Britain's ozone levels near all-time low, scientists say

Arctic's record cold winter helps deplete radiation shield

David Adam, science correspondent
Saturday March 19, 2005
The Guardian

Scientists warned yesterday that levels of protective ozone over Britain are approaching record lows.

According to a monitoring centre in Germany, the ozone layer above Britain was reduced to half its normal thickness yesterday, and could get worse today. Ozone shields us from the sun's harmful ultraviolet radiation, which can cause skin cancer and cataracts.

Markus Rex, head of a European ozone monitoring programme in Potsdam, said a combination of the coldest Arctic winter on record and the current high pressure weather system over the north Atlantic had created ideal conditions for ozone loss.

"These two processes play together, and together they result in low ozone layer concentrations over the UK. It could get worse, it depends on what happens over the next month."

Dr Rex said the ozone layer over Britain was about 2.5mm thick yesterday, down from the usual 4mm-5mm. Anything below 2mm counts as a hole. An ozone layer half as thick will let in four times as much ultraviolet radiation. Scientists measure ozone thickness in Dobson units, 100 equalling one mm.

"It is a significant effect but it is not a completely unusual situation," he said. "And even if we have a large increase in UV, the exposure is still much smaller than in summer when the sun is at its highest."

Ozone depletion is a largely forgotten problem since the Montreal protocol successfully reduced levels of CFC chemicals in the atmosphere, after British scientists in Antarctica reported they were destroying ozone. But the chlorine-containing compounds take decades to degrade, and scientists say thinning of the ozone layer will probably get worse before it gets better.

Ozone levels decline over the Arctic and Antarctic in their respective springs as the returning sunlight kicks off the destructive chemical reac tions high in the atmosphere. Low temperatures accelerate this loss and most attention until now has been on the colder Antarctic, where a hole in the ozone layer has opened each spring since the 1980s.

Following an unusually cold Arctic winter - which some have linked to global warming - European scientists raised the alarm about northern ozone loss in January.

The European commission said: "Should further cooling of the Arctic stratosphere occur, increasing ozone losses can be expected for the next couple of decades. A hole in the ozone layer can lead to intensified harmful UV radiation affecting inhabited polar regions and Scandinavia, possibly down to central Europe. This could have consequences for human health as well as for biodiversity."

European scientists have been regularly checking ozone concentrations and weather conditions 12 miles up in the stratosphere using satellites, balloons and aircraft. They are particularly concerned about the formation over the Arctic winter of large, ice-laden clouds in and around the ozone layer. These polar stratospheric clouds, which form inside a swirling mass of cold air called the polar vortex, interfere with the chemistry of the high atmosphere and help to free chlorine from CFCs and speed ozone breakdown. The clouds that formed over the most recent winter are the largest seen over the Arctic for 20 years.

"The polar vortex is where the action takes place and where the situation gets bad," Dr Rex said. "If the vortex remains stable over the next few weeks, we would certainly expect to see much more ozone loss."

High pressure reduces ozone levels because it squeezes the protective gas away from the top of the troposphere, the lowest region of the atmosphere where most of the weather occurs.

The Arctic polar version moves around, unlike the Antarctic version that remains in the same place each year. This prevents concentrated ozone loss in a localised area, as occurs in the southern hemisphere each spring, but it makes UV exposure on the ground less predictable. The polar vortex was over Britain and parts of central Europe last week but has now passed further north. It could return next week, but some forecasts suggest it will start to break up.

John Pyle, an atmospheric chemist with the European ozone research coordinating unit in Cambridge, said: "It is interesting to us scientists and it is of concern, but we are not yet at the stage we have been at in the Antarctic for the last 15 years. I certainly would not want to create a scare story over it. If people ask if should they be staying indoors, the answer is no."

Rapid ozone loss begins when the temperature high in the stratosphere drops to around - 78C - until this year, only occasionally reached in the Arctic ozone during winter.

Much of the springtime ozone loss is usually made up by the formation of new ozone in the tropics, which is transported to the Arctic. "The chemical ozone loss should have been balanced by ozone transport," Dr Rex said. "This increase has been chewed away by the chemical ozone losses this winter."