The recovery is a result of the 1987 Montreal Protocol banning chlorine pollutants from the atmosphere, said Betsy Weatherhead, a researcher with the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration. But by the end of the century, ozone levels could be slightly higher or slightly lower than before 1980 because of high natural variability and human caused changes like warming temperatures, said Weatherhead.

A paper by Weatherhead and Signe Bech Andersen of the Danish Meteorological Institute in Copenhagen is featured on the cover of the May 4 issue of Nature.

"We now have some confidence that the ozone layer is responding to the decreases in chlorine levels in the atmosphere due to the leveling off and decrease of CFCs, and most of the improvements are in agreement with what we had hoped for with the Montreal Protocol in place," she said. "But we are not out of the woods yet, and the ozone recovery process still faces a number of uncertainties."

At high latitudes, for example, warmer temperatures at Earth's surface can trigger colder conditions in the lower stratosphere and promote the formation of polar stratospheric clouds, which can contribute to severe ozone depletion. "During the next few years, ozone levels in the Arctic will be strongly influenced by stratospheric temperature, possibly resulting in delayed recovery or record-low observations," the authors wrote in Nature.

The new study shows a larger than expected recovery of ozone in the northern mid-latitudes in recent years, she said. The increase may be partially a result of natural variability, includ
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3-May-2006