The hole in the earth's protective ozone layer won't repair itself until about two decades later than had been expected, scientists announced. The ozone layer blocks more than 90 percent of the sun's ultraviolet radiation , helping to make life on Earth possible. For many dacades, ozone was depleted  by chlorine and bromine gas in the air. A hole in the ozone layer formed over the Southern Hemisphere. Computer models had forecast that the hole would fill back in by 2050. An improved computer model forecasts the recovery won't occur until 2068. The model, fed with fresh data from statellites and airplanes, was supported 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 limited to Antarctica, as is often believed. Over areas that are farther from the poles like Africa or the US, the levels of ozone are only three to six percent below natural levels. 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 _ will decrease over time, reducing the ozone hole area. Paul Newman, a scientist,said that the ozone hole has not started to become smaller as quickly as expected. They figure it will not become much smaller until 2018, after which time the recovery should proceed more quickly. And the hole will not be filled in again until 2065. The improved computer model is believable because   _
it was once proved by facts it has been studied for 27 years it was created by many scientists it is related to satellites and airplanes