**WORKSHOP REPORT**

**CCSP WORKSHOP ON THE OZONE DEPLETION ASSESSMENT 2006**

**State of the Knowledge and Next Steps in the Research Domain**

1.  **[STATE OF THE KNOWLEDGE]{.underline}**

    1.  **Ozone-Depleting Substances**

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a)  The total combined abundances of anthropogenic ozone-depleting gases
    in the troposphere continue to decline from the peak values reached
    in the 1992-1994 time period. The shorter-lived gases (e.g., methyl
    chloroform and methyl bromide) continue to provide much of the
    decline in total combined effective abundances of anthropogenic
    chlorine-containing and bromine-containing ozone-depleting gases in
    the troposphere and will soon be insignificant in the atmosphere.
    The early removal of the shorter-lived gases means that later
    decreases in ozone-depleting substances will likely be dominated by
    the atmospheric removal of the longer-lived gases.

b)  The combined stratospheric abundances of the ozone-depleting gases
    show a downward trend from their peak values of the late 1990s,
    which is consistent with surface observations of these gases and a
    time lag for transport to the stratosphere.

c)  Brominated (and chlorinated) very short-lived substances are
    believed to make a significant contribution to total stratospheric
    bromine (and chlorine) and its effect on stratospheric ozone.
    Bromine is now estimated to be approximately 60 times as effective
    as chlorine in global ozone depletion, on a per-atom basis.
    Atmospheric total bromine concentrations peaked in the lower
    atmosphere around 1998 and are now decreasing.

d)  Hydrochlorofluorocarbons (HCFCs) continue to increase in the
    atmosphere. In 2004 they contributed 6% to the lower atmospheric
    chlorine burden, compared to 5% in 2000. HCFC-22 is the most
    abundant of the HCFCs and is increasing at 3.2% per year.

e)  During the recent period of near-constant abundances of
    ozone-depleting gases, variations in meteorology have been
    particularly important in influencing the behavior of ozone over
    much of the polar and extrapolar (60°S-60°N) regions.

    1.  **Polar Ozone**

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a)  Springtime polar ozone depletion continues to be severe in cold
    stratospheric winters. Meteorological variability has played a
    larger role in the observed variability in ozone, over both poles,
    in the past few years.

b)  The Antarctic ozone hole now is not as strongly influenced by
    moderate decreases in ozone-depleting gases, and the unusually small
    ozone holes in some recent years (e.g., 2002 and 2004) are
    attributable to dynamical changes in the Antarctic vortex.

c)  Arctic ozone depletion exhibits large year-to-year variability,
    driven by meteorological conditions. Over the past four decades,
    these conditions became more conducive to severe ozone depletion
    because of increasingly widespread conditions for the formation of
    polar stratospheric clouds during the coldest Arctic winters. This
    change is much larger than can be expected from the direct radiative
    effect of increasing greenhouse gas concentrations. The reason for
    the change is unclear, and it could be because of long-term natural
    variability or an unknown dynamical mechanism.

    1.  **Global Ozone (60^0^S-60^0^N)**

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a)  The decline in abundances of extrapolar stratospheric ozone seen in
    the 1990s has stopped.

b)  The contribution of polar ozone depletion to midlatitude ozone
    depletion is substantial; the extent of the contribution is
    estimated to be about one-third in the Northern Hemisphere and
    one-half in the Southern Hemisphere. The larger contribution in the
    Southern Hemisphere is to be expected, given the larger polar ozone
    depletion in the Antarctic relative to the Arctic region, and may
    explain the differences in magnitude and seasonality of the
    long-term ozone changes in midlatitudes between the two hemispheres.

c)  Observations together with model studies suggest that the
    essentially unchanged column ozone abundances averaged over
    60°S-60°N over roughly the past decade are related to the near
    constancy of stratospheric ozone-depleting gases during this period.

d)  Total column ozone over the tropics (25°S-25°N) remains essentially
    unchanged.

    1.  **Stratospheric Temperature and Surface UV Radiation**

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a)  The stratospheric cooling observed during the past two decades has
    slowed in recent years. Large interannual variability in polar
    stratospheric temperatures complicates the interpretation of
    temperature trends.

b)  Measurements from some stations in unpolluted locations indicate
    that UV irradiance (radiation levels) has been decreasing since the
    late 1990s, in accordance with observed ozone increases. However, at
    some Northern Hemisphere stations UV irradiance is still increasing,
    as a consequence of long-term changes in other factors that also
    affect UV radiation.

c)  In polar regions, high UV irradiances lasting for a few days have
    been observed in association with episodes of low total ozone.

