This image, provided by Nasa, captures this year’s ozone hole over Antarctica, which has been declared by scientists in the US as the biggest and deepest on record.

"From September 21 to 30, the average area of the ozone hole was the largest ever observed, at 10.6 million square miles (27.4 square kilometres)," said Paul Newman, of Nasa’s Goddard Space Flight Centre outside Washington.

The ozone layer, a form of oxygen in the upper atmosphere which shields Earth from the sun’s harmful ultraviolet rays, thins out over the South Pole each year. This is largely due to human-made compounds releaing chlorine and bromine gases into the stratosphere, eating into the barrier and causing the hole.

If the stratospheric weather conditions had been normal, the ozone hole would be expected to reach a size of about 8.9 million to 9.3 million square miles (23 million to 24 million square kilometres), about the surface area of North America, Nasa said in a statement.

The size of the ozone hole varies year to year depending on temperature variations in the Antarctic stratosphere. Colder temperatures result in larger and deeper ozone holes, while warmer temperatures lead to smaller ones. This year, the lower stratosphere was about 5 degrees Celsius (9 degrees Fahrenheit) cooler than average.

As a result of international bans on ozone-depleting substances, concentrations of harmful chemicals in the lower atmosphere have been declining since 1995.

However, many of the chemicals have extremely long lifetimes once released into the air. While there are year-to-year variations, scientists expect a slow recovery of the ozone layer by the year 2065.

The blue and purple colours indicate where there is the least ozone, and the greens, yellows, and reds are where there is more of a presence.

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