Hole in the sky: Europe, Canada and Russia at risk as continent-sized 'second hole' in ozone layer opens over the Arctic

The ozone loss over the Arctic was so severe this year that for the first time a 'hole' in the ozone layer, similar to the one over the Antarctic, appeared.

At a level of around 13 miles above the ground, 80 per cent of the ozone was lost, potentially exposing people on Earth's surface to harmful ultraviolet-B rays from the sun, which can cause sunburn and skin cancer.

The loss happened not because of increased use of ozone-destroying chemicals - now banned, and rarely used - but because cold high-altitude weather made the existing chemicals 'more active'.

An ozone hole five times the size of California opened over the Arctic this spring, matching ozone loss over Antarctica for the first time on record, scientists said today. The white line marks the area within which chemical ozone destruction takes place

The paper, published in science journal, Nature, said: 'Chemical ozone destruction over the Arctic in early 2011 was — for the first time on record — comparable to that in the Antarctic ozone hole.'

Data showing ozone destruction was released in April, but this is the first full analysis of the effect in the Arctic. Scientists cannot predict whether the effect will persist - but under certain conditions, predict 'severe ozone loss'.

'Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded,' said the report.

A Nasa-led study has documented an unprecedented depletion of Earth's protective ozone layer above the Arctic last winter and spring caused by an unusually prolonged period of extremely low temperatures in the stratosphere

This year's loss was recorded in the initial analysis carried out by the World Meteorological Organization. It wrote: 'Depletion of the ozone... has reached an unprecedented level over the Arctic because of the continuing presence of ozone-depleting substances in the atmosphere and a very cold winter in the stratosphere.'

When man-made substances such as CFCs are broken down by ultraviolet rays, chlorine is released - reacting with ozone and depleting the protective layer around the earth. Chlorine monoxide (measured in the map above) is present in one stage of the reaction. 

The current loss is thanks to the polar vortex, a weather system that circulates over the Arctic. The weather in the stratosphere remained cold for an unusually long period, and cold air spread over a larger area than normal. 

Weather in the stratosphere is often very different to the weather we experience. 

Dr Michelle Santee of Nasa's Jet Propulsion laboratory said, 'It was continuously cold from December to April, and that has never happened before in the Arctic.' 

Levels of harmful UV-B rays rose, but were not sustained. It's still not clear what risks this new hole has posed or will pose to human health.

While chlorine compounds such as CFCs have been phased out in most countries, they persist in the upper atmosphere for decades. 

Scientists predict that the current 'trend' for cold winters in the stratosphere will continue, and that the ozone hole may continue. 'Over the last decades, cold winters have been getting colder,' said Dr Santee. 'If other cold ones happen while chlorine levels are high, we'd anticipate severe ozone loss.'