    1.  **Impact of Climate Change**

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a)  Future increases of greenhouse gas concentrations will contribute to
    the average cooling in the stratosphere.

b)  Chemical reaction rates in the atmosphere are dependent on
    temperature, and thus the concentration of ozone is sensitive to
    temperature changes.

c)  Changes to the temperature and circulation of the stratosphere
    affect climate and weather in the troposphere.

d)  Updated datasets of stratospheric water vapor concentrations now
    show differences in long-term behavior.

    1.  **The Future of the Ozone Layer and Surface UV Radiation**

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a)  Over the past 10 years, changes in equivalent effective
    stratospheric chlorine (EESC) have likely contributed to the slowing
    of midlatitude total column ozone decline and the leveling off of
    column ozone.

b)  The decline in EESC has not caused the large annual variations
    observed in Arctic ozone depletion. These large interannual
    variations, driven by changes in meteorology, are likely to preclude
    the detection of the first stage of recovery.

c)  It is unlikely that total ozone averaged over the region 60°S-60°N
    will decrease significantly below the low values of the 1990s,
    because the abundances of ozone-depleting substances have peaked and
    are in decline. Although the decrease in ozone-depleting substances
    is the dominant factor in the expected return of ozone levels to
    pre-1980 values. Changes in climate will influence whether, when,
    and to what extent ozone will return to pre-1980 values in different
    regions.

d)  The Antarctic ozone hole is expected to continue for decades.
    Antarctic ozone abundances are projected to return to pre-1980
    levels around 2060-2075, roughly 10-25 years later than estimated in
    the 2002 Assessment.

e)  Large ozone losses will likely continue to occur in cold Arctic
    winters during the next 15 years.

f)  Human activities are expected to affect stratospheric ozone through
    changes in tropospheric emissions of trace gases. Enhanced methane
    emission is expected to enhance ozone production in the lower
    stratosphere, whereas an increase in nitrous oxide emission is
    expected to reduce ozone in the middle and high stratosphere. Also,
    changes in non-methane hydrocarbons and NOx emissions are expected
    to affect the tropospheric concentrations of hydroxyl radical and,
    hence, impact the lifetimes and concentrations of stratospheric
    trace gases such as methane and organic halogen species.

g)  Climate change will also influence surface UV radiation through
    changes induced mainly to clouds and the ability of the Earth\'s
    surface to reflect light.

h)  Future changes of stratospheric water vapor concentrations are
    uncertain. If water vapor concentrations increase in the future,
    there will be both a radiative and a chemical effect. Modeling
    studies suggest increased water vapor concentrations will enhance
    odd hydrogen (HOx) in the stratosphere and subsequently influence
    ozone depletion. Increases in water vapor in the polar regions would
    raise the temperature threshold for the formation of polar
    stratospheric clouds, potentially increasing springtime ozone
    depletion.

    1.  **Implications for Policy Formulation**

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    a.  Our basic understanding that anthropogenic ozone-depleting
        substances have been the principal cause of the ozone depletion
        over the past decades has been strengthened.

    b.  The Montreal Protocol is working: There is clear evidence of a
        decrease in the atmospheric burden of ozone-depleting substances
        and some early signs of stratospheric ozone recovery.

    c.  Long-term recovery of the ozone layer from the effects of
        ozone-depleting substances is expected to span much of the 21st
        century and is estimated to occur later than projected in the
        previous Assessment (2002).

    d.  Failure to comply with the Montreal Protocol would delay, or
        could even prevent, recovery of the ozone layer.

    e.  The role of very short-lived halogenated substances (especially
        brominated) in stratospheric ozone depletion is now believed to
        be of greater importance than previously assessed.

    f.  Understanding the interconnections between ozone depletion and
        climate change is crucial for projections of future ozone
        abundances.

.

2.  **[NEXT STEPS IN THE RESEARCH DOMAIN]{.underline}**

    1.  **CCSP Strategic Plan**

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a)  The general research agenda put forth in Strategic Plan continues to
    be relevant. There are, however, some areas that warrant additional
    attention. See below.

