An international agreement to phase out use of chemicals that damage the ozone layer appears to be working. A new report finds that ozone-depleting substances in the atmosphere are down by 10 to 15 percent, and that the ozone layer is by and large getting thicker.
The reason is that nations have followed through on commitments made under the Montreal Protocol and related pacts to phase out use of chloroflurocarbons (CFCs) and halons, according to the new assessment (PDF) released this week by the United Nations Environment Program and the World Meteorological Organization. About 300 scientists contributed to the report.
The ozone layer is a thin film of gas in the stratosphere. It protects the Earth from the Sun's ultraviolent rays, which can cause skin cancer, eye damage, and other forms of ill health for both animal and plant life on Earth.
CFCs and halons were common in products like refrigerators, fire-fighting foams and aerosol spray cans. But from the early 1970s onward, evidence mounted that UV radiation broke down these compounds in the mid-stratosphere (about six miles above the Earth's surface), resulting in the release of chlorine and bromine atoms that break down ozone (O3) molecules. The U.S. Environmental Protection Agency estimates that one chlorine atom can rip apart over 100,000 ozone molecules.
After the Montreal Protocol came into effect in 1989, countries began phasing out manufacture and use of ozone-destroying substances. There have been signs in the past 10 to 15 years that the atmosphere's "ozone column" is thickening in places, suggesting that the ban is working. The new report estimates that by 2050, the ozone layer in the Arctic and middle latitudes should return to roughly the condition it was in in 1980. Because natural atmospheric conditions cause air pollutants to concentrate over the poles, the seasonal ozone hole over Antarctica each spring (which has caused changes in the summer climate of the Southern Hemisphere) will take longer to heal.
A side benefit of CFC reduction is that it may be helping to blunt the progress of global warming, since CFCs are also powerful greenhouse gases. The assessment estimates that in 2010, lowered emissions of ozone depleters equated to keeping around 10 metric gigatons of CO2 out of the atmosphere, “which is about five times larger than the annual emissions reduction target” for 2008-2012 under the Kyoto Protocol climate treaty.
There are some warnings in the report as well. Some of the compounds being swapped in for ozone depleters – such as hydrofluorocarbons (HFCs) – are also potent greenhouse gases. If their use increases as predicted, they will contribute quite a lot to surface temperature rises.
As well, carbon tetrachloride (CCl4) levels remain unexpectedly high, even though the substance was banned under the Montreal Protocol. Participants in the treaty reported no new emissions of CCI4, which was used in fire fighting and dry cleaning, between 2007 and 2012. NASA credits the high levels to "unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl4 sources."
The UN assessment also warns that the options available for stopping future damage to the ozone layer are becoming limited as most of the most straightforward actions play themselves out. These have ranged from ending the production of ozone-harming substances, to the destruction of "banks" of destructive chemicals and upgrading to appliances that don't contain CFCs. Presumably more ingenuity will be required on humanity's part to continue making progress -- by coming up with new, safe chemicals and technologies -- as well as not repeating the mistakes of the past.