Ozone Hole History

The Scientific Assessment of Ozone Depletion: 2010 (World Meteorological Organization: Global Research and Monitoring Project–Report No. 52) issued (without date) by Ozone Secretariat of the United Nations Environment Programme (available in sections or complete pdf file) deals with, among other issues involving ozone-depleting substances (ODSs), the so-called Ozone Hole above Antarctica, the discovery of which did much to conduce the Montreal Protocol. Among the findings are:

  • The spring time (October) mean column ozone within the hole for the past 15 years has remained 40% below 1980 value and that the erythemal UV (ultraviolet radiation that causes skin burns) at the South Pole between 1991 and 2006 was 55–85% larger than the estimated values for the years 1963–1980.
  • The ozone hole has affected the surface climate of the Southern Hemisphere by changing the surface winds over the summer. In addition to warming the Antarctic peninsula the “changes in the winds have also been linked to regional changes in precipitation, increases in sea ice around Antarctica, warming of the Southern Ocean, and a local decrease in the ocean sink of CO2.”
  • Antarctic springtime ozone does not yet show a statistically significant increasing trend. This is because meteorological variability (year-to-year) is much larger than the expected response to the small ODS decreases in the Antarctic so far.
  • The hole is expected to decrease over the course of the century although “small episodic Antarctic ozone holes may occur even through the end of the century.”

While the Montreal Protocol has not yet seemed to have had a significant impact in reducing the ozone hole in Antarctica, it appears to have made a small impact on total emissions of greenhouse gas. Most OCDs are significant greenhouse gases with a long life. The UN report estimates that “In 2010, the decrease of annual ODS emissions under the Montreal Protocol is estimated to be about 10 gigatonnes of avoided CO2-equivalent emissions per year, which is about five times larger than the annual emissions reduction target for the first commitment period (2008–2012) of the Kyoto Protocol.” Nevertheless the substitutes for OCDs are also greenhouse gases. “As the majority of ODSs have been phased out, demand for hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) substitutes for the substances controlled under the Montreal Protocol has increased; these are also greenhouse gases.  HCFCs deplete much less ozone per kilogram emitted than chlorofluorocarbons (CFCs), while HFCs are essentially non-ozone depleting gases.”

The measurements contained in the maps above were made by NASA’s Total Ozone Mapping Spectrometer instruments from 1979–2003 and by the Royal Netherlands Meteorological Institute Ozone Monitoring Instrument from 2004–present. The units of ozone concentration are “Dobson Units.” NASA’s Ozone Hole Watch describes Dobson Units (DUs) as follows:

“One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth). Expressed another way, a column of air with an ozone concentration of 1 Dobson Unit would contain about 2.69×1016 ozone molecules for every square centimeter of area at the base of the column. Over the Earth’s surface, the ozone layer’s average thickness is about 300 Dobson Units or a layer that is 3 millimeters thick.”

The maps come from NASA’s Earth Observatory page, which describes them as follows:

“On September 17, 1979 (top left), the first year in which ozone was measured by satellite, the ozone level was at 194 Dobson Units. On October 7, 1989 (top right), the year that the Montreal Protocol went into force, ozone dropped to 108 DU. On October 9, 2006 (bottom left), ozone measured 82 DU. By October 1, 2010, the value was back up to 118 DU.

“The lowest value (deepest hole) ever recorded was 73 Dobson Units on September 30, 1994, while the broadest hole occurred on September 29, 2000, when the ozone-depleted area stretched 29.9 million square kilometers. The record for mean size of the ozone hole—the greatest extent over a one-month window—was September 7 to October 13, 2006, when the hole reached 26.2 million square kilometers. The mean ozone hole in 2010 was 22.2 million square kilometers.”

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