Scientific Assessment of Ozone Depletion 2018: Overview

Motivation and scope

The Montreal Protocol (MP) Scientific Assessment Panel (SAP) is charged with providing scientific information for the Montreal Protocol on Substances that Deplete the Ozone Layer. This information provides the scientific basis for decisions made by the Parties. A major element of this responsibility is the quadrennial Scientific Assessment of Ozone Depletion – mandated under the MP's Article 6. The SAP Co-Chairs are responsible for leading this report. This discussion paper outlines the planning for the 2018 assessment, the eighth since 1991.

The 2018 assessment terms of reference were provided by the Parties at the 27th Meeting of the Parties in Dubai in 2015. The terms, included as an attachment to this document, are broadly defined under Article 3 of the Vienna Convention which is concerned with ozone depletion and the role of ozone-depleting substances (ODSs). Science topics will also include updates of those found in previous assessments; for example, ODSs and other trace gases, global and polar ozone, stratospheric temperatures and circulation, ultraviolet radiation, the interrelation between climate changes and global ozone, and the policy implications of Montreal Protocol decisions.

An amendment to the Montreal Protocol to regulate the production and consumption of hydrofluorocarbons (HFCs) was adopted at the 28th Meeting of Parties in Kigali, Rwanda, on 15 October 2016. The Kigali amendment will avoid large emissions of HFCs over the coming decades as the developing world advances and expands its use of various compounds in its refrigeration, air conditioning, foam, and other economic sectors. HFCs are ODS substitute compounds that only weakly impact ozone, but could contribute substantially to anthropogenic climate forcing. The amendment increases the importance of the SAP examining current and projected emissions of HFCs and their impacts in the new assessment.

Assessment documents

Current plans include the production of three documents:

  • Core assessment. The core assessment comprises separate chapters on selected topics as noted above. The chapter content would focus on the most important developments since the 2014 assessment and avoid reviewing the topic or providing a tutorial. Each chapter will begin with a bulleted chapter summary and include ample references to the scientific literature and a number of figures. This approach closely follows that of previous assessments. The core assessment length is expected to be in the range 300 to 500 pages.
  • Executive Summary. The Executive Summary begins by drawing from the chapter summary bullets to create a comprehensive narrative of high-level messages in language suitable for the Parties. This section is followed by more scientific details on the high-level topics and a section on policy implications. Finally, the chapter summary bullets are reprinted for ease of reference. The Executive Summary will adopt features found in the 2014 Assessment for Decision Makers, such as graphical illustrations, tables and sidebars, to help communicate needed information for the Parties and other interested readers
  • Twenty Questions and Answers on Ozone Layer Depletion booklet. The 20 Questions booklet explains the ozone depletion story in a question/answer format ranging from 'What is ozone and where is it in the atmosphere?' to 'Is depletion of the ozone layer the principal cause of global climate change?' The answers are crafted with language suitable for policy makers, educators, students and the interested public and illustrated with simplified graphics. The first booklet was produced for the 2002 assessment. We anticipate that the 2018 edition will be a limited update of the 2014 edition, involving addition of data to time-dependent graphics and revisions or additions to the text to reflect new knowledge or understanding since the 2014 assessment.

All of the assessment documents – core assessment, Executive Summary, and 20 Questions booklet – will be printed as separate documents and together in one document. Quantities will be limited and distributed as requested to the assessment authors and reviewers, the Parties, other interested scientists, and those in non-governmental organizations (NGOs), etc. In addition, all assessment content will be available on the web at one or more URLs along with graphics in downloadable forms. The web distribution follows the approach taken for the last several assessments. In 2014, the core assessment was only in electronic form (UNEP/WMO Scientific Assessment of Ozone Depletion: 2014), but based upon feedback from the community the entire 2018 assessment will appear in printed form.

Chapter Topics

The chapter structure for the 2018 assessment contains six chapters, listed with a brief suggested synopsis:

  • Chapter 1. Ozone-Depleting Substances
    Focus: Long-lived and short-lived substances that deplete the ozone layer (e.g., CFCs, HCFCs, methyl bromide) and other molecules of special interest to the Parties (e.g., N2O, other substitutes, very short-lived substances). Sections would address ODS lifetimes, trends, and budgets based on observations and models. Updated information on the carbon tetrachloride budget would be included from the recent SPARC report.
  • Chapter 2. Hydrofluorocarbons (HFCs)
    Focus: The lifetimes, trends, and budgets of leading HFCs based on observations and models and scenarios of future consumption, emissions, and radiative forcing. Of special interest are the provisions of the recent agreement to phasedown HFC production and consumption under the Montreal Protocol.
  • Chapter 3. Global stratospheric ozone: Past, present, and future
    Focus: Examining the trends in global ozone and future projections taking into account climate change, greenhouse gas increases, aerosols, dynamical changes, the solar cycle, and other non- halogen effects within the vertical distribution of ozone. Attribution of past ozone changes and evidence of recovery are included in this chapter through the analysis of a range of model simulations (3D and climate models).
  • Chapter 4. Polar stratospheric ozone: Past, present, and future
    Focus: The status of ozone layer depletion in both the Arctic and Antarctic winter/spring seasons, building upon recent observations, field studies, and theory. Topics of interest are response of the Antarctic ozone hole to reductions in stratospheric chlorine loading and future greenhouse gas increases and climate change; and the influence of temperature changes and tropospheric composition on future Arctic ozone depletion.
  • Chapter 5. Stratospheric ozone changes and climate
    Focus: The influence of stratospheric changes (ozone, temperature, circulation, etc.) on surface climate and tropospheric chemistry, and the influence of climate change and tropospheric composition changes (observed and projected) on the ozone layer. Topics also include stratospheric water vapor, stratospheric aerosols and sulfur species, and temperature feedbacks to both chemistry and dynamics.
  • Chapter 6. Scenarios and Information for Policymakers
    Focus: Projected future scenarios of effective equivalent stratospheric chlorine (EESC) and HFCs. Future scenarios would be developed (via interaction with the Technology and Economic Assessment Panel) for emissions of controlled substances expected under the Protocol and other assumptions.
  • Other topics
    Potential influences on stratospheric ozone such as volcanoes, rockets and aircraft, geoengineering, and tropospheric composition changes (including biogenic emissions) are topics of interest that could be addressed as sections in one or more chapters.


  1. Assessment Team

    Co-Chairs David Fahey (USA), Paul Newman (USA), John Pyle (UK), and Bonfils Safari (Rwanda)

    Assessment Coordinator Sarah Doherty (USA)

    Steering Committee (see Organization Chart)

    Chapter Authors and Review Editors (see Organization Chart)

    Publication and graphics team Debe Dailey-Fisher (USA), Ann Reiser (USA)

  2. Terms of reference from the Parties at MOP27 in Dubai in November 2015

    Decision XXVII/6 (Paragraph 7):
    To request the Scientific Assessment Panel "to undertake, in its 2018 report, a review of the scientific knowledge as dictated by the needs of the parties to the Montreal Protocol, as called for in the terms of reference for the panels, taking into account those factors stipulated in Article 3 of the Vienna Convention, including estimates of the levels of ozone-layer depletion attributed to the remaining potential emissions of ozone-depleting substances and an assessment of the level of global emissions of ozone-depleting substances below which the depletion of the ozone layer could be comparable to various other factors such as the natural variability of global ozone, its secular trend over a decadal timescale and the 1980 benchmark level"

  3. 2014 Assessment

    The most recent ozone assessment was delivered in 2014. It is comprised of the main document with 5 chapters, the Assessment for Decision Makers (ADM) PDF file and the Twenty Questions and Answers about the Ozone Layer: 2014 Update.

  4. Assessment timeline

    • 2016
      • Discussion Paper circulated for comments by scientific community (November - December)
    • 2017
      • Lead Authors and Review Editors established (January 2017)
      • Lead Authors assemble chapter teams and early preparation steps begin (January - February)
      • Lead Authors submit their detailed Chapter outlines for Steering Committee review (1 March)
      • Steering committee reviews and approves detailed Chapter outlines and assigns page allocations (31 March)
      • Drafting of chapters begins (April)
      • 1st meeting of Co-Chairs, Lead Authors, Steering Committee, Review Editors (April)
      • Chapter first order draft (FOD) preparations (April - September)
        • Individual chapter author team meetings (convened by Lead Authors in May - June)
        • Chapter summary bullets and complete outlines submitted for review (15 July)
        • FOD completed (15 September)
      • Email review of FOD begins (late September)
      • Drafting of FOD Executive Summary (ES) begins (October)
      • Chapter FOD reviews due (November)
      • Chapter Editor summaries sent to Lead Authors (7 November)
    • 2018
      • Second order draft (SOD) of chapters completed (31 January)
      • Review of SOD begins (early February)
      • Open review meeting of SOD (early March)
        • Presentation of FOD ES
        • All cited papers must be accepted for the published literature Panel Review meeting (Les Diablerets).
        • Steering committee meets to prepare ES SOD after meeting.
      • Third order (TOD) and final chapter drafts submitted (1 May)
      • SOD ES distributed to Reviewers (1 April)
      • Panel Review meeting to discuss and finalize ES (15-20 July, Les Diablerets, Switzerland)
      • ES finished and submitted to Ozone Secretariat after Les Diablerets and approximately one month prior to 30th Meeting of Parties (MOP) of the Montreal Protocol (Fall 2018)
      • Presentation of Executive Summary to the MOP (Fall 2018)
      • Final submission of assessment core document to Ozone Secretariat (31 December 2018)