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A Hole in the Sky: Ozone Depletion - Montreal Protocol Science

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Article 6 of the Montreal Protocol requires that the ratifying nations base their decision-making on scientific information assessed and presented by an international panel of ozone experts. This panel includes the World Meteorological Organization (WMO), the United Nations Environment Programme (UNEP), the European Commission, and two U.S. organizations—the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA).

In March 2003 the WMO published the panel's latest findings in Scientific Assessment of Ozone Depletion: 2002. This is the fifth scientific assessment of the world's ozone condition. Previous assessments were published in 1989, 1991, 1994, and 1998. The reports are based on analysis of data collected from satellites, aircraft, balloons, and ground-based instruments and the results of laboratory investigations and computer modeling.

The scientific assessment published in 2003 presents the following major findings:

  • For the period 1997–2001 Earth's average stratospheric ozone concentration was approximately 3 percent less than the concentration measured prior to 1980 (the baseline condition).
  • Ozone loss has occurred primarily at Earth's poles and over mid-latitude regions of the Northern and Southern hemispheres. No statistically significant ozone loss has been recorded over tropical regions near Earth's equator.
  • Ozone depletion over the Southern hemisphere occurs year-round. Depletion over the Northern hemisphere is more seasonal, worsening during winter and spring months. Ozone depletion over Antarctica (the "ozone hole") has historically been a springtime phenomenon. However, data collected in recent years show that the ozone hole is persisting for a longer time each year, not disappearing until early summer.
  • Ozone concentrations measured over Antarctica during occurrences of the ozone hole consistently average 40–50 percent less than concentrations measured prior to 1980.
  • The average size of the Antarctic ozone hole increased during the 1980s and 1990s. Although the growth rate was slower during the 1990s than it was during the 1980s, scientists are not sure yet that the size of the hole has reached a maximum.
  • Ozone depletion has been recorded at certain times over the North Pole during the last decade. However, scientists do not believe that an Arctic ozone hole that returns on a regular basis is likely to form due to the highly variable meteorological conditions at the North Pole.
  • Measurements taken at various sites around the world indicate an increase of 6–14 percent in the amount of ultraviolet radiation reaching the ground since the early 1980s. However, lack of long-term data and an abundance of other factors besides ozone that affect UV levels (such as cloudiness) make it difficult to determine the statistical significance of the recorded UV increases.
  • The total concentration of ozone-depleting chemicals in the troposphere (lower atmosphere) has declined since peaking in 1992–1994. Chlorine concentrations measured in 2000 were 5 percent lower than those recorded in 1992–1994. Bromine concentrations increased by 3 percent per year over the same time period. The increase in bromine is blamed on halon emissions. Atmospheric concentrations of methyl chloroform have declined significantly. CFC-11 and CFC-113 concentrations are also decreasing. CFC-12 concentrations continue to increase, although at a slower rate each year than in previous decades.
  • Continued reductions in ozone-depleting chemicals in the stratosphere should lead to complete recovery of Earth's ozone layer during the twenty-first century. Computer models suggest that falling atmospheric ozone concentrations should level off by the year 2012 and then begin to increase to their normal levels. The annual occurrence of the Antarctic ozone hole is expected to cease by the year 2050.
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