copenhagen Adjustment
and Amendment

1994

Scientific Assessment of Ozone Depletion: 1994. WMO No. 37.

1995

Vienna Adjustment

1997

Montreal Adjustment
and Amendment

1998
1999

Scientific Assessment of Ozone Depletion: 1998. WMO No. 44.
Beijing Adjustment
and Amendment

2002

Scientific Assessment of Ozone Depletion: 2002. WMO No. 47.

2006
2007

Scientific Assessment of Ozone Depletion: 2006. WMO No. 50.
Montreal Adjustment

2010

Scientific Assessment of Ozone Depletion: 2010. WMO No. 52.

2014
2016

Scientific Assessment of Ozone Depletion: 2014. WMO No. 55.
Kigali Amendment

2018

Scientific Assessment of Ozone Depletion: 2018. WMO No. 58.

2021

Report on Unexpected Emissions of CFC-11. WMO No. 1268.

2022

Scientific Assessment of Ozone Depletion: 2022. GAW No. 278.

3

INTRODUCTION
The 1985 Vienna Convention for the Protection of the Ozone
Layer is an international agreement in which United Nations
States recognized the fundamental importance of preventing
damage to the stratospheric ozone layer. The 1987 Montreal
Protocol on Substances that Deplete the Ozone Layer and its
succeeding amendments, adjustments, and decisions were subsequently negotiated to control the consumption and production of anthropogenic ozone-depleting substances (ODSs) andsome hydrofluorocarbons (HFCs). The Montreal Protocol Parties
base their decisions on scientific, environmental, technical, andeconomic information that is provided by their assessment panels. The Protocol requests quadrennial reports from its Scientific
Assessment Panel that update the science of the ozone layer.
This Executive Summary (ES) highlights the key findings of the
Scientific Assessment of Ozone Depletion: 2022, as put together by an international team of scientists. The key findings of eachof the seven chapters of the Scientific Assessment have beencondensed and formulated to make the ES suitable for a broad
audience.
Ozone depletion is caused by human-related emissions of
ODSs and the subsequent release of reactive halogen gases,
especially chlorine and bromine, in the stratosphere. ODSs include chlorofluorocarbons (CFCs), bromine-containing halonsand methyl bromide, hydrochlorofluorocarbons (HCFCs), carbon tetrachloride (CCl4), and methyl chloroform. The substances
controlled under the Montreal Protocol are listed in the various
annexes to the agreement (CFCs and halons under Annex A and
B, HCFCs under Annex C, and methyl bromide under Annex E)2.
These ODSs are long-lived (e.g., CFC-12 has a lifetime greaterthan 100 years) and are also powerful greenhouse gases (GHGs).
As a consequence of Montreal Protocol controls, the stratospheric concentrations of anthropogenic chlorine and bromine are
declining.
In addition to the longer-lived ODSs, there is a broad classof chlorine- and bromine-containing substances known as very
short-lived substances (VSLSs) that are not controlled under theMontreal Protocol and have lifetimes shorter than about 6 months.
For example, bromoform (CHBr3) has a lifetime of 24 days, whilechloroform (CHCl3) has a lifetime of 149 days. These substances
are generally destroyed in the lower atmosphere in chemical reactions. In general, only small fractions of VSLS emissions reachthe stratosphere where they contribute to chlorine and bromine
levels and lead to increased ozone depletion.
The Montreal Protocol’s control of ODSs stimulated the2development of replacement substances, firstly HCFCs and thenHFCs, in a number of industrial sectors. While HFCs have only aminor effect on stratospheric ozone, some HFCs are powerful
GHGs. Previous Assessments have shown that HFCs have been
increasing rapidly in the atmosphere over the last decade andwere projected to increase further as global development continued in the coming decades. The adoption of the 2016 Kigali
Amendment to the Montreal Protocol (see Annex F) will phasedown the production and consumption of some HFCs and avoid
much of the projected global increase and associated climatechange.
Observations of atmospheric ozone are made by instruments on the ground and on board balloons, aircraft, and satellites. This network of observations documented the decline ofozone around the globe, with extreme depletions occurring over
Antarctica in each spring and occasional large depletions in the
Arctic, and they allowed us to report some indications of recovery in stratospheric ozone in the 2014 and 2018 Assessments.
The chemical and dynamical processes controlling stratosphericozone are well understood, with ozone depletion being fundamentally driven by the atmospheric abundances of chlorine andbromine.
Strong declines in the emissions of ODSs starting in the late1980s led to a decline in the abundances of chlorine and bromine
starting around the turn of the century. As a result, the first indications of ozone recovery are emerging. In addition to ODSs, model
simulations demonstrate that stratospheric ozone concentrations
are also affected by the chemical and climate effects of greenhouse gases. In particular, increasing concentrations of the GHGs
carbon dioxide (CO2) and methane (CH4) during this century willcause global ozone levels to increase beyond the natural levelof ozone observed in the 1960s, primarily because of the cooling of the upper stratosphere and a change of the stratosphericcirculation. On the other hand, the chemical effect of increasing
concentrations of nitrous oxide (N2O), another GHG, is to deplete
stratospheric ozone.
This 2022 Assessment is the tenth in a series that is provided to the Montreal Protocol by its Scientific Assessment Panel.
Completely new to this Assessment is Chapter 6, on the potential effects on ozone of the intentional addition of aerosols
to the stratosphere, known as stratospheric aerosol injection(SAI). SAI has been proposed as a potential method to reduceclimate warming by increasing sunlight reflection; an unintended consequence of SAI is that it could also affect stratosphericMontreal Protocol Handbook, 2018.

5

temperatures, circulation and ozone production and destruction
rates and transport. This new chapter assesses our understandingof these effects based on the SAI strategy and under different climate warming scenarios, as well as identifying sources of uncertainty in these impacts.

In the other six chapters of this Assessment, many of ourprevious Assessment findings are strengthened and new results
are presented. A clear message of the 2022 Assessment is that
the Montreal Protocol continues to be effective at reducing theatmospheric abundance of ODSs.

Terminology Used in the Executive Summary
Ozone-depleting substance (ODS)
An ODS is a chemical that depletes stratospheric ozone.
Under the Montreal Protocol, most of the widely used ODSs,
with the exception of nitrous oxide (N2O), are controlled
under Annexes A, B, C, and E. These include, among others,
chlorofluorocarbons (CFCs), carbon tetrachloride (CCl4),
methyl chloroform (CH3CCl3), halons, methyl bromide(CH3Br) and hydrochlorofluorocarbons (HCFCs). TheseODSs typically have sufficiently long atmospheric lifetimes
to reach the stratosphere after being emitted at the surface.
Methyl bromide is the shortest-lived of the controlled substances and has natural and anthropogenic sources. OtherODSs are not yet controlled under the Montreal Protocol.

Ozone depletion potential (ODP) / ODP
weighting
The ozone depletion potential (ODP) of a substance is a metric for determining the relative strength of that chemical’sability to destroy ozone. The ODP of a substance is definedas the ratio of the change in global ozone for a given massemission of the substance to the change in global ozone forthe same mass emission of CFC-11 (CFCl3). In order to be able
to compare the potential impact on stratospheric ozone ofchanges in the emissions of different gases, gases are often
weighted by their ODP and given as “ODP-weighted emissions”, so that the units of these emissions are “Mt CFC-11
equivalent”.

Halogenated very short-lived substances(VSLSs)Halogenated very short-lived substances (VSLSs) have atmospheric lifetimes less than 0.5 year and yet make a contribution to stratospheric chlorine or bromine levels. As
short-lived ODSs, a large fraction of VSLS emissions are destroyed in the troposphere, limiting the fraction of emissions
that reaches the stratosphere and causes ozone depletion.
VSLS emissions that occur in regions with rapid transport
to the stratosphere will make an enhanced contribution to
stratospheric halogen levels. Hence, the ODP of a VSLS is6generally dependent on assumptions about the emission
source region and time of the year of the emissions. VSLSs
are not controlled under the Montreal Protocol.

Equivalent effective chlorine (EECl) and equivalent effective stratospheric chlorine (EESC)
EECl is a metric for representing ODS levels in the troposphere. It is calculated based upon the surface atmosphericconcentrations of individual ODSs, their number of chlorine
and bromine atoms, and the relative efficiency of chlorine
and bromine at ozone depleting ozone.
EESC is, similarly, a metric for representing ODS levels in thestratosphere. It is calculated based upon the same three factors as EECl, as well as accounting for the time required forthe substances to reach different stratospheric regions andbreak down to release their chlorine and bromine atoms.
As EESC continues to decrease in response to Montreal
Protocol provisions, stratospheric ozone is expected toincrease.
In this Assessment, neither EECl nor EESC include chlorine
and bromine from very short-lived substances (VSLSs).

Feedstocks and banks
A “feedstock” is a substance used to synthesize one ormore other chemicals through a process of chemicaltransformation.
The “bank” of a given substance represents the amountof that substance that has been produced, is contained inequipment or products, and has not yet been released tothe atmosphere. Banks include substances contained in refrigeration and air conditioning equipment, foams, and fire
protection systems. Without intervening actions, some fraction of the substances contained will be gradually released
during the equipment or product’s lifetime, and some will be
released at or after the end-of-life of equipment or products.
With an intervention at the end of life, chemicals may becollected, stored, and destroyed, thereby preventing theirrelease into the atmosphere. All releases to the atmosphere,
as well as destruction, result in a decrease in the bank size.

Global warming potential (GWP) / GWP
weighting
The global warming potential (GWP) is a metric for determining the relative contribution of a substance to climate warming. GWP is defined as the ratio of the radiative forcing for agiven mass emission of a substance relative to the same massemission of CO2 summed over a given time period (typically
20 or 100 years). In this Assessment, a 100-yr time window isimplied unless otherwise stated. As such, the GWP of CO2 isdefined to be unity. In order to be able to compare the potential impact on climate of changes in the emissions of different gases, the emissions are often weighted by their GWP
and given as “GWP-weighted emissions”, so that the units ofthese emissions are “Gt CO2-equivalent”.

Representative concentration pathways(RCPs) & shared socioeconomic pathways(SSPs)
Representative concentration pathways (RCPs) and sharedsocioeconomic pathways (SSPs) were developed by theclimate change research community to describe a range ofplausible societal futures out to the year 2100. SSPs are themain scenarios assessed in the 6th Intergovernmental Panelon Climate Change (IPCC) reports, replacing the previous
generation of scenarios, the RCPs. For better comparabilityof the projections using SSPs with projections done for theprevious IPCC Assessment Report (AR5), we include a description of the RCPs. Both define a timeline of emissions ofgreenhouse gases (GHGs) expressed in units of GtCO2-eq
and describe a range of plausible future climate pathways.

The four RCP pathways, RCP-2.6, RCP-4.5, RCP-6.0, and
RCP-8.5, are labeled by the approximate radiative forcing at
2100 (e.g., RCP-2.6 has a global mean radiative forcing from
GHGs in 2100 of 2.6 W m–2). RCP-2.6 assumes that GHG
emissions peaked before 2020; RCP-4.5 assumes a peakaround 2040; RCP-6.0 assumes a peak around 2080; andRCP-8.5 assumes no peak before 2100. Each scenario includes certain socioeconomic assumptions about fossil fueluse and other aspects related to GHG emissions and otherfactors that affect climate, such as aerosol emissions and land
use change. The SSPs adopt similar radiative forcing values
at 2100 for consistency with the RCPs, but differ in their exact
composition and emission trajectories. For example, methane trajectories are quite different between the two scenario
frameworks.
Like the RCPs, the SSPs define a timeline of atmosphericconcentrations of GHGs expressed in units of GtCO2-eq.
SSPs are developed based on a range of socioeconomic
development trajectories, coupled with the expected global
mean radiative forcing from GHGs in 2100. The SSPs include
scenarios for “Sustainability” (SSP1), “Middle of the Road”(SSP2), “Regional Rivalry” (SSP3), “Inequality” (SSP4) and“Fossil-fueled Development” (SSP5) pathways. So, for example, SSP2-4.5 is a “Middle of the Road” pathway that ends in2100 with a radiative forcing of 4.5 W m–2. The SSPs also include a new very low emissions scenario which is consistent
with staying below 1.5 °C of warming. Here, projections of
future ozone abundances are given for the greenhouse gas
trajectories given in specific SSPs.

7

EXECUTIVE
SUMMARYExecutive Summary

1
ABUNDANCES AND TRENDS INOZONE-DEPLETING SUBSTANCES (ODSs)
Our confidence in the achievements of the Montreal Protocol continues to be based on sustained networks of measurements of long-lived source gas abundances covering several decades. These measurements allow the determinationof global abundances, their interhemispheric differences and their trends. The data allow us to derive emissions that canbe compared with emissions derived from data reported to the UN Environment Programme, when combined with lifetime
information and atmospheric modelling.

Changes in tropospheric chlorine andbromine over 2016–2020• The atmospheric abundances of both tropospheric chlorine (Cl) and bromine (Br), from long-lived ozone-depleting substances (ODSs) controlled under theMontreal Protocol, continued to decline (Figure ES-1).
The observed rate of decline in tropospheric chlorine due tosubstances controlled under the Montreal Protocol was 15.4
± 4.1 ppt Cl yr –1 (Table ES-2), which is close to the baseline
projection from the 2018 Assessment.
• Tropospheric chlorine from very short-lived gases,
whose sources are mainly anthropogenic and which are
not controlled under the Montreal Protocol, increasedby 2.1 ± 0.6 ppt Cl yr –1.
• The observed rate of decline in tropospheric bromine
due to controlled substances was 0.18 ± 0.05 ppt Br
yr –1, which is close to the baseline projection from the2018 Assessment. The majority of this decrease originatedfrom decreases in halon abundances.

Total stratospheric chlorine and bromine• Total chlorine entering the stratosphere from controlled and uncontrolled ODSs declined by 420 ± 20
ppt (11.5%) between the 1993 peak (3660 ppt) and2020 (3240 ppt) (Figure ES-2). This long-term decrease
was largely driven by decreasing abundances of CH3CCl3 and
CFCs.
• HCl is the major chlorine component in the upper stratosphere. Its abundance in this region decreased on average by 0.5 ± 0.2 % yr –1 during 1997–2020. The long-term
decrease is consistent with the decline in total chlorine entering the stratosphere.

• Total bromine entering the stratosphere from controlled and uncontrolled ODSs declined by 3.2 ± 1.2 ppt(14.5%) between the 1999 peak (22.1 ppt) and 2020
(18.9 ppt). This long-term decrease was largely driven by
decreasing abundances of CH3Br and halon-1211.
• Total stratospheric bromine, as derived from brominemonoxide (BrO) observations, has decreased by 0.18
± 0.04 ppt Br yr –1 (0.8% yr –1) since 2003. This decreaseis consistent with the decline in total bromine entering in thestratosphere.

CFC-11• Global CFC-11 emissions declined after 2018, droppingto 45 ± 10 Gg in both 2019 and 2020. This drop suggests the elimination of most of the unexpected emissions occurring in the years after 2012 (Figure ES-3).
• A large fraction of the unexpected emissions originatedfrom eastern China. This finding is based on available regional observations from multiple sites. The decline of CFC-11
emissions from eastern China since 2018 explains 60 ± 30%
of the observed global emission decrease.

CFC-12• Global CFC-12 abundances continued to decreaseduring 2016–2020. Estimates of global CFC-12 emissions
were 33 ± 21 Gg yr –1 in 2016 and 25 ± 20 Gg yr –1 in 2020.
• CFC-12 emissions from eastern China decreased from
3.3 ± 1.4 Gg yr –1 in 2016 to 0.5 ± 0.5 Gg yr –1 in 2019.
This decrease is likely associated with the decline in CFC-11
production.

9

Executive Summary

CFC-11 eq emissions (Mt yr –1)

ODSs and Ozone Timelines(a) CFC-11 equivalent emissions
1.2

0.8

0.0

(b) Equivalent effective chlorine

4

EECl (ppb)

HCFCs
Halons
CFC-113, CFC-114,
CFC-115, CH3CCl3
CCl4
CFC-12
CFC-11
CH3Cl
CH3Br

0.4

3
2
1
0
1960

1980

2000

2020

2040

2060

2080

2100

Year

Total ozone (DU)(c) Annual global total ozone, 60°S – 60°N
300
1980 level

290

Observations
Mean of observations

280

Future Projection Scenarios:
Low climate forcing (SSP1-2.6)
Medium climate forcing (SSP2-4.5)
High climate forcing (SSP3-7.0)
Combined uncertainties

270

Total ozone (DU)350(d) October Antarctic total ozone, 70°S – 90°S

300

1980 level

250

200

150
1960

1980

2000

2020

2040

2060

2080

Year
(see caption on facing page)

10

2100

Executive Summary

Other CFCs

Halons and methyl bromide (CH3Br)• Global abundances of CFC-13, CFC-112a, CFC-113a, CFC114a, and CFC-115 increased from 16.0 ± 0.3 ppt in 2016
to a total of 17.2 ± 0.3 ppt ppt Cl in 2020. These changes suggest stable or increasing emissions. Atmosphericobservations confirm that eastern Asia is a substantial source
region.

• Bromine from halons has decreased from a peak of 8.5± 0.1 ppt in 2006 to 7.3 ± 0.1 in 2020. Halon-1211, halon2402, and halon-1202 abundances continued to decline
between 2016 and 2020. The rate of change of halon-1301
remained indistinguishable from zero. In 2020 it was the mostabundant halon in the atmosphere.

Carbon tetrachloride (CCl4)• Methyl bromide (CH3Br) abundances have varied annually between 6.5 ppt and 6.9 ppt during 2016–2020
with no clear overall trend. Most anthropogenically produced CH3Br has been phased out except for quarantine andpre-shipment (QPS) fumigation, leaving natural emissions as
the dominant source. Reported QPS consumption has been
relatively stable for more than two decades.

• The atmospheric abundance of CCl4 continued to decrease at slower rates than expected, which could be
due to underestimated emissions from feedstock production and usage. Global CCl4 emission estimates basedon atmospheric observations are now more accurate than in
the last Assessment due to an improved lifetime estimate, andwere on average 44 ± 15 Gg yr –1 in both 2016 and 2020.
• Emissions of CCl4 in eastern China over the period
2013–2019 show year-to-year variability likely related to CFC-11 production. Emissions increased after 2013,reaching 11.3 ± 1.9 Gg yr –1 in 2016, and decreased to 6.3 ±1.1 Gg yr –1 in 2019.

Hydrochlorofluorocarbons (HCFCs)• Tropospheric chlorine from HCFCs has continued to increase, reaching 320 ± 3 ppt in 2020. The annual average
growth rate of chlorine from HCFCs decreased from 5.9 ± 1.3
ppt yr –1 reported in the 2018 Assessment to 2.5 ± 1.0 ppt yr –1
during 2016–2020.
• Global emission estimates of HCFC-22 show evidenceof a decline in 2020 after a period of relatively constantemissions. HCFC-142b emissions continued to decline,and HCFC-142b abundances have started to decrease.
In contrast, HCFC-141b as well as several low-abundance HCFCs (HCFC-31, HCFC-124, HCFC-133a, and the
newly detected HCFC-132b) show stable or increasing
emissions.

Halogenated very short-lived substances(VSLS)• Dichloromethane (CH2Cl2), the main component of VSLS
chlorine, continued to increase between 2016 and 2020
with a slightly lower growth rate than prior to 2016. This
increase primarily results from growing CH2Cl2 emissions in
Asia.
• Tropospheric chlorine based on measurements of VSLS
source gases increased by about 10 ppt between 2016
and 2020. The estimated input of chlorine from VSLSs to thestratosphere also increased by about 10 ppt and amounts to
130 ± 30 ppt in 2020, contributing about 4% of the total chlorine input (Figure ES-2).
• Chlorinated VSLSs contribute 4% to the total stratospheric chlorine input in 2020 (Figure ES-2). The VSLS
chlorine input is estimated as 130 ± 30 ppt in 2020 comparedto 120 ± 40 ppt in 2016.
• Brominated VSLSs, with mainly natural sources, contribute 5 ± 2 ppt to stratospheric bromine and show nolong-term changes.

Figure ES-1. Timeline of: a) CFC-11-equivalent emissions, b) equivalent effective chlorine (EECl), c) global total ozone,
and d) October Antarctic total ozone. Annual CFC-11-equivalent emissions are computed for the ODSs shown in the legend by
multiplying mass emissions of a substance by its ODP (panel a). Historical emissions are derived from the measured atmosphericabundances of individual ODSs. The future projections of emissions assume full compliance with the Montreal Protocol and use
standard methodologies based on reported production, inventory estimates of the banks, and release rates. The annual abundances of EECl, shown for the global surface, are based on surface abundances (measured or derived from projected emissions
and lifetimes) of the chlorine- and bromine-containing substances (panel b). The bromine abundances are weighted by a factor of
65 to account for the greater efficiency of bromine in ozone destruction reactions in the atmosphere. Global total column ozone
represents an annual average over 60°N to 60°S latitudes (panel c) and Antarctic total column ozone represents an Octoberaverage over 70°S to 90°S latitudes (panel d). Panels (c) and (d) include a comparison of chemistry-climate model results (blacklines with gray shadings indicating uncertainty ranges) and available observations (purple lines). The chemistry-climate model
projections assume compliance with the Montreal Protocol and an increase in greenhouse gases following either the SSP1-2.6(low climate forcing), SSP2-4.5 (medium climate forcing), or SSP3-7.0 (high climate forcing) scenario, which diverge in 2014. In
panel (b),the white line with an arrow marks when EECl returns to its 1980 value. The uncertainty shown in panels (c) and (d) represents the 1-sigma standard deviation about the multi-model mean (MMM), either added to the SSP3-7.0 MMM (upper limit) or
subtracted from the SSP1-2.6 MMM (lower limit). [Data sources are: panel (a) mixing ratios in Figure 7-5 and Table 7-6 and ODPs
and lifetimes in Table A-1; panel (b) following Figure 7-5 and Table 7-6 with an alpha factor of 65; panel (c) Figure 3-24; and panel(d) Figure 4-24.]

