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Can Ozone Depletion be Reversed?

Introduction

Can the hole in the ozone layer be repaired? Yes. If concentrations of ozone-destroying chemicals are reduced, the natural balance between ozone creation and destruction can be restored. However, this might require the complete elimination of CFCs, halons, carbon tetrachloride, methyl chloroform, and methyl bromide. In late 1991, scientists estimated that even with the current global schedule to eliminate ozone-destroying substances, the ozone layer would not return to 'normal' (pre-1980 chlorine levels) until the middle of the 21st century. Nevertheless, the 1998 World Meteorological Organiszation Scientific Assesment of Ozone Depletion observed that abundance of ozone-depleting compounds in the lower atmosphere (below the stratosphere) is now slowly declining from a peak in 1994.

Phasing Out CFCs

The initial concern about the ozone layer in the 1970s led to a ban on the use of CFCs as aerosol propellants in several countries, including the U.S. However, production of CFCs and other ozone-depleting substances grew rapidly afterwards as new uses were discovered. Through the 1980s, other uses expanded and the world's nations became increasingly concerned that these chemicals would further harm the ozone layer. In 1985, the Vienna Convention was adopted to formalise international co-operation on this issue. Additional efforts resulted in the signing of the Montreal Protocol in 1987. After the original Protocol was signed, new measurements showed worse damage to the ozone layer than was originally expected. In 1992, reacting to the latest scientific assessment of the ozone layer, the Parties decided to completely end production of halons by the beginning of 1994 and of CFCs by the beginning of 1996 in developed countries.

Between 1986 and 1991, world-wide consumption of CFCs-11, CFC-12 and CFC-113 decreased by 40%, ahead of the schedule outlined in the Montreal Protocol and faster even than called for in the more ambitious 1990 London Amendment to the Montreal Protocol. Manufacturers, who were earlier convinced that CFCs were unique and irreplaceable, were finding themselves moving quickly to alternative processes and chemicals. Hydrocarbons have replaced CFCs as aerosol propellants and as blowing agents for foams, and as cleansing solvents in electronics manufacturing. HCFCs, which have much smaller ozone depleting potentials have replaced CFCs for refrigeration and air conditioning.

Are International Agreements Enough?

Without the Montreal Protocol, continued use of CFCs and other compounds would have tripled the stratospheric abundance of chlorine and bromine by 2050. Because of measures taken under the Protocol, emissions of ozone-depleting substances are already falling. Under current agreements, the stratospheric concentrations of chlorine and bromine are expected to reach their maximum within a few years and then slowly decline, although concentrations of chlorine are already falling in the troposphere. With evidence that international agreements to phase out the use of ozone depleting chemicals appear to be working, both NASA (the North American Space Agency) and NOAA (the National Oceanographic and Atmospheric Administration) in the United States of America have expressed confidence that, all other things being equal, the stratospheric ozone layer should return to normal by the middle of the next century. The recovery of the ozone layer will be gradual because of the long times required for CFCs to be removed from the atmosphere; some take as long as several hundred years. Nevertheless, the likelihood remains that deep ozone holes will continue to form annually in the polar regions, well into the next century. This situation will persist until stratospheric chlorine levels decrease.

According to the Intergovernmental Panel on Climate Change (IPCC), even if the control measures of the 1990 London and 1992 Copenhagen Amendments were to be implemented by all nations, the abundance of stratospheric chlorine and bromine will increase over the next several years. The Antarctic ozone hole, caused by industrial halocarbons, will therefore recur each spring. In addition, since these gases are also responsible for the observed reduction in middle and high latitude stratospheric ozone, the depletion at these latitudes is predicted to continue unabated for at least 5 to 10 years.