In the first three months of this year, something unprecedented happened in the skies over the Arctic. A large hole appeared in the ozone layer, far bigger than any seen there before.
The Arctic ozone layer suffers a little damage every winter, but the effect is normally short-lived. "This is a clear step beyond that," says Neil Harris of the University of Cambridge. As the measurements came in, ozone researchers began to debate whether the loss could be compared to that seen over the Antarctic. "It's the first time we've even discussed that question," says Harris.
Between 18 and 20 kilometres up, over 80 per cent of the existing ozone was destroyed. "The loss in 2011 was twice that in the two previous record-setting Arctic winters, 1996 and 2005," says Nathaniel Livesey of the Jet Propulsion Laboratory in Pasadena, California.
The hole was similar in size to those seen in Antarctica in the 1980s. The Antarctic hole has continued to grow since then, and is far larger today.
The Arctic ozone hole will have allowed more ultraviolet radiation than before through, but it is unlikely anyone has been seriously harmed, says Bruce Armstrong of the University of Sydney, Australia. "Occasional ozone depletion episodes such as this would add very little to the underlying population's risk of UV-related cancer."
Ozone killer
The question now vexing atmospheric scientists is why the hole grew so large, and whether it will open again. Livesey and his colleague Michelle Santee say the hole formed because the stratosphere remained cold for several months longer than usual. The cold air allowed water vapour and nitric acid to condense into polar stratospheric clouds, which catalyse the conversion of chlorine into chemically active forms that destroy ozone.
But we don't know why the stratosphere stayed cold for so long.
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