Myth: Ozone Depletion Occurs Only In Antarctica
News about the ozone hole that forms over Antarctica each October has spread around the world. First formed in the early 1980s, the ozone hole can be as big as the U.S. and as deep as a 66% loss of ozone. However, less-well-known is that ozone depletion has been measured everywhere outside the tropics, and that it is, in fact, getting worse.
In March, 1999, over 260 of the world's top atmospheric researchers released their latest findings in a volume titled Scientific Assessment of Ozone Depletion: 2002 . Ozone levels vary by season and latitude. The report concluded that in the middle latitudes (most of the populated world), ozone levels have fallen about 10% during the winter and 8% in the summer. Since 1979, they have fallen about 4% per decade when averaged over the entire year. Depletion is generally worse at higher latitudes, i.e. further from the Equator. The Executive Summary to the Assessment is available from the WMO.
Ozone levels at Arosa, Switzerland clearly show a sharp drop beginning in the early 1970s. The graph to the left shows long-term ozone levels over Arosa, Switzerland. Although ozone levels rise and fall in natural cycles, the average level remained constant from 1926 until 1973. Beginning in 1973, however, and continuing through 2001, ozone levels have dropped at a rate of 2.3 percent / decade. Both the 11-year sunspot cycle and volcanic eruptions such as Mount Pinatubo in 1991 can affect total ozone levels temporarily. However, it is clear that at Arosa, neither had a long-term impact. The sharp decline beginning in 1973 demonstrates that the world is not experiencing a natural cycle in ozone levels. In fact, this decline matches the high rate of growth in the use of CFCs and other ozone-depleting substances.
Ozone depletion is not constant through the year. More detailed monthly data from the Swiss Institute for Atmospheric Scienceshow that most of the depletion occurs in winter and spring.
In addition, satellite measurements of ozone depletion over North America demonstrate ozone depletion occuring since 1979. Note that in the graph to the left, column ozone over the Seattle area was 391 Dobson units (DU). In 1994, however, ozone levels had dropped to 360 DU. Los Angeles saw a similar drop, from 368 DU to 330 DU. Finally, the Miami area ozone levels fell from 303 DU to 296 DU.
In general, ozone depletion is greater at higher latitudes. Thus, the decrease near Seattle will be greater than near Los Angeles, while Miami will see the smallest depletion of the three cities. However, southern cities also have much higher incidence of UVB light; even with less depletion, the net increase in UVB can be greater. While exact calculations cannot be made from this graph, it demonstrates that the ozone layer is being damaged over much of the globe, not just over Antarctica.
Finally, measurements over various cities in Canadashow ozone depletion. These measurements confirm that ozone depletion is not limited to Antarctica.