WASHINGTON, Oct. 25 (Xinhua) -- The hole above Antarctica in Earth's protective ozone layer this year was slightly smaller than average in recent decades, U.S. space agency NASA said Friday.
The September-October 2013 average size of the hole was 21 million square kilometers, while the average size measured since the mid-1990s when the annual maximum size stopped growing is 22.5 million square kilometers, NASA said in a statement.
However, it's too early to determine whether a healing of the hole has begun, the space agency said.
"There was a lot of Antarctic ozone depletion in 2013, but because of above average temperatures in the Antarctic lower stratosphere, the ozone hole was a bit below average compared to ozone holes observed since 1990," said Paul Newman, an atmospheric scientist and ozone expert at NASA's Goddard Space Flight Center.
The single-day maximum area this year was reached on Sept. 16 when the maximum area reached 24 million square kilometers, about equal to the size of North America. The largest single-day ozone hole since the mid-1990s was 29.9 million square kilometers) on Sept. 9, 2000, it said.
The ozone hole in the stratosphere is a seasonal phenomenon that starts to form in August and September. However, as the result of the 1987 Montreal Protocol, an international treaty to protect the ozone layer by phasing out production of ozone- depleting chemicals, levels of most ozone-depleting chemicals in the atmosphere have gradually declined, thus the size of the hole has stabilized, with variation from year to year driven by changing meteorological conditions.