For the first time, seasonal ozone loss in the stratosphere over the Arctic qualifies as a full-blown “ozone hole.” Although winter-spring ozone holes over the Antarctic, associated with extreme cold and chlorofluorocarbons and other ozone-depleting chemicals, have been documented since the 1980s, ozone loss over the milder Arctic had been far less dramatic. An international team led by Gloria L. Manney at NASA’s Jet Propulsion Laboratory now shows that in 2011 the amount of ozone loss in the Arctic was comparable to that in some early Antarctic ozone holes. (Nature, DOI: 10.1038/nature10556). The cause, they believe, stems from an unusually lengthy cold period—conditions that promote formation of stratospheric clouds, providing surfaces on which reactions that convert chlorine into ozone-destroying species such as ClO take place. The researchers comment that atmospheric behavior in the Arctic is still poorly understood, and thus scientists are unable to predict if or when another major ozone loss will occur there.