The hole in Earth's protective ozone layer won't repair itself until about two decades later than had been expected, scientists announced yesterday.

The ozone layer blocks more than 90 percent of the sun's ultraviolet radiation, helping to make life as we know it on Earth possible. For many decades, ozone was depleted by chlorine and bromine gas in the air, produced by man-made chlorofluorocarbons. A hole in the ozone layer formed over the Southern Hemisphere.

Efforts to curb those chemicals have in recent years led to optimism that ozone would rebuild.

Computer models had predicted the hole would fill back in by 2050.

An improved computer model, from scientists at NASA, NOAA and the National Center for Atmospheric Research, predicts the recovery won't occur until 2068. The model, fed with fresh data from satellites and airplanes, was verified by the fact that it accurately reproduced ozone levels in the Antarctic stratosphere over the past 27 years.

The ozone hole is actually more of a broad region with less ozone than ought to occur naturally. It is not confined to Antarctica, as is often believed.

"Over areas that are farther from the poles like Africa or the U.S., the levels of ozone are only three to six percent below natural levels," explained the leader of the new study, Paul Newman, a research scientist at NASA's Goddard Space Flight Center. "Over Antarctica, ozone levels are 70 percent lower in the spring. This new method allows us to more accurately estimate ozone-depleting gases over Antarctica, and how they will decrease over time, reducing the ozone hole area."

Newman and his colleagues also found that the ozone hole has not started to shrink as much as anticipated. They figure it will not start shrinking a lot until 2018, after which time the recovery should proceed more quickly.

The new study confirms earlier work Newman and colleagues reported last year, in which they said the hole wouldn't replenish until 2065.

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