A.R. Ravishankara, Paul A. Newman, John A. Pyle, and Ayité-Lô Ajavon
Scientists have known for many decades that the stratospheric ozone layer screens harmful ultraviolet radiation (UV) from the Earth's surface. Therefore, it has also been known that the ozone layer protects against adverse effects on humans (e.g., skin cancer and cataracts), the biosphere (e.g., inhibiting plant growth and damaging ecosystems), and physical infrastructure of the modern era (e.g., degradation of materials). In the early 1970s, scientists recognized that human actions could deplete this protective layer in connection with nitrogen oxide emissions from a proposed fleet of supersonic aircraft flying in the stratosphere. Around that time, it was shown that human-produced chlorofluorocarbons (CFCs) that had been manufactured (and emitted to the atmosphere) had remained in the atmosphere because of their stability. Soon afterward, scientists warned that these CFCs that are stable in the lower atmosphere would get to the stratosphere, where they could deplete the ozone layer. They also warned that the depletion would be large if CFC emissions continued unabated. Various national and international assessments that estimated the impact of CFCs on the ozone layer were carried out. For example, using the then-state-of-the-art models of the atmosphere, a 1981 Assessment sponsored by the World Meteorological Organization (WMO) and agencies of the United States of America estimated that up to ~15% of the column ozone would be depleted by the middle of the 21st century if the CFC emissions went unabated at 1974 emission levels under certain assumptions about other emissions and changes (WMO, 1982). Studies also predicted a decrease in ozone of 5-10% if a fleet of 500 supersonic aircraft emitting nitrogen oxides were to fly routinely in the stratosphere.
In 1985, massive ozone losses in measured column abundances during the Antarctic spring (the ozone hole) were reported and CFCs were implicated for the loss. Extensive research efforts showed that CFCs and other ozone-depleting substances (ODSs) containing chlorine and bromine were the cause. Further, measured global ozone abundances showed a decrease between 0.5% and 1.5% by 1980. Thus, ozone depletion was not just a phenomenon expected by the middle of the 21st century, but was already occurring. As a result of these findings on ozone depletion, stratospheric science rapidly evolved during the latter part of the 20th century, allowing understanding, diagnosis, and prediction of the evolution of the ozone layer; these rapid scientific developments provided a sound basis for the critical policy decisions that followed.
Faced with the potential impact of human-produced long-lived halogenated chemicals on stratospheric ozone, the Vienna Convention for the Protection of the Ozone Layer was enacted in 1985 to protect human health and the environment against adverse effects resulting from modification of the ozone layer. The recognition that CFC use was increasing, and scientific evidence that this increase would cause large ozone depletions, led in 1987 to the Montreal Protocol on Substances that Deplete the Ozone Layer, a protocol that regulated and slowed the production of designated ODSs. As new scientific knowledge became available over the next two decades, the Protocol has been amended and adjusted to provide additional protection for the ozone layer. The Montreal Protocol is now more than 20 years old and has been ratified by all of the world's nations.
The Montreal Protocol, at its inception, established three expert panels – the Scientific Assessment Panel (SAP), the Environmental Effects Assessment Panel (EEAP), and the Technology and Economic Assessment Panel (TEAP). These panels provide the basis for science-based decision making via periodic assessment reports. The SAP's primary focus is to provide an assessment of ozone layer science, including information about the abundances and emissions of ozone-depleting substances, ultraviolet radiation changes, along with additional information concerning policy options for consideration by the Parties to the Protocol. In addition, the SAP reports also aid other customers: various nations, by providing information needed for their decision making; industry, by providing a basis for technology choices; the broad science community, the EEAP, and the TEAP, with the latest information about the ozone layer science; the ozone research community, with information on the current science and gaps in knowledge; and the general public, including students and educators, with key information about this complex issue. The "Twenty Questions and Answers About the Ozone Layer" and its predecessors, which are companions to the SAP assessment reports, also help by providing clear, easy-to-understand communication of the ozone layer issues to the Parties and the general public. Further, every four years, the Cochairs of the three Assessment Panels compile a Synthesis Report based on the findings of their individual Assessment reports. These Assessments – individual Assessments and the Synthesis Report – together provide the latest information to the Parties to the Protocol.
