Although we're making progress in cutting down on the use of ozone-depleting chemicals, the milestone doesn't mean we've solved the problem yet, the agencies cautioned. These compounds can last for several decades in the atmosphere and are extremely damaging to ozone during that time, breaking it down in huge quantities. "But it's important to recognize that what we're seeing this year is due to warmer stratospheric temperatures".
Ozone lies in the stratosphere, seven to 25 miles about the Earth's surface.
The Montreal Protocol, a landmark global environmental treaty that took effect in 1988, has reduced CFC emissions worldwide. Taking action to reduce the amount of CFCs in the atmosphere has had a real impact. The elevated temperatures have slowed down the ozone depletion, keeping the hole at its small size, NASA explained in a video below.
Scientists at NASA's headquarters in Greenbelt, Md., explained that the change in temperatures that triggered warmer air in the Arctic, contributing to ozone gap shrinkage, was related to a "normal yearly phenomenon and not climate change".
These chemical reactions are maximised on the surface of high flying clouds, but milder than average conditions in the stratosphere above Antarctica this year inhibited cloud formation and persistence, according to a NASA statement. Since then, they have slowly declined but remain high enough to produce significant ozone loss.
This would be outstanding in a typical year.
Warming in the shape of "sudden stratospheric warming" events, were unusually strong this year, NOAA adds. About 19.3km above the Earth's surface, temperatures during September were 29 degrees warmer than average, NASA reported, "which was the warmest in the 40-year historical record for September by a wide margin".
In addition, these weather systems also weakened the Antarctic polar vortex, a ribbon of high-speed air encircling the South Pole, which typically concentrates the coldest air near or over the Pole itself. Winds dropped from a normal 161 miles per hour to about 67 miles per hour (259 kph to 108 kph), NASA reported. So, the ozone hole over Antarctica tends to be much bigger in the southern winter.
In the late 1970s, scientists discovered ozone levels in the atmosphere were dropping due to the heavy use of certain refrigerants, solvents, propellants, and foam-blowing agents. The space agency revealed that the hole in the ozone layer is the smallest since its discovery in 1982, and now, the hole had shrunk into 3.9 million square miles. This swiftly resulted in a binding worldwide treaty that many experts consider the most successful environmental agreement to date.