The annual ozone hole reached its peak extent of 6.3 million square miles (16. 4 million square kilometers) on Sept. 8, and then shrank to less than 3.9 million square miles (10 million square kilometers) for the remainder of September and October, according to NASA and NOAA satellite measurements. During years with normal weather conditions, the ozone hole typically grows to a maximum area of about 8 million square miles in late September or early October.

"It's great news for ozone in the Southern Hemisphere," said Paul Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But it's important to recognize that what we're seeing this year is due to warmer stratospheric temperatures. It's not a sign that atmospheric ozone is suddenly on a fast track to recovery."

Ozone is a highly reactive molecule comprised of three oxygen atoms that occurs naturally in small amounts. Roughly seven to 25 miles above Earth's surface, in a layer of the atmosphere called the stratosphere, the ozone layer is a sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants.