Image caption The Arctic ozone hole lay over over populated regions for parts of winter and spring

Ozone loss over the Arctic this year was so severe that for the first time it could be called an "ozone hole" like the Antarctic one, scientists report.

About 20km (13 miles) above the ground, 80% of the ozone was lost, they say.

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.

It is currently impossible to predict if such losses will occur again, the team writes in the journal Nature.

Early data on the scale of Arctic ozone destruction were released in April, but the Nature paper is the first that has fully analysed the data.

"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.

Why [all this] occurred will take years of detailed study Michelle Santee, JPL

"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."

Ozone-destroying chemicals originate in substances such as chlorofluorocarbons (CFCs) that came into use late last century in appliances including refrigerators and fire extinguishers.

Their destructive effects were first documented in the Antarctic, which now sees severe ozone depletion in each of its winters.

Their use was progressively restricted and then eliminated by the 1987 Montreal Protocol and its successors.

The ozone layer blocks ultraviolet-B rays from the Sun, which can cause skin cancer and other medical conditions.

Longer, not colder

Winter temperatures in the Arctic stratosphere do not generally fall as low as at the southern end of the world.

Image caption Ozone destruction takes place within polar stratospheric clouds, with chlorine the main culprit

No records for low temperature were set this year, but the air remained at its coldest for an unusually long period of time, and covered an unusually large area.

In addition, the polar vortex was stronger than usual. Here, winds circulate around the edge of the Arctic region, somewhat isolating it from the main world weather systems.

"Why [all this] occurred will take years of detailed study," said Dr Santee.

"It was continuously cold from December through April, and that has never happened before in the Arctic in the instrumental record."

The size and position of the ozone hole changed over time, as the vortex moved northwards or southwards over different regions.

Some monitoring stations in northern Europe and Russia recorded enhanced levels of ultraviolet-B penetration, though it is not clear that this posed any risk to human health.

While the Arctic was setting records, the Antarctic ozone hole is relatively stable from year to year.

This year has seen ozone-depleting conditions extending a little later into the southern hemisphere spring than usual - again, as a result of unusual weather conditions.

Chlorine compounds persist for decades in the upper atmosphere, meaning that it will probably be mid-century before the ozone layer is restored to its pre-industrial health.

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