A European Space Agency satellite has measured record low levels of ozone over the Arctic, creating a situation similar to the ozone hole over the Antarctic.

The Envisat satellite measures trace gases in the atmosphere as well as temperature of the sea surface. What it found was that in March of this year the levels of ozone over the Arctic, especially over the area between northwestern Russia and Greenland, were much lower than they were the previous year. Ozone levels were down in 1997, which was also an exceptionally cold winter. But the levels measured in 2011 were a record, according to the ESA.

It was caused by unusual weather in the winter of 2010-2011. The winds around the pole, called the polar vortex, were unusually strong. That isolated the air above the North Pole more than usual, and kept the region colder than it would ordinarily be.

A similar phenomenon occurs over Antarctica, but though that case the elevation of the land and the fact that it is surrounded by water (rather than the reverse, as in the Arctic) creates a region of relatively isolated air.

As Arctic spring approached in March, sunlight hit that colder air. The light caused the release of atoms of chlorine and bromine from chlorofluorocarbons (CFCs) in the atmosphere, which deplete ozone by combining with ozone molecules and breaking them into chlorine or bromine monoxide and ordinary oxygen.

Ozone forms a layer that sits about 25 kilometers (15 miles) above the Earth's surface. Ozone blocks ultraviolet radiation that can cause skin cancer and cataracts. The ozone layer was depleted for decades by CFCs left over from when they were a common refrigerant and propellant in aerosol sprays. CFCs have been largely phased out after the Montreal Protocol was signed by most nations in 1989.

One issue is whether the ozone depletion is linked to rising global temperatures, or whether a warmer world could help keep ozone in the atmosphere. A cooling stratosphere, which is what happens when the lower atmosphere gets warmer, could lead to more polar clouds at those altitudes, which would lead to ozone loss. But a warmer earth could also cause more air from the southern latitudes to mix with polar air, bringing more ozone with it and mitigating ozone depletion. Which mechanism it is will require more study, according to Mark Weber, from the University of Bremen in Germany.

While the ozone layer was depleted this year over the Arctic, that trend is actually likely to get better, as the reduction in ozone caused by the use of CFCs in the past has leveled off and the ozone layer is expected to recover by 2050.

Average Arctic total ozone for the month of March between 1979 and 2011, based on assimilated observations of several satellite instruments. Credits: KNMI