The ozone hole over the South Pole is roughly a third smaller than its average in recent years and it has split in two, according to researchers at the U.S. National Oceanic and Atmospheric Administration and NASA. Last month, the hole (actually a thinning of the layer of ozone that protects the Earth from ultraviolet rays) measured 6 million square miles. By contrast, for the last six years the hole averaged around 9 million square miles, and in 2001, it reached its maximum size of 10.2 million square miles — larger than North America. Scientists attributed this year’s improvement to warmer-than-normal temperatures over the Antarctic. (The chemical reactions that trigger compounds such as chlorofluorocarbons to begin destroying ozone bonds are temperature-dependent.) Ozone-destroying chemicals have been banned by international treaties, and some scientists expect the hole to begin repairing itself soon and to close entirely by as early as 2050.