11

Executive Summary

Halogen Gases Entering the Stratosphere

2500
2000
1500
1000

+185%
–27%
–16%

–2.8%

500
Naturalsources

0

–0.4%
1993

Very short-lived
gases
Methyl chloroform,–99%
Other gases, +11%
CFC-113
HCFCs
Carbontetrachloride
CFC-11

CFC-1220

15

Chlorine

18.9 ppt–15%–25%

Other halons+19%

Halon-1301–22%

Halon-1211

–71%

10

5

Methylchloride2020

22.1 ppt

ODSs from human activity

3000

–11%
+63%
–16%

Ozone-depleting substances (ODSs)
from human activity

Total chlorine abundance (ppt)

3500

Total

3240 ppt

0

nochangeNatural sources

3660 ppt

Total bromine abundance (ppt)

Total

no
change1999

Methyl bromide
human, natural

Very short-lived
gases

2020

BromineFigure ES-2. Chlorine and bromine input to the stratosphere for a reference year (1993 for chlorine and 1999 forbromine) and for 2020, for different species and classes of compounds. The reference year is the year of maximum chlorineor bromine loading of the troposphere. Mole fractions of long-lived gases were mostly derived from surface observations fromglobal networks (AGAGE and NOAA), except for CH3Cl before 1995, when observations from both networks were unavailable
and values were filled with the simulations from the scenario A1 of the previous Ozone Assessment, which are based on
measurements of firn air. The VSLS contributions for bromine are included as a constant 5 ppt, as in previous Assessments. The
VSLS chlorine contribution is based on the VSLS input from a model constrained by observed surface boundary conditions. Total
VSLS Cl input derived in this way is 80 ppt in 1993 and 130 ppt in 2020. For chlorine, HCFCs include HCFC-22, HCFC-141b, and
HCFC-142b; “other” includes contributions from minor CFCs (CFC-13, CFC-112, CFC-113a, CFC-114+CFC-114a, and CFC-115)
and halon-1211. For bromine, “other halons” is the sum of bromine contained in halon-1202 and halon-2402. Methyl chloride iscounted as having purely natural sources, despite some indications of anthropogenic contributions. The contribution of natural
sources to CH3Br mole fractions was estimated as a constant 5.5 ppt, based on the published firn air and ice core measurements,
whereas the anthropogenic contribution was estimated by the global surface mole fractions measured by AGAGE and NOAA
minus 5.5 ppt. [See also Figure 1-15]• New evidence suggests that iodine from mostly natural
sources is entrained into the stratosphere, contributing
0.3–0.9 ppt VSLS iodine in particulate or gas-phaseform. No observational trend estimates exist.

Other gases that influence stratosphericozone and climate• Three major greenhouse gases—CH4, N2O, and CO2—causechanges in stratospheric chemistry and dynamics that canaffect O3. An increase in N2O depletes ozone, and increases
in CH4 and CO2 tend to increase global stratospheric column
ozone. These gases have increased over the industrial
era and continue to increase, and are thus additional
factors, beyond ODSs, that control stratospheric O3
trends.
• Anthropogenic N2O emissions in 2020, when expressed
as a CFC-11-equivalent, were more than two times the

12

ODP-weighted emissions from all CFCs in that year, andmore than 20% of the CFC emissions in 1987, when thelatter were at their peak.
• The abundances of many non-ODS, non-HFC, highly fluorinated substances (e.g., SF6, perfluorocarbons, SO2F2,
NF3) have continued to increase. While these species do
not deplete ozone, they are very strong greenhouse gaseswith long atmospheric residence times. Total direct radiative
forcing due to anthropogenic emissions from these species
increased from 0.013 W m–2 in 2016 to 0.014 W m–2 in 2020.
• Decarbonization of the fossil fuel industry through atransition to molecular hydrogen (H2) could lead to large
increases in atmospheric H2. Estimates from the few existing studies point to relatively small impacts of H2 on
future global stratospheric ozone. Global abundances of
H2 increased by about 70% since preindustrial times and havevaried between 530 and 550 ppb since the late 20th century.

Executive Summary
Table ES-2. Contributions of ODSs controlled under the Montreal Protocol to tropospheric chlorine and bromine in 2020,
and annual average trends between 2016 and 2020.
Contribution to
tropospheric chlorine andbromine in 20203 (ppt Cl/Br)

Changes in tropospheric chlorine
and bromine (ppt Cl or Br per year)
from 2016 to 2020

Controlled chlorine substances by group
Chlorofluorocarbons (CFCs)

1925−12.9 ± 2.0

Methyl chloroform (CH3CCl3)

4.2−0.90 ± 0.04

Carbon tetrachloride (CCl4)308−3.8 ± 1.0

Hydrochlorofluorocarbons (HCFCs)

319+2.5 ± 0.4

Halon-1211

3.16−0.1 ± 0.02

Total chlorine from controlled substances

2560−15.1 ± 2.4

Controlled bromine substances by group
Halons

7.3−0.11 ± 0.02

Methyl bromide (CH3Br)4

6.6−0.07 ± 0.02

Total bromine from controlled substances

13.9−0.18 ± 0.05

3

Values are annual averages.

Some anthropogenic uses of CH3Br are exempted from Montreal Protocol controls, and CH3Br has natural sources,
which results in a natural background concentration.
4

Annual emissions or production (Gg yr –1)

Figure ES-3. CFC-11 global emissions
CFC-11 Annual Emissions and Production
and reported production. Shown are120
emissions of CFC-11 derived from AGAGENOAA observations
2002-2012 mean
(Advanced Global Atmospheric Gases
AGAGE observations
2006 projection
Experiment; red) and NOAA (National
Reported production2012 projection
100
Oceanic and Atmospheric Administration;blue) global network measurements of
CFC-11 abundances (see also Figure 1-3
80of the Assessment) and a model using a
CFC-11 lifetime of 52 years. Also shownis the production history reported to the
UN Environment Programme for all uses
60
(green), the average of annual emissions
over the 2002–2012 period (horizontal
purple line) extended to 2020 (dashed40
purple line), and scenario projections
based on observations through 2006 or
through 2012 (grey dotted and dashed20
lines). These emission projections are calculated using standard methodologies
based on reported production, inventory0
estimates of the bank, and an empirically
1995
2000
2005
2010
2015
2020
determined release fraction from the bankYearover the seven years before 2006 or 2012,
which is then applied to subsequent years(see Chapters 1 and 7). Uncertainties in emissions, shown as vertical lines on the data points, include the influence of measurement and model representation uncertainties, and do not include the influence of dynamical variability. The uncertainties are
smaller than those presented in Figure 1-3, because uncertainties related to factors constant across the whole time period, suchas lifetimes and calibration scale, have been omitted.

13

Executive Summary2
HYDROFLUOROCARBONS (HFCs)

Hydrofluorocarbons (HFCs) do not contain ozone-depleting chlorine or bromine. Similar to long-lived CFCs and HCFCs,
some HFCs have high global warming potentials. The Kigali Amendment to the Montreal Protocol, which was adopted in2016 and came into force in 2019, sets schedules for the phase-down of production and consumption of specific HFCs. The
radiative forcing due to HFCs is currently small, and the Kigali Amendment was designed to avoid uncontrolled radiative
forcing growth in coming decades. HFCs were included as one group within the basket of gases of the 1997 Kyoto Protocol
and as a result some countries supply annual emission estimates of HFCs to the United Nations Framework Convention on
Climate Change (UNFCCC). The Kigali Amendment initiated additional reporting of production and consumption of HFCs
and the emissions of HFC-23. HFC-23 is considered separately primarily because it is emitted to the atmosphere largely as
a by-product of HCFC-22 production. This reporting will become more complete as more Parties ratify this Amendment.

Observed HFC abundances and associatedemissions• Global atmospheric abundances and emissions of most
HFCs are increasing. CO2-equivalent emissions of HFCs derived from observations increased by 18% from 2016 to 2020.
• Global HFC emissions derived from atmospheric observations are larger than those reported by Annex I
Parties to UNFCCC. The gap between these estimateshas grown since the previous Assessment. In 2019,
Annex I UNFCCC reporting accounted for approximately one
third of the global total emissions derived from atmosphericobservations.
• It is not possible to attribute a substantial fraction of
global HFC emissions to individual countries due tolimitations in the global monitoring networks and reporting. Observationally based emission estimates areavailable for some non-Annex I countries. When these areadded to UNFCCC Annex I reports, around 40% of global
total CO2-equivalent emissions (excluding HFC-23) remain
unexplained.
• Global emissions of HFC-23 derived from atmosphericobservations increased since the previous Assessment,inconsistent with new information suggesting asubstantial rise in abatement independent of Kigali
Amendment controls. The estimated global emissions of
HFC-23 were 17.2 ± 0.8 kt yr –1 in 2019. This value is substantially higher than the emissions of 2.2 kt yr –1 in that year derived
from activity-based estimates. These activity-based estimates
are derived from UNFCCC emission reports, information on

14

production and abatement submitted under the Montreal
Protocol, and the estimated effect of national regulations.
• Observational evidence suggests that changes are
occurring in the use of certain HFCs and their replacements, HFOs (hydrofluoroolefins), because of national
regulations, market developments, and actions related
to the implementation of the Kigali Amendment.
º The 2017–2019 CO2-eq. emissions of HFCs are approximately 20% lower than those projected in the scenario
without national regulations or the controls of the Kigali
Amendment.
º HFOs are increasing in the atmosphere, consistent with
their increasing use in place of HFCs. Measurements
show that atmospheric background abundances of two
HFOs at one central European site have increased by anorder of magnitude from 2016 to 2020.
• The formation in the atmosphere of trifluoroacetic acid
(TFA) is expected to increase in the coming decades due
to increased use of HFOs and HCFOs. TFA, a breakdown
product of some HFCs, HCFCs, HFOs and HCFOs, is a persistent chemical with potential harmful effects on animals,
plants, and humans. The concentration of TFA in rainwaterand ocean water is, in general, significantly below known toxicity limits at present. Potential environmental impacts of TFA
require future evaluation due to its persistence.

Projections of HFCs and temperature contributions• Since the previous Assessment, updated projections

Executive Summary

HFC emissions

0.5

Scenarios:

86

Without control measures (2018)
Kigali Amendment (2018)
Kigali Amendment(2022 update)
Zero production
2023-onTemperature contribution (°C)

Emissions (Gt CO2-eq yr–1)

10

4

2

0
2000

2020

2040

2060

2080

2100

Year

Surface temperature HFCs
Scenarios:

0.4

Without control measures (2018)
Kigali Amendment (2018)
Kigali Amendment (2022 update)
Zero production 2023-on

0.3

0.2

0.1

0.0
2000

2020

2040

2060

2080

2100

Year

Figure ES-4. HFC emissions (left) and their impact on global average surface temperature (right). Shown is a scenario
without global HFC control measures (the ‘baseline’ scenario from the 2018 Assessment, blue area) and the 2018 and 2022 scenarios assuming full compliance with the Kigali Amendment (orange and pink, respectively). Also shown is a scenario assumingthat the global production of HFCs ceased in 2020 (black dashed line). For comparison, the total warming from all greenhousegases is projected to be 1.4 °C to 4.4 °C by the end of the 21st century, relative to 1850–1900, following IPCC (2021) projections.
The contribution from HFC-23 emissions is not included here.

have been made of HFC emissions assuming adherence
to the Kigali Amendment (excluding HFC-23). The projected emissions and the associated radiative forcing
and temperature change are smaller than estimatedpreviously. The revised projections are based on extended atmospheric observations from 2014 to 2020, updated
UNFCCC national emission inventory reports, updated activity data from Annex I countries, and new consumption datafrom some non-Annex I countries.
• Concerted efforts to improve the energy efficiency ofrefrigeration and air conditioning equipment could leadto reductions in greenhouse gas emissions of the sameorder as those from the global implementation of theKigali Amendment. These estimated benefits of improving
energy efficiency are highly dependent on the greenhousegas emission rate from power generation and the pace of decarbonization in the energy sector.

• Following the controls of the Kigali Amendment, HFCemissions (excluding HFC-23) in 2050 are projected tobe 0.9–1.0 Gt CO2-eq. yr –1 in the updated 2022 Kigali
Amendment scenario, compared to 4.0–5.3 Gt CO2eq yr –1 in the 2018 scenario without control measures(Figure ES-4). The corresponding radiative forcing in 2050
due to HFCs is 0.09–0.10 W m–2 with adherence to the Kigali
Amendment, compared to 0.22–0.25 W m–2 without control
measures. Annual average surface warming from HFCs is expected to be 0.04 °C in 2100 under the updated 2022 Kigali
Amendment scenario, compared to 0.3–0.5 °C without control measures.
• Emissions of HFC-23 are expected to grow in the coming
decades unless abatement during HCFC-22 production
is increased. This growth is based on an anticipated continued increase in HCFC-22 production, primarily for feedstockuse, which is allowed under the Montreal Protocol.

15

Executive Summary

3
STRATOSPHERIC OZONEThe Montreal Protocol and its Amendments and Adjustments have been effective in decreasing the abundance ofODSs in the atmosphere. The clearest signs of corresponding ozone recovery are seen in the upper stratosphere and in
the Antarctic lower stratosphere in spring. ODS-related ozone recovery is difficult to detect in other regions due to large
natural variability and confounding factors, such as climate change and changes in tropospheric ozone. In the Arctic, forexample, severe ozone loss occurs only under cold stratospheric conditions (e.g., in spring 2011 and most recently in spring2020). An Arctic ozone trend is difficult to detect given the much larger variability than in the Antarctic. Episodic volcanic
eruptions and, recently, intense wildfires can increase stratospheric aerosol substantially and hence have the potential toperturb stratospheric ozone. The effects of the Australian wildfires of 2019/2020 and of the large Hunga Tonga-Hunga
Ha’apai volcanic eruption in 2022 on ozone are not assessed here and are an area of active research. Ozone in the tropical lower stratosphere shows little response to changes in ODSs, because halogen-driven ozone depletion is comparatively small in this region.

Antarctic and Arctic ozone• Recovery of Antarctic stratospheric ozone continues toprogress. New results since the 2018 Assessment support
the findings reported at that time that the Antarctic ozone
hole has generally diminished in size and depth since theyear 2000. New analyses provide additional evidence that
September is the period when stratospheric ozone over
Antarctica shows the largest sensitivity to decreasing ODSs,
and when Antarctic ozone recovery rates are the strongest
and the most statistically significant.
• Antarctic ozone holes observed between 2019 and2021 exhibited substantial variability in size, strength,
and longevity. This behavior is largely dynamically driven, is consistent with our understanding, and does notchallenge the evidence for the emergence of recovery.
The 2019 ozone hole was the smallest since 2002. In contrast,
both 2020 and 2021 had relatively large and long-lasting latespring ozone holes.
• In the Arctic, observed trends in ozone remain small
compared to the large year-to-year variability. Thisprecludes the identification of a statistically significant
trend in Arctic ozone over the 2000–2021 period.
• Arctic total ozone reached exceptionally low values
in spring 2020. A very stable, cold, and long-lived stratospheric polar vortex enabled halogen-catalyzed chemicalozone loss that exceeded the previous record-breaking loss
observed in spring 2011. The strong vortex also inhibited
dynamical replenishment of polar ozone. The evolution of

16

high-latitude ozone in 2020 is successfully reproduced bymodel simulations, further substantiating our understandingof polar ozone chemistry.

Global ozone
Changes to date in total column ozone• Aggregated ground- and space-based observations
indicate an increase of 0.3% decade –1 (with a 2-sigma
uncertainty of at least ±0.3% decade –1) in near-global(60°S–60°N) total column ozone over the 1996–2020
period. This trend is consistent with model simulations andour scientific understanding of the processes controllingozone. Over the same period, trends over broad latitude
bands are as follows:º Southern Hemisphere mid-latitude (35°S–60°S), total
column ozone has increased (0.8 ± 0.7% decade–1).
º Northern Hemisphere mid-latitudes (35°N–60°N),
total column ozone trends are negligible (0.0 ± 0.7%
decade–1).
º Tropical (20°S–20°N) total column ozone shows no clear
trend (0.2 ± 0.3% decade–1), likely because lower stratospheric ozone is decreasing while tropospheric ozone isincreasing, both unrelated to changes in ODSs.
The latitudinal pattern of these total column ozone trends islargely consistent with our scientific understanding and is reproduced in the latest set of chemistry-climate models.

Executive Summary

Observed and Simulated Ozone Trends from 2000–2020
35°S–60°S

20°S–20°N

35°N–60°N

1

10

30

Altitude (km)Pressure (hPa)

40

20
100

–6 –4 –2 0

2

4

6

–6 –4 –2 0

2

4

6

–6 –4 –2 0

2

4

6

Trend (% per decade)Observations
Average
Uncertainty range

Model Simulations
Average
Unceratinty range

Figure ES-5. Ozone trends in the stratosphere from 2000 to 2020 for three latitude bands. Ozone trends derived fromsatellite observations (thick magenta lines, with uncertainty ranges given by thin magenta lines) are compared to trends fromchemistry-climate models (black lines, with uncertainty ranges given by the grey envelopes). The largest increase has occurredin the upper stratosphere across all three regions, where observations and models are in best agreement. The maximum positive
trend of about 2% per decade occurs near 40 km altitude. Uncertainties in the trends become larger below 25 km, where observations in the mid-latitudes suggest a decrease while models suggest an increase. [See also Figures 3-11 and 3-12]• Present day (2017–2020) total column ozone as measured from space-based and ground-based observations remains lower than the 1964–1980 average, byº about 2% for the near global average (60°S–60°N)º about 4% in the Northern Hemisphere mid-latitudes(35°N–60°N)º about 5% in the Southern Hemisphere mid-latitudes(35°S–60°S)º about 1% in the tropics (20°S–20°N).
Within uncertainties associated with natural variability and instrumental accuracy, these values are essentially the same asgiven in the previous Assessment for the 2014–2017 average.

Changes to date in vertically resolved ozone
Vertically resolved trends (Figure ES-5) are very similar to
those given in the last Assessment. With longer records and updates to merged datasets, uncertainties have been reduced.
• Measurements show unambiguous increases in upper
stratospheric ozone for 2000–2020 outside of the polar
regions. Positive trends have a range of 1.5–2.2% decade–1 at
mid-latitudes in both the Northern and Southern Hemispheres
and 1.1–1.6% decade–1 in the tropics.
• Upper stratospheric ozone increases are due to a combination of decreases in ozone-depleting substancesand decreases in stratospheric temperature driven byincreases in CO2. New model simulations reaffirm this finding from the 2018 Assessment.
• There are multiple lines of evidence from both observations and models for a small, though uncertain, decrease(1–2% decade –1, with uncertainty up to ±5% decade –1)
in tropical lower stratospheric ozone over 2000–2020.
This decrease is consistent with climate change-driven acceleration of the large-scale circulation and has a small impact
on total column ozone. Chemical ozone loss from chlorine
and bromine is comparatively minor in the tropical lower
stratosphere.
• Observations suggest small decreases in lower stratospheric ozone in the mid-latitudes of both hemispheresfor 2000–2020, while chemistry-climate model simulations suggest small increases. Ozone in mid-latitudes haslarge year-to-year variability; thus trends have large uncertainties, and they are not robust across all datasets and models. The observed decrease is more evident in the Northern
Hemisphere.
• Outside of polar regions, attribution of total column
ozone trends during the period of slow ODS decline
requires knowledge of changes in ozone in both the troposphere and stratosphere. For instance, there is evidence
that the lack of a change in total column ozone in the tropics
reflects an increase in tropospheric ozone that compensates
for the ozone decrease in the tropical lower stratosphere.

17

Executive Summary

Future ozone changes
As reported in the last Assessment, the key drivers of future
stratospheric ozone levels continue to be declining ODSs coupled with CO2-driven cooling in the upper stratosphere and astrengthening of the Brewer-Dobson circulation. Total column
ozone will also be affected by changes in the tropospheric ozone
burden.
• New estimates for the year of return of total column
ozone outside of polar regions to 1980 values arebroadly consistent with the last Assessment. Also similar to the 2018 Assessment, these modeled return dates
vary considerably depending on the assumed future
greenhouse gas scenario. Total column ozone returns to1980 values sooner for scenarios that assume larger emissionsof greenhouse gases than scenarios with smaller greenhousegas emissions. Broadly, the return dates for a middle-ofthe-road (SSP2-4.5) scenario are consistent with previous
Assessments:º around 2040 for near global mean (60°S–60°N) annually
averaged column ozone;º around 2045 for Southern Hemisphere (60°S–35°S) annually averaged column ozone; andº around 2035 for Northern Hemisphere (35°N–60°N)
annually averaged column ozone.
• For scenarios that assume strong reductions in the emission of tropospheric ozone precursors, the resulting
reductions in tropospheric ozone can be important for
total column ozone trends. Under such scenarios, total
column ozone in the tropics is projected to remain below
the 1980 values until at least 2100. As discussed in the last
Assessment, tropical total column ozone under high greenhouse gas (GHG) scenarios will be below 1980 values in 2100
due to circulation-driven changes affecting lower stratospheric ozone.
• The Antarctic ozone hole is expected to gradually close,with springtime total column ozone returning to 1980
values shortly after mid-century (about 2065). Updated

18

chemistry-climate model projections suggest that ozone hole
recovery may depend on the future climate change scenario,
with projections of return around 2050 for the low climatechange mitigation scenarios. This sensitivity of Antarctic recovery to climate change scenario differs from the findings inprevious Assessments and may be due to the use of a smaller number of updated models, as well as the models being
forced with different evolutions of GHGs.
• Arctic springtime total ozone is expected to return to1980 values slightly before mid-century (about 2045).
Substantial Arctic ozone loss will occur in cold winters/
springs as long as ODS concentrations are well above naturallevels. While dynamical changes associated with increasing
GHGs lead to an earlier recovery of Arctic ozone, increasing
stratospheric water vapor abundances and CO2-driven cooling of the lower stratosphere may increase the potential forthe formation of polar stratospheric clouds in dynamically undisturbed Arctic winters, leading to ozone loss.
• The unreported production of CFC-11 over 2012–2019
is estimated to delay polar ozone return to 1980 values
by up to 3 years. For global total column ozone, the delay is
about 1 year.
• Exceptional events can temporarily perturb chemical and dynamical processes that affect stratosphericozone amounts. Since the last Assessment, these include
the 2019/2020 wildfires in Australia, the eruption of the
Hunga Tonga-Hunga Ha’apai volcano, and disruptions to thequasi-biennial oscillation of the tropical winds. In particular,
intense wildfires have become more frequent. Their potential
impacts on the stratosphere are not yet well quantified andare a subject of active research.
• The impending loss of vertically resolved, global spaceborne measurements of ozone-related atmosphericconstituents (e.g., reactive chlorine, water vapor, andlong-lived transport tracers) will impede the ability tomonitor and explain changes in the stratospheric ozone
layer in the future.

Executive Summary

4
STRATOSPHERIC OZONE CHANGE
AND ITS INFLUENCE ON CLIMATE
Stratospheric ozone has a wide-ranging influence on the Earth system. Antarctic ozone depletion caused an expansion of the tropics and a poleward shift of the jet stream and storm tracks in the Southern Hemisphere that lead to pronounced changes in summertime surface climate, as summarized in the previous Assessments. Continuing ozone recovery
and increases in atmospheric greenhouse gas (GHG) concentrations will be key drivers of future Southern Hemisphereclimate changes. The relative importance of ozone recovery for future Southern Hemisphere climate will depend on themagnitude and rate of atmospheric GHG concentration changes.

Evolution of stratospheric climate• The estimated rate of long-term cooling in the global
middle and upper stratosphere (0.6 K decade –1) is similar to previous Assessments. The evolution of stratospherictemperatures continues to follow the behavior expected fromthe well understood effects of natural and anthropogenic forcings. The long-term trends are primarily driven by changing
CO2 and stratospheric ozone. Global temperature in the lower
stratosphere has been near constant since the late 1990s.
• In the future, increasing GHGs and the effects of ozone
recovery would have opposing effects on stratospherictemperature and circulation. For a moderate GHG emission scenario (RCP6.0), stratospheric cooling and the acceleration of the global stratospheric transport circulation (theBrewer-Dobson Circulation) driven by increasing GHGs dominate over opposing effects from ozone recovery. Under bothmoderate (RCP4.5/SSP2-4.5) and high emission (RCP8.5/
SSP5-8.5) scenarios, the delayed breakdown of the austral
springtime polar vortex that was driven by ozone depletion in
the late 20th century will persist due to the effect of increasing
GHGs.

