AN ENCYCLOPEDIA OF SUSTAINABILITY
Heading: Environment Topic: Ozone depletion
While ozone, composed of three oxygen atoms instead of two in the normal oxygen in air, is a serious air pollutant in the troposphere near the ground surface, there is an ozone layer high in the stratosphere that is both created by and filters out dangerous ultraviolet radiation from the sun. This layer about 12-45 km above the ground level shields the Earth's surface from the Sun's damaging ultraviolet (UV-B) rays. Exposure to increased UV-B radiation at the Earth's surface is known to result in skin cancer, and unpredictable damage to plants, algae, the food chain and the global ecosystem. Without the protective ozone layer, most living things could not support direct sunlight.
Scientist first suggested in 1974 that man-made chloroflourocarbons (CFCs), widely used in aerosol cans and as a refrigerant, might cause ozone depletion, for which they later won a Nobel Prize. There is now conclusive proof that CFCs and similar chemicals are the cause of ozone depletion in the stratosphere, since chemicals found there could come from no other source (Russell et al., 1996). The reduction and elimination of production of many ozone-depleting substances in industrialized countries under the Montreal Protocol to protect the ozone layer is a major international environmental accomplishment (EEA, 1995). A decrease in levels of ozone depleting substances in the lower atmosphere has already been recorded (Montzka, 1996). However the damage to the ozone layer continues to accelerate, thinning twice as fast as predicted, for reasons scientists cannot explain (MacKenzie, 1995). The next ten years are expected to be the most vulnerable (Albritton, 1995).
The hole in the ozone layer over the Antarctic reached its largest size until that time in September 1998. It grew by more than 15 percent, exposing not only Antarctica but a large area of the Pacific and Atlantic oceans and the southern tip of South America to harmful ultraviolet radiation. The ozone reduction was more serious because the polar vortex of high level winds around a cold low-pressure centre was larger than usual, facilitating increased ozone destruction ( WMO, 1998). Major ozone layer losses are now occurring over the northern hemisphere as well, with serious losses since the winter of 1991-92 and a record hole in 1996 lasting two months that doubled carcinogenic ultraviolet rays over an area covering Scandinavia and extending from Greenland to Western Siberia (WMO, 1996). The lowest reading over Britain in 1996 showed a 47 percent reduction from the March average (Pearce, 1996). There was a 35 percent loss over Siberia in 1995, reductions of 10-15 percent over Europe as far south as Spain (Bojkov, 1995), and a loss of 5-18 percent in the U.S. (Komhyr et al., 1994). The greenhouse effect, which causes stratospheric cooling, may be contributing to ozone hole formation, and may also slow recovery even after ozone depleting substances start declining (Pearce, 1996; MacKenzie, 1995).
Satellite measurements in September 2000 revealed that the stratospheric ozone “hole” over the Antarctic had a reached a record 28.3 million square kilometres (some one million sq. km more than the previous record, in 1998). Earlier in the year, ozone depletion over northern latitudes also reached record levels, leading to predictions of a second ozone hole over the Arctic; such an event would expose many millions of people to dangerous doses of ultraviolet-B radiation.
The danger is that ozone-destroying chemicals are long-lasting and take time to travel up to the stratosphere. Chemicals released years ago are still present in the atmosphere and are contributing to today's peak concentrations.
Meanwhile, global climate change is thought to be slowing the ozone layer's healing process. The warming of the atmosphere near the ground causes the stratosphere to become even colder. Cold stratospheric temperatures, particularly during the early Antarctic spring, catalyze the chemical processes that destroy ozone molecules (UNEP, 2000).
The results of the WMO/UNEP scientific assessment of ozone depletion, released in 1998, confirmed the effectiveness of the Montreal Protocol on Substances that Deplete the Ozone Layer (WMO/UNEP, 1998). A full recovery of the Earth's protective ozone shield could occur by the middle of the 21st century if the Protocol is fully implemented. However, even though the Protocol is working well to reduce the use and release of ozone-depleting substances, the life of chemicals already released in the atmosphere will keep the depletion going for years to come.
The combined total abundance of ozone-depleting compounds in the troposphere (the lowest part of the atmosphere) peaked in 1994 and is now slowly declining. However, total concentrations of bromine are still increasing. In the northern polar latitudes, in six out of the last nine boreal winter-spring seasons, ozone has declined during some months by 25 % to 30 % below the 1960s average. In the Antarctic, the appearance of the ozone hole during the austral springs has continued unabated, with ozone column losses usually exceeding 50 % during the months of September and October. Only over the middle latitudes in both the northern and southern hemispheres has the ozone decline slowed in comparison with the previous scientific assessment in 1994. If measures had not been taken in accordance with the Montreal Protocol and its Amendments and Adjustments, the ozone decline would have been much stronger and would have continued for many more decades. Ozone losses in the stratosphere may have caused part of the observed cooling of the lower stratosphere in the polar and upper middle latitudes (about 0.6 degrees centigrade per decade since 1979). The increase of ozone in the troposphere since pre-industrial times is estimated to have contributed 10 % to 20 % of the warming due to the increase in long-lived greenhouse gases during the same period. The abundance of ozone-depleting substances in the stratosphere is expected to peak by the year 2000. However, when changing atmospheric conditions are combined with natural ozone variability, detecting the start of the ozone layer recovery may not be possible for perhaps another 20 years (WMO/UNEP, 1998).
REFERENCES AND SOURCES
Komhyr, W.D., R.D. Grass, R.D. Evans, R.K. Leonard and D.M. Quincy. 1994. "Unprecedented 1993 ozone decrease over the United States from Dobson spectrophotometer observations." Geophysical Research Letters 21:201-204. 1 February 1994.
Russell, James M., M.Z. Luo, R.J. Cicerone and L.E. Deaver. 1996. "Satellite confirmation of the dominance of chloroflourocarbons in the global stratospheric chlorine budget. Nature 379:526-529. February 1996.
Based partly on materials originally prepared for UN System-wide Earthwatch