Over the past two decades, the ozone depletion assessments have provided information updates roughly every four years and have been interspersed with a few brief reports on special topics that addressed urgent needs of the Parties to the Protocol. As knowledge of ozone layer science has increased, the assessments have built a vast amount of knowledge. Now, the SAP is addressing some key remaining issues regarding the ozone layer and its future development. They include the following:
The SAP's goal is to provide clear scientific answers to these questions. These questions provide the major thrust of the research in this area and are at the center of the current Assessment.
This current document provides the latest assessment of the science of the ozone layer. Below, we very briefly summarize our understanding of the science going into this Assessment. We summarize the findings of the most recent previous report of 2006 and note the key issues for the present Assessment.
Emissions of ODSs were increasing at a substantial rate before the Montreal Protocol was enacted in 1987. As a result of the Protocol, emissions of most of the major ODSs – the chlorofluorocarbons (CFCs) and methyl chloroform (CH3CCl3) – began decreasing soon thereafter. Because of the long lifetimes of CFCs, their atmospheric abundances continued to increase in the early 1990s even as their emissions were decreasing. However the abundance of the short-lived methyl chloroform responded quickly, as expected, and started to decrease in the atmosphere. Originally, some of the CFC replacements were the so-called transition substitutes (hydrochlorofluorocarbons, HCFCs); they contained chlorine but were shorter lived than the CFCs they replaced. This substitution led to a lower accumulation of the HCFCs and a smaller fraction of their emissions being transported to the stratosphere. Subsequently, the HCFCs were also selected for phase-out, and non-chlorine containing substitutes are now being phased in. Because of these changes, the sum of the abundances of chlorine and bromine ODS species in the troposphere, as measured by equivalent chlorine (ECl), reached a peak in the 1994-1995 time period and has continued to decrease thereafter. The majority of the decrease in the ECl is attributed to the rapid decline of emissions of the short-lived methyl chloroform and, to a lesser extent, methyl bromide. The tropospheric abundance of ECl by the end of 2005 was shown in the previous Assessment to have decreased to roughly 92% of its maximum value seen during the period between 1992 and 1994 (i.e., about a 8% decline in roughly 14 years); these values will be updated in this report.
Balloon, aircraft, and satellite observations, and the interpretation of those observational data, show clearly that stratospheric abundances of chlorine and bromine are also decreasing. The vertical and temporal variations of the ODS species are generally consistent with our understanding of atmospheric dynamics and stratospheric chemical processes, though there are some quantitative differences between observations and calculations. Improvements in quantification of these variations are expected. These improvements will enable an even better definition of the stratospheric distribution and trends of the ODSs as well as their degradation products, which will enable a better quantification of their individual role in ozone layer depletion.
The CFCs, as well as some halons (which are sources of bromine to the stratosphere), have lifetimes ranging from several decades to a few centuries. Hence, the decline of stratospheric chlorine and bromine levels to values observed before 1980 will take decades.
As noted above, CFCs have been replaced by non-ozone depleting technologies, by substitutes that deplete less ozone (e.g., hydrochlorofluorocarbons or HCFCs), and by non-ozone depleting substances (e.g., hydrofluorocarbons or HFCs). The atmospheric levels of these less-depleting and non-depleting substitutes have grown rapidly over the last decade. HCFCs typically have shorter atmospheric lifetimes and lower Global Warming Potentials (GWPs) than CFCs, but HFC substitutes for HCFCs typically have comparable, and in a few cases even longer, atmospheric lifetimes and comparable or larger GWPs; but they have Ozone Depletion Potential (ODP) values of essentially zero. The increases observed for HCFCs and HFCs reflect their widespread use as ODS replacements and our understanding of their atmospheric lifetimes.