Influence on tropospheric and surface climate• New evidence suggests that ozone recovery hascaused changes in the observed trends of the Southern
Hemisphere atmospheric circulation between theozone depletion and recovery periods. Model simulations
support the attribution of these changes to ozone recovery.
These results provide evidence that Southern Hemispherecirculation trends have responded to the recovery of Antarctic
ozone due to the Montreal Protocol (see Figure ES-6).
• While there are no detectable surface impacts of longterm Arctic ozone changes, new evidence shows thatfor individual years low springtime Arctic ozone canamplify existing stratospheric circulation anomalies andtheir influence on tropospheric circulation and surface
climate.

Influence on the Southern Hemisphere oceanand cryosphere• New evidence confirms that ozone depletion is unlikely
to have driven the observed high-latitude sea-surface
temperature cooling and changes in Antarctic sea ice
since 1979. There is no robust link between ozone depletionand net Southern Ocean carbon uptake, which exhibits large
decadal variations.

Radiative forcing from past ODS, HFC, andstratospheric ozone changes• The calculated total direct radiative forcing due to CFCs,
HCFCs, halons, CCl4 and CH3CCl3 decreased by 0.006 W
m–2 since 2016 and was 0.337 W m–2 in 2020. This forcing
is approximately 16% of the radiative forcing of CO2. CO2equivalent emissions (in Gt CO2-eq yr –1) in 2020 were, for species where estimates are available, 0.7 ± 0.4 for CFCs, 0.7 ±0.1 for HCFCs, 0.09 ± 0.03 for CCl4 and CH3CCl3 combined,
and 0.02 ± 0.01 for halons.
• The best estimate of radiative forcing from stratospheric ozone changes over 1850–2011 is –0.02 W m–2, withan uncertainty of ± 0.13 W m–2. Hence, the combined radiative forcing from ODSs and historical stratospheric ozone
changes is positive (around 0.3 Wm–2), consistent with previous Assessments.
• Radiative forcing from measured HFCs continues to increase. The radiative forcing due to the HFCs reached 0.044± 0.006 W m–2 in 2020, an increase of around one-third since

19

Executive SummarySouthern Hemisphere Circulation Changes
During Ozone Depletion and Recovery
34° S
Reanalysis
Trend line

Reanalysis
Trend line

Hadley Cell edge

Jet position

48° S

50° S

35° S

36° S

52° S

1980

1990

2000

Year

2010

37° S
1980

1990

2000

Year

2010

Figure ES-6. Antarctic ozone recovery has caused changes in the observed trends of the Southern Hemisphere atmosphericcirculation between the ozone depletion and recovery periods (see Figure ES-1 panel d). Shown are the positions (in latitude) ofa) the Southern Hemisphere midlatitude jet and b) Hadley Cell edge, which indicates the poleward extent of the subtropical dry
zone, in austral summer (December–February). Solid lines are 3-year running means derived from four different meteorologicalreanalyses. The dashed lines are piece-wise linear trends computed over the two periods 1980–2000 and 2000–2017. [Figure
adapted from Figure 5-14].

2016. The most important contributor to HFC radiative forcing
was HFC-134a (44%), and HFC-125 (18%) overtook HFC-23(15%) as the second largest contributor. Together, the HFCs
represent approximately 2% of the radiative forcing of CO2.
Total CO2-equivalent emissions in 2020 were 1.22 ± 0.05 Gt
CO2-eq yr –1.

Climate impacts of the control of ODSs by theMontreal Protocol• New studies support previous Assessments that the de-20

cline in ODS emissions due to the implementation of theMontreal Protocol avoids an additional global warmingof approximately 0.5–1 K by mid-century compared toan extreme scenario with an uncontrolled increase in
ODSs of 3–3.5% per year and the resulting changes inozone. New evidence suggests an additional avoided warming by mid-century due to prevention of UV radiation damage
to the terrestrial carbon sink, as such damage would cause
additional CO2 to remain in the atmosphere.

Executive Summary

5
STRATOSPHERIC AEROSOL INJECTION
AND POTENTIAL IMPACTS ON OZONE
Global warming has now reached approximately 1.2 °C above preindustrial levels. Climate model scenarios considered by IPCC (2021) indicate continued future warming in the next few decades even with ambitious mitigation and decarbonization, leading to further climate impacts. Stratospheric Aerosol Injection (SAI) has the potential to limit the risein global surface temperatures by increasing the concentrations of particles in the stratosphere. These particles reflect afraction of sunlight back to space, in a process similar to that evident after large volcanic eruptions. However, SAI comeswith significant risks and can cause unintended consequences. The 2022 Assessment is the first to dedicate a chapter toassess the potential impacts on stratospheric ozone in possible SAI scenarios in the coming decades based on the limited
number of model simulations that have been performed to date.
• Stratospheric Aerosol Injection (SAI) has the potential toreduce global mean temperatures. However, SAI cannot fully offset the widespread effects of global warming and produces unintended consequences, including
effects on ozone. Details of these effects depend on thespecifics of the SAI scenario and SAI injection strategy.

º Very few Earth System Models resolve complex stratospheric processes, including detailed aerosol microphysics coupled with chemistry, radiation, and dynamics. In addition, the sparsity of current existing model
simulations limits the confidence in the quantification ofmany impacts.

º In different SAI scenarios, the modeled effects of SAI on
future ozone depend on the specific details of future climate change, and on the amount, timing and duration of
SAI applied. Offsetting an ever-increasing global warming with an ever-increasing SAI (“strong SAI”) has beenshown to lead to increasing environmental risks.

º Injection rates vary between 8 and 16 Mt of sulfur dioxide (SO2) per year to cool the Earth by 1 °C (an injection
amount approximately equivalent to that of the Mount
Pinatubo eruption in 1991), based on simulations withseven Earth System Models.

º In a world with limited mitigation of greenhouse gas
emissions, global mean temperatures continue to increase significantly in the future (Figure ES-7, black line).
This future warming would be reduced by aggressive
decarbonization (orange line). An SAI peakshaving scenario offsets the overshoot of surface temperature above
a certain threshold until greenhouse gases have been
reduced (purple line).
º Different SAI strategies, such as the altitude and latitudeof injection, and type of material, have been developed
to mitigate some of the unintended climate impacts of
SAI. In modelling studies, the principal injected material
is sulfur. Different strategies would have different effects
on stratospheric ozone.
• Model simulations of SAI reveal large differences insurface cooling per unit sulfur injected, which are attributed to differences in representing key processes.
Explosive volcanic eruptions serve as natural analogs toaid evaluation of these models.

º Explosive volcanic eruptions sporadically emit millions of
tonnes of SO2 into the stratosphere and provide useful,albeit imperfect, natural analogs for evaluating the global
models used to conduct SAI simulations.
• The net effects of large-scale SAI on stratospheric ozone
are mainly driven by i) increases in aerosol surface area,ii) stratospheric halogen and nitrogen concentrations,and iii) aerosol-induced heating of the stratosphere,
which change both stratospheric ozone chemistry andstratospheric dynamics. These simulated changes arestrongly model dependent.
º Enhanced stratospheric sulfate aerosol increases stratospheric heterogeneous chemical reaction rates, leading
to enhanced or depleted stratospheric ozone dependingon altitude, latitude, and season. Details depend on the
SAI-induced aerosol surface area distribution, the current
stratospheric halogen and nitrous oxide concentrations,and SAI-induced changes in stratospheric water vapor.
º Increased sulfate aerosols in SAI scenarios heat the lower
tropical stratosphere by 4.6 ± 2.7 °C per 1°C surface

21

Executive Summary

Peakshaving Scenario for
Stratospheric Aerosol Injection (SAI)

Global surface temperature

Scenarios:
Limited/no mitigation:
high-end global warming
Aggressive mitigation
and CO2 removal (CDR):
low-end global warming
Peakshaving: SAI withaggressive mitigation
and CDR
Temperature offset
due to SAI
Assumed stabilization
temperature

Time in decades

cooling based on results from different models andinjection scenarios. Resulting changes in stratosphericcomposition and transport depend on the details of theinjection strategy and are strongly model dependent.
• Additional ozone depletion due to SAI is simulated inspring over Antarctica, with magnitudes dependent on
the injection rate and timing. Simulations of strong SAI
show an increase in total column ozone (TCO) in mid-latitudes (40–60°N) in the winter Northern Hemisphere.
º For October over Antarctica, SAI simulations that achievea global mean surface cooling of 0.5 °C in the first 20
years, show a reduction of TCO of around 58 ± 20 DU,
assuming 2020–2040 halogen conditions. This reduction brings TCO values close to the observed minimum inthe 1990s. Less ozone loss would be expected for a later
SAI start date, when halogen concentrations are projected to be lower.
º Beyond the first 20 years, the continued application ofstrong SAI, to offset almost 5 °C of warming by 2100, reduces Antarctic ozone in October by similar amounts (5522Figure ES-7. Schematic diagram representing the concept of “Peakshaving”.
Different lines illustrate global mean surface temperatures for future scenarios: alimited or no mitigation scenario leading
to strong future global warming (blackline); a so-called “overshoot scenario”
that assumes strong mitigation and Carbon Dioxide Removal (CDR), and leads
to a temporary overshoot of global mean
temperatures above sustainable limits forsome time (orange); a Peakshaving scenario that applies temporary SAI to theovershoot scenario in order to preventthe increase in global mean temperaturefrom exceeding these sustainable limits(purple line). The blue arrows represent
the approximate relative magnitude of
the temperature impact of the applied
SAI.

± 20 DU) throughout the 21st century despite declining
abundances of ozone-depleting substances (ODS). In
this case, ozone hole recovery from ODSs is delayed by
between 25 and 50 years. A peakshaving scenario potentially leads to less ozone depletion.
º Under stronger SAI scenarios, ozone is significantly
enhanced in NH mid-latitudes in winter owing to stratospheric heating from injected sulfur, which leads to increased equator to poleward transport of ozone.
º Ozone loss within the Arctic polar vortex has not yet
been robustly quantified for SAI.
• The injection of aerosols other than sulfate is expected
to change the effects on ozone via associated changes
in heterogeneous chemistry, dynamics and transport.
Aerosol types that are more chemically inert and absorb lesssolar radiation may reduce chemical and dynamical impacts
on stratospheric ozone respectively. However, the laboratory
studies and climate model simulations sufficient to quantify
these effects have yet to be performed.

Executive Summary

6
POLICY-RELEVANT SCENARIOSAND INFORMATION
Changes in total column ozone and in average radiative forcing in response to various control measures using alternative scenarios and bounding test cases are shown in Figure ES-8. The baseline scenario used here assumes full compliance with the Montreal Protocol. The hypothetical alternative scenarios assessed here include the elimination of banks,
production, and emissions of gases that are both controlled and uncontrolled by the Montreal Protocol and are intended
to demonstrate the impacts on climate and ozone relevant to policy actions.

• The unexpected emissions of CFC-11 over 2012–2019
have led to a delay in the return of mid-latitude EESC to1980 abundances by about 1 year. The reduction in emissions since 2018, based on global and regional observations,have prevented a longer delay.
• The CFC-11 production that led to these observed unexpected emissions has most likely increased global
banks. Assuming these emissions were associated withthe production of insulating foams, it is estimated that theyaccount for 25% to 45% of the unreported production. Thissuggests a potential increase in the CFC-11 bank of 146–1320
kt from unreported production between 2012 and 2019. For
reference, a 1000 kt increase in the 2020 bank would further
delay the return of mid-latitude EESC to 1980 levels by almost
4 years (Figure ES-8).
• If it were possible to eliminate all future long-lived anthropogenic ODS emissions in 2023, this would bringforward the return of mid-latitude EESC to 1980 abundances by about 16 years and increase the average ofglobal stratospheric ozone in the period 2023–2070 byabout 2 DU. This provides an upper limit for the reduction of
EESC through control measures. These emissions are dominated by the release from current banks, with additional contributions from controlled future production and consumption of ODSs, production for feedstock use, and quarantine
and preshipment uses of CH3Br.
• The projected return of mid-latitude EESC is delayed by
6 years compared with the previous Assessment due
mostly to larger assessed banks in the current baseline
scenario.The larger bank estimates primarily arise from theuse of a new modelling approach to assess the banks.
• Total production of controlled substances for feedstock
use is increasing. If all future feedstock-related emissions were eliminated, this would bring forward thereturn of mid-latitude EESC to 1980 levels by almost 4

years when compared to the baseline scenario. Reported
feedstock production has increased by 75% by mass overthe last decade. Assuming that the fraction of emissions related to feedstock production has not changed, emissions
have increased accordingly. Additionally, feedstock usage
has led to the emission of a range of ODS by-products andintermediates.
• The CCl4 emissions from feedstock production and use
currently dominate the ODS influence on ozone fromall feedstocks. The elimination of these CCl4 emissions
accomplishes much of the projected 4-year accelerated
return in EESC noted above. This usage of CCl4 is expected
to continue increasing primarily because of its application in
the growing production of HFOs, and could roughly double
CCl4 abundances in 2100 compared to the baseline scenario.
• If future emissions of methyl bromide (CH3Br) from quarantine and preshipment (QPS) applications could be
eliminated, this would accelerate the return of mid-latitude EESC by about 2 years. Production for QPS use has remained nearly unchanged over the last two decades. It now
constitutes almost 99% of the reported production of CH3Br,
with critical use exemptions (CUEs) making up the remaining
reported production. The importance of QPS CH3Br has been
noted in previous Assessments.
Abundances of several gases not controlled by the Montreal
Protocol have been increasing due primarily to anthropogenic
emissions and have direct effects on stratospheric ozone, for example dichloromethane (CH2Cl2) and N2O.
• Emissions of CH2Cl2, the dominant anthropogenic
VSLS chlorine gas, continue to increase and augmentozone-depleting chlorine in the atmosphere. Future
projections are uncertain due to the highly variable emissions
over the past few years. If CH2Cl2 emissions continue at theircurrent level, they will continue to deplete approximately

23

Executive Summary
1 DU of global, annual average ozone. Elimination of theseemissions would rapidly reverse this depletion (Figure ES-8).
• A 3% reduction in anthropogenic N2O emissions, averaged over 2023–2070, leads to an increase in global
ozone of about 0.5 DU averaged over the same period,

and a decrease of about 0.40 W m–2 in radiative forcing,averaged over 2023–2100 (Figure ES-8). This reductionis the amount obtained when comparing the baseline N2O
scenario (SSP2-4.5) to the strongest N2O mitigation scenarioof the SSPs (SSP1-1.9).

Change in Ozone in Alternative Scenarios
CFCs
Halons
HCFCs
HCFCs
CH3Br
CCl4

High-GWP HFCs

(0)
(0)

–0.2

–0.1

0

0.1

0.2

0.3

0.4

0.5

Elimination of production (excl.
feedstocks) from 2023 onwards
Elimination of emissions (excl.
feedstocks) from 2023 onwards
Feedstocks increased
Feedstocks eliminatedHFC-23Elimination of emissions
from 2023 onwards

N2O

Reduction of emissions to SSP1-1.9

CFC-11
CFC-11

Increased banks size in 2020 of 1000 Gg
Reduced emission of 280 Gg from 2011–2019

CH2Cl2

No emissions from 2023 onwards(excluding HFC-23)(range of estimates)

Bank recapture anddestruction in 2023

0.6

Change in total column ozone (%)
Decreasing ozone

Increasing ozone

Change in Radiative Forcing in Alternative Scenarios
CFCs
Halons
HCFCs
HCFCs
CH3Br
CCl4

High-GWP HFCs

(0)

–0.06

–0.04

–0.02

0.0

Elimination of production (excl.
feedstocks) from 2023 onwards
Elimination of emissions (excl.
feedstocks) from 2023 onwards
Feedstocks increased
Feedstocks eliminatedHFC-23Elimination of emissions
from 2023 onwards

N2O

Reduction of emissions to SSP1-1.9

CFC-11
CFC-11

Increased banks size in 2020 of 1000 Gg
Reduced emission of 280 Gg from 2011–2019

CH2Cl2

No emissions from 2023 onwards(excluding HFC-23)–0.08

Bank recapture anddestruction in 2023

0.02

Change in average radiative forcing (W m–2)
Decreasing climate forcing

Increasing climate forcing

Figure ES-8. Impacts of various alternative scenarios and test cases on total column ozone (averaged over 2020 through2070) and radiative forcing of climate (averaged over 2023 through 2100) compared with the baseline scenario. The
scenarios and cases include reduced N2O emissions (SSP1-1.9 scenario), elimination of emissions for HFCs, HFC-23, CH2Cl2, andCCl4 (excluding emissions from feedstock production and usage) starting in 2023, elimination of future production of CH3Br and
HCFCs starting in 2023 (excluding feedstock production and usage), and elimination and destruction of banks of halons, HCFCs
and CFCs in 2023. Also considered are the unexpected CFC-11 emissions over 2012–2019 (assumed to be 280 Gg in total), an
additional 1000 Gg in the 2020 CFC-11 bank, elimination of all feedstock-related emissions starting in 2023, and a case in whichfeedstock-related emissions are allowed to grow at their current growth rates through 2030 and are then held constant. Potential
climate benefits from improved energy efficiency in the refrigeration and air conditioning sector are not included here, and arethought to have the potential to have an impact much larger than that of any of the scenarios and cases considered here. For
reference, current total column ozone depletion is about 2% when averaged over 60°S–60°N, and the current radiative forcing
from CO2 is about 2 W m–2.

24

Chapter Summaries

SCIENTIFIC SUMMARIESOF THE CHAPTERS

Chapter 1:
Update on Ozone-Depleting Substances (ODSs)
and Other Gases of Interest to the Montreal Protocol
This chapter concerns atmospheric changes in ozone-depleting substances (ODSs), such as chlorofluorocarbons (CFCs),
halons, chlorinated solvents (e.g., carbon tetrachloride [CCl4]
and methyl chloroform [CH3CCl3]) and hydrochlorofluorocarbons (HCFCs), which are controlled under the Montreal Protocol.
Furthermore, the chapter updates information about ODSs notcontrolled under the Protocol, such as methyl chloride (CH3Cl)
and very short-lived substances (VSLSs). In addition to depletingstratospheric ozone, many ODSs are potent greenhouse gases.
Mole fractions of ODSs and other species are primarily
measured close to the surface by global or regional monitoring
networks. The surface data can be used to approximate a mole
fraction representative of the global or hemispheric troposphericabundance. Changes in the tropospheric abundance of an ODS
result from a difference between the rate of emissions into theatmosphere and the rate of removal from it.
• The total amount of chlorine and bromine from ODSs
that were controlled under the original Montreal
Protocol is continuing to decline, as the overall emissions are smaller than the rate at which these ODSs aredestroyed. Abundances of many of the first-stage replacement compounds, HCFCs, are now increasing very
slowly or not at all.

Tropospheric Chlorine (Cl)
Total tropospheric chlorine is a metric used to quantify the
combined globally averaged abundance of chlorine in the troposphere due to the major chlorine-containing ODSs. The contribution of each ODS to total tropospheric chlorine is the productof its global mean tropospheric mole fraction and the number ofchlorine atoms it contains.
• Total tropospheric chlorine from ODSs continued todecrease between 2016 and 2020. Total troposphericchlorine in 2020 was 3220 ppt (where ppt refers to parts pertrillion as a dry air mole fraction), about 1.8% lower than in2016 and 12% lower than its peak value in 1993. Of the 2020
total, CFCs accounted for about 60%, CH3Cl for about 17%,and CCl4 and HCFCs each for about 10%. The contribution
from CH3CCl3 has now decreased to 0.1%. Very short-lived
source gases (VSL SGs), as measured in the lower troposphere, contributed approximately 3.5%.
º During the period 2016–2020, the observed rate of
decline in tropospheric chlorine due to controlled substances was 15.1 ± 2.45 ppt Cl yr −1, which is larger thanduring the 2012–2016 period (12.8 ± 0.8 ppt Cl yr −1).
This rate of decrease was close to the projections in theprevious Assessment. The net rate of change was the result of a slightly slower than projected decrease in CFCs
and a slower HCFC increase than in the 2018 A1 projection scenario.
º When substances not controlled under the Montreal
Protocol are also included, the overall decrease in tropospheric chlorine was 15.1 ± 3.6 ppt Cl yr −1 during
2016–2020. This is larger than the rate of decline during
the 2012–2016 period (3.6 ± 4.7 ppt Cl yr −1) and comparable to the rate of decline in controlled substances.
Changes in the predominantly anthropogenic dichloromethane (CH2Cl2) and the largely natural CH3Cl largely
canceled each other out, resulting in almost no net
change in Cl from uncontrolled substances during thisperiod.
• Starting around 2018, the rate at which the CFC-11 mole
fraction was declining in the atmosphere accelerated
again, following a slowdown since 2013. These recentchanges are largely due to a decrease in emissions originating mostly from northeastern China. Assuming noUncertainties in absolute changes of atmospheric abundances were derived using the 1 standard deviation measurement uncertainties (where appropriate combined as thesquare root of the sum of their squares) and the bootstrap algorithm described in Barreto and Howland (2006). Similar to the procedure described in Leedham Elvidge et al.
(2018), and to represent atmospheric variability, data was converted to a dataset comprised of 1) original data, 2) original data minus measurement uncertainty and 3) original
data plus measurement uncertainty. This dataset was then resampled (with replacement) 1000 times to derive a standard deviation that is a realistic representation of the uncertainty of the entirety of the original data.
525

Chapter Summaries
impact from changes in atmospheric circulation, global emissions increased from about 57 Gg yr −1 (= kt yr−1) in 2012 toaround 78 Gg yr −1 in 2017; after 2018, they then decreased,
to approximately 47 Gg yr −1 in 2020. Emissions from northeastern China explain 60 ± 40% of the 2012–2018 increase
and 60 ± 30% of the subsequent decrease. There is evidence
that other recent significant emission regions include theArabian and Indian subcontinents. If these renewed global
emissions are associated with uses that substantially increase
the size of the CFC-11 bank, further emissions resulting fromthis production would be expected in the future.
• During 2016–2020, mole fractions of CFC-12 decreased
by about 2.8%, which is comparable to the decreaseduring 2012–2016 (~2.3%). Estimates of global CFC-12
emissions in 2016 and 2020 were similar within uncertainties,
at 33 ± 21 Gg yr –1 and 25 ± 20 Gg yr –1, respectively. CFC-11
and CFC-12 are often co-produced, and atmospheric observations have confirmed a decrease in CFC-12 emissions from
northeastern China from 3.3 ± 1.4 Gg yr –1 in 2016 to 0.5 ± 0.5
Gg yr –1 in 2019.
• The CFC-113 global mole fraction has continued to decrease, but emissions remained constant within uncertainties
at around 6 ± 6 Gg yr –1 between 2016 and 2020.
• Mole fractions of CFC-114 remained stable during
2016–2020, whereas those of CFC-13, CFC-113a, and
CFC-115 continued to rise, and mole fractions of CFC112a and CFC-114a exhibited positive growth after previously showing near-zero change. Total Cl from the latter
five CFCs increased from 16.0 ± 0.3 ppt in 2016 to a total of
17.2 ± 0.3 ppt Cl in 2020. These findings likely indicate an increase or stabilization of the emissions of these relatively low
abundance compounds. While some of these emissions are
known to originate from eastern China, the primary processes
responsible are unknown.
• The rate at which CCl4 has declined in the atmosphereremains slower than expected from its reported use asa feedstock and its removal rate from the atmosphere,
which indicates ongoing emissions of around 44 ± 15
Gg yr −1. This is likely, at least in part, due to feedstock emissions from the production of chloromethanes and perchloroethylene and from chlor-alkali plants. Global CCl4 emission
estimates based on atmospheric observations are now moreaccurate than in the last Assessment due to an improved lifetime estimate.
• Emissions of CCl4 in eastern China over the period
2013–2019 show year-to-year variability likely related to CFC-11 production. Emissions increased after 2013,reaching 11.3 ± 1.9 kt yr –1 in 2016, and decreasing to 6.3 ±1.1 kt yr –1 in 2019.
• Total tropospheric chlorine from HCFCs has continued
to increase, reaching 320 ± 3 ppt in 2020. There is evidence of a slowdown of this increase, as the annual average
growth rate of total chlorine from HCFCs decreased from 5.9± 1.3 ppt yr –1 during 2012–2016 to 2.5 ± 0.4 ppt yr –1 during
2016–2020.
• Combined emissions of the major HCFCs have declined
since the previous Assessment. Emissions of HCFC-22
and HCFC-142b likely declined between 2016 and 2020,26

while emissions of HCFC-141b, after an initial drop, likely rose
year-on-year since 2017, amounting to a total rise of ~4.5 Ggduring 2017–2020. These findings are consistent with a sharp
drop in reported HCFC consumption after 2012, particularly
from Article 5 countries.
• Continued emissions of the compounds HCFC-124, HCFC31, HCFC-132b, and HCFC-133a have been inferred fromatmospheric measurements. HCFC-132b is yet another newly
detected HCFC, and its atmospheric mole fractions, whilecurrently small, continue to increase.

