London

Environmental scientists have struggled for years to dispel a popular misconception that global warming and the thinning ozone layer are one and the same. But researchers who have just completed the largest ever survey of Arctic stratospheric ozone now warn that the two processes are, indeed, connected.

The bad news is that climate change could prolong ozone depletion in the Arctic by many years — despite the success of the Montreal Protocol of 1987 in reducing emissions of ozone-destroying chemicals.

Preliminary results, released this week, indicate that Arctic ozone loss is back at the alarming levels seen in the mid-1990s, when three successive winters, from 1994–95 to 1996–97, raised serious concerns. This year, at altitudes of around 18 kilometres, more than 60 per cent of the ozone was destroyed.

If the polar vortex — a stable mass of cold air that forms above the Arctic each winter — gets cold enough, icy clouds form in the arid stratosphere. These polar stratospheric clouds harbour compounds containing chlorine and bromine, which destroy ozone.

For the past few months, that process has been under unprecedented scrutiny as more than 500 scientists have used a suite of research aircraft, balloons, satellites and ground-based instruments to monitor ozone depletion in the polar vortex.

The survey, which has just finished, was a collaboration between the European Commission, which funded the THESEO 2000 programme, and the United States, which called its project SOLVE. As Nature went to press, THESEO 2000 scientists were attempting a final balloon flight.

Into the vortex: NASA ‘spy’ planes have detected renewed ozone depletion in the Arctic stratosphere. Credit: NASA

As part of the survey, an ER-2 aircraft — a civilian version of the U2 spy plane, from the US space agency NASA — flew from Kiruna in Sweden into the vortex, recording the concentration of ozone-destroying compounds and a wide spectrum of chemicals including nitrous oxide, carbon dioxide and methane.

Using these measurements, scientists were able to ‘fingerprint’ discrete parcels of stratospheric air, and then sample them repeatedly over a period of time with a range of instruments. These analyses revealed steady declines in ozone levels from January.

The extent of ozone destruction in a given year depends on how cold the vortex gets, and how long it lasts. In both 1997–98 and 1998–99, the vortex was a few degrees warmer than in the middle of the decade, and broke up before the spring, when increasing daylight hastens the chemical reactions that destroy ozone. As result, the Arctic retained most of its ozone.

The damage can also be limited by the presence in the stratosphere of nitric acid, which combines with chlorine and prevents it attacking ozone. But if ice particles in the polar stratospheric clouds sink down from the stratosphere, they take nitric acid with them. That happened this year. “There was extensive denitrification in the polar vortex,” says Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The worry is that global warming will make this year's pattern more common. Most modellers believe that the Arctic stratosphere will cool as the lower atmosphere warms. Certainly, winters like this year's, in which the polar vortex has been cold and long-lasting, seem to be becoming more frequent. “One year does not prove a case,” says Newman. “But we have seen quite a few years lately in which the stratosphere has been colder than normal.”

http://www.ozone-sec.ch.cam.ac.uk

http://cloud1.arc.nasa.gov/solve/