Global atmospheric column ozone amounts decreased over the decades from the 1970s to the 1990s, with a decrease amounting to 3.5% between average 1964-1980 and 2002-2005 values. Springtime Antarctic ozone levels slowly decreased in the 1970s and exhibited rapid decreases in the 1980s and early 1990s. In the 14-20 km layer of the Antarctic stratosphere, where most of the ozone resides, virtually all of the ozone is now destroyed every year in the late August to early October period. Large Arctic ozone depletions have also been observed in the spring in some years during the last two decades, but Arctic ozone depletion is modulated strongly by variability in atmospheric dynamics, transport, and temperature. The very high levels of chlorine and bromine from ODSs directly cause the observed large polar ozone depletions (both over the Antarctic and the Arctic).
Atmospheric ozone levels (often measured as a column amount) exhibit well-known and understood variations in space and time. Ozone amounts are influenced not only by the concentrations of ODSs but also by atmospheric transport (winds), incoming solar radiation, aerosols (fine particles suspended in the air), and other natural compounds. Given natural variability, methods used to measure stratospheric ozone must be consistent and very stable over decades if they are to be used to detect the changes expected over these long periods due to the changes in ODS abundances. Based on observations from ground-based instruments and satellites, it is clear that global ozone levels reached a minimum in the mid-1990s. Since then the levels have not decreased further nor have they increased substantially. Similarly, the Antarctic ozone hole continues to be no worse than in the mid-1990s but there also has been no discernible improvement, consistent with predictions from previous assessments. Both annual global ozone and the springtime Antarctic ozone levels continue to vary from year to year because of meteorological variability. There is no discernible ozone depletion over the tropics outside of the natural background variations. Vertically, ozone depletion is most evident in the lower and upper stratosphere, with minimal changes in the mid-stratosphere.
In the last few decades, ozone levels in the stratosphere have responded to volcanic eruptions that have injected large amounts of sulfur dioxide into the stratosphere, which then forms sulfate aerosols in this region. These sulfate aerosols enhance the ozone depletion by chlorine from ODSs. The very large ozone depletions induced by the presence of aerosols following the eruptions of Mt. Pinatubo (1991) and El Chichón (1982) are very clearly seen in the records in the Northern Hemisphere. The influence of these eruptions persisted for several years. As the stratosphere recovered from the volcanic emissions, there were corresponding changes in ozone. The ozone response depends on the effective abundances of chlorine and bromine in the stratosphere. Thus, response to future volcanic eruptions will likely be smaller because chlorine/bromine concentrations will be smaller (see Figure P-1). The mechanisms for these changes are qualitatively understood, but some uncertainties remain in their quantification.
The observed levels of ozone described above and the vertical, latitudinal, and seasonal structure of their temporal trends, as well as the spatial and temporal variability, are consistent with our combined understanding of the atmospheric motions (transport), the chemistry, and the level of ODSs in the atmosphere. Even though some details of chemical and dynamical processes are uncertain, atmospheric models have been largely successful in reproducing observed ozone levels and their temporal and spatial variations. The link between ODSs and ozone depletion was clearly established in the 1989 Ozone Assessment (WMO, 1991) and that conclusion has only been strengthened since then.
Ultraviolet radiation (UV) from the Sun is divided into wavelength bands. UV-B is the band that leads to serious medical problems. Fortunately, the majority of the UV-B is absorbed by ozone. The surface UV-B and UV-A levels (expressed as the UV Index) are directly related to the amount of overhead ozone. Other factors such as clouds, aerosols, ground reflectivity, and other tropospheric pollutants also influence surface UV-B. The data outside of the polar regions shows that, consistent with the observed small ozone depletion, there have not been large increases in surface UV-B over the last few decades. The relatively small increases of surface UV-B in the midlatitudes, which are expected based on the observed ozone decline, are responsible for small changes in the UV background level, which are superposed by other strong effects, such as changes in cloudiness. However, since medical impacts are UV-dose related, the UV changes due to ozone depletion are nonetheless important. In contrast, over Antarctica, and on occasion in other parts of the high latitudes in the Southern Hemisphere, large increases in UV-B have been seen; they are clearly associated with the ozone hole or the remnants of the ozone hole passing over the measurement sites.