Tropospheric Bromine (Br)Total tropospheric bromine is defined in analogy to total
tropospheric chlorine. Even though the abundance of bromine
is much smaller than that of chlorine, it has a significant impact
on stratospheric ozone because it is around 60–65 times more
efficient than chlorine as an ozone-destroying catalyst.
• Total tropospheric bromine from controlled ODSs
(halons and methyl bromide [CH3Br]) continued to decrease, and was 13.9 ppt by 2020, 3.2 ppt below thepeak levels observed in 1999. From 2012 to 2016, total
controlled bromine declined at a rate of 0.15 ± 0.14 ppt Br yr−1
(about 1% yr−1). This rate increased to 0.18 ± 0.05 ppt Br yr−1
during 2016–2020, with halons contributing about 60% to
the overall decline.
• The mole fractions of halon-1211, halon-2402, andhalon-1202 continued to decline between 2016 and2020. There was no significant change in the mole fraction of halon-1301 between 2016 and 2020. This ODS
is, at ~3.3 ppt, now the most abundant halon in the atmosphere. Emissions of halon-2402, halon-1301, and halon1211, as derived from atmospheric observations, declined orremained stable between 2016 and 2020.
• CH3Br annually averaged mole fractions showed little
net change between 2016 and 2020. The small increase(2–3%) observed between 2015 and 2016 was compensated by a small decrease (4%) largely taking place
during 2016–2017. The 2020 mole fraction was around
6.6 ppt, a reduction of 2.6 ppt from peak levels measured
between 1996 and 1998. Reported quarantine and pre-shipment (QPS) consumption was relatively stable from 1996 to
2020.

Halogenated Very Short-Lived Substances(VSLSs)
VSLSs are defined as trace gases whose local lifetimes areshorter than 0.5 years and have non-uniform tropospheric abundances. These local lifetimes typically vary substantially overtime and space. Of the very short-lived source gases (VSL SGs)
identified in the atmosphere, brominated and iodinated species
are predominantly of oceanic origin, while chlorinated species
have significant anthropogenic sources. VSLSs that reach thestratosphere will release the halogen they contain almost immediately and will thus play an important role for lower-stratospheric ozone in particular. Due to their short lifetimes and theiratmospheric variability, the quantification of their contribution ismuch more difficult and has much larger uncertainties than for
long-lived compounds.

Chapter Summaries• Total tropospheric chlorine from VSL SGs in the background lower atmosphere is dominated by anthropogenic sources. It continued to increase between 2016
and 2020, but its contribution to total stratosphericchlorine remained small. Global mean chlorine from VSLSs
in the troposphere has increased from about 103 ppt in 2016
to about 113 ppt in 2020. The relative contribution of VSLS
to the stratospheric chlorine input amounted to 4% in 2020,
compared to 3.6% in 2016.

• Total bromine input to the stratosphere of 18.9 ppt isderived for 2020 by combining 13.9 ppt from longlived gases and 5 ppt from VSLSs not controlled under
the Montreal Protocol. The total input declined by 14.5%
between 1999 peak values and 2020. Anthropogenic emissions of all brominated long-lived gases are controlled, but asCH3Br also has natural sources, more than 50% of the brominereaching the stratosphere is now estimated to be from sources not controlled under the Montreal Protocol.

• Dichloromethane (CH2Cl2), with predominantly anthropogenic sources, is the main contributor to total
chlorine from VSLSs. It accounted for the majority of
the change in VSLS chlorine between 2016 and 2020.
The CH2Cl2 global mean abundance reached approximately
40–45 ppt in 2020, which is more than a doubling comparedto the early part of the century. The rate of increase slowed
after 2016 but remained substantial. Regional CH2Cl2 emissions from Asia most likely account for most of this increase
and more than offset a small decrease in European and North
American emissions.

• Total stratospheric bromine, derived from observationsof bromine monoxide (BrO), has decreased at a rate ofabout 0.8% yr −1 since 2003. This decline is consistent withthe decrease in total tropospheric organic bromine, based on
measurements of CH3Br and the halons. There is no indication of a long-term change in natural sources of stratosphericbromine.

• Brominated VSLSs contribute 5 ± 2 ppt to stratospheric bromine; this constitutes about 27% of total stratospheric bromine in 2020. The main sources for brominated
VSLSs are natural, and no long-term change is observed. Dueto the decline in the abundance of controlled bromine compounds, the relative contribution of VSLSs to total stratospheric bromine increased by about 1% since 2016.
• New evidence suggests that natural iodinated VSLSs
contribute 0.3–0.9 ppt iodine to the stratosphere. A
rapid shift in the partitioning between gas-phase and particulate iodine has been detected in the upper troposphere. Thismechanism can enable iodine entrainment into the stratosphere in particulate form in addition to the entrainment in gas
form. No observational trend estimates exist.

Stratospheric chlorine and bromine
In the stratosphere, chlorine and bromine can be released
from organic source gases to form inorganic species, which participate in ozone depletion. In addition to estimates of the stratospheric input derived from the tropospheric observations, measurements of inorganic halogen loading in the stratosphere areused to determine trends of stratospheric chlorine and bromine.
• The total chlorine input to the stratosphere for 2020
was 3240 ppt, which is 11.5% below the 1993 peakvalue, equivalent to a decline of 420 ± 20 ppt. This longterm decrease was largely driven by decreasing abundancesof CH3CCl3 and CFCs. The chlorine input for 2020 is derived
from measurements of long-lived ODSs at the surface and estimates of stratospheric entrainment of VSLSs.
• Hydrogen chloride (HCl) is the major reservoir of inorganic chlorine (Cly). Middle-stratosphere profile andtotal column measurements of HCl show a long-term decrease for the period 1997–2020 of around 0.5 ± 0.2%
yr −1. If the evaluations are constrained to the shorter period
2005–2020 the satellite records show a rate of decrease ofaround 0.3 ± 0.2% yr −1. This latter rate of decline in stratospheric HCl for the more recent period is in good agreementwith expectations from the decline in tropospheric chlorine,
which slowed after 2000.

Equivalent Effective Stratospheric Chlorine(EESC)
EESC is the chlorine-equivalent sum of chlorine and bromine
derived from ODS tropospheric abundances, weighted to reflect
their expected depletion of stratospheric ozone. The growth anddecline in EESC depend on a given tropospheric abundance
propagating to the stratosphere with varying time lags (on theorder of years) associated with transport to different regions of
the stratosphere. Therefore, the EESC abundance, its peak timing, and its rate of decline are different in different regions of thestratosphere.
• By 2020, EESC had declined from peak values by about
11% for polar winter conditions and by about 15% for
mid-latitude conditions. This drop to 1607 ppt is 37% of thedecrease required for EESC in mid-latitudes to return to the1980 benchmark level. In polar regions, the drop to 3710 ppt
is about 23% of the decrease required to return to the 1980
benchmark level. However, regional estimates have indicatedthat EESC might be higher in some parts of the stratosphere,with an additional 200–300 ppt predominantly originatingfrom CH3Cl and CH3Br. Contributions from the ozone-depleting VSLSs and nitrous oxide (N2O) are currently not includedin EESC calculations.

Tropospheric and Stratospheric Fluorine (F)While fluorine has no direct impact on stratospheric ozone,
many fluorinated gases are strong greenhouse gases, and theiremissions are often related to the replacement of chlorinatedsubstances controlled under the Montreal Protocol. For this reason, trends in fluorine are also assessed in this report.
• The main sources of fluorine in the troposphere and in
the stratosphere are CFCs, HCFCs, and HFCs (hydrofluorocarbons). In contrast to total chlorine, total fluorine
in the troposphere continued to increase between 2016
and 2020, at a rate of 1.71% yr −1. This increase shows thedecoupling of the temporal trends in fluorine and chlorine dueto the increasing emissions of HFCs (see Chapter 2). The ODS
contribution to the fluorine budget has started to decline, sothat the fluorine trend due to ODSs alone became negative
after 2016. In contrast, the fluorine trend due to HFCs has constantly increased, causing the total fluorine trend to increase
as well. The Northern Hemisphere stratospheric abundance27

Chapter Summaries
of inorganic fluorine has continued to increase at a rate ofabout 0.8% yr −1 since 2004.

Effect of ODSs on climate• The total direct radiative forcing of CFCs continues tobe distinctly higher than that of HCFCs, with CFCs contributing around 68% of the total forcing from ODSs.
Radiative forcing from CFCs has dropped by 0.007 W m–2
since 2016 to about 0.257 W m−2 in 2020, while radiative
forcing from HCFCs increased from 0.062 W m−2 to 0.064 W
m–2 from 2016 to 2020. The total direct radiative forcing due
to CFCs, HCFCs, halons, CCl4, and CH3CCl3 was 0.337 W m–2
in 2020 (approximately 16% that of CO2).
• CO2-equivalent emissions of CFCs and HCFCs wereagain approximately equal in 2020. Based on 100-yeartime horizon global warming potentials (GWPs), the CO2equivalent emissions (in Gt CO2-eq yr –1) in 2020 were, for species where estimates are available, 0.7 ± 0.4 for CFCs, 0.7 ±0.1 for HCFCs, 0.09 ± 0.03 for CCl4 and CH3CCl3 combined,
and 0.02 ± 0.01 for halons. The CO2-equivalent emissions
from the sum of CFCs, HCFCs, halons, CCl4, and CH3CCl3 remained similar to the value reported in the last Assessment at
approximately 1.5 Gt CO2-eq in 2020.

Other gases that affect ozone and climate• Mole fractions of many other gases that affect bothozone and climate (including the three major greenhouse gases CH4, N2O, and CO2) have changed since
the last Assessment. The atmospheric abundance of methane (CH4) has continued to increase following a period ofstagnation in the early 2000s. The drivers of the changing
trend are likely largely anthropogenic.
• Mole fractions of N2O, which is an ODS, continue togrow in the atmosphere, with growth rates exceedingsome of the highest projections. When expressed as a
CFC-11-equivalent, anthropogenic N2O emissions in 2020
were equal to more than two times the ODP-weighted emissions from all CFCs in that year. When compared to the CFC
emission peak from 1987, those 2020 anthropogenic N2Oemissions were equal to more than 20% of the ODP-weighted
emissions from CFCs in that year. Almost half of the N2O emissions in recent years are anthropogenic in origin.
• The global mole fractions of many non-ODS, non-HFC,
highly fluorinated substances have continued to grow(e.g., sulfur hexafluoride [SF6], carbon tetrafluoride [CF4],
hexafluoroethane [C2F6], sulfuryl fluoride [SO2F2], and nitrogen trifluoride [NF3]). These species contributed 0.014 W m–2
to anthropogenic radiative forcing in 2020. In contrast, theabundance of the sulfur-containing compound sulfur dioxide(SO2) has not changed substantially, while carbonyl sulfide(COS) has shown a small negative trend.
• Molecular hydrogen (H2) is included in the Assessment
for the first time, due to its potential future effects on
stratospheric ozone. The decarbonization of the fossil
fuel industry could lead to drastically increasing atmospheric mole fractions of H2. The resulting future effects
on ozone are currently not well understood but are expected to be small. Atmospheric abundances of H2 haveincreased from ~330 ppb during the mid-to-late 1800s to thepresent levels of 530–550 ppb in the late 20th and early 21st
centuries.

Chapter 2:Hydrofluorocarbons (HFCs)
Hydrofluorocarbons (HFCs) have been increasingly produced and used in applications such as refrigeration, air-conditioning, and foam blowing following the phasedown of ozone-depleting substances (ODSs). In addition to emissions resulting
from these uses, some HFCs, particularly HFC-23, are released
as by-products during the manufacture of other compounds.
While being benign for the stratospheric ozone layer and generally having lower radiative efficiencies than the most abundantODSs, long-lived HFCs are potent greenhouse gases. Therefore,
HFCs were included in the basket of substances controlled by
the 1997 Kyoto Protocol under the United Nations Framework
Convention on Climate Change (UNFCCC). Subsequently, certain HFCs were brought into the Montreal Protocol frameworkby the Kigali Amendment in 2016. The Kigali Amendment, which28

came into force in January 2019 for parties who ratified theAmendment, seeks to limit the production and consumption of
a selection of HFCs. For HFC-23, the Kigali Amendment seeks tolimit emissions formed as a by-product of HCFC (hydrochlorofluorocarbon) and HFC production to the extent practicable using
approved technologies.
For all the most abundant HFCs (HFC-134a, HFC-23, HFC32, HFC-125, and HFC-143a) and some of the more minor HFCs,
atmospheric observations have been available for several years
or decades. Observations in the remote atmosphere can be used
to derive “top-down” global emissions. These emissions can becompared to the sum of “bottom-up” estimates derived from accounting methods for Annex I parties to the UNFCCC, who are
required to report their emissions annually. For some parts of theChapter Summaries
world, atmospheric observations exist in sufficient density to derive top-down emissions estimates at regional scales. These canbe compared to bottom-up estimates reported by the countries
in these regions.
Based on the historical emissions trends derived from atmospheric data and estimates of future consumption, projections of
future emissions can be derived under different policy scenarios.
These emissions scenarios can be used to estimate the climate
impact of various HFC policies in terms of future radiative forcing
and temperature change.
The key findings of this chapter are as follows:• Global mean abundances of each of the major HFCs haveincreased since 2016. Radiative forcing due to the HFCs
reached 44.1 ± 0.6 mW m–2 in 2020, an increase of around
one-third since 2016. HFC-134a remained the largest contributor to the overall radiative forcing due to HFCs (44%), andHFC-125 (18%) overtook HFC-23 (15%) as the second-largest
contributor.
• Total CO2-equivalent HFC emissions inferred from observations increased through 2020. The total carbon
dioxide-equivalent emissions (CO2-eq, calculated using 100year global warming potentials, GWPs) due to HFCs was 1.22
± 0.05 Pg CO2-eq yr –1 in 2020 (1 Pg = 1 Gt), 19% higher thanin 2016. Of this total, HFC-134a was responsible for approximately 30%, HFC-125 for 28%, HFC-23 for 20%, and HFC143a for 15%. Emissions of the majority of the most abundant
HFCs grew between 2016 and 2020, except for HFC-143a,
HFC-152a, HFC-365mfc, and HFC-43-10mee, for which emissions remained roughly constant. In 2020, global total CO2eq emissions due to HFCs were 60–70% higher than those of
CFCs (chlorofluorocarbons) or HCFCs.
• The gap between total CO2-eq HFC emissions reported
by Annex I countries to the UNFCCC and global estimates derived from atmospheric data has grown. The
emissions reported by Annex I countries in common reporting
format (CRF) are approximately constant in the period 2015–2019, while atmospheric observations in the backgroundatmosphere suggest continued growth in global total emissions. In 2019, UNFCCC reports accounted for only 31% (including HFC-23 in the analysis) or 37% (excluding HFC-23) ofthe global total CO2-eq emissions derived from observations.
Regional top-down emissions estimates for Europe, the USA,
and Australia are similar to reported bottom-up emissions, suggesting that underreporting by these Annex I countries likely
does not explain this discrepancy. Inverse modeling studies
have been carried out for China and India (both non-Annex I
countries) and find that around one-third of the emissions gap(excluding HFC-23) could be explained by sources in thesecountries. However, approximately 40% of global total HFC
CO2-eq emissions (excluding HFC-23) remain unaccounted
for by Annex I reports or top-down estimates for non-Annex I
parties. Top-down regional emissions estimates are available
from only a relatively small number of countries based on theexisting measurement network, whereas global top-down estimates reflect the aggregate of all emissions (for longer-lived
HFCs). Therefore, the unattributed emissions probably occur
in countries that are not monitored by atmospheric measurements and/or that do not report to the UNFCCC in CRF.

• The global inferred CO2-eq HFC emissions are less thanthe emissions in the 2018 Assessment HFC baseline
scenario. They are about 20% lower for 2017–2019. It
is too early to link this directly to the provisions of the Kigali
Amendment, since the first step in the scheduled phasedown
was in 2019. The lower emissions can be explained by lower
reported consumption in several countries following national
regulations.
• The ratio of global HFC-23 emissions inferred from atmospheric observations to reported HCFC-22 production has increased between 2010 and 2019, despite
reports of substantial new emissions abatement since2015. Top-down estimates of global HFC-23 emissions were17.2 ± 0.8 Gg yr –1 in 2019 (1 Gg = 1 kt). This is substantiallylarger than a bottom-up estimate of 2.2 Gg yr –1 derived from
UNFCCC reports for Annex I countries (1.6 Gg yr –1), HCFC22 production reported to the United Nations Environment
Programme (UNEP), and national abatement programs in
India and China. The contribution to the global atmospheric HFC-23 budget of photolysis of trifluoroacetaldehyde(CF3CHO), a minor degradation product of some fluorinatedcompounds, is assessed to be negligible.
• Some HFCs and unsaturated HFCs (hydrofluoroolefins[HFOs]) degrade in the environment to produce trifluoroacetic acid (TFA), a persistent toxic chemical. HFO1234yf has been increasingly used to replace HFC-134a as
a mobile air conditioner (MAC) refrigerant. Measurements
show that atmospheric background abundances of HFO1234yf at Jungfraujoch, Switzerland have grown from less than
0.01 ppt before 2016 to annual median levels of 0.10 ppt in
2020. At the 2020 level, the oxidation of HFO-1234yf is likely
producing a comparable, or potentially larger, amount of TFA
than the oxidation of HFC-134a locally near Jungfraujoch.
The measured and model simulated concentrations of TFA
from the use of HFO-1234yf and other relevant HFOs, HFCs,
HCFCs, and hydrochlorofluoroolefins (HCFOs) is, in general,significantly below known toxicity limits at present. However,the production of TFA in the atmosphere is expected to increase due to increased use of HFOs and HCFOs. Potential
environmental impacts of TFA require future evaluation due toits persistence.
• Projected emissions of HFCs based on current trends inconsumption and emissions, national policies in several countries, and the Kigali Amendment are lower thanthose projected in the 2018 Assessment. The 2020–2050
cumulative emissions in the 2022 updated Kigali Amendment
scenario are 14–18 Pg CO2-eq lower than the corresponding
scenario in the previous Assessment. The 2050 radiative forcing in a scenario that assumes no controls on HFCs, is 220–
250 mW m–2 (termed the Baseline scenario in the previous
Assessment). Radiative forcing in 2050 is reduced to 90–100
mW m–2 in the 2022 Kigali Amendment scenario, 30 mW m–2
lower than projected in the 2018 Kigali Amendment scenario. The new scenario follows national controls on the consumption and production of HFCs in non-Article 5 countries,
reflects lower reported consumption in China, is based on
updated historical information on the use of HFCs in non-Article 5 countries, uses observed mixing ratios through 2020
as a constraint, and includes assumptions about reduced useof HFCs for commercial and industrial refrigeration. The new29

Chapter Summaries
scenario also assumes that all countries adhere to the provisions of the Kigali Amendment.
• Under the provisions of the Kigali Amendment, current
trends in consumption and emissions, and national policies, the contribution of HFCs to global annual averagesurface warming is projected to be 0.04 °C in 2100. Thisis substantially lower than under the scenario without HFC
control measures, for which a contribution of 0.3–0.5 °C wasprojected.

• Concerted efforts to improve energy efficiency of refrigeration and air-conditioning equipment could leadto reductions in greenhouse gas emissions of the sameorder as those from global implementation of the Kigali
Amendment. These estimated benefits of improving energy
efficiency are highly dependent on greenhouse gas emissions
from local electric grids and the pace of decarbonization in
the energy sector.

Chapter 3:
Update on Global Ozone: Past, Present, and FutureThis chapter presents our current understanding of global
ozone outside of the polar regions. The increase of ozone-depleting substance (ODS) concentrations caused the large ozone decline observed from the early satellite era (circa 1980) to the mid1990s. Since the late 1990s, concentrations of ODSs have been
declining due to the successful implementation of the Montreal
Protocol and its Amendments and adjustments. Since the last
Assessment, the longer observational records show a small increase in near-global total column ozone (TCO) with reduced uncertainty, but this trend is not yet statistically significant. A small
increase in TCO is seen in the Southern Hemisphere (SH) mid-latitudes but not yet in the Northern Hemisphere (NH) mid-latitudes
or tropics. Different processes operating at different altitudes
complicate the attribution of the overall total column trend.
However, a significant increase in upper-stratospheric ozone
noted in the previous Assessment continues, driven by declines inozone-depleting substances and increases in greenhouse gases(GHGs). Model simulations support our overall understanding ofthese trends.
Over this century, we expect an increase in global stratospheric ozone as the concentrations of ODSs decline. The future
evolution for different latitudes and vertical levels depends onthe future concentrations of GHGs and precursors of tropospheric ozone. These other influences may lead to TCO levels thatremain below 1980 values in some regions, even after concentrations of ODSs have declined to pre-1980 levels.

