By Liz Kalaugher, at the EGU General Assembly in Vienna
Spring 2011 has seen the largest-ever degree of ozone loss over the northern hemisphere, journalists at the EGU General Assembly in Vienna heard this morning.
This year about 40% of the ozone column above the Arctic has disappeared, breaking the previous record of 30%. The cause? An unusual persistence of cold temperatures in the stratosphere into March, allowing longer lifetimes for the polar stratospheric clouds that enable conversion of pollutant gases into ozone-destroying chlorine.
The ozone hole that forms above Antarctica each spring will start to recover in about 20 years. That’s according to David Hofmann of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory, who spoke at a press briefing at this year’s EGU meeting in Vienna, Austria.
Hofmann and colleagues have been taking ozone measurements from balloons above Antarctica since 1986. Their calculations of the rate at which ozone disappears as the sun rises above Antarctica in early September indicate that there are no signs yet of ozone hole recovery.
Indeed, Hofmann reckons that recent fluctuations in the size of the ozone hole each year have been down to meteorology. But on the plus side, for the last 6 or 7 years there’s been no evidence of a continuing decline – ozone loss rates have stabilized and the “patient isn’t getting any worse”.
“If you’re sick you’re very happy when you start to get better but it still takes a long time,” said Hofmann, continuing the human patient analogy.
When the sun first makes an appearance in the region on September 7th it kicks off the ozone-destroying properties of halogen molecules that have congregated on cold cloud surfaces during the winter month. By mid-October the resulting reactions have destroyed almost all the ozone in the band 14-21 km above the Earth. Later in the season, as the stratosphere warms and the clouds disappear, the ozone mostly returns.
During this first month of sunlight, Hofmann’s team found that the ozone concentration declines by roughly 12% a day. “It’s like uncompounding interest,” he said. And, in the band 16-18 km high, the ozone loss rate peaks at 15-20% a day.
Between 1986 and the early 2000s, this loss rate increased but since then it has stayed about the same. That’s probably due to a gradual decline in the amount of halogens in the atmosphere, following legislation restricting the use of chlorofluorocarbons, but the gases have a long lifetime.
As for the future, the team’s model suggests that spring ozone loss rates will be roughly constant for 15 to 20 years. Then the ozone hole will begin to recover and will approach normal levels in 2060-2070.
“Three or four years ago I would have said we will probably detect the patient getting better in two to three years and recovering in 30-40 years,” said Hofmann. But now the scientists have realised that recovery will be slower. “For us scientists who have to go to Antarctica and measure stuff it’s job security,” he added.
As generally seems to be the case, climate change complicates the picture further. As the troposphere warms, the stratosphere will cool to maintain the Earth’s energy balance. This will create more clouds in the stratosphere and more activated chlorine to destroy ozone. “Global warming will probably keep the ozone hole going a couple of years longer,” said Hofmann.