The changes in UV-B levels are consistent with our understanding of UV transmission and the other factors that influence UV-B at the surface.
The change in the atmospheric ODS concentrations is the most important factor in the ozone layer changes that have occurred over the past half a century and also in the predicted return of the ozone layer to levels that existed prior to 1980. However, many other aspects of the Earth system are also changing. These include changes in climate and tropospheric composition.
Climate change influences the stratosphere in many ways. The primary influence is a cooling of the mid- to upper stratosphere due to increases in carbon dioxide (CO2) via radiation to space, which is a well-understood process. This cooling has been clearly seen in measured temperatures. The cooling influences the ozone loss rates in the stratosphere – increasing it in the lower stratosphere and decreasing it in upper stratosphere. At the same time the warming in the troposphere accelerates processes of ozone formation. Further, climate change has an effect on transport between the stratosphere and the troposphere and within the stratosphere, and in turn, climate will influence the recovery of ozone layer from the effects of ODSs.
Tropospheric changes also influence stratospheric ozone levels. For example, an increased abundance of methane (CH4) in the troposphere will result in more methane being transported to the stratosphere, where methane interacts with chlorine compounds, converting active chlorine that destroys ozone to inactive hydrogen chloride (HCl) that does not destroy ozone. Changes in methane also lead to changes in water vapor in the stratosphere, with important consequences. Similarly, changes in nitrous oxide (N2O) also influence ozone destruction. Other tropospheric changes of interest include processes leading to increases in sulfur in the stratosphere. In some cases, changes of these tropospheric processes may be related to climate change. For instance, climate change may affect biogeochemical cycles and cause an increase in tropospheric concentrations of certain species as well as the transport rate between the troposphere and the stratosphere. The latter may be particularly important for the very short-lived species.
The timeline of the ozone evolution from the pre-ODS era to roughly 2100 was presented in the 2006 Assessment to facilitate discussion on recognition and attribution of the recovery of the ozone layer. This approach provided a pathway for interim conclusions on this issue, but many issues remained unresolved. They include: How should recovery be defined? What time period is appropriate as a baseline against which we can measure recovery? How do we separate ozone changes due to ODSs from those due to changes in climate and tropospheric composition? How do we describe and attribute future changes in levels of ozone? Given the natural variability, at which point will one be confident of the recovery from ODS effects? This Assessment addresses some of these issues and concepts (see Prologue Box 2 on Recovery Issues).
As noted above, increases in CO2 in the atmosphere have led to a clear decrease in upper stratospheric temperature. This temperature trend is a very clear signature of the radiative influence of increasing CO2 abundances. Changes in the stratosphere – be it the temperature decrease due to CO2 increases or ozone layer depletion due to ODSs – are an integral part of the changes to the Earth system. Further, these changes in the stratosphere influence what happens at the surface. Therefore, the influence of stratospheric changes on surface climate is an important issue.
Ozone is a greenhouse gas that greatly influences the Earth's energy budget. Therefore, ozone changes – depletion in the stratosphere due to ODSs, recovery from the depleted state as ODSs decline, and tropospheric ozone changes – also influence climate. Further, many of these ODSs that deplete the ozone layer are also greenhouse gases. Consequently, they influenced Earth's climate in the past as their abundances increased and will continue to do so, albeit to a lesser extent, as their abundances decrease in response to compliance with the Montreal Protocol. Furthermore, many of the substitutes for CFCs and HCFCs are also potent greenhouse gases and their contribution to climate change will depend on the their potency for warming and their emission rates.
These are some of the emerging issues that have been covered only briefly in the past due to a primary focus on ozone depletion issues. As research on the influence of stratospheric changes on the overall climate has emerged, the current Assessment is devoting more attention to this topic.