Changes to date in total column ozone• Aggregated ground- and space-based observations
indicate an increase of 0.3% decade–1 (with a 2-sigma
uncertainty of at least ±0.3% decade–1) in near-global(60°S–60°N) TCO over the 1996–2020 period. This trend
is consistent with model simulations and our scientific understanding of the processes controlling ozone.
• Over the same 1996–2020 period, the TCO trends in broad
latitude bands are as follows:30º SH mid-latitude (60–35°S) TCO has increased (0.8 ±
0.7% decade–1).
º NH mid-latitude (35–60°N) TCO trends are negligible(0.0 ± 0.7% decade–1).
º Tropical (20°S–20°N) TCO shows no clear trend (0.2 ±
0.3% decade–1), likely because stratospheric ozone isdecreasing while tropospheric ozone is increasing, both
unrelated to changes in ODSs.
The latitudinal pattern of these TCO trends is largely consistent with our scientific understanding and is reproduced in thelatest set of chemistry-climate models (CCMs).
• Present-day (2017–2020) TCO as measured from spacebased and ground-based observations remains lower than
the 1964–1980 average byº about 2% for the near-global average (60°S–60°N),º about 4% in the NH mid-latitudes (35–60°N),º about 5% in the SH mid-latitudes (35–60°S), andº about 1% in the tropics (20°S–20°N).
Within uncertainties associated with natural variability and instrumental accuracy, these values are essentially the same asgiven in the previous Assessment for the 2014–2017 average.

Changes to date in vertically resolved ozone
Vertically resolved trends are very similar to those given in
the last Assessment. However, with longer records and updated
merged datasets, recovery trends are now statistically significant in more locations.
• Measurements show unambiguous increases in upper-stratospheric ozone for 2000–2020. Positive trends
have a range of ~1.5–2.2% decade–1 at mid-latitudes in boththe Northern and Southern Hemispheres and ~1–1.5% decade–1 in the tropics.

Chapter Summaries• Upper stratospheric ozone increases are due to a combination of decreases in ODSs and decreases in stratospheric temperature driven by increases in carbon dioxide (CO2). New CCM simulations affirm this finding from thelast Assessment.

sooner for scenarios that assume larger emissions of GHGs
than scenarios with smaller GHG emissions. The return dates
for a middle-of-the-road (SSP2-4.5) scenario are:

• There are multiple lines of evidence from both observations and models for a small though uncertain decrease(1–2% decade–1, with uncertainty up to ±5% decade–1)
in tropical lower stratospheric ozone over 2000–2020.
This decrease is consistent with climate change-driven acceleration of the large-scale circulation and has a small impact on
TCO. Chemical ozone loss from chlorine and bromine is comparatively minor in the tropical lower stratosphere.

º around 2045 for SH (60–35°S) annually averaged column ozone; and• Observations suggest small decreases in lower stratospheric ozone in the mid-latitudes of both hemispheresfor 2000–2020, while chemistry-climate model simulations suggest small increases. Ozone in mid-latitudes haslarge year-to-year variability; thus, trends have large uncertainties, and they are not robust across all datasets and models. The observed decrease is more evident in the Northern
Hemisphere.
• Attribution of TCO trends during the period of slow ODS
decline requires knowledge of changes in ozone in boththe troposphere and stratosphere. For instance, there isevidence that the lack of a change in TCO in the tropics reflects an increase in tropospheric ozone that compensates forthe ozone decrease in the tropical lower stratosphere. Thisdecrease, due to a climate change-driven acceleration of thelarge-scale circulation, is expected based on modeling studies. Depletion due to ODSs, on the other hand, is very minor
in the tropical lower stratosphere. Nevertheless, analyses ofthese changes using different observational datasets indicate
significant remaining uncertainty.

Future ozone changes
Projections of future stratospheric ozone are available
from new model simulations that follow new emissions scenarios: the shared socioeconomic pathways (SSPs). These scenarios all assume compliance with the Montreal Protocol and its
Amendments and adjustments for ODSs but span a wider range
in future GHG and pollutant emissions pathways than the scenarios used in the previous Assessment, although there are fewer
models from which to draw results. As in the last Assessment, thekey drivers of future stratospheric ozone levels continue to be
declining ODS concentrations coupled with CO2-driven coolingin the upper stratosphere and a strengthening of the BrewerDobson circulation. TCO will also be affected by changes in the
tropospheric ozone burden.
• New estimates for the year of return of near-global TCO
to its 1980 value are broadly consistent with the last
Assessment. Also similar to the last Assessment, thesemodeled return dates vary considerably depending on
the assumed future scenario. TCO returns to its 1980 valueº around 2040 for near global mean (60°S–60°N) annually
averaged column ozone;º around 2035 for NH (35–60°N) annually averaged column ozone.
• For scenarios that assume strong reductions in theemission of tropospheric ozone precursors, the resulting reductions in tropospheric ozone can be important
for TCO trends. Under such scenarios, TCO in the tropics is
projected to remain below the 1980 values until at least 2100.
As discussed in the last Assessment, tropical TCO under high
GHG scenarios will be below 1980 values at 2100 due to circulation-driven changes affecting lower stratospheric ozone.
• Future ozone recovery and the expected strengthening of the Brewer–Dobson circulation will most likely
increase the proportion of ozone of stratospheric origin
in the troposphere. A new analysis has quantified the contribution of stratosphere-to-troposphere transport of ozone inmodels under scenarios with limited GHG mitigation (RCP6.0
and RCP8.5). While stratosphere-to-troposphere transportremains highly variable between models and is strongly scenario-dependent, the projected increase is robust, suggesting increases of stratospheric ozone in the troposphere of
10–50% over the 21st century, depending on the model andscenario. Nonetheless, in situ chemistry involving air pollutants remains the largest production term for the simulated
tropospheric ozone budget.
• The unreported production of CFC-11 over 2012–2019 (seeChapter 1) is estimated to delay global TCO recovery to 1980
levels by ~1 year.

Emerging Issues• Exceptional events can temporarily perturb chemical and dynamical processes that affect stratosphericozone amounts. Since the last Assessment, these include
the 2019/2020 wildfires in Australia, the eruption of the
Hunga Tonga-Hunga Ha’apai volcano, and disruptions to thequasi-biennial oscillation of the tropical winds. In particular,
intense wildfires have become more frequent. Their potential
impacts on the stratosphere are not yet well quantified andare a subject of active research.
• The impending loss of vertically resolved, global spaceborne measurements of ozone-related atmosphericconstituents (e.g., reactive chlorine, water vapor, andlong-lived transport tracers) will impede the ability tomonitor and explain changes in the stratospheric ozone
layer in the future.

31

Chapter Summaries

Chapter 4:
Polar Stratospheric Ozone: Past, Present, and Future
The chemical and dynamical processes controlling polarozone are well understood. Polar ozone depletion is fundamentally driven by anthropogenic chlorine and bromine, withthe severity of the chemical loss each year in both polar regions
strongly modulated by meteorological conditions (temperatures
and winds) and, to a lesser extent, by the stratospheric aerosol
loading and the solar cycle. As noted in previous Assessments,
the stratospheric halogen concentration resulting from the emissions of ozone-depleting substances (ODSs) reached its peak in
the polar regions around the turn of the century and has been
gradually declining since then in response to actions taken under
the Montreal Protocol and its Amendments and adjustments.
The 2018 Assessment reported for the first time that signs of
the onset of ozone recovery from the effects of ODSs had been
detected over the Antarctic. More varied and more robust signsof the onset of recovery are now beginning to emerge; as theobservational record lengthens, ozone hole recovery trends are
expected to continue to become clearer against the backgroundof natural variability. Nevertheless, the Antarctic ozone holewill continue to be a recurring phenomenon until the middle ofthe century, although with a decreasing average size and someinterannual variability. The Arctic is more dynamically variable,
precluding identification of a significant increase in Arctic ozone.
Cold conditions conducive to substantial stratospheric ozone
loss occur in some Arctic winter/spring seasons and are expected to continue to do so, interspersed with warmer years withlittle or no ozone depletion. Chemistry–climate model (CCM)
projections largely confirm previous studies that, in both hemispheres, springtime polar total column ozone (TCO) will returnto 1980 historical levels around the middle of this century. For
the Antarctic, the timing of this return depends mainly on the
declining stratospheric halogen concentrations from decreasing
ODS emissions, and the impact of climate change is small. In the
Arctic, TCO is expected to return to 1980 levels earlier than in the
Antarctic. This is because in the Arctic, springtime stratosphericozone has a stronger dependence on the future greenhouse gas(GHG) emissions scenarios.

Observed changes in polar ozone• The Antarctic ozone hole continued to appear eachspring during the 2018–2021 period. The occurrence andcharacter of recent ozone holes are consistent with the current concentrations of ODSs and their small overall downward
trend.
• Recent Antarctic ozone holes exhibited substantial
interannual variability in size, strength, and longevity: the 2019 ozone hole was the smallest since 2002,
whereas 2020 saw a deep ozone hole of record duration. In 2019, a strong minor sudden stratospheric warming
disrupted the evolution of the ozone hole, leading to the earlytermination of chemical ozone depletion and relatively high
TCO. In contrast, in 2020 and 2021, weak atmospheric wave

32

activity resulted in exceptionally persistent polar vortices.
Despite decreasing ODS concentrations, the unusual dynamical state of the stratosphere in 2020 and 2021 induced large
and long-lasting late spring ozone holes.
• Recovery of Antarctic stratospheric ozone continues toprogress. New results since the 2018 Assessment support
the findings reported at that time that the Antarctic ozone holehas diminished in size and depth since the year 2000. The remarkable Antarctic ozone holes in 2019, 2020, and 2021 do
not challenge the findings of the emergence of recovery.
• Arctic total ozone reached exceptionally low values
in spring 2020. A very stable, cold, and long-lived stratospheric polar vortex enabled halogen-catalyzed chemicalozone loss exceeding that observed during the previous record-breaking spring of 2011. The strong vortex also inhibited
dynamical replenishment of ozone. The evolution of high-latitude ozone in 2020 is successfully reproduced by model
simulations, further substantiating our understanding of polarozone chemistry.
• No statistically significant signature of recovery in Arctic
stratospheric ozone over the 2000–2021 period has yet
been detected. Observed Arctic ozone trends remain small
compared to the year-to-year dynamical variability.

Understanding of factors controlling polarozone• An updated vortex-wide climatology of polar stratospheric cloud (PSC) occurrence and composition basedon satellite data enabled advances in the understandingof particle formation mechanisms and trends. Evidence
that heterogeneous nucleation on preexisting ice particles
or foreign nuclei, such as meteoritic particles, is the typicalformation process for the nitric acid trihydrate (NAT) particles that lead to denitrification has been strengthened. PSC
occurrence in the Arctic early winter significantly increased
between the 1980s (1978–1989) and the recent past (2006–2018), while in the Antarctic, PSC occurrence was very similarin the two periods.
• The broad range of polar springtime TCO in recent years
in both hemispheres is largely explained by differencesin the magnitude of the dynamical forcing. Both the weak
Antarctic ozone hole in 2019 and the record-low Arctic ozone
in spring 2020 resulted from atypical dynamical conditions in
the respective winters. Although exceptional, the evolutionof polar ozone in both years was in line with current understanding of the chemical and dynamical factors controlling its
abundance.
• September, and especially the first half of that month,is the period when the impact of ODSs on stratospheric ozone over Antarctica can be quantified with theChapter Summaries

greatest certainty, and thus it represents the most suitable time window for monitoring ozone recovery. Until
recently, most studies of Antarctic ozone depletion trends
focused on longer time windows or later ones that included
the months of October and November. New analyses indicatethat September ozone has the largest sensitivity to decreasing ODSs, and September observations show the strongest
and the statistically most significant Antarctic ozone recovery
rates.

• The timing of the recovery of the ozone hole may beaffected by anthropogenic climate change, with the
MMM from updated CCM projections recovering approximately 15 years earlier for both SSP3-7.0 and
SSP5-8.5 GHG scenarios. This sensitivity of Antarctic returndate to different climate change scenarios was not evident inprojections presented in previous Assessments. The small setof CMIP6 models included in this Assessment makes interpretation of this scenario sensitivity difficult.

• Model simulations with historical emissions scenarios
indicate that decreasing atmospheric amounts of ODSs
can explain the observed increase in Antarctic springtime ozone over the last two decades. Model simulations
indicate that if ODS concentrations had remained at the peakvalues attained in the late 1990s, recent polar springtime
ozone loss in both hemispheres would have been ~20 DU(~10%) larger than currently observed. Model simulationsof unabated ODS emissions (i.e., allowing for a 3–3.5% yr –1
increase in emissions since the mid-1980s) indicate that conditions similar to those currently observed over Antarctica
would have occurred in the Arctic in years with unusually stable and long-lived stratospheric vortices, such as 2011 and2020.

• Arctic springtime total ozone is expected to returnto 1980 values near mid-century (about 2045, with arange between 2029 and 2051), based on full compliance with the Montreal Protocol and assuming thebaseline estimate of the future evolution of GHGs (SSP24.5). This return date is around a decade later than projectedby simulations in the previous Assessment using a different
set of models and scenarios, but with considerable overlapof the large range. The timing of the recovery of Arctic TCO
in spring will be affected by anthropogenic climate change.
Consistent with previous Assessments, the new model simulations confirm that in the Arctic, dynamical changes induced byenhanced GHG concentrations cause an earlier return of TCO
to historical values than do reductions in ODSs alone.

• Future commercial supersonic or hypersonic aircraft
fleets would cause stratospheric ozone depletion. Bothtypes of aircraft would potentially release substantial amountsof water vapor and nitrogen oxides (NOx) into the stratosphere, with concomitant strong effects on stratosphericozone arising primarily through enhancement of NOx catalyticozone destruction at cruise altitudes. This could reduce total
column ozone by as much as 10%, depending on aircraft typeand injection altitude, and would be most pronounced in the
Northern Hemisphere polar region in spring and fall.

• Future ozone depletion will be substantial in the Arctic
during cold winters/springs as long as ODS concentrations are well above natural levels. The projected strong
increase in GHGs will cause cooling in the stratosphere. Thiseffect, coupled with increases in stratospheric humidity from
GHG warming of the tropical tropopause and increases in future tropospheric CH4 emissions, will increase the potential
for formation of PSCs in Arctic winter, leading to ozone loss.

Future evolution of polar ozone• The Antarctic ozone hole is expected to gradually close.
September multi-model mean (MMM) TCO from updated CCM projections, based on full compliance with theMontreal Protocol and assuming the baseline estimateof the future evolution of GHGs (SSP2-4.5), returns to1980 values shortly after mid-century (about 2066,
with a range between 2049 and 2077, arising from the
spread in modeled dynamical variability). The October
TCO MMM returns two years earlier, with a similar uncertainty
range.

• Noncompliant production (e.g., of CFC-11) could delay
the recovery of ozone to 1980 values by several years
by slowing the rate of decline of stratospheric chlorine.
The magnitude of the delay depends on the total additionalemissions. Additional emissions of 120–440 Gg of CFC-11
over the period 2012–2019 are estimated to delay the return to 1980 levels for Antarctic column ozone by 0.5–3.1
years. Emissions of uncontrolled very short-lived substances(VSLSs; e.g., chloroform [CHCl3], dichloromethane [CH2Cl2])
could also extend the timeframe for polar ozone recovery
by the same mechanism, with the impact dependent on theamount of chlorine delivered to the stratosphere. The futuremagnitudes of emissions from noncompliant production andanthropogenic VSLSs are highly uncertain.

33

Chapter Summaries

Chapter 5:
Stratospheric Ozone Changes and Climate
Since the last Assessment, new research has continued toquantify, attribute and improve the understanding of long-termchanges in stratospheric climate. New studies are assessed thatquantify the effects of ozone-depleting substances and ozone
changes on the climate system, including atmospheric temperatures and circulation, the ocean and the cryosphere. The new results support the main conclusions from the previous Assessment.

Changes in stratospheric climate

continue to be opposite in sign from modeled
trends. However, new observationally based estimatessince the last Assessment bring observed trends closer
to modeled trends.
• Polar Vortex Trends and Variability: Recent extreme
polar vortex events in both hemispheres caused strong
variations of polar ozone. However, currently there is noevidence for a systematic trend toward more frequent
polar vortex disruptions in either hemisphere.

• Stratospheric Temperature: The global middle andupper stratosphere continues to cool at a rate of ~– 0.6
K decade–1 because of growing levels of well-mixed
greenhouse gases (GHGs; primarily carbon dioxide[CO2]) and evolving stratospheric ozone in response tochanging ozone-depleting substances (ODSs). Lowerstratospheric temperatures have been near constant since
the late 1990s. The overall evolution is consistent with thewell-understood effects of ozone, ODSs, GHGs, stratosphericaerosols, and solar variability. This is in agreement with previous Assessments.

º Two sudden stratospheric warming (SSW)7 events havebeen observed in the Southern Hemisphere (SH) since
the start of comprehensive satellite records in 1979.
New model studies show that this is consistent withmodel simulations, and no change in SSW frequency isnecessary to explain this occurrence rate. The delay of
the austral polar vortex breakup date, which in the past
was driven by ozone depletion, is not expected to fully
reverse by the end of the 21st century, due to the opposing effect of GHG increases under moderate and highemission scenarios.

• Stratospheric Water Vapor: Since the last Assessment,
the understanding of processes that influence water
vapor entry into the stratosphere has strengthened.
Interannual variations in lower-stratospheric water vapor are
quantitatively consistent with observed tropical tropopause
temperatures, with small contributions from monsoon circulations and overshooting convection. Models predict small
multi-decadal increases in tropopause temperature and lower-stratospheric water vapor as a response to GHG increases,
but these changes are still not evident within the variability ofthe observational records.

º In the Northern Hemisphere (NH), new studies confirm that changes in SSW frequency and in polar vortex
strength are not robustly detected in the historical record, and future changes are not robust across models.

• Brewer-Dobson Circulation6 (BDC):º The BDC in the lower stratosphere has accelerated
in recent decades and is predicted to continue toaccelerate in the future given continued increases
in GHG abundances. This result is confirmed by models, observations, and reanalyses. New studies since
the last Assessment confirm the attribution of the BDC
acceleration by models to increases in GHGs and ODSinduced ozone depletion over the last decades of the20th century. Model simulations indicate that the declineof ODSs and subsequent recovery of ozone should haveacted to reduce the rate of BDC acceleration after theyear 2000, but there is not yet sufficient analysis to determine whether this change has been detectable outsideof the natural variability in the BDC.
º Estimates of past BDC trends in the middle andupper stratosphere based on observations• Quasi-Biennial Oscillation (QBO)8: Since the last
Assessment, there is more confidence that the amplitude of the QBO will weaken in the future as a result ofacceleration of the BDC, but there is still large uncertainty
about any change in its periodicity and the associated ozone
variability.
º New model studies infer that further disruptions of the
QBO, such as occurred in 2016 and 2019, might become
more likely as a result of increasing GHGs.

Ozone and ODS effects on climate• Ozone and ODS Radiative Forcing (RF): New estimatesconfirm previous Assessments in that the RF from ODSs,
including the indirect effect on ozone abundances, hasbeen positive over the second half of the 20th century,
contributing to anthropogenic GHG forcing. The newest
best estimate of stratosphere-adjusted RF over the period
1850–2011 from stratospheric ozone changes is –0.02 W m–2,
with an uncertainty of ± 0.13 W m–2. The range in this RF remains smaller than the RF from ODSs (0.337 W m–2). However,new studies reveal uncertainties in the estimation of radiative
forcing, due to 1) rapid adjustments arising from troposphericcirculation changes and 2) uncertainties in modeled ozone

The global zonal mean circulation that transports mass, heat, and tracers in the stratosphere.
Based on an adapted SSW definition in the Southern Hemisphere; see Chapter 5, Section 5.2.6.1.
8
Quasi-periodic (period ~28 months) oscillation of stratospheric equatorial winds from easterly to westerly.
6734

Chapter Summaries
trends. Since the late 1990s, the RF from ODSs and changes instratospheric ozone abundances has remained approximately
constant as a consequence of the Montreal Protocol.

under high GHG emissions scenarios but little change oreven an equatorward shift of the jet in austral summer
under low GHG emissions scenarios.

• ODS Effects on Climate: There is new evidence since the last
Assessment that suggests that the direct radiative effects ofODSs on climate not only contributed to global warming but
also enhanced Arctic amplification9 in the late 20th century.

• Ozone-Induced Impacts on the SH Ocean and
Cryosphere:• Role of Stratospheric Ozone in the Climate Response to
GHG Forcing: Evidence suggests that GHG-induced ozone
changes act to dampen the GHG-induced surface temperature warming. New estimates since the last Assessment confirm that this climate feedback by stratospheric ozone is negative but smaller than previously estimated. In addition, thereis new evidence for an influence of stratospheric ozone on the
tropospheric and stratospheric circulation response to GHGs
via ozone-circulation coupling.
• Relevance of Stratospheric Ozone-Circulation Coupling
for Trends and Interannual Variability:º Two-way ozone-circulation coupling modulates theeffects of ozone depletion and recovery on SH stratospheric circulation trends, as well as stratospheric interannual variability in the tropics and extratropics in bothhemispheres.
º There have been no detectable effects of long-term
ODS-driven ozone trends in the Arctic on troposphericand surface climate. Yet, new evidence shows that forindividual years low springtime Arctic ozone can amplify existing stratospheric circulation anomalies and theirsubsequent influence on tropospheric circulation andsurface climate.
• Signature of Ozone Recovery
Hemisphere Circulation:

in

the

Southernº Antarctic ozone depletion led to pronounced
changes in the SH atmospheric circulation, as summarized in the previous Assessments. New evidence
suggests that the recovery of Antarctic ozone is now
evident as changes in SH atmospheric circulation trends
between the ozone depletion and recovery eras (the eras
before and after roughly the year 2000, respectively).
The observed changes in circulation trends are significant at stratospheric altitudes but on the fringe of significance in the troposphere; model simulations support the
hypothesis that the changes in atmospheric circulation
trends are driven by the onset of ozone recovery.
º Climate simulations suggest that in the future the effectsof ozone recovery will compete with the effects of GHG
increases on SH tropospheric circulation changes, resulting in a poleward shift of the mid-latitude jet in all seasons9º Ocean and Sea Ice: Observed upper Southern Ocean
warming and freshening since the 1950s is driven primarily by increasing GHGs. Stratospheric ozone depletion plays a secondary role in the warming. In agreementwith previous Assessments, ozone trends are unlikely tohave driven the observed high-latitude sea surface temperature cooling and weak sea ice changes since 1979.
Ocean eddies continue to remain a source of uncertainty
in the ocean’s response to wind changes.
º Carbon Uptake: The Southern Ocean carbon uptake
exhibits strong decadal variations. Ozone changes areunlikely to have substantially contributed to the observed
net change in Southern Ocean carbon uptake, consistent
with the conclusion from the previous Assessment.
º Antarctic Ice Sheet: New modeling evidence suggeststhat stratospheric ozone depletion could potentiallyhave influenced the surface mass balance of the Antarctic
ice sheet by enhancing precipitation over the continent
in the latter part of the 20th century. However, the underlying processes whereby stratospheric ozone depletion
influences continentwide precipitation are poorly constrained; further, observed Antarctic surface mass balance shows large variability.

Climate impacts of the Montreal Protocol• New evidence since the last Assessment shows that the
decline in ODS emissions due to the implementation of theMontreal Protocol has already had an influence on SH circulation trends due to the stabilization and slow recovery of the
Antarctic ozone hole, leading to a change in trends in the austral summer tropospheric circulation.
• Recent modeling studies estimate that the Montreal Protocol
has already resulted in the avoidance of 0.17 ± 0.06 K global
surface warming and 0.45 ± 0.23 K of Arctic surface warming
in 2020, and will likely avoid about 0.5–1 K (0.79 ± 0.24 K) of
global surface warming by the mid-21st century compared to a
scenario with uncontrolled ODS emissions.
• New evidence since the last Assessment suggests that theMontreal Protocol has also potentially avoided an additional
0.5–1.0 K globally averaged surface warming by the end of
the 21st century by protecting the terrestrial carbon sink fromultraviolet (UV) radiation damage, which would cause additional CO2 to remain in the atmosphere.

Arctic amplification refers to the ratio of Arctic warming (60–90°N) to global warming over a given time period.

35

Chapter Summaries

Chapter 6:
Stratospheric Aerosol Injection and
Its Potential Effect on the Stratospheric Ozone Layer
Since the 2018 Ozone Assessment global warming has continued, having now reached approximately 1.2 °C above preindustrial levels. All climate model scenarios considered by IPCC
(2021) indicate continued future warming beyond 1.5 °C above
the preindustrial level, a limit that has been proposed to prevent
further detrimental impacts. Ambitious mitigation and decarbonization efforts are required to minimize the likely overshootof temperatures above this limit and to stabilize global surface
temperatures in the future. However, with a temperature overshoot, irreversible impacts on the climate system may still occur.
Stratospheric aerosol injection (SAI) has been suggested as a potential mechanism for reflecting sunlight back to space, thereby
offsetting some of the surface warming. Evidence from explosive
volcanic eruptions and various model simulations has shownthat increasing stratospheric sulfate aerosols can substantiallycool the planet. SAI and other solar radiation modification (SRM)
approaches may therefore be the only option to keep the global
surface temperature below the limit of 1.5 °C. The amount andduration of SAI required would depend on how fast atmosphericgreenhouse gas (GHG) concentrations are lowered through mitigation and decarbonization efforts.
While SAI could reduce some of the impacts of global warming, it cannot restore past climatic conditions and would very
likely cause unintended consequences, including changes instratospheric ozone concentrations. To date, Earth system models (ESMs) have performed simulations to provide information on
the climate impacts, benefits, and risks of SAI. Little research has
been done to quantify the effects of SAI on the stratospheric composition and total column ozone (TCO) in a multi-model setting,and even fewer studies have examined the effects of aerosol
types other than sulfate. While existing studies do not suggest a
deepening of the ozone hole beyond that already experienced,
current shortcomings in model representation of required processes limit confidence in the results.
This new chapter of the Ozone Assessment assesses the impacts of SAI on stratospheric ozone through SAI-related changes
in stratospheric chemistry and transport. The dependence of SAI
effects on future climate change scenarios and injection strategies, as well as uncertainties in our current understanding andmodel shortcomings, are assessed. Side effects and risks beyond
the effects on stratospheric ozone are only briefly covered. It is
well recognized that any potential future deployment of SAI isfundamentally linked to complex moral, ethical, and governanceissues. These aspects are of critical importance but beyond thescope of this chapter, which will focus solely on physical science.

Framing SAI scenarios and strategies• Based on the observed cooling after large volcanic
eruptions and various model studies, stratosphericaerosol injection (SAI) has the potential to reduce global
mean temperatures. However, SAI cannot fully offset

36

the widespread effects of global warming and produces
unintended consequences, including effects on ozone.
Details of these effects depend on the specifics of the
SAI scenario and injection strategies. SAI uses stratospheric aerosols to reflect sunlight back to space, thereby coolingthe planet. A straightforward offsetting of global warmingfrom greenhouse gases (GHGs) cannot be achieved because
SAI reduces a fraction of the incoming sunlight, which isseasonally and latitudinally dependent, while GHGs interact
with terrestrial radiation and warm the planet more uniformly
across latitudes and seasons. In addition, aerosol heating of
the lowermost stratosphere by SAI using sulfate would resultin further residual impacts, including changes in regional temperatures, precipitation, and stratospheric ozone. Details of
the future climate scenario, the SAI scenario (i.e., the degreeof SAI cooling applied), and applied SAI strategy (i.e., thespecifics of injection location, timing, and material for achieving predefined climate goals) determine the specifics of theresulting impacts and risks.
º Changes in future ozone using SAI depend on details of future climate change and the degree of SAI
cooling applied. The three different SAI scenarios
considered in this report (Figure 6-2, reproducedhere) result in significantly different future ozone.
The “peakshaving” scenario (Panel A in Fig. 6-2) assumes
delayed and then aggressive mitigation and carbon dioxide removal (CDR). SAI offsets the overshoot of thesurface temperature target until greenhouse gases havebeen sufficiently reduced. The “strong SAI” scenario(Panel B) assumes a limited or no-mitigation high-warming future scenario, requiring continuously increasing
SAI to keep surface temperatures from exceeding theclimate target (dashed line). The “medium SAI” scenario(Panel C) assumes a limited or no-mitigation high-warming future scenario in which global warming is reduced
to that of a moderate mitigation scenario (red line) by
the deployment of SAI. A qualitative illustration of therequired injection amounts for each scenario is shown inPanel D. The impacts on ozone of many other possible
SAI scenarios have not been comprehensively studied todate. These scenarios currently do not include any socioeconomic feedbacks related to SAI.
º In model simulations, different injection strategies
have been developed to mitigate some of the unintended climate impacts of SAI. For the same scenario,
the specifics of the injection strategy, including location,
timing, and material, can be adjusted to better achieve
desired global and regional climate targets and minimize
regional changes. Some models include a feedback control algorithm to modulate annual stratospheric sulfur injections in order to reach predefined climate temperatureChapter Summaries

A) Peakshaving:B) Strong SAI:

Aggressive mitigation and CO2 removal (CDR) plus
SAI to prevent target temperature overshootReduction of high end global warming to a
specific target temperature

Limited/no mitigation:
high-end global warming

Aggressive mitigation
and CO2 removal (CDR):
low-end global warming

Assumedstabilization
temperature

Temperature offset
due to SAIGlobal surface temperature

Global surface temperature

Limited/no mitigation:
high-end global warming

Assumedstabilization
temperature

Temperature offset
due to SAIPeakshaving: SAI withaggressive mitigation and CDRTime in decades

Time in decadesC) Medium SAI:D) Injection amount of precursor
aerosol for SAI scenarios

Reduction of high end global warming tomoderate global warming

Strong SAI:High to low forcing

Assumedstabilization
temperature

Temperature offset
due to SAIConventional mitigation:
moderate global warming

Medium SAI:High tomoderate
forcing

Injection amountGlobal surface temperature

Limited/no mitigation:
high-end global warming

Peakshaving

0.0Time in decades

Figure 6-2. Schematic diagram representing the concept of three policy-relevant SAI scenarios: peakshaving
scenario, strong SAI scenario, and medium SAI scenario. Different lines illustrate global mean surface temperatures
for future scenarios: a limited or no
mitigation scenario leading to strong
future global warming (black line); a
so-called “overshoot scenario” thatassumes strong mitigation and Carbon
Dioxide Removal (CDR), and leads to atemporary overshoot of global mean
temperatures above sustainable limits
for some time (orange); a peakshaving
scenario that applies temporary SAI
to the overshoot scenario in order toprevent the increase in global mean
temperature from exceeding thesesustainable limits (purple line); and amoderate warming scenario (red). The
blue arrows represent the approximate
relative magnitude of the temperature
impact of the applied SAI. The bottom
right panel shows the stratosphericinjection that is applied under each ofthese three scenarios.

Time in decades

goals and other impact-relevant targets. Adjustments ofsulfur injection to account for climate feedback help inmanaging uncertainties and limiting some of the side effects of SAI. Different strategies change the effectivenessof SAI and its effects on stratospheric ozone.

SAI effects on radiation and temperature• Multi-model comparisons reveal large uncertainties inforcing and surface cooling per unit of sulfur injected,
which are attributed to differences in model complexity
in representing key processes and details of SAI strategies. Using sulfate aerosol, the efficacy of the radiative forcing ranges between –0.04 and –0.1 W m–2 per Tg SO2 yr –1,
and the resulting surface cooling ranges from 0.04 to 0.14 °C
per Tg SO2 yr –1 based on a multi-model analysis. Continuous
annual injection rates vary between 8 and 16 Tg of SO2 yr –1
to cool the Earth by 1 °C; this range is approximately equivalent to the estimated injection amount from Mount Pinatubo
in 1991, which resulted in less than 0.5 °C global surface
cooling. The significant uncertainties associated with thesevalues are attributed to differences in model representationsof stratospheric chemistry, transport, radiation, and aerosol
microphysical processes, including differences in model resolution. The choices of SAI injection location, timing, and material influence the resulting stratospheric aerosol mass, opticaldepth, and surface area density (SAD), which determine bothcooling efficacy and impacts on stratospheric ozone.

Mechanisms for SAI impacts on ozone• Despite the limited number of model studies, some robust impacts of SAI on ozone have been identified. The
combined effects of large-scale, long-term SAI on ozone
are driven by 1) an increase in aerosol surface area, 2)
stratospheric halogen concentrations, and 3) aerosol-induced heating of the stratosphere, which changes
both stratospheric ozone chemistry and stratospheric
dynamics. SAI impacts on total column ozone (TCO) are regionally and seasonally dependent and result in ozone reduction in spring over Antarctica due to the increase in chemicalozone depletion. In contrast, an increase in TCO is possible(with increasing SAI amount) in the tropics, as well as in thewinter Northern Hemisphere (NH) in mid- and high latitudes,
due to increased tropical chemical ozone production rates
and increased poleward transport.
º Enhanced stratospheric sulfate aerosol increases
stratospheric heterogeneous chemical reaction
rates and can enhance or deplete ozone dependingon the altitude, latitude, and season. Net chemical ozone production rates decrease in the lower polar

37

Chapter Summaries
stratosphere in winter and spring where halogen andhydrogen catalytic cycles are most important but increase in the tropical mid-stratosphere where the nitrogen cycle is most important. The magnitude and sign ofozone changes depend on the details of the SAI aerosol
distribution and the current stratospheric halogen andnitrous oxide concentrations, as well as on any changes
in stratospheric water vapor due to changes in transport
and temperature that occur in response to SAI.
º Enhanced stratospheric sulfate aerosol also impacts stratospheric temperature, transport, andchemistry, causing a general increase of ozone
concentrations in the tropics and mid- to high latitudes through enhanced transport from the tropics to high latitudes. Increased sulfate aerosols in SAI
scenarios heat the lower tropical stratosphere by 4.6 ±2.7 °C per ​​1 °C surface cooling, with variation acrossmodels and injection strategy. The heating induced byaerosols changes the vertical and horizontal transport in
the stratosphere and polar vortex dynamics and leads toan acceleration of the lower branch of Brewer-Dobson
Circulation (BDC). The stronger transport of ozone tohigh latitudes with SAI can overcompensate for theeffects of ozone depletion, especially in the Northern
Hemisphere winter in the strong SAI scenario. Heatingof the tropopause results in increases in stratosphericwater vapor. For any given scenario, the impacts of SAI
on stratospheric temperature, transport, and dynamics
are strongly model dependent.

SAI impacts on ozone in the future• Future changes in TCO resulting from SAI would be inaddition to changes driven by future climate conditions and stratospheric halogen burden, as describedin Chapters 3, 4, and 5. The SAI-related TCO changes
depend on the required SAI injection rate, which is different for the three defined SAI scenarios (Figure 6-2).
Compared to conditions without SAI, significant TCO
reductions are expected in October over Antarctica for
any SAI applications within the 21st century that are sufficient to appreciably impact climate warming.
º In October over Antarctica, aerosol injection rates
sufficient to achieve a 0.5 °C global cooling overthe period 2020–2040 result in a reduction of TCO
of around 58 ± 20 DU compared to no SAI. Smaller
initial injection rates to achieve cooling of 0.2 °C
between 2020 and 2040 result in a modeled reduction in TCO of 17 ± 9 DU. Large injection rates basedon the peakshaving and strong SAI scenarios starting in2020 bring TCO close to the minimum values observed
between 1990 and 2000, while smaller injection rates in
the medium SAI scenario lead to less TCO reduction. The
initial phase-in of SAI leads to relatively larger reductionsin TCO over Antarctica in spring compared to a casewithout SAI because of nonlinearities in microphysicalprocesses.
º In October over Antarctica, the magnitude of TCO
changes in the second half of the 21st century increase with increasing injection rates. Injection
rates and the resulting TCO reductions are scenario,

38

strategy, and model dependent. Under the strong
SAI scenario, with injections starting in 2020, model
simulations suggest that Antarctic TCO is reduced byaround 55 ± 20 DU in October throughout the 21st century and the ozone hole recovery is delayed between
25 and 50 years. In this case, the effect of continually
increasing injections is offset by the simultaneously declining chlorine burden in response to Montreal Protocol
provisions. SAI, therefore, counters some of the superrecovery of TCO above 1980 values driven by increasing
greenhouse gases. The medium SAI scenario results in asmaller TCO reduction of between 9 and 29 DU (basedon three models), and the peakshaving scenario results
in no significant ozone loss by 2100 due to SAI (based on
one model).
º In the Arctic in spring, SAI starting in 2020 toachieve global cooling of 0.5 °C by 2040 results in
TCO reductions between 13 DU ± 10 DU and 22 ± 21
DU compared to no SAI, with no significant changes after 2040, based on results from two different
models. The change in TCO for smaller initial injection rates is not significant. In the Arctic, chemicalchanges are in part offset by changes in dynamics, resulting in smaller SAI-induced changes of TCO compared to
Antarctica. As a result, SAI only slightly offsets the superrecovery of TCO in a high-GHG scenario. Modeled impacts on TCO in the Arctic under the medium SAI scenario are smaller and not significant. These results, which are
based on ensemble means of zonal and monthly mean
TCO comparisons, do not reflect possible larger regional ozone changes that may occur within the Arctic polar
vortex for years with warm and cold vortex conditions.
º In NH mid-latitudes in winter, increasing SAI toward the end of the century in both the strong andmedium SAI scenarios can lead to a significant TCO
increase relative to that in a scenario with no GHG
mitigation and without SAI. In both SAI scenarios,
the increased heating in the tropical lower stratospherecauses increased transport of ozone from the tropics tomid- and high latitudes, resulting in a greater increasein TCO with injection amount. SAI, therefore, enhances
the super recovery of TCO for a high-GHG scenario. No
significant TCO changes occur in NH mid-latitudes in thepeakshaving scenario.

Other side effects, risks, and limitations of SAI• Limited aerosol injections in a peakshaving scenario
minimize SAI-induced side effects and climate risks,
including reductions in global precipitation, while climate impacts and risks increase in scenarios with lessmitigation and more SAI. A portfolio of climate responses,
including effective mitigation and decarbonization, limits theamount of SAI needed to maintain the global surface temperature below specific targets. Since SAI offsets the warmingfrom atmospheric GHGs, limiting SAI would reduce the risks
associated with a potential abrupt termination of SAI. Such an
abrupt termination would result in a rapid (within 10 years) return of climate to the non-SAI climate base state if SAI was notrestarted. Other side effects induced by SAI, such as Eurasian
winter warming and associated precipitation impacts and aChapter Summaries
significant weakening of the Asian monsoon, depend on theamount of SAI. Ocean acidification depends mostly on atmospheric carbon dioxide (CO2) concentrations and is impacted
only to a small extent by SAI.

SAI using aerosols other than sulfates• The use of aerosols other than sulfate is expected
to change the effects on ozone via changes in heterogeneous chemistry and dynamics and transport.
Comprehensive climate model simulations to quantify
these effects have yet to be performed. Other aerosol
types that absorb less solar radiation would heat the tropicallower stratosphere much less than sulfate. They are also potentially more chemically inert and less impactful on stratosphericozone. Materials that have been considered include calcium
carbonate, titanium dioxide, aluminum oxide, and diamond.
The effects on ozone are less certain for these alternate materials owing to the paucity of laboratory and modeling studies
investigating them and the lack of natural analogs.

Evaluation of models• The study of SAI is aided by natural analogs. Volcanic
eruptions and pyrocumulonimbus events are useful,albeit imperfect, natural analogs for assessing SAI.
Present-day Earth system models may not accurately simulate
the effects of stratospheric aerosol perturbations on ozone
and other side effects. Remote sensing and in situ observations of volcanic eruptions and pyrocumulonimbus (pyroCb)
formation provide essential information on the stratosphericevolution of injected sulfur dioxide and resultant sulfate aerosol, which can be used to assess and improve SAI models.
However, remote and in situ observations valuable for evaluating the effects of injected aerosols on the ozone layer are
generally lacking. SAI scenarios with continuous aerosol (precursor) injections will produce different stratospheric aerosol
distributions than pulse injections that occur with natural analogs; therefore, accurately simulating these natural events isa necessary but not sufficient constraint on model fidelity inrepresenting SAI.

Chapter 7:
Scenarios and Information for Policymakers
In its evaluation of future scenarios, this chapter uses reduced
complexity models to calculate future impacts on ozone and climate. These models supplement the results from more complex
models discussed in Chapters 3–6, with the added advantage thatthe simpler framework allows exploration of a greater number ofscenarios and sensitivity experiments.

Post-Kigali Information of Interest• The Kigali Amendment to the Montreal Protocol, alongwith regional and national regulatory and voluntary actions taken before Kigali entered into force, is expected
to substantially limit future climate forcing by HFCs.
Assuming global compliance with the Kigali Amendment, it
is expected that HFCs will cause a peak radiative forcing ofabout 100 mW m–2 by mid-century. This may be comparedto some past projections of forcing absent the Kigali Amendment or regulation under another convention, the highest
being in excess of 400 mW m–2 in 2050, with substantial increases after that. Given the regional and national regulatory
and voluntary actions taken before Kigali entered into force,and assuming global adherence to the Kigali Amendment
to the Montreal Protocol, the contribution of HFCs to global
annual average warming is projected to be 0.04 °C in 2100
(Chapter 2), with a continued decline after that time.
• The elimination of all long-lived HFC emissions (including HFC-23) from 2023 onward represents an extreme

example of the potential opportunities for future HFC
reductions and would reduce the average radiative
forcing over 2023–2100 by 79 mW m–2, with additional
benefits continuing after 2100. This is more than twice thebenefit of eliminating all controlled ODS emissions from thebaseline scenario and would reduce the warming attributable
to all HFCs to less than 0.01 °C by 2100. Of the 79 mW m–2,
51 mW m–2 arises from future production and usage of longlived HFCs (excluding HFC-23), 16 mW m–2 comes from future
emissions from current banks, and 11 mW m–2 comes fromemissions of HFC-23.
• If emissions of HFC-23, a potent greenhouse gas, remain
at the current relative level compared with HCFC-22 production, HFC-23 has the potential to cause about half ofthe climate forcing (30 mW m–2) of all the other HFCs,
combined, by 2100. HFC-23 is emitted into the atmospheremainly as a by-product from the production of HCFC-22.
Its emissions relative to the amounts of HCFC-22 produced
have not changed much in recent years and are higher thanwould be expected if state-of-the-art destruction had been
performed during the HCFC-22 production process. While
the Kigali Amendment to the Montreal Protocol requiresthat HFC-23 be “destroyed to the extent practicable,” thisrequirement and the connected reporting of emissions went
into effect only on 1 January 2020, and thus reporting is still incomplete and the global response is unclear. Through 2019,39

Chapter Summaries
the emissions of HFC-23 as a fraction of HCFC-22 production
indicate that a considerable part of the produced HFC-23 wasstill being released unabated into the atmosphere.
• Other sources of HFC-23 emissions to the atmosphere
may exist and could contribute to its atmospheric burden. There could be contributions to HFC-23 abundances
through formation and loss during the production of tetrafluoroethene (TFE) and from the incineration of HCFC-22.
Furthermore, direct emissions could grow from the use of
HFC-23 in low-temperature refrigerants, although it is not the
only refrigerant used in this application.
• The Kigali Amendment’s control of high-GWP HFCs is
expected to lead to overwhelmingly positive climate
benefits. Nevertheless, there is a potential for certain
negative side effects. Hydrofluoroolefins (HFOs) are increasingly used for replacing high-global warming potential(GWP) HFCs in refrigeration, foam blowing, and various otherapplications. This replacement leads to less climate change.
However, high-volume usage of CCl4 (carbon tetrachloride) as
a feedstock in the production of HFOs, a usage and production not controlled by the Montreal Protocol, could lead tosustained elevated abundances of CCl4 if current techniques
are continued and some fraction of feedstock production continues to be emitted. A second side effect is that HFO-1234yf
emitted into the atmosphere will be fully converted to the stable trifluoroacetic acid (TFA; see below).
• Trifluoroacetic acid (TFA), which is produced in the atmosphere from the degradation of HFCs, HCFCs, HFOs,
and HCFOs, is not expected to harm the environment
over the next few decades, although some regional
concerns have been raised; periodic evaluation of thisassessment is suggested, as important gaps in our understanding remain. This assessment is based on updated
estimates of the TFA formation from current atmospheric concentrations of HFCs and HCFCs (hydrochlorofluorocarbons)and their projected decline, as well as the expected increasing abundance of HFOs as HFC and HCFC replacements
within the next years. With long-lived HFCs being replacedwith high-TFA-producing, short-lived HFOs, more TFA will beformed in the atmosphere. Because of the shorter lifetime of
HFOs, this TFA is expected to be deposited nearer to the location of emissions. Other anthropogenic sources of TFA, suchas the incineration of polytetrafluoroethene (PTFE), could also
contribute. In view of changing and potential unknown sources, concentrations of TFA should be monitored for changes
in different parts of the environment, with a special focus on
highly populated regions and on the remote ocean.

Updates on the Climate Impact of Gases Controlled by the Montreal Protocol• In the baseline scenario, future emissions of HFCs (excluding HFC-23), HFC-23, HCFCs, and CFCs contribute
approximately 68, 11, 9, and 9 mW m–2 to radiative
forcing, respectively, averaged over the 2023–2100
period. Of the 68 mW m–2 from HFCs, 51 mW m–2 arise fromfuture production. For reference, CO2 (carbon dioxide) emissions from fossil fuel usage over this time period are projected
to contribute an average of about 3250 mW m–2 in the SSP24.5 scenario. The total radiative forcing from CFCs, HCFCs,

40

and their HFC replacements is projected to continue to remain roughly constant for the next decade or two. After about
2040, the ODS and HFC restrictions of the Montreal Protocol,
if adhered to, are expected to ensure a continued decline
in the total RF from ODSs and their replacements. Previous
expected increases in RF driven by projected HFC increases
throughout the century are now mitigated by assumed compliance with the Kigali Amendment.
• The effective radiative forcing of the halocarbons has
been revised to encompass lower values due to a larger range of estimated negative forcing from the ozone
depletion they cause. This offset of the halocarbon direct
radiative forcing remains highly uncertain.