    1.  **Residence Time**

b)  Atmospheric residence times of ODSs need to be re-evaluated. If
    chemistry/climate models are to project future recovery then they
    should include feedback of changing lifetime due to GHG-forced speed
    up of residual circulation. This will not work unless observed
    lifetimes are consistent with models.

c)  The uncertain size of ODS banks could be improved with better
    lifetime estimates.

d)  Uncertain decomposition rates in the stratosphere ('fractional
    release rates') influence reactive halogen recovery projections.

    1.  **Ozone and Climate**

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a)  Coupling between ozone and climate (particularly for the SAM). Need
    to include ozone in climate models (key to future projections):

    -   How ozone has influenced Southern Hemisphere climate in the past
        few decades? What about the future?

    -   Need to better understand enhanced warming in the Antarctic
        Peninsula linked to ozone (and GHGs).

b)  How should the critical linkages between ozone and climate be
    addressed?

-   Better understanding of observation: nature and drivers of vortex
    changes

-   Better representation in future projections: requires interactions
    with climate modelers/analysts to develop improved tools for
    near-term

c)  The stratosphere has been key to recent advances in vertical
    temperature profiles but there are remaining issues and they are
    important for climate change.

d)  There is a need to better understand present and past ozone and
    temperature observations:

-   3D models will play an increasingly important role

e)  Stronger Brewer/Dobson circulation? How will this warm the polar
    stratosphere, and how will it influence ozone depletion in both
    polar regions?

f)  Climate effect of CFC (banks) and HCFCs and on the natural fluxes of
    ODSs particularly CH~3~Cl and CH~3~Br, but also very short-lived
    ODSs

    1.  **"Signal to Noise" Problem**

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a)  To confirm the effectiveness of the Montreal Protocol, we need to
    determine the ozone changes due to ODS. But the "signal to noise"
    problem confounds this. A few of the key issues are: defining an
    appropriate baseline from the short and noisy data; and, assessing
    the extent to which other signals, including climate change, are
    playing a role. So far, the noise is not well characterized

b)  The "signal to noise" problem also complicates the question of
    whether the ozone increase at midlatitude is due to a decrease in
    halogen-induced loss and/or transport as seen in recent studies.

    1.  **Emissions**

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a)  The degree of compliance with the Protocol (HCFC use).

b)  Trends in non-regulated uses (QPS, Feedstock, CUEs of CH~3~Br and
    others), and uncharacterized sources (CH~3~Br, CCl~4~).

c)  Trends in non-regulated GHGs emission and other gases with potential
    effects on ozone chemistry (e.g. NOx, NOy, Sox...).

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3.  **[Decision Support and Policy Related Aspects of Ozone
    Assessment]{.underline}**

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1.  **Policy-relevant science questions or issues**

    a.  Definition of "recovery" and implications of using 1980 levels
        of O~3~ as a baseline (Ravi, Shepherd, Newman, Solomon)

        -   Poor record of O~3~ variability pre-1980

        -   Possibility of meaning of "super-recovery"

        -   Uneven "recovery"

    b.  Accounting for impacts of non-regulated or poorly regulated
        ODSs, including uncharacterized or unregulated sources,
        short-lived species, HCFCs, and new substances (Montzka, Ko,
        Thompson, McFarland, Andersen)

    c.  "Banked" materials -- how to deal with these banks, service
        needs into the future, implications for accidental or deliberate
        release, creative thinking about economically and
        environmentally efficient clean-up, rationale for destruction
        under the precautionary principle (Montzka, Ko, Thompson,
        Andersen)

    d.  Better linking between climate and ozone, including benefits for
        climate and ozone layer realized as a result of rapid phase-out
        of ODSs and effects of GHGs on ozone (Andersen, Newburg,
        Solomon)

    e.  Need support (conceptual and monetary) for long-term
        observations and monitoring of chemical cycles, impacts of
        climate change, and chemistry of emerging compounds (Kurylo)

        -   CCSP Strategic Plan may help set the tone for the importance
            of ongoing support for observations

        -   Continual monitoring was a key concern of many parties at
            Nairobi MOP

        -   Money is not the only problem -- also losing expertise and
            institutional memory when people retire or move to other
            projects