The major findings of the 2010 Assessment are given in the Executive Summary that follows this Prologue. To place these findings in context and show the changes in our knowledge over the past four years, we provide below the summary of the 2006 Assessment (WMO, 2007). Further, for ease of comparison, the findings from the 2006 Assessment are grouped according to where they are covered in the 2010 Assessment; i.e., the 2006 Assessment is mapped on to the 2010 Assessment's structure.
A major finding of the previous Assessment in 2006, the tenth in a series of Assessments dating back to 1981, was that the Montreal Protocol was working as intended. Some specific findings of the 2006 Assessment are summarized in the schematic shown as Figure P-1.
The high-level findings of the previous Assessment (WMO, 2007) include the following.
Findings of the 2006 Assessment that are related to "Ozone-Depleting Substances (ODSs) and Related Chemicals" covered in Chapter 1 of the 2010 Assessment:
Findings of the 2006 Assessment that are related to "Stratospheric Ozone and Surface Ultraviolet Radiation" in the past and our understanding of its changes covered in Chapter 2 of the 2010 Assessment:
Findings of the 2006 Assessment that are related to "Future Ozone and Its Impact on Surface UV" covered in Chapter 3 of the 2010 Assessment:
Findings of the 2006 Assessment that are related to the influence of "Stratospheric Changes and Climate" covered in Chapter 4 of the 2010 Assessment:
Findings of the 2006 Assessment that are related to "A Focus on Options and Information for Policymakers" covered in Chapter 5 of the 2010 Assessment:
Much new information has been generated since the 2006 Assessment. Further, the information needs of the Parties to the Protocol have also changed. The specific requests of the Parties to the SAP are given in the Preface of this Assessment. Of particular note are the questions related to the influence of stratospheric changes on Earth's climate. This is somewhat of a new issue to the SAP and thus demands a chapter of its own.
This Assessment is an update to previous Assessments, and in particular the 2006 Assessment. However, as noted above, the changes in ozone and UV are not rapid and there are no new major findings in this area. To reflect this updating approach and consolidation of information, the structure of this Assessment differs from the most recent reports. In this Assessment, Chapter 1 deals with all issues related to ODSs; they include long-lived and very short-lived halocarbons as well as the replacements for the ODSs. In particular, it covers the trends and abundances of the replacements for ODSs that are greenhouse gases (but not ODSs), such as HFCs that are being discussed by the Parties to the Protocol for regulation. Chapter 2 deals with all observations of ozone and surface UV to date and our understanding of these observations, including a discussion of the current state of polar ozone. Chapter 3 focuses primarily on the future response of the ozone layer and UV-B radiation to reduced halocarbon emissions and other changes in an effort to focus on the question: What should one anticipate for ozone layer depletion and its consequences? It also picks up the issue of the definition and recognition of the recovery of the ozone layer first discussed in the 2006 Assessment. Of particular note are the issues related to the influence of stratospheric changes on climate. This issue was briefly described in the 2006 Assessment, which mostly focused on the influence of climate change on the recovery of the ozone layer. Because of the emergence of information on the influence of the stratospheric changes on Earth's climate, we have added a new chapter – Chapter 4 – to address this topic. Chapter 4 focuses on the two-way connection between stratospheric changes and climate changes. This places the effects of halocarbon-induced ozone depletion on climate in the broader context of other stratospheric changes. Chapter 5 is expanded to include not only the policy options, often posed in hypothetical terms, available for further action but also other information relevant to the Parties to the Protocol.
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WMO (World Meteorological Organization), Scientific Assessment of Stratospheric Ozone: 1989, Global Ozone Research and Monitoring Project-Report No. 20, Geneva, Switzerland, 1991. [Referred to as the 1989 Assessment.]
WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project-Report No. 47, Geneva, Switzerland, 2003. [Referred to as the 2002 Assessment.]
WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project-Report No. 50, 572 pp., Geneva, Switzerland, 2007. [Referred to as the 2006 Assessment.]