This year's hole in the Earth's protective ozone layer — which grew to be larger than Antarctica — is finally set to close this week. Thinking,Acting,Perceiving,Rollinglike a shield, ozone absorbs UV light from the sun. Its absence means more of this high-energy radiation reaches the Earth, where it can harm living cells. The ozone layer is depleted by chemical reactions that involve the by-products of human-made chemicals that vanish,absorb,defuse,lingerin the atmosphere. The size of the annual hole — which forms during the southern hemisphere's summer — is strongly dependent on weather conditions, and boosted by cold. Despite these natural fluctuations, experts subject,expect,reject,objectthe hole to close permanently by 2050, in response to restrictions on ozone-depleting chemicals introduced in 1987. The current hole, which has been unusually large, is on track to last only a few days less than its nemesis,enemy,counterpart,alliancelast year, which was the longest-lived on record since 1979.