Box 1. How can we tell if the Montreal Protocol is working?

The ozone hole grew during the 1980s as emissions of chlorofluorocarbons and other gases built up in Earth's atmosphere and ozone-depleting chemistry took place at ever-faster rates. By the late 1990s, enough nations had signed the Montreal Protocol to drastically reduce global emissions of most of the key gases. How can we be confident that these actions are working?

Monitoring these gases in our atmosphere provides direct proof. The great bulk of industrial chemicals that contribute to changes in the ozone layer are remarkably inert and so survive in the atmosphere for decades or even centuries. The mix of factors that controls the build-up and decline of such gases — production, emission and lifetime — can be illustrated by data for one major chlorofluorocarbon, CFC-11 (see inset).

Global production and emission for CFC-11 (see inset and refs 7,12) are based upon industrial estimates, and are thought to be accurate to about 15%. Production is not necessarily the same as emission in a given year because of delayed release from existing materials, such as thermal foam insulation in older buildings or refrigerators⁷. Consumers and policy-makers who wish to accelerate the recovery of the ozone should consider responsible recovery of such materials, as is done when the materials in discarded refrigerators are recycled rather than shredded.

The third factor, lifetime, is reflected in the comparison of the observed and calculated abundances of CFC-11. Observations from both hemispheres^{8, 9, 10} (data points) are shown alongside the calculated abundances based on global emission estimates (curves) and adopting atmospheric lifetimes of 1, 10, 50, 150 or 500 years. These show that the atmospheric lifetime of CFC-11 is about 50 years. If it were much shorter, the gas's abundance could never have built up to its present value and would have dropped more steeply by now; much longer and the concentrations would have peaked at higher values and would be continuing to increase slowly today despite the greatly reduced emissions of recent years.

The slow decline in abundances now being observed by scientists worldwide supports, within data uncertainties, the true 50-year lifetime of CFC-11 and illustrates the success of the Montreal Protocol. More evidence that the protocol is working comes from the gradual closing of the gap between Northern and Southern Hemisphere abundances measured over the past decade. During the build-up of emissions in the 1970s and 1980s there were far larger releases of CFC-11 in the more heavily populated Northern Hemisphere, but as emissions declined after 1990, atmospheric mixing acted to even out the global distribution. Similar behaviour has been documented for many other chlorofluorocarbons. These data show that the Montreal Protocol is working, while emphasizing the need for patience as we seek signs of genuine recovery in the ozone layer over future decades.

S.S.