The hole in the Earth’s ozone layer that formed over the Antarctica this September was about 20 percent smaller than last year, scientists say. Early indications are that the ozone hole may be healing, says one top scientist. 

The hole in the Earth’s ozone layer that forms over Antarctica each September grew to about 8.9 million square miles this September before starting to recover, according to scientists from NASA and the National Oceanic and Atmospheric Administration (NOAA) who monitor the annual phenomenon.

In 2015 the ozone hole grew to 10.9 million square miles, 2 million square miles larger than this year, before returning to relatively normal summer levels. Its larger size last year was due to colder-than-average temperatures in the stratosphere that amplified the destruction of ozone by sunlight reacting with chlorine and bromine from man-made chemicals, scientists said. In 2016, warmer stratospheric temperatures constrained the growth of the ozone hole.

“This year we saw an ozone hole that was just below average size,” said Paul A. Newman, chief scientist for Earth Sciences at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.  “What we’re seeing is consistent with our expectation and our understanding of ozone depletion chemistry and stratospheric weather.”

At its peak on September 28, the ozone hole extended across an area nearly three times the size of the continental United States. The average area of the hole observed since 1991 has been roughly 10 million square miles.

A research team led by Susan Solomon, a professor of atmospheric chemistry and climate science at MIT, found multiple lines of evidence for the healing. The findings were published Thursday in Science.

Solomon’s team found that, in recent years, the hole is not eclipsing the 12-million-square-kilometer threshold until later in the southern spring, which indicates that the September hole is shrinking. In fact, the researchers believe the ozone hole has shrunk by more than 4 million square kilometers. Furthermore, the hole is not as deep as it used to be.

“The fact that the ozone hole is opening later is really the key here,” Solomon told National Geographic. “It is opening later, it is smaller, and its depth is depleted. All of the measurements are independent, and when they all point to this [healing], it is hard to imagine any other explanation.”

Ozone, which occurs naturally in small amounts in the atmosphere, is comprised of three oxygen atoms as opposed to the two that make up the much more abundant molecular oxygen. High in the stratosphere, roughly 6 to 30 miles above the surface, the ozone layer acts like sunscreen, shielding the Earth from potentially harmful ultraviolet radiation that can cause skin cancer, cataracts, and suppress immune systems, as well as damage plants. Ozone is also one of the primary greenhouse gasses that regulate the Earth’s temperature.

This time-lapse photo shows a weather balloon and ozonesonde rising into the Antarctic sky during the 2016 research season. Credit: Christian Krueger/Ice Cube.

First detected in 1985, the Antarctic ozone hole forms during the Southern Hemisphere’s late winter months of August and September as the sun’s rays return after months of polar night. The sunlight initiates catalytic reactions that produce chemically active forms of chlorine and bromine concentrated over the South Pole during winter. These reactions rapidly destroy ozone molecules.

In addition to the area of the ozone hole, scientists also measure the concentration of ozone that would be found in a column of atmosphere extending from the surface to the edge of space. The most common unit for measuring ozone concentration is the Dobson Unit, which is the number of ozone molecules that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 32 degrees Fahrenheit at an atmospheric pressure equivalent to the Earth’s surface.

This year, the ozone layer reached a minimum concentration of 114 Dobson Units on October 1. In 2015, the ozone layer reached a minimum of 101 Dobson units on October 4.  During the 1960s, before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 260 to 320 Dobson units.