Scientific Assessment of Ozone Depletion: 2002

Twenty Questions and Answers About the Ozone Layer

2002 Assessment Twenty Questions and Answers About the Ozone Layer cover
Twenty Questions and Answers About the Ozone Layer

Ozone is a very small part of our atmosphere, but its presence is nevertheless vital to human well-being.

Most ozone resides in the upper part of the atmosphere. This region, called the stratosphere, is more than 10 kilometers (6 miles) above Earth's surface. There, about 90% of atmospheric ozone is contained in the "ozone layer," which shields us from harmful ultraviolet light from the Sun.

However, it was discovered in the mid-1970s that some human-produced chemicals could destroy ozone and deplete the ozone layer. The resulting increase in ultraviolet radiation at Earth's surface can increase the incidences of skin cancer and eye cataracts.

Following the discovery of this environmental issue, researchers focused on a better understanding of this threat to the ozone layer. Monitoring stations showed that the abundances of the ozone-depleting chemicals were steadily increasing in the atmosphere. These trends were linked to growing production and use of chemicals like chlorofluorocarbons (CFCs) for refrigeration and air conditioning, foam blowing, and industrial cleaning. Measurements in the laboratory and the atmosphere characterized the chemical reactions that were involved in ozone destruction. Computer models employing this information could then predict how much ozone depletion was occurring and how much more could occur in the future.

Observations of the ozone layer itself showed that depletion was indeed occurring. The most severe and most surprising ozone loss was discovered to be over Antarctica. It is commonly called the "ozone hole" because the ozone depletion is so large and localized. A thinning of the ozone layer also has been observed over other regions of the globe, such as the Arctic and northern middle latitudes.

The work of many scientists throughout the world has provided a basis for building a broad and solid scientific understanding of the ozone depletion process. With this understanding, we know that ozone depletion is occurring and why. And, most important, it has become clear that, if ozone-depleting gases were to continue to accumulate in the atmosphere, the result would be more ozone layer depletion.

In response to the prospect of increasing ozone depletion, the governments of the world crafted the 1987 United Nations Montreal Protocol as a global means to address a global issue. As a result of the broad compliance with the Protocol and its Amendments and Adjustments and, importantly, the industrial development of more "ozonefriendly" substitutes for the now-controlled chemicals, the total global accumulation of ozone-depleting gases has slowed and begun to decrease. This has reduced the risk of further ozone depletion. Now, with continued compliance, we expect recovery of the ozone layer in the late 21st century. International Day for the Preservation of the Ozone Layer, 16 September, is now celebrated on the day the Montreal Protocol was agreed upon.

This is a story of notable achievements: discovery, understanding, decisions, and actions. It is a story written by many: scientists, technologists, economists, legal experts, and policymakers. And, dialogue has been a key ingredient.

To help foster a continued interaction, this component of the Scientific Assessment of Ozone Depletion: 2002 presents 20 questions and answers about the often complex science of ozone depletion. The questions address the nature of atmospheric ozone, the chemicals that cause ozone depletion, how global and polar ozone depletion occur, and what could lie ahead for the ozone layer. A brief answer to each question is first given in italics; an expanded answer then follows. The answers are based on the information presented in the 2002 and earlier Assessment reports. These reports and the answers provided here were all prepared and reviewed by a large international group of scientists1.

