For the first time in 30 years, the gaping hole in the ozone layer above Antarctica is showing signs of healing.
Every year since it was discovered in 1985, scientists have watched the hole grow bigger from one Antarctic spring to the next, eventually covering 10.9 million square miles in 2015.
Now researchers are noting an encouraging trend. Though the hole still exists and reached a record size last year, it is forming at a slower rate, according to a report published Thursday in the journal Science.
Thanks to human actions to curb the use of ozone-destroying chlorofluorocarbons, or CFCs, the hole has started growing later in the spring, the study’s authors said, and they can foresee a time, around the middle of the century, when it’s gone.
“We are starting to see signs of improvement over Antarctica,” said Paul Newman, an atmospheric scientist at NASA’s Goddard Space Flight Center who monitors the hole but was not involved in the study.
Earth wears the ozone layer like a thick blanket. The invisible gas blocks the majority of the sun’s harmful ultraviolet radiation. Without that shield, life as we know it would not be possible.
The ozone layer sits in the stratosphere, a region of the atmosphere that begins about 10 miles above the ground — higher than planes can fly — and extends an additional 20 miles above that.
The hole in the ozone layer comes and goes over the course of each year. The conditions for creating it begin in late August, and it reaches its maximum size in October. Scientists tracking the state of the hole have typically focused on its size at that latter point.
The new study takes a closer look at what happens earlier in the spring, when the hole begins to develop.
The stratosphere above Antarctica is a particularly dangerous place for ozone to be when winter gives way to spring, said study leader Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology.
“Antarctica is really the coldest place on Earth,” Solomon said. The extremely cold temperatures cause thin, wispy clouds to form high in the stratosphere, creating a perfect place for the byproducts of CFCs — hydrochloric acid and chlorine nitrate — to touch down.
When they react on the surface of these clouds, they release chlorine gas. Then the sun, which has just returned to the South Pole, provides energy that splits the gas into two single chlorine atoms. These atoms steal oxygen from the ozone and break it down.
“Spring is the Goldilocks time,” Solomon said. “There’s enough sunlight to drive the chemistry, and you have cold enough temperatures” for the clouds to keep CFC byproducts in the mix.
The longer the process continues, the more ozone is destroyed.
By October, the ozone layer “has been punched out of existence,” Newman said.
Even before the hole was discovered in the 1980s, scientists had realized that man-made CFCs could be damaging to the atmosphere.
CFCs are nontoxic and nonexplosive chemicals that work very well as refrigerants, propellants and solvents. As a result, they were widely used in air conditioning systems, cans of hair spray and a variety of other consumer and industrial products.
That changed with the signing of the Montreal Protocol in 1987. The international treaty called on countries to phase out the use of most CFCs by 1996.
The ban went into effect at the start of 1989, but scientists didn’t see any immediate reduction in the concentration of CFCs in the stratosphere. The stability of these chemicals — once touted as one of their biggest benefits — meant they could remain in the atmosphere for 50 to 100 years.
Ozone is measured by sensors attached to large balloons that are released into the air. They rise at a speed of roughly 11 mph, taking measurements as they go and transmitting data back to a ground station. They can travel about 22 miles up before bursting and drifting back to earth.
Satellites provide additional information on ozone-layer thickness by recording the UV light that is reflected off of it.
Solomon and her colleagues examined data going back to 1970 to see whether ozone levels had improved in the early part of the hole-forming season.
Although they didn’t see that much difference from one October to the next, they could see that between 2000 and 2014, ozone levels during September had improved. The progress was in line with predictions made by computer models designed to simulate the impact of CFC reductions.
Then came 2015. The size of the ozone hole in October broke a record, even though levels of CFC byproducts in the atmosphere were still falling.
“Could it be that the volcanoes are holding back the ozone from recovering?” Solomon said.
When the researchers considered the effects of the sulfur particles sent into the atmosphere by volcanoes — particularly Calbuco in southern Chile, which erupted in April 2015 — they could see that the answer was yes. In fact, the Calbuco eruption increased the size of the ozone hole in September by 2.7 million square miles.
Without the volcano, the researchers said the hole would have continued to heal.
“We took action, and here we are 30 years later,” Solomon said, “seeing that that action has produced the positive result that we hoped for.”
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