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arctic_ozoneAn ozone hole has formed over the Arctic. Like in the Antarctic so much ozone has dissolved it can now be called an "ozone hole".

The BBC reports
that scientist have calculated that at 20 kilometers above ground a total of 80% of the ozone is lost.

The cause was an unusually long spell of cold weather at altitude. In cold conditions, the chlorine chemicals that destroy ozone are at their most active.

"Winter in the Arctic stratosphere is highly variable - some are warm, some are cold," said Michelle Santee from Nasa's Jet Propulsion Laboratory (JPL).

"But over the last few decades, the winters that are cold have been getting colder.

"So given that trend and the high variability, we'd anticipate that we'll have other cold ones, and if that happens while chlorine levels are high, we'd anticipate that we'd have severe ozone loss," Santee said.

Read more on Nature website here.