Ozone-Depleting Substances (ODSs) and
Their Replacements: Impacts on Ozone andClimateBelow, we discuss potential trajectories of equivalent effective stratospheric chlorine (EESC; a proxy for ozone depletion)and radiative forcing (a proxy for climate change) that result
from our current understanding of the emissions of individual
gases or groups of gases and the processes that lead to theseemissions. We reference these potential changes to the so-calledbaseline scenario, which should be considered a plausible future
pathway for these gases that is consistent with the controls of theMontreal Protocol. The specific assumptions made in the baseline scenario can be extremely important to the results. Notethat the combined impact of changing assumptions is not always
simply the addition of each of the changes. It is also importantto recognize that the return date of EESC to 1980 levels is quitesensitive to any change in the EESC concentration because of
the relatively small rate at which the EESC is projected to decline
around the middle of this century. While a change in the returndate to 1980 EESC levels measured in tenths of years or even a
few years cannot be discerned in the atmosphere, primarily due
to natural variability, this metric can be useful for comparing various alternative ODS scenarios.
It should also be noted that the EESC formalism adopted
here is the same one that was applied in Appendix 6C of the 2018
Assessment and reflects our improved scientific understanding of
EESC (see Section 7.3). This alters the time evolution of EESC anddates when EESC returns to 1980 levels when compared with theolder approach used in the main part of Chapter 6 of the 2018
Assessment, but it has little effect on the relative impacts of thevarious alternative future scenarios. If EESC comparisons are
made with the 2018 Assessment, it is most appropriate to compare to those found in Appendix 6C rather than Table 6-5 of that
Assessment.
• Changes in the current baseline scenario lead to a delayin the return of mid-latitude and polar EESC to 1980 levels by 4 years and 7 years, respectively, compared withthe baseline scenario in the previous Assessment. Thisis due mainly to a larger assessed CFC-11 bank, and toa lesser degree, to a larger assessed CFC-12 bank. The
larger bank for CFC-11 does not include any explicit increase
due to unreported production over the past decade, as thatamount is highly uncertain.
• The unexpected emissions of CFC-11 declined after
2018. The continued elimination of this emission andChapter Summaries
the production that has caused it will prevent a substantial impact on ozone and climate. Cumulative unexpected
emissions over 2012–2019 have been estimated at 120–440
Gg. Since then, these annual emissions have diminished substantially from their peak amount. The integrated emissions
over this period are calculated to lead to a delay in the returnof mid-latitude EESC to 1980 levels by about one year and tocause an additional radiative forcing of 2 mW m–2 averaged
over 2023–2100. It is unclear how much of the productionthat led to these emissions has gone into banks, as opposed
to having already been emitted. If the unexpected emissions
over 2012–2019 were associated with the production of insulating foams, it is estimated that they would have accounted
for 25% to 45% of the unreported production, with the rest
(146–1320 Gg) going into the CFC-11 bank. The impact of
any increase in the bank can be estimated from knowing thata hypothetical 1000 Gg added to the 2020 bank delays thereturn of mid-latitude EESC to 1980 levels by almost four years
and leads to an additional averaged radiative forcing over
2023–2100 of about 6 mW m–2.
• The hypothetical elimination of all future ODS emissions would bring forward the return of mid-latitude
and polar EESC to 1980 levels by 16 years and 19 years,
respectively, and increase the average of global stratospheric ozone levels in the period 2020–2070 by about
2 DU. It would also reduce average radiative forcing by 31
mW m–2 averaged over 2023–2100. These emissions are
dominated by the release from current banks, with a smaller
contribution from future production of ODSs that is controlled
by the Montreal Protocol and emissions associated with production intended for feedstock purposes. Estimates of banksizes are highly uncertain though; the bank approach used in
the scenarios here has resulted in substantially larger 2020
banks than estimated in the previous Assessment.
• In the baseline scenario, future emissions from current
CFC banks contribute more to EESC than do emissions
from either HCFC banks or halon banks. However, given
the uncertainty in estimates of current bank sizes, these differences are likely not statistically significant. An elimination of
the emissions from the CFC banks are calculated to bring forward the return of mid-latitude EESC to 1980 levels by about
5 years. In this chapter, there is no evaluation made regarding
the accessibility of various banks in terms of recapture anddestruction.
• In the baseline scenario, future emissions from current
HCFC banks contribute more to climate change than dofuture emissions from either CFC banks or halon banks.
However, the differences in the climate impacts between thebanks of HCFCs and CFCs are likely not statistically significant.
Again, there is no evaluation made regarding the accessibilityof various banks in terms of recapture and destruction.
• Elimination of future emissions of methyl bromide(CH3Br) from quarantine and preshipment (QPS) applications, not controlled by the Montreal Protocol,
would accelerate the return of mid-latitude and polar
EESC to 1980 levels by about two years and would increase globally averaged total ozone by 0.2 DU when
averaged over 2020–2070. Production for QPS use hasremained relatively stable over the last two decades and now
constitutes almost 99% of reported production of CH3Br,

since emissions from other uses have declined dramatically.
Non-QPS applications of CH3Br were completely phased out
in 2015, except for approved critical use exemptions (CUEs).
These CUEs have declined by a factor of ~200 since 2005 andmake up the remaining ~1% of reported production. CH3Br
has little direct impact on climate.
• Otherwise-controlled ODSs have increasingly beenused as feedstocks. With estimated emission rates of
2–4% (4.3% for CCl4) from the produced ODSs, thishas resulted in estimated emissions associated withODS feedstock applications of 37–59 Gg (15–19 ODPGg) in 2019. The influence on ozone of these emissions
was dominated by emissions from the feedstock useof CCl4. When compared to the baseline scenario, in
which these emissions continue at current levels, anelimination of emissions associated with feedstock use
would bring forward the return of mid-latitude andpolar EESC to 1980 levels by about 4 and 5 years, respectively. Between 2009 and 2019, the mass of ODSs usedas feedstocks, which is not controlled under the Protocol,
increased by 75%. When expressed in units of Gg ODP (Ggmultiplied by the ozone depletion potential), the increase in
feedstock-linked production was only 41% over the same period, as HCFC-22, with a relatively low ODP, was responsible
for the highest growth. Eliminating all these emissions in the
future would reduce averaged radiative forcing by 6 mW m–2
compared with the baseline scenario.
• Of the feedstock production reported, estimated emissions from CCl4 and HCFC production dominate theimpact on climate over the coming decades. These twogroups lead to an increased average radiative forcing
over 2023–2100 of 5 mW m–2 in the baseline scenario.
The size of this climate effect is dependent on the assumptions
made in the baseline scenario regarding feedstock production growth.
• CCl4 feedstock production and usage increased by afactor of about two within the last decade. If CCl4 emissions associated with these allowed uses continue togrow through 2030 as they have been growing overthe past decade, future CCl4 atmospheric concentrations will decline more slowly and will be about twice
as high (+20 ppt) in 2100 than in the baseline scenario,
in which feedstock-related emissions remain constant.
As reported in the 2018 Assessment, CCl4 emissions inferredfrom atmospheric observations continue to be considerably
higher than those estimated from feedstock uses, as reported
to the United Nations Environment Programme (UNEP), andother known sources. CCl4 emissions related to its feedstock
production and usage have been assessed to be 4.3% of the
produced amount of CCl4, with a relatively large associateduncertainty. Calculated as ODP-weighted emissions, theemissions from feedstock use of CCl4 in 2019 was 11.2 ODPGg yr –1, or 60–74% of all feedstock-related emissions. This isimportant, as the usage of CCl4 is projected to continue toincrease because of its application in the growing productionof HFOs in the replacement of the long-lived HFCs. An elimination of all future CCl4 emissions associated with feedstock
usage would reduce radiative forcing by about 2 mW m–2
compared with the baseline scenario when averaged over
2023–2100.

41

Chapter Summaries• In addition to CCl4, the most important contributions to
ODP-weighted emissions from other ODSs used as feedstock are from CFC-113 and CFC-114 (2.3–4.6 ODP-Gg),from HCFC-22 (0.5–1.1 ODP-Gg), and from the sum ofother HCFCs (0.1–0.3 ODP-Gg), with the highest contribution from HCFC-142b. These are based on estimatedemissions of 2–4% relative to the production amount. The
increased use of HCFC-22 and other HCFCs as feedstocks forfluoropolymer production within the last decades is expected
to continue into the future. On the other hand, the usage offeedstock chemicals for the production of HFCs will likely decline because of the Kigali Amendment.
• The production and usage of short-lived chlorinatedsolvents is not controlled by the Montreal Protocol, andsome are used in large amounts. Their impact on stratospheric ozone, and their ODPs, vary depending on theseason and location of emissions and could grow in the
future even as emissions from long-lived ODSs decline.
More than 1600 Gg of CHCl3 (chloroform) are used as feedstock in the production of HCFC-22. Emissions from CHCl3
used as a feedstock are comparable to its solvent emissions.
CH2Cl2 (dichloromethane), TCE (trichloroethene), and PCE(perchloroethene) are also used as feedstock chemicals, although their emissions are dominated by emissive uses (e.g.,
from solvents).
• Sustained increases in anthropogenic chlorinated very
short-lived substance (VSLS) emissions, as seen forCH2Cl2 over the last two decades, would lead to morestratospheric ozone depletion in the future. While observed growth rates of CH2Cl2 have been highly variable andfuture projections are believed to be highly uncertain, emissions have continued to increase since the last Assessment. If
emission rates remain constant at their present level into the
future, CH2Cl2 is projected to deplete 0.8–1.7 DU averaged
over 2020 to 2070 compared to a case of zero future emissions. Any reduction in the production and consumption ofCH2Cl2 would have a rapid impact on ozone, since this VSLS isboth emitted soon after production and is cleansed out of thestratosphere within a few years.
• A reduction in future N2O emissions from that in thebaseline scenario (SSP2-4.5) to that in the SSP scenario
with the strongest N2O mitigation (SSP1-1.9) results in a
0.5 DU increase in ozone averaged over 2020 to 2070,
or about one-quarter of the impact of eliminating all
emissions from controlled ODSs beginning in 2023. Thisemission reduction also leads to a radiative forcing reductionof 43 mW m–2 averaged over 2023–2100. The magnitude ofthis N2O reduction represents a decrease in anthropogenic
N2O emissions of 3% compared with the baseline scenario
when averaged over 2020 –2070.

Impacts of Mitigation Options and Particular
Scenarios
Figure 7.1 (also shown as Figure ES-8 in this document)shows the ozone and climate-relevant changes that would
occur if various actions were to be taken. These changes areshown as the differences in global total column ozone averaged over 2020–2070 and in radiative forcing averaged
over 2023–2100, both relative to the baseline scenario,

42

which includes the Kigali Amendment controls for HFCs in
Annex F, Group 1. The options available to hasten the recoveryof the ozone layer are somewhat limited, mostly because past actions have already been very successful at reducing emissions of
ODSs and their replacements.
• For the ODSs, the single most effective ozone depletion andclimate change mitigation option, not considering technicalfeasibility, is bank recapture and destruction of the CFC banks;
however, large uncertainties in the CFC-11 and CFC-12 banks
have been reported in the literature, with the recent production associated with the unexpected emissions of CFC-11 further adding to uncertainties in the bank sizes. Furthermore,no assessment has been made here regarding the fraction of
the banks that are accessible for capture or the fraction thatare active.
• For CH3Br, elimination of production for currently uncontrolled QPS applications is shown.
• For CCl4, the impact of eliminating emissions from controlled
production starting in 2023 is shown.
• For CH2Cl2, an uncontrolled ozone-depleting gas with an atmospheric lifetime of ~180 days, future emissions continue tohave the potential to lead to more ozone depletion than emissions from many of the other alternative scenarios explored
here. CH2Cl2 is emitted mainly from Asia, and emissions andconcentrations have been growing steadily in recent years.
• For N2O, the impacts of a strong mitigation scenario (SSP1-1.9)
are compared to the base-line scenario (SSP2-4.5).
• For HFCs, the impact of a hypothetical complete global
phaseout of production (excluding HFC-23) starting in 2023
is shown. An additional scenario is included in which HFC-23
emissions are reduced to virtually zero, consistent with thecurrent best practice of incineration, rather than the assumed
emissions rate of 1.6% of HCFC-22 production included in thebaseline scenario, in order to show the effect of nearly eliminating by-product emissions.

Updates on Impacts of Greenhouse Gases
and Other Processes on Future Stratospheric
Ozone
In this section, we summarize potentially important impacts
on the future of the ozone layer that could result from anthropogenic activity not associated with ODS or replacement production and consumption and thus that is not controlled by theMontreal Protocol. Net stratospheric cooling, which is projected
in many scenarios due to increases in greenhouse gas concentrations, is predicted to lead to increases in upper-stratosphericozone at all latitudes, with a more complex pattern of ozone
changes in the lower stratosphere, including a decrease at tropical latitudes driven by changes in dynamics and transport; theseprocesses are discussed in detail in Chapters 3 and 4. Potential
climate intervention activities that may affect ozone are discussed in Chapter 6.
• Our ability to accurately predict future changes in theozone layer continues to be limited more by uncertainties in the future levels of CO2, CH4 (methane), and N2O
than by uncertainties in the levels of ODSs. Global mean
tropospheric warming, as well as stratospheric cooling, willChapter Summaries
drive ozone changes through both atmospheric circulationand chemistry, while changing CH4 and N2O will lead to further changes in the chemistry associated with stratosphericozone. Future ozone levels depend on the path of greenhouse gas emissions and aerosol abundances, as well as thesensitivity of the climate system to these emissions.
• Rocket launches presently have a small effect on total
stratospheric ozone (much less than 0.1%). However,rocket systems using new propellants (e.g., hydrogenand methane) could exert a substantial influence in the
future. The future scenarios of space industry emissions consider the potential for a significant increase in launch rates,
the adoption of new launch-vehicle propellants, and an increase in middle-atmosphere aerosol and the production ofNO (nitrogen monoxide) by reentering space debris. Many of
the impacts of rocket activity involve chemistry and radiativeinteractions that are poorly understood and, in some cases,
not yet studied. Furthermore, the planned development of
massive low-Earth orbit satellite constellations (megaconstellations) could cause particulates resulting from space debris

reentry to become comparable to that from launch emissions;little is known about the impacts of reentry particles, and theiraccumulation in the stratosphere has not been modeled. The
uncertainties in these processes and in any potential new
emission sources limit the confidence level of predictions ofpresent and future impacts of space industry emissions on
stratospheric ozone. Periodic assessment and critical knowledge gap identification are warranted.
• The influence of hydrogen as an energy carrier on
stratospheric ozone remains uncertain. Hydrogen-basedenergy will likely play a role in a future non- or reduced-fossil
economy. However, if it is not a dominant energy carrier, it isunlikely that it will significantly affect ozone. This statement
should be reevaluated periodically.
• The impacts of supersonic aircraft on stratosphericozone are discussed in Chapter 4.
• Climate intervention approaches that affect the stratospheric ozone layer are discussed in Chapter 6.

43

Authors, Contributors, and Reviewers

AUTHORS, CONTRIBUTORS, AND REVIEWERS
Co-Chairs of the Scientific Assessment Panel (SAP) of the Montreal Protocol
and Assessment Co-Chairs
David W. Fahey
Paul A. Newman
John A. Pyle
Bonfils Safari

NOAA Chemical Sciences Laboratory
NASA Goddard Space Flight Center
University of Cambridge and the National Centre for Atmospheric Science (NCAS)
University of Rwanda, College of Science and Technology

USA
USA
UK
RwandaAssessment Scientific Steering Committee
David W. Fahey
Paul A. Newman
John A. Pyle
Bonfils Safari
Julie Arblaster
Lucy Carpenter
Jianxin Hu
Ken Jucks
David A. Plummer

NOAA Chemical Sciences Laboratory
NASA Goddard Space Flight Center
University of Cambridge and the National Centre for Atmospheric Science (NCAS)
University of Rwanda, College of Science and Technology
Monash University
University of YorkPeking University, College of Environmental Sciences and Engineering
NASA Headquarters
Environment and Climate Change Canada, Climate Research Division

USA
USA
UK
RwandaAustralia
UK
China
USA
CanadaAssessment Coordinator
Sarah J. Doherty

University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory

USA

Graphics and Layout CoordinatorChelsea R. Thompson

NOAA Chemical Sciences Laboratory

USA

Coordinating Lead Authors
Chapter 1: Update on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Johannes C. Laube
Susann Tegtmeier

Forschungszentrum Jülich, Institute for Energy and Climate Research: Stratosphere (IEK-7)
University of Saskatchewan, Institute of Space and Atmospheric Studies

Germany
CanadaChapter 2: Hydrofluorocarbons (HFCs)
Qing Liang
Matt Rigby

NASA Goddard Space Flight Center
University of Bristol, School of Chemistry

USA
UK

Chapter 3: Update on Global Ozone: Past, Present, and Future
Birgit Hassler
Paul J. Young

Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
Lancaster University

Germany
UK

Chapter 4: Polar Stratospheric Ozone: Past, Present, and Future
Martyn P. Chipperfield
Michelle L. Santee

University of Leeds
NASA Jet Propulsion Laboratory, California Institute of Technology

UK
USA

Chapter 5: Stratospheric Ozone Changes and Climate
Hella GarnyHarry Hendon

Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
Bureau of Meteorology and Monash University

Germany
Australia

Chapter 6: Stratospheric Aerosol Injection and Its Potential Effect on the Stratospheric Ozone Layer
James Haywood
Simone Tilmes

University of Exeter and Met Office Hadley Centre
National Center for Atmospheric Research (NCAR), Atmospheric Chemistry Observations & Modeling

UK
USA

Chapter 7: Scenarios and Information for Policymakers
John S. Daniel
Stefan Reimann

NOAA Chemical Sciences Laboratory
Swiss Federal Laboratories for Materials Science and Technology (Empa)

USA
Switzerland

Annex A: Summary of Abundances, Lifetimes, ODPs, REs, GWPs, and GTPs
James B.BurkholderØivind Hodnebrog

NOAA Chemical Sciences Laboratory
CICERO Center for International Climate Research

USA
NorwayTwenty Questions and Answers About the Ozone Layer: 2022 Update
Ross J. Salawitch

44

University of Maryland College Park

USA

Authors, Contributors, and Reviewers
Co-Authors
Chapter 1: Update on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Rafael Pedro Fernandez
Jeremy Harrison
Lei HuPaul Krummel
Emmanuel Mahieu
Sunyoung Park
Luke Western

Institute for Interdisciplinary Science (ICB), National Research Council (CONICET)National Centre for Earth Observation (NCEO)
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Global Monitoring Laboratory
Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere,
Climate Science Centre
University of Liège
Kyunpook National University, School of Earth System Sciences, Department of Oceanography
University of Bristol, School of Chemistry

Argentina
UK
USA
Australia
BelgiumSouth Korea
UK

Chapter 2: Hydrofluorocarbons (HFCs)
Xuekun Fang
Dave Godwin
Jens Mühle
Takuya Saito
Kieran Stanley
Guus J. M. Velders

Zhejiang University
U.S. Environmental Protection Agency, Stratospheric Protection Division
University of California San Diego, Scripps Institution of Oceanography
National Institute for Environmental Studies
Institute of Atmospheric and Environmental Sciences
National Institute for Public Health and the Environment (RIVM) & Utrecht University

China
USA
USA
Japan
Germany
Netherlands

Chapter 3: Update on Global Ozone: Past, Present, and Future
William T. Ball
Robert Damadeo
James Keeble
Elaine Maillard Barras
Viktoria F. Sofieva
Guang Zeng

Delft University of Technology, Department of Geoscience and Remote Sensing
NASA Langley Research Center
National Centre for Atmospheric Science (NCAS)
Federal Office of Meteorology and Climatology MeteoSwiss
Finnish Meteorological Institute (FMI)
National Institute of Water and Atmospheric Research (NIWA)

Netherlands
USA
UK
Switzerland
Finland
New Zealand

Chapter 4: Polar Stratospheric Ozone: Past, Present, and Future
Simon P. Alexander
A. T. J. de Laat
Doug E. Kinnison
Jayanarayanan Kuttippurath
Ulrike Langematz
Krzysztof Wargan

Australian Antarctic Division
Royal Netherlands Meteorological InstituteNational Center for Atmospheric Research (NCAR)
Centre for Oceans, Rivers, Atmosphere and Land Services (CORAL), Indian Institute ofTechnology Kharagpur
Freie Universtät Berlin
Science Systems and Applications Inc. (SSAI) at NASA Goddard Space Flight Center

Australia
Netherlands
USA
India
Germany
USA

Chapter 5: Stratospheric Ozone Changes and Climate
Marta Abalos
Gabriel Chiodo
Ariaan Purich
William J. Randel
Karen L. Smith
David Thompson

Universidad Complutense de Madrid, Department of Earth Physics and Astrophysics, Facultad
de CC. Fisicas
ETH Zürich, Institute for Atmospheric and Climate Science
ARC Centre of Excellence for Climate Extremes and Climate Change Research Centre
National Center for Atmospheric Research (NCAR)
University of Toronto Scarborough, Department of Physical and Environmental Sciences
Colorado State University

Spain
Switzerland
Australia
USA
CanadaUSA

Chapter 6: Stratospheric Aerosol Injection and Its Potential Effect on the Stratospheric Ozone Layer
Anthony JonesFrank Keutsch
Anton Laakso
Ulrike Niemeier
Anja Schmidt
Daniele Visioni
Pengfei Yu

Met Office
Harvard University, Department of Chemistry and Chemical Biology
Finnish Meteorological Institute (FMI), Atmospheric Research Centre of Eastern Finland
Max Planck Institute for Meteorology
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA); Ludwig
Maximilian University of Munich, Meteorological Institute; and University of Cambridge, Yusuf
Hamied Department of Chemistry
Cornell University, Sibley School of Mechanical and Aerospace Engineering
Jinan University, Institute for Environmental and Climate Research

UK
USA
FinlandGermany
GermanyUSA
China

Chapter 7: Scenarios and Information for Policymakers
Paul Ashford
Eric Fleming
Ryan Hossaini
Megan Lickley
Robyn Schofield
Helen Walter-Terrinoni

Anthesis Consulting Group
Science Systems and Applications, Inc. (SSAI) at NASA Goddard Space Flight Center
Lancaster University
Massachusetts Institute of Technology
The University of Melbourne
The Air Conditioning, Heating, and Refrigeration Institute (AHRI) and Technology and Economic
Assessment Panel (TEAP) of the Montreal Protocol

UK
USA
UK
USA
Australia
USA

Twenty Questions and Answers About the Ozone Layer: 2022 Update
Sarah J. Doherty
David W. Fahey
Eric Fleming
Laura McBride

University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory
Science Systems and Applications, Inc. (SSAI) at NASA Goddard Space Flight Center
University of Maryland, College Park

USA
USA
USA
USA

45

Authors, Contributors, and Reviewers
Richard McKenzie
Karen H. Rosenlof
Chelsea R. Thompson

National Institute of Water and Atmospheric Research (NIWA)NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory

New Zealand
USA
USA

Contributing Authors
Chapter 1: Update on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Elliot AtlasPeter BernathGeoff Dutton
Lucien Froidevaux
Ryan Hossaini
Timo Keber
Theodore K. Koenig
Stephen A. Montzka
Jens Mühle
Simon O’Doherty
David E. Oram
Klaus Pfeilsticker
Maxime Prignon
Birgit Quack
Matt Rigby
Meike Rotermund
Takuya Saito
Isobel J. Simpson
Dan SmaleMartin K. Vollmer
Dickon Young

