The ozone hole over the South Pole is taking longer to recover than previously thought, according to a recent study by the National Aeronautics and Space Administration (NASA) and two other federal agencies. The study, carried out using a new computer modeling system, projects that the protective ozone layer high over Antarctica (in the stratosphere) will take nearly 20 years longer to fully mend than scientists had estimated. Earlier projections indicated that the ozone layer would recover by 2050; the new findings put recovery in 2068.

Ozone depletion occurs throughout the stratosphere but is pronounced at the poles, where the ozone layer becomes so thin at times that it is nearly nonexistent--that is, a "hole" forms. The largest hole appears over Antarctica during the Southern Hemisphere's spring and exposes nearly the entire continent to higher doses of the sun's ultraviolet radiation. Human activity takes most of the blame: Air currents carry the anthropogenic chemical compounds chlorofluorocarbons (CFCs) and halons into the stratosphere, which produce chlorine and bromine gases that, in turn, destroy ozone.

The modeling system combines several data sources. These include estimates of future Antarctic chlorine and bromine levels based on current amounts (from observations by NASA satellite, National Oceanic and Atmospheric Administration ground-level stations, and National Center for Atmospheric Research aircraft); likely future emissions; the time it takes for the transport of those emissions into the Antarctic stratosphere; and assessments of future weather patterns over Antarctica. The 1987 Montreal Protocol and other international agreements banned the production of CFCs and other chemicals linked to ozone depletion, but these compounds and their effects still persist in the environment. Scientists with the NASA project say that the hole over Antarctica has not shrunk significantly since the ban went into effect, and the ozone layer may not begin to show real progress toward recovery until 2018.

The stratospheric ozone layer (not to be confused with harmful ground-level ozone, a primary ingredient of smog) prevents 90-99 percent of the sun's ultraviolet radiation from reaching the Earth's surface. Without such protection, such radiation can cause skin cancer, genetic damage, and eye damage and can harm marine life.