Credit: KNMI/ESA

There are mixed tidings for Earth's protective ozone layer this year. Although the seasonal Antarctic ozone hole has already swollen to the size of Europe, the ozone layer worldwide seems to be on the road to recovery.

Ozone molecules in the atmosphere shield Earth from harmful ultraviolet radiation, but man-made chemicals called chlorofluorocarbons (CFCs) eat away at the protective layer. Last week, the European Space Agency's Envisat satellite recorded an exceptionally large seasonal ozone hole forming over the South Pole (shown right in dark blue). Only the holes of 1996 and 2000 have been larger at this time of year.

But everywhere except the polar regions, the ozone layer seems to have stopped thinning as early as 1996, a study now suggests. CFC use was phased out by the 1987 Montreal Protocol, and researchers have been monitoring ozone levels for signs of recovery.

The latest study relies on a sophisticated regression analysis of ozone measurements from satellite- and ground-based instruments taken between 1978 and 2002 (G. C. Reinsel et al. J. Geophys. Res. 110, D16306; 2005). It found evidence for a modest but lasting ozone recovery, particularly at high latitudes.

“Our findings do offer hope,” says Elizabeth Weatherhead, an environmental researcher at the University of Colorado at Boulder and one of the study's co-authors. “But we are still in a depleted state, and there has also been very little improvement in lessening of ultraviolet radiation since the mid-1990s.”

Last spring, scientists reported the biggest ozone losses ever recorded over the Arctic, probably due to unusually low winter temperatures (see Nature 435, 6; 200510.1038/435006b).

Experts are divided as to whether the turnaround in ozone levels can be attributed to the Montreal Protocol, and if so to what extent. But most researchers agree that because CFCs last for a long time in the atmosphere, ozone levels should have continued to fall until the end of the last century. Some scientists therefore argue that if ozone destruction peaked as early as 1996, other changes in atmospheric dynamics must also be at work.