1. A draft of this component of the Assessment was reviewed and discussed by the 74 scientists who attended the Panel Review Meeting for the 2002 ozone assessment (Les Diablerets, Switzerland, 24-28 June 2002). In addition, subsequent contributions, reviews, or comments were provided by the following individuals: A.-L.N. Ajavon, D.L. Albritton, S.O. Andersen, P.J. Aucamp, G. Bernhard, M.P. Chipperfield, J.S. Daniel, S.B. Diaz, E.S. Dutton, C.A. Ennis, P.J. Fraser, R.-S. Gao, R.R. Garcia, M.A. Geller, S. Godin-Beekmann, M. Graber, J.B. Kerr, M.K.W. Ko, M.J. Kurylo, M. McFarland, G.L. Manney, K. Mauersberger, G. Mégie, S.A. Montzka, R. Müller, E.R. Nash, P.A. Newman, S.J. Oltmans, M. Oppenheimer, L.R. Poole, G. Poulet, M.H. Proffitt, W.J. Randel, A.R. Ravishankara, C.E. Reeves, R.J. Salawitch, M.L. Santee, G. Seckmeyer, D.J. Siedel, K.P. Shine, C.C. Sweet, A.F. Tuck, G.J.M. Velders, R.T. Watson, and R.J. Zander.


    1. What is ozone and where is it in the atmosphere?
      Figure Q1-1 [eps] [small jpeg] [large jpeg]
      Figure Q1-2 [eps] [small jpeg] [large jpeg]
    2. How is ozone formed in the atmosphere?
      Figure Q2-1 [eps] [small jpeg] [large jpeg]
    3. Why do we care about atmospheric ozone?
      Figure Q3-1 [eps] [small jpeg] [large jpeg]
    4. Is total ozone uniform over the globe?
      Figure Q4-1 [eps] [small jpeg] [large jpeg]
    5. How is ozone measured in the atmosphere?
      Figure Q5-1 [eps] [small jpeg] [large jpeg]

    1. What are the principal steps in stratospheric ozone depletion caused by human activities?
      Figure Q6-1 [eps] [small jpeg] [large jpeg]
    2. What emissions from human activities lead to ozone depletion?
      Figure Q7-1 [eps] [small jpeg] [large jpeg]
    3. What are the reactive halogen gases that destroy stratospheric ozone?
      Figure Q8-1 [eps] [small jpeg] [large jpeg]
      Figure Q8-2 [eps] [small jpeg] [large jpeg]
      Figure Q8-3 [eps] [small jpeg] [large jpeg]
    4. What are the chlorine and bromine reactions that destroy stratospheric ozone?
      Figure Q9-1 [eps] [small jpeg] [large jpeg]
      Figure Q9-2 [eps] [small jpeg] [large jpeg]
    5. Why has an "ozone hole" appeared over Antarctica when ozone-depleting gases are present throughout the stratosphere?
      Figure Q10-1 [eps] [small jpeg] [large jpeg]
      Figure Q10-2 [eps] [small jpeg] [large jpeg]

    1. How severe is the depletion of the Antarctic ozone layer?
      Figure Q11-1 [eps] [small jpeg] [large jpeg]
      Figure Q11-2 [eps] [small jpeg] [large jpeg]
    2. Is there depletion of the Arctic ozone layer?
      Figure Q12-1 [eps] [small jpeg] [large jpeg]
    3. How large is the depletion of the global ozone layer?
      Figure Q13-1 [eps] [small jpeg] [large jpeg]
    4. Do changes in the Sun and volcanic eruptions affect the ozone layer?
      Figure Q14-1 [eps] [small jpeg] [large jpeg]

    1. Are there regulations on the production of ozone-depleting gases?
      Figure Q15-1 [eps] [small jpeg] [large jpeg]
    2. Has the Montreal Protocol been successful in reducing ozone-depleting gases in the atmosphere?
      Figure Q16-1 [eps] [small jpeg] [large jpeg]

    1. Does depletion of the ozone layer increase ground-level ultraviolet radiation?
      Figure Q17-1 [eps] [small jpeg] [large jpeg]
      Figure Q17-2 [eps] [small jpeg] [large jpeg]
    2. Is depletion of the ozone layer the principal cause of climate change?
      Figure Q18-1 [eps] [small jpeg] [large jpeg]

    1. How will recovery of the ozone layer be detected?
    2. When is the ozone layer expected to recover?
      Figure Q20-1 [eps] [small jpeg] [large jpeg]