Shackleton Centenary Expedition

Ozone hole over Antarctica covered area size of North America November 04 2008

ozone loss was at its worst in 2006 when the hole covered more than 11.4m square miles.jpg

The ozone hole over Antarctica covered an area as big as North America at its peak in September, according to the latest figures.

Ozone loss was at its worst in 2006 when the hole covered more than 11.4m square miles. Nasa satellites measured the maximum area of this year's ozone hole at 10.5m square miles and four miles deep on 12th September.

"Weather is the most important factor in the fluctuation of the size of the ozone hole from year-to-year," said Bryan Johnson, a scientist at NOAA's Earth System Research Laboratory in Boulder, Colorado which monitors ozone, ozone-depleting chemicals, and greenhouse gases around the globe.

"How cold the stratosphere is and what the winds do determine how powerfully the chemicals can perform their dirty work."

The cycle starts in May, according to the NOAA, as Antarctica moves into a period of 24-hour-a-day darkness. Rotating winds the size of the continent create a vortex of cold, stable air centred near the South Pole that isolates CFCs over the continent.

When spring sunshine returns in August, the sun's ultraviolet light sets off a series of chemical reactions inside the vortex that consume the ozone. The colder and more isolated the air inside the vortex, the more destructive the chemistry.

By late December when the southern hemisphere summer is at its height, the vortex has crumbled and the ozone has returned until the process begins again the following winter.

NOAA atmospheric chemist Stephen Montzka said: "The decline of these harmful substances to their pre-ozone hole levels in the Antarctic stratosphere will take decades. We don't expect a full recovery of Antarctic ozone until the second half of the century."

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