University of Miami, Rosenstiel School of Marine and Atmospheric Science
Old Dominion University, Department of Chemistry & Biochemistry
NOAA Global Monitoring Laboratory
NASA Jet Propulsion Laboratory
Lancaster University
Institute for Atmospheric and Environmental Sciences
Peking University, College of Environmental Sciences and Engineering and State Key Joint Laboratory ofEnvironmental Simulation and Pollution Control
NOAA Global Monitoring Laboratory
University of California San Diego, Scripps Institution of Oceanography
University of Bristol, School of Chemistry
University of East Anglia, School of Environmental Sciences
University of Heidelberg, Institut für Umweltphysik
Chalmers University of Technology, Department of Earth, Space and Environment
GEOMAR, Helmholtz Centre for Ocean Research Kiel
University of Bristol, School of Chemistry
University of Heidelberg, Institut für Umweltphysik
National Institute for Environmental Studies
University of California Irvine, Department of Chemistry
National Institute of Water and Atmospheric Research (NIWA)
Swiss Federal Laboratories for Materials Science and Technology (Empa)
University of Bristol, School of Chemistry

USA
USA
USA
USA
UK
Germany
China
USA
USA
UK
UK
Germany
SwedenGermanyUK
GermanyJapan
USA
New Zealand
Switzerland
UK

Chapter 2: Hydrofluorocarbons (HFCs)
Peter BernathNada Derek
Vladimir Orkin
Stefan Reimann
Isobel J. Simpson
Luke Western

Old Dominion University, Department of Chemistry & Biochemistry
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
National Institute of Standards and Technology (NIST)
Swiss Federal Laboratories for Materials Science and Technology (Empa)
University of California Irvine, Department of Chemistry
University of Bristol, School of Chemistry

USA
Australia
USA
Switzerland
USA
UK

Chapter 3: Update on Global Ozone: Past, Present, and Future
Matt Amos
Niramson Azouz
Melanie Coldewey-Egbers
Lawrence Coy
Simone Dietmüller
Sandip S. Dhomse
Sophie Godin-Beekman
Daan Hubert
Mahesh Kovilakam
Paul A. Newman
Clara Orbe
Irina Petropavlovskikh
William J. Randel
Wolfgang Steinbrecht
Monika E. Szelag
Kleareti Tourpali
Corinne Vigouroux
Mark Weber

Lancaster University, Lancaster Environment Centre
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS)
Deutsches Zentrum für Luft und Raumfahrt (DLR), Remote Sensing Technology InstituteScience Systems and Applications, Inc. (SSAI) at NASA Goddard Space Flight Center
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
University of Leeds, School of Earth and Environment
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Centre National de laRecherche Scientifique (CNRS), Sorbonne University
Royal Belgian Institute for Space Aeronomy (BIRA-IASB)
Science Systems and Applications, Inc. (SSAI) at NASA Langley Research CenterNASA Goddard Space Flight CenterNASA Goddard Institute for Space Studies and NASA Goddard Space Flight Center
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Global Monitoring Laboratory
National Center for Atmospheric Research (NCAR)
Meteorological Observatory Hohenpeissenberg Deutscher Wetterdienst (DWD)
Finnish Meteorological Institute (FMI)
Aristotle University of Thessaloniki, Laboratory of Atmospheric Physics
Royal Belgian Institute for Space Aeronomy (BIRA-IASB)
Universität Bremen, Institute of Environmental Physics

UK
FranceGermanyUS
Germany
UK
FranceBelguim
USA
USA
USA
USA
USA
Germany
Finland
GreeceBelguim
Germany

Chapter 4: Polar Stratospheric Ozone: Past, Present, and Future
Sandip S. Dhomse
Jens-Uwe Grooß
James Keeble
Zachary D. Lawrence
Gloria L. Manney
Rolf Müller
Eric NashPaul A. Newman
David A. Plummer
Sarah Safieddine

46

University of Leeds
Forschungszentrum Jülich
University of Cambridge, Department of Chemistry
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Physical Sciences Laboratory and NorthWest Research Associates
NorthWest Research Associates and New Mexico Institute of Mining and Technology
Forschungszentrum Jülich
Science Systems and Applications, Inc. (SSAI)
NASA Goddard Space Flight Center
Environment and Climate Change Canada, Climate Research BranchLaboratoire Atmosphères, Observations Spatiales (LATMOS)/IPSL, Sorbonne Université, Université deVersailles Saint Quentin-en-Yvelines (UVSQ), Centre National de la Recherche Scientifique (CNRS)UK
Germany
UK
USA
USA
Germany
USA
USA
CanadaFranceAuthors, Contributors, and Reviewers
Ines TritscherPeter von der Gathen
Mark Weber
Ingo Wohltmann

Forschungszentrum Jülich
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research
Universität Bremen, Institute of Environmental Physics
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research

Germany
Germany
Germany
Germany

Chapter 5: Stratospheric Ozone Changes and Climate
James A. Anstey
Blanca Ayarzagüena
Antara Banerjee
Martyn P. ChipperfieldMartin Dameris
Ramiro Checa Garcia
Rishav Goyal
Paul A. Newman
Felix Plöger
Lorenzo Polvani
William J. Randel
Karen H. Rosenlof
Anja Schmidt
William Seviour
Keith Shine
Neil SwartPaul J. Young

Environment and Climate Change Canada, Canadian Centre for Climate Modelling and Analysis
Universidad Complutense de Madrid
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
University of Leeds
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
University of Vienna
University of New South Wales, Climate Change Research Centre
NASA Goddard Space Flight Center
Forschungszentrum JülichColumbia University, Department of Applied Physics and Applied Mathematics
National Center for Atmospheric Research (NCAR)NOAA Chemical Sciences Laboratory
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA); Ludwig
Maximilian University of Munich, Meteorological Institute; and University of Cambridge, Yusuf
Hamied Department of Chemistry
University of Exeter
University of Reading, Department of Meteorology
Environment and Climate Change Canada, Canadian Centre for Climate Modelling and Analysis
Lancaster University

CanadaSpain
USA
UK
Germany
Austria
Australia
USA
Germany
USA
USA
USA
Germany
UK
UK
CanadaUK

Chapter 6: Stratospheric Aerosol Injection and Its Potential Effect on the Stratospheric Ozone Layer
John Dykema
Anthony Jones
Anton Laakso
Catherine Anne Wilka

Harvard University, John A. Paulson School of Engineering and Applied Sciences
Met OfficeFinnish Meteorological Institute (FMI), Atmospheric Research Centre of Eastern Finland
Stanford University

USA
UK
Finland
USA

Chapter 7: Scenarios and Information for Policymakers
Laura McBride
Martin N. Ross
Sunyoung Park
Susann Tegtmeier
Ross J. Salawitch
David Sherry
Guus J. M. Velders

University of Maryland, College Park
The Aerospace Corporation
Kyungpook National University, School of Earth System Sciences, Department of Oceanography
University of Saskatchewan, Institute of Space and Atmospheric Studies
University of Maryland, College Park
NSA Ltd (Nolan Sherry & Associates)
National Institute for Public Health and the Environment (RIVM) & Utrecht University

USA
USA
South Korea
CanadaUSA
UK
Netherlands

Annex A: Summary of Abundances, Lifetimes, ODPs, REs, GWPs, and GTPs
Brian McDonald
Vladimir Orkin
Vassilis C. Papadimitriou
Daniel Van Hoomissen

NOAA Chemical Sciences Laboratory
NIST Chemical Science and Technology Laboratory
University of Crete, Department of Chemistry and University of Colorado, Cooperative Institutefor Research in Environmental Sciences (CIRES)
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory

USA
USA
GreeceUSA

Review Editors
Chapter 1: Update on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Andreas Engel
Bo YaoInstitute for Atmospheric and Environmental Sciences, University of Frankfurt
Fudan University

Germany
China

Chapter 2: Hydrofluorocarbons (HFCs)
Stephen A. Montzka
Martin K. Vollmer

NOAA Global Monitoring Laboratory
Swiss Federal Laboratories for Materials Science and Technology (Empa)

USA
Switzerland

Chapter 3: Update on Global Ozone: Past, Present, and Future
Jessica Neu
Wolfgang Steinbrecht

NASA Jet Propulsion Laboratory, California Institute of Technology
Meteorological Observatory Hohenpeissenberg Deutscher Wetterdienst (DWD)

USA
Germany

Chapter 4: Polar Stratospheric Ozone: Past, Present, and Future
Susan Solomon
Mark Weber

Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences
Universität Bremen, Institute of Environmental Physics

USA
Germany

Chapter 5: Stratospheric Ozone Changes and Climate
Amy H. Butler
Amanda Maycock

NOAA Chemical Sciences Laboratory
University of Leeds, School of Earth and Environment

USA
UK

47

Authors, Contributors, and Reviewers
Chapter 6: Stratospheric Aerosol Injection and Its Potential Effect on the Stratospheric Ozone Layer
Valentina Aquila
Karen H. Rosenlof

American University, Department of Environmental Science
NOAA Chemical Sciences Laboratory

USA
USA

Chapter 7: Scenarios and Information for Policymakers
Lambert Kuijpers
Donald J. Wuebbles

A/gent b.v. Environmental Consultancy
University of Illinois, Department of Atmospheric Sciences

Netherlands
USA

Reviewers
Hideharu Akiyoshi
Mads P. Sulbaek Andersen
Stephen O. Andersen
Tim Arnold
Alkiviadis F. Bais
Govindasamy Bala
Antara Banerjee
Tina Birmpili
Adam Bourassa
Peter Braesicke
Amy H. Butler
Natalia CalvoLong Cao
Martin Dameris
Sean Davis
Anne Douglass
Vitali Fioletov
Paul Fraser
Lucien Froidevaux
Jan Fuglestvedt
Rolando Garcia
Chaim Garfinkel
Nathan P. Gillett
Sophie Godin-Beekman
William Goetzler
Neil HarrisPeter Hitchcock
Peter Irvine
Martin Jucker
David Karoly
Alexey Yu. Karpechko
Jooil Kim
Andrew Klekociuk
Ben Kravitz
Lambert Kuijpers
Susan Gabriela Lakkis
Jean-Francois LamarqueSunday Leonard
Eun-pa Lim
Jintai Lin
Pu LinNathaniel Livesey
Diego Loyola
Doug MacMartin
Sasha Madronich
Michela Maione
Archie McCulloch
Sophia Mylona
Hiroaki Naoe
Simon O’Doherty
David OramMarisol Osman
Prabir Patra
Andrea Pazmiño
Thomas Peter
Damaris Kirsch Pinheiro
Marta Pizano

48

National Institute for Environmental Studies (NIES)
California State University, Northridge
Institute for Governance and Sustainable Development (IGSD)
National Physical Laboratory and University of Edinburgh
Aristotle University of Thessaloniki, School of Physics, Laboratory of Atmospheric Physics
Indian Institute of Science, Center for Atmospheric and Oceanic Sciences
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
UNEP Multilateral Fund Secretariat for the Implementation of the Montreal Protocol
University of Saskatchewan, Institute of Space and Atmospheric Studies
Karlsruher Institut für Technologie (KIT)NOAA Chemical Sciences Laboratory
Universidad Complutense de Madrid, Department of Earth Physics and Astrophysics
Zhejiang University, Department of Atmospheric Sciences, School of Earth Sciences
Deutsches Zentrum für Luft und Raumfahrt (DLR)
NOAA Chemical Sciences Laboratory
NASA Goddard Space Flight Center (emeritus)
Environment and Climate Change Canada
Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and AtmosphereNASA Jet Propulsion Laboratory, California Institute of Technology
Centre for International Climate and Environmental Research (CICERO)
National Center for Atmospheric Research (NCAR)
Hebrew University of Jerusalem
Environment and Climate Change Canada
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Centre National de laRecherche Scientifique (CNRS), Sorbonne University
Guidehouse
Cranfield University
Cornell University, Department of Earth and Atmospheric Sciences
University College London, Department of Earth Sciences
Climate Change Research Centre, , University of New South Wales
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Finnish Meteorological Institute (FMI)
University of California San Diego, Scripps Institution of Oceanography
Australian Antarctic Division
Indiana University
A/gent b.v. Environmental Consultancy
Pontificia Universidad Católica Argentina, Facultad de Ingeniería y Ciencias Agrarias and Universidad
Tecnológica Nacional, Facultad Regional Buenos
National Center for Atmospheric Research (NCAR)
UN Environment Programme (UNEP) Scientific & Technical Advisory Panel to the Global Environment
Facility (STAP-GEF),
Bureau of Meteorology
Peking University, School of Physics, Department of Atmospheric and Oceanic Sciences
Princeton University, Program in Atmospheric and Oceanic Sciences and NOAA Geophysical Fluid
Dynamics Laboratory
NASA Jet Propulsion Laboratory, California Institute of Technology
Deutsches Zentrum für Luft und Raumfahrt (DLR)
Cornell University
National Center for Atmospheric Research (NCAR)
University of Urbino, Department of Pure and Applied Sciences and National Research Council of Italy,
Institute for Climate and Atmospheric Sciences
University of Bristol, Atmospheric Chemistry Research Group (retired)
UN Environment Programme (UNEP) Ozone Secretariat
Meteorological Research Institute (MRI)
University of Bristol
University of East Anglia
Karlsruher Institut für Technologie (KIT)Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Centre National de la Recherche
Scientifique (CNRS), Université de Versailles Staint Quentin-en-Yvelines (UVSQ)ETH Zürich
Federal Univeristy of Santa Maria, Chemical Engineering Department, Center of Technology
Consultant

Japan
USA
USA
UK
GreeceIndia
USA
CanadaCanadaGermany
USA
Spain
China
Germany
USA
USA
CanadaAustralia
USA
NorwayUSA
IsraelCanadaFrance
USA
USA
USA
UK
Australia
Australia
Finland
USA
Australia
USA
Netherlands
Argentina
USA
Kenya
Australia
China
USA
USA
Germany
USA
USA
Italy
UK
Kenya
Japan
UK
UK
Germany
Japan
FranceSwitzerland
BrazilColombia

Authors, Contributors, and Reviewers
Felix Plöger
Michael Prather
A.R. Ravishankara
Claire Reeves
Laura Revell
Rob Rhew
Jadwiga Richter
Harald Rieder
Martin Riese
Alan Robock
Alfonso Saiz-Lopez
Nihar ShahJonathan Shanklin
Keith Shine
Michael Sigmond
Björn-Martin Sinnuber
Keith Solomon
Gabi Stiller
Kane Stone
Susan Strahan
William Sturges
Matt Tully
Jean-Paul Vernier
Timothy Wallington
Shingo Watanabe
Ray WeissDavid Wilmouth
Shigeo Yoden
Durwood Zaelke
Yanli Zhang

Forschungszentrum Jülich
University of California Irvine, Earth System Science Department
Colorado State University
University of East Anglia (emeritus)
University of Canterbury
University of California, Berkeley
National Center for Atmospheric Research (NCAR)
University of Natural Resources and Life Sciences, Vienna
Forschungszentrum Jülich
Rutgers University
Spanish National Research Council, Instituto de Química Física Rocasolano, CSIC
Lawrence Berkeley National Laboratory
British Antarctic Survey (BAS) (Emeritus Fellow)
University of Reading, Department of Meteorology
Environment and Climate Change Canada, Canadian Centre for Climate Modelling and Analysis
Karlsruher Institut für Technologie (KIT)
University of Guelph, Center for Toxicology
Karlsruher Institut für Technologie (KIT), IMK-ASF
Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences
University of Maryland, Baltimore County and NASA Goddard Space Flight Center
University of East Anglia
Bureau of Meteorology
National Institute of Aerospace and NASA Langley Research Center
Ford Motor Company, Research & Advanced Engineering
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
University of California San Diego, Scripps Institution of Oceanography
Harvard University
Kyoto University
Institute for Governance and Sustainable Development (IGSD)
Chinese Academy of Science (CAS), Guangzhou Institute of Geochemistry (GIG)

Germany
USA
USA
UK
New Zealand
USA
USA
Austria
Germany
USA
Spain
USA
UK
UK
CanadaGermany
CanadaGermany
USA
USA
UK
Australia
USA
USA
Japan
USA
USA
Japan
USA
China

Editorial TeamSarah J. Doherty
Chelsea R. Thompson
Jacquelyn Crossman
Mark Essig
Thomas K. Maycock
Andrea L. McCarrick
Brooke C. Stewart

University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory
MPF-ZAI
North Carolina State University, Cooperative Institute for Satellite Earth System Studies
North Carolina State University, Cooperative Institute for Satellite Earth System Studies
North Carolina State University, Cooperative Institute for Satellite Earth System Studies
North Carolina State University, Cooperative Institute for Satellite Earth System Studies

USA
USA
USA
USA
USA
USA
USA

Administrative and Technical Support
Catherine Burgdorf Rasco
Ronda Knott
Douglas Ohlhorst
Albert Romero
Kathy A. Thompson
Richard Tisinai
Catherine Weable

University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory
NOAA Office of the Chief Administrative Officer
Science Systems and Applications, Inc. (SSAI)
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
NOAA Chemical Sciences Laboratory

USA
USA
USA
USA
USA
USA
USA

49

Authors, Contributors, and Reviewers

Final Author Meeting

Geneva, Switzerland, 25 – 29 July 2022
Stephen O. Andersen
Valentina Aquila
Julie Arblaster
Tina Birmpili
Peter Brasicke
Amy H. Butler
Lucy Carpenter
Gabriel Chiodo
Martyn P. Chipperfield
Robert Damadeo
John S. Daniel
Sarah J. Doherty
Andreas EngelDavid W. Fahey
Vitali Fioletov
Paul Fraser
Hella Garney
Sophie Godin-Beekman
Marco Gonzales
Neil Harris
Birgit Hassler
James Haywood
Harry Hendon
Jianxin Hu
Ken Jucks
Doug E. Kinnison
Ronda Knott
Lambert Kuijpers
Johannes C. Laube
Qing Liang
Megan Lickley
Bella Maranion
Amanda Maycock
Stephen A. Montzka
Rolf Mueller
Jens Mühle
Sophia Mylona
Stoyka Netcheva
Jessica Neu
Paul A. Newman
Simon O’Doherty
David A. Plummer
Michael Prather
John A. Pyle
William J. Randel
A. R. Ravishankara
Stefan Reimann
Matt Rigby
Karen H. Rosenlof
Bonfils Safari
Ross J. Salawitch
Michelle L. Santee

50

Institute for Governance & Sustainable Development (IGSD)
American University, Department of Environmental Science
Monash University
UNEP Multilateral Fund Secretariat for the Implementation of the Montreal Protocol
Karlsruher Institut für Technologie (KIT)NOAA Chemical Sciences Laboratory
University of YorkETH Zürich, Institute for Atmospheric and Climate Science
University of Leeds
NASA Langley Research Center
NOAA Chemical Sciences Laboratory
University of Colorado, Cooperative Institute for Research in Environmental Sciences (CIRES)
at NOAA Chemical Sciences Laboratory
University of Frankfurt, Institute for Atmospheric and Environmental Sciences
NOAA Chemical Sciences Laboratory
Environment and Climate Change Canada (ECCC)
Climate Science Centre, Commonwealth Scientific and Industrial Research Organization (CSIRO)
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Centre National de laRecherche Scientifique (CNRS), Sorbonne University
United Nations Environment Programme (UNEP) Ozone Secretariat (retired)
Cranfield University
Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA)
University of Exeter and Met Office Hadley Centre
Bureau of Meteorology and Monash University
Peking University, College of Environmental Sciences and Engineering
NASA Headquarters
National Center for Atmospheric Research (NCAR)NOAA Chemical Sciences Laboratory
A/gent b.v. Environmental Consultancy
Forschungszentrum Jülich, Institute for Energy and Climate Research: Stratosphere (IEK-7)
NASA Goddard Space Flight Center
Massachusetts Institute of Technology
U.S. Environmental Protection Agency (EPA) and Technology and Economic Assessment Panel (TEAP)of the Montreal Protocol
University of Leeds, School of Earth and Environment
NOAA Global Monitoring Laboratory
Forschungszentrum Jülich
University of California San Diego, Scripps Institution of Oceanography
United Nations Environment Programme (UNEP) Ozone Secretariat
World Meteorological Organization (WMO)
NASA Jet Propulsion Laboratory, California Institute of Technology
NASA Goddard Space Flight Center
University of Bristol
Environment and Climate Change Canada, Climate Research Division
University of California Irvine, Earth System Science Department
University of Cambridge and the National Centre for Atmospheric Science (NCAS)
National Center for Atmospheric Research (NCAR)
Colorado State University
Swiss Federal Laboratories for Materials Science and Technology (Empa)
University of Bristol, School of Chemistry
NOAA Chemical Sciences Laboratory
University of Rwanda, College of Science and Technology
University of Maryland College Park
NASA Jet Propulsion Laboratory, California Institute of Technology

USA
USA
Australia
CanadaGermany
USA
UK
Switzerland
UK
USA
USA
USA
Germany
USA
CanadaAustralia
Germany
FranceCosta Rica
UK
Germany
UK
Australia
China
USA
USA
USA
Netherlands
Germany
USA
USA
USA
UK
USA
GermanyUSA
Kenya
Switzerland
USA
USA
UK
Canada
USA
UK
USA
USA
Switzerland
UK
USA
RwandaUSA
USA

Authors, Contributors, and Reviewers
Megumi Seki
Keith Shine
Susan Solomon
Wolfgang Steinbrecht
Susan Strahan
Susann Tegtmeier
Chelsea R. Thompson
Kathy A. Thompson
Simone Tilmes
Matt Tully
Guus J. M. Velders
Daniele Visioni
Martin Vollmer
Helen Walter-Terrinoni
Catherine Weable
Mark Weber
Ray Weiss
Luke Western
Donald J. Wuebbles
Bo YaoPaul J. Young

United Nations Environment Programme (UNEP) Ozone Secretariat
University of Reading, Department of Meteorology
Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences
Meteorological Observatory Hohenpeissenberg Deutscher Wetterdienst (DWD)
University of Maryland, Baltimore County
University of Saskatchewan, Institute of Space and Atmospheric Studies
NOAA Chemical Sciences Laboratory
Science Systems and Applications, Inc. (SSAI)
National Center for Atmospheric Research (NCAR), Atmospheric Chemistry Observations & Modeling
Bureau of Meterology
National Institute for Public Health and the Environment (RIVM) & Utrecht University
Cornell University, Sibley School of Mechanical and Aerospace Engineering
Swiss Federal Laboratories for Materials Science and Technology (Empa)
The Air Conditioning, Heating, and Refrigeration Institute (AHRI) and Technology and Economic
Assessment Panel (TEAP) of the Montreal Protocol
NOAA Chemical Sciences Laboratory
Universität Bremen, Institute of Environmental Physics
University of California San Diego, Scripps Institution of Oceanography
University of Bristol
University of Illinois, Department of Atmospheric Sciences
Fudan University
Lancaster University

Kenya
UK
USA
Germany
USA
CanadaUSA
USA
USA
Australia
Netherlands
USA
Switzerland
USA
USA
Germany
USA
UK
USA
China
UK

Highlights of the Executive Summary
Tina Birmpili
Neil Harris
Karen H. Rosenlof
Susan Solomon
Donald J. Wuebbles

UNEP Multilateral Fund Secretariat for the Implementation of the Montreal Protocol
Cranfield University
NOAA Chemical Sciences Laboratory
Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences
University of Illinois, Department of Atmospheric Sciences

Canada
UK
USA
USA
USA

51