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The Big Answer

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Ozone Depletion

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Between 1979 and 1986, the earth's ozone layer, which protects all plant and animal life from harmful ultraviolet radiation and helps regulate temperature, diminished 5 percent. While increased production of chlorofluorocarbons is blamed, it is uncertain if other pollutants might also be affecting it. Though many industrialized nations have phased out production in the last few years, there is no promise it will be completely halted. Although twice a group of 24 nations has met and pledged first (in 1987), that they would halve their CFC (clorofluorocarbon) production. And then in June 1990, 90 countries agreed to ban CFCs entirely by 2000, giving developing nations until 2010 to ban them.

However, in 1991 it was discovered that the ozone layer was being destroyed twice as fast as scientists had previously thought. In fact, in April of 1991 EPA chief William Reilly announced that ozone over the United States had been depleted 4 to 5 percent (far more than the 1.5 percent depletion noted in late 1987). Europe, the C.I.S. and northern Asia have similar losses, while areas at latitudes like Sweden and the Hudson Bay have 8 percent losses.

What this means is 200,000 additional deaths from skin cancer in the United States in the next 50 years (along with 400,000 already expected), more cataracts, weakened immune systems, crop damage and disrupted plankton reproduction, the mainstay of the marine food chain.

The hole in the ozone layer
The hole in the ozone layer

And it will get worse. The EPA's original estimates of 10 to 12 percent ozone loss over the next 20 years have been thrown out because they're too low. CFCs stay in the atmosphere for decades. Even with efforts to find substitutes for ozone-depleting substances whereby developing countries could ban CFCs by 2000 along with developed nations, too many CFCs are already on their way to the stratosphere for earlier bans to have much effect.

When CFCs reach the upper atmosphere, they are exposed to ultraviolet rays, which causes them to break down into substances that include chlorine. The chlorine reacts with the oxygen atoms in ozone and tears apart the ozone molecule. One chlorine atom can destroy over a hundred thousand ozone molecules, according to the U.S. Environmental Protection Agency.

The seasonal hole in the ozone layer over Antarctica was—in 2012—at its second-smallest point in the past 20 years, according to new research from NASA and the National Oceanic and Atmospheric Administration (NOAA). Could this be the first real sign that the ozone layer is recovering after decades of destruction? Too early to tell. (Source: Discover magazine) See also Ozone depletion.

The average thickness of the ozone layer is about 300 Dobson Units, or some 3 millimeters (the height of two pennies stacked together). The ozone hole is the region where concentrations drop below 220 Dobson Units.

Ozone Hole Area in millions of kilometers squared, and Minimum Ozone measured in Dobson Units, which is the most common unit for measuring ozone concentration

Yearmillions km2Dobson Units
1981 0.6209.5
1984 10.1163.6
1985 14.2146.5
1986 11.3157.8
1987 19.3123.0
1988 10.0171.0
1989 18.7127.0
1990 19.2124.2
1991 18.8119.0
1992 22.3114.3
1993 24.2112.6
1994 23.6 92.3
1996 22.7108.8
1997 22.1108.8
1998 25.9 98.8
1999 23.2102.9
2000 24.8 98.7
2001 25.0100.9
2002 12.0157.4
2003 25.8108.7
2004 19.5123.5
2005 24.4113.8
2006 26.6 98.2
2007 22.0116.2
2008 25.2114.1
2009 22.0107.5
2010 19.4128.0
2011 24.7106.2
2012 17.9139.1
(Source: NASA)