Ozone Layer Stabilizing But Not Recovered

by Alana Herro

After experiencing severe losses between 1979 and 1996, Earth’s ozone layer has ceased its precipitous decline, according to scientists with the U.S. National Oceanic and Atmospheric Administration.1 The average amount of ozone in the stratosphere in 2002–05 was similar to the average measured in 1998–2001, although it was still 3.5 percent below 1964–80 averages.2 (See Figure 1).

Meanwhile, at its annual peak, the “hole” in the ozone layer above Antarctica grew to 27.5 million square kilometers in 2006—close to the 28.7 million square kilometers reached in 2000.3 (See Figure 2.) Severe ozone losses are expected there for at least two more decades.4

The ozone layer protects Earth from harmful ultraviolet (UV) radiation by absorbing many of the sun’s UV rays. But the release into the atmosphere of certain chemicals, such as chlorofluorocarbons (CFCs) and methyl bromide, disrupts the ozone creation cycle, thinning this delicate shield. CFCs have been widely used for refrigeration purposes, aerosol propellants, and blowing agents.

In humans, high levels of UV radiation can cause sunburn and malignant melanoma, lesions and cataracts, and suppression of the immune system; in plants, they can cause DNA damage.5 When the Antarctic ozone hole widens, people in southern Chile and Argentina are advised to avoid direct sunlight to minimize their health risks.6

Much of the success in stabilizing atmospheric ozone levels can be attributed to the Montreal Protocol on Substances That Deplete the Ozone Layer, a treaty adopted in 1987 to reduce the release of ozone-depleting substances (ODS). As a result of scheduled ODS phaseouts in industrial and developing countries, CFC use decreased 96 percent between 1986 and 2005, to 41,200 tons, while methyl bromide use dropped to some 12,500 tons from 37,000 tons in 1995.7 (See Figure 3.) ODS persist in the stratosphere for many years, however, so decreased use does not immediately mean decreased accumulation. Meanwhile, use of hydrochlorofluorocarbons, a less-damaging CFC substitute that still contributes to some ozone loss, increased steadily— from less than 15,000 tons in 1992 to nearly 32,000 tons in 2005.8

Roughly 90 percent of the ozone in the atmosphere is found in the stratosphere, from 10–16 to 50 kilometers above Earth’s surface; the rest occurs in the troposphere (from the surface to 10–16 kilometers above).9 By 2005, total ODS levels in the troposphere had dropped 8–9 percent from their peak in 1992–94; though stratospheric ODS levels peaked in the late 1990s, reductions there are somewhat less because it takes a few years for near-surface trends to be reflected.10 The stabilization of the ozone layer has stopped the rise in surface UV radiation in unpolluted areas outside the poles and in some areas led to a slight decline in radiation.11

Production and use of harmful ODS has not ended completely, however. Exemptions for some ODS, such as methyl bromide for agricultural purposes, are slowing progress.12 Other challenges include the ongoing illegal trade in CFCs, growing legal production of ODS in developing countries, and the continued use of older refrigerators and other products that contain the chemicals.13

The consensus of most researchers is that ozone concentrations over Earth’s non-polar regions will return to pre-1980 levels between 2040 and 2050.14 Ozone concentrations over the Arctic are expected to reach pre-1980 levels at the same time or earlier, while those over the Antarctic are unlikely to do so until 2060–75 (and that is assuming continuing phaseout of ODS).15 The ozone hole is expected to remain large for at least a decade or so and will continue to fluctuate with meteorological conditions (it is larger in colder winters, for instance).16

Cyclic changes in UV radiation emitted by the sun affect ozone levels, since radiation initiates stratospheric ozone formation.17 A typical solar cycle can contribute to a 1–2 percent variation in total ozone levels.18 Volcanic eruptions deplete the ozone layer as well, by emitting large amounts of sulfur dioxide, which convert to aerosols that aid chlorine destruction of ozone.19 Neither of these factors, however, plays as large a role in ozone stability as the release of ODS does.20

Includes the following charts and graphs
Annual Global Mean Total Ozone Values, 1979-2005
Yearly Maximum Ozone Hole Size, 1979-2006
Consumption of Ozone-Depleting Substances, 1986-2005

Notes
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