2.  **"Twenty Questions and Answers about the Ozone Layer" has been
    well-received and serves as an informal and formal tool for
    communicating with public and policy makers (e.g., use by EPA as a
    part of reporting under Clean Air Act) (Fahey, Land)**

    a.  Attempts to tell the ozone "story" and to counter technical
        misunderstandings and misinformation (Fahey)

    b.  Translation into languages other than English will raise the
        potential for information to be more widely read and dispersed
        (Fahey, Thompson)

    c.  The document is broadly structured to best serve all of the
        parties at MOP -- it may be worthwhile to engage US stakeholders
        (Congress, etc.) via CCSP to determine how they use the document
        and if there are any needs that it does not meet (Thompson,
        Land)

    d.  Format may be considered for other assessment activities such as
        SAPs (e.g., IPCC WGI created a FAQ, as did EEAP) (Bornman)

    e.  There is a continuing need for documents that educate newcomers
        and non-specialists in the assessment process (McFarland)

3.  **Balancing assessments between providing information useful and
    relevant to policy makers and remaining policy neutral (Kurylo,
    McFarland)**

    a.  Need for credible, scientific, policy-neutral reports that
        identify the problems and various solutions or paths forward
        (Thompson)

4.  **Interactions between stakeholders and assessment team**

    a.  Must be early and continual (Kurylo)

    b.  In some cases, scientists have a better/more clear understanding
        of an issue than policy makers realize -- thus scientists and
        science communicators need to do a better job of getting the
        message across

    c.  Often times existing materials can be helpful in answering
        policy makers questions, and CCSP may be able to assist in
        eliciting policy makers' questions and matching these questions
        to materials already available

5.  **How O~3~ Assessment is used by government**

    a.  EPA (Land)

        -   Tracking progress and "healing" of the ozone layer -- are
            regulatory programs effective?

        -   Informing the planning process for new and revised
            regulatory and voluntary programs

        -   Regulatory impact assessments

        -   Data and figures used in EPA reports to public and industry

        -   Developing environmental indicators

        -   Planning and identifying research needs (Newburg)

        -   PART reports

        -   Addendum to reporting under §603 of the Clean Water Act

        -   Developing policy positions for the US delegation to the MOP

        -   Placing O~3~ programs in the context of climate change

    b.  State (Thompson)

        -   Providing context and continuing emphasis on the ozone issue
            (fairly regular turnover of delegation members)

        -   Often not obvious how reports are used -- but they are read
            and science remains an important driver

        -   Support for US contributions to Montreal Protocol
            Multilateral Fund (\>\$500M)

6.  **How O~3~ Assessment is used by industry (McFarland)**

    a.  1985 assessment played an important role in DuPont's support of
        international convention on ozone

    b.  Subsequent assessments were important in DuPont's decision to
        phase out production of CFCs and HCFCs

7.  **US leadership of assessment processes**

    a.  Susan Solomon will not be leading WG1 for AR5 and both US
        co-leads of O~3~ Assessment have retired -- thus there is a need
        for US to rapidly step forward to fill these positions and
        retain a leading role (McFarland)

8.  **Interactions between and among assessments**

    a.  Ozone and IPCC are on different time schedules, so sometimes it
        is possible to feed model outputs and other information between
        the processes, while other times the modelers and others are
        overtaxed (Solomon)

        -   A potential role for WCRP (with assistance from WCRP) may be
            to look at what models are needed for ozone and IPCC and how
            the two timetables affect the way that models are built and
            analyzed (Solomon)

    b.  Eventually there may be a need or ability to better phase or
        combine O~3~ and IPCC assessments, but current MP timetable
        requires an ozone assessment in 2010 -- and any decision to
        change timing and scope is the responsibility of the policy
        community (Solomon, McFarland)

    c.  SAP 2.4 has been able to draw on IPCC, TEAP, Ozone Assessment,
        and new peer reviewed publications, allowing the time/ability to
        craft a U.S. focused chapter in the report (Kurylo)

9.  **Evolution of assessments**

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a)  The relationship between science assessments and policy should be
    carefully considered. On the one hand, it is important for the
    outcomes of science assessments to be independent of policy
    pressures. On the other hand, it is important for assessments to
    address both policy-relevant issues as well as outcomes from
    curiosity-driven science.

b)  The most recent IPCC assessment has brought us beyond global mean
    temperature to the doorstep of considering the whole earth system,
    which could be a new frontier for AR5 (Solomon)

c)  We may see more emphasis on sector-based special reports (e.g.,
    aerosols, aviation) (Solomon)