Ozone Loss: The Arctic

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.

"Winter in the Arctic stratosphere is highly variable - some are warm, some are cold, but over the last few decades, the winters that are cold have been getting colder.Why this occurred will take years of detailed study.” said Michelle Santee from Nasa's Jet Propulsion Laboratory (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.

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

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.