The ozone layer that protects life from the sun's ultraviolet radiation has thinned to record low levels over the Arctic, the UN says.

The atmosphere over the Arctic lost 40 per cent of its ozone between the beginning of winter and late March, the World Meteorological Organization, or WMO, reported Tuesday. The organization is the UN's weather agency, and its report was based on ozone measurements around the Arctic from satellites, balloons and ground stations.

The previous record was a loss of 30 per cent over the whole winter.

As of Wednesday, the ozone loss was still continuing, reported Kaley Walker, an atmospheric physicist at the University of Toronto. She is part of the international group of scientists monitoring ozone over the Arctic.

300-arctic-ozone

Polar stratospheric clouds such as these over Sweden play an important role in ozone destruction over the poles. ((Reuters))

As of mid-March, the ozone hole already stretched from part way up Baffin Island over to Russia, she told CBC's Quirks & Quarks in an earlier interview. However, this week, it was pushed more towards Russia than the Canadian Arctic.

Later in the season, the ozone-depleted air above the Arctic may move toward lower latitudes, the WMO warned, and in that case people living in the northern hemisphere "can expect increased ultraviolet radiation as compared to the normal for the season."

While some ultraviolet radiation has been linked to skin cancer, cataracts and damage to the immune system, the WMO says the sun is still relatively low in the sky, and this limits the amount of radiation that passes through the atmosphere.

Leftover chemicals, climate change

The record ozone loss has been caused by ozone-destroying chemicals such as chlorofluorocarbons and halons left over from before the Montreal Protocol. That was the international agreement signed in 1987 to phase out those chemicals, which were used in products such as refrigerators, aerosol spray cans, fire extinguishers.

How ozone is lost

In winter, a stable mass may form over one of the Earth's poles. If it doesn't move, it becomes very cold because winter days over the poles feature 24 hours of darkness. That allows for the formation of polar stratospheric clouds — clouds made of ice and sometimes frozen sulphuric or nitric acid — where ozone-depleting chemicals accumulate.

When the sun returns in the spring, those chemicals are released, says Canadian atmospheric physicist Kaley Walker.

"The colder it is, the more clouds, the more chlorine we release, the more ozone we destroy."

The ozone loss continues until the stable air mass breaks up. As of April 6, that had not yet happened over the Arctic.

Levels of ozone-depleting chemicals in the atmosphere are starting to come down, Walker said. However, it may take decades for their concentrations to subside to pre-1980 levels as was agreed in the Montreal Protocol.

UN officials project the ozone layer outside the polar regions will recover to pre-1980 levels sometime between 2030 and 2040.

Meanwhile, climate change is producing conditions in the atmosphere that enhance ozone-destroying chemical reactions.

Such conditions have occurred over Antarctica every winter for a long time — because Antarctica is a land mass and it is colder than the Arctic — producing the well-known ozone hole there. But until recently, they occurred less frequently above the Arctic because of the effects of the Arctic Ocean.

Now greenhouse gases are trapping heat close to the surface of the Earth, preventing it from reaching the stratosphere above, where the ozone layer is found. Colder temperatures in the stratosphere boost ozone depletion.

On the upside, Walker said, CFCs are themselves powerful greenhouse gases, and banning them has averted some of the warming that might otherwise have occurred.

Arctic ozone
Move the slider up and down to compare the ozone above the Arctic in NASA images from March 2010 and March 2011