WASHINGTON, Nov. 3 (Xinhua) -- The 2017 ozone hole that forms over Antarctica each September was the smallest observed since 1988, two U.S. government agencies have found.
The smallest ozone hole in 2017, however, is due to natural variability, not a signal of rapid healing, scientists from the U.S. space agency NASA and the U.S. National Oceanic and Atmospheric Administration (NOAA) said.
Measurements from NASA satellites showed the ozone hole reached its peak extent on Sept. 11, covering an area about two and a half times the size of the United States -- 7.6 million square miles (19.7 million square kilometers) in extent -- and then declined through the remainder of September and into October.
NOAA ground- and balloon-based measurements also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988.
"The Antarctic ozone hole was exceptionally weak this year," Paul Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center said in a statement this week.
"This is what we would expect to see given the weather conditions in the Antarctic stratosphere."
Scientists said an unstable and warmer Antarctic vortex -- the stratospheric low pressure system that rotates clockwise in the atmosphere above Antarctica -- helped minimize polar stratospheric cloud formation in the lower stratosphere, thus preventing chemical reactions that destroy ozone from happening.
The average area of the daily ozone hole maximums observed since 1991 has been roughly 10 million square miles (25.9 million square kilometers).
NASA and NOAA said the current ozone hole area is still large because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.
Scientists expected the Antarctic ozone hole to recover back to 1980 levels around 2070 because an international deal signed 30 years ago began regulating ozone-depleting compounds.
The ozone layer in the stratosphere, roughly seven to 25 miles (11 to 40 kilometers) above Earth's surface, acts like sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants.