The wound to the ozone layer above Antarctica, a menace to most of the marine food chain, is showing signs of healing. A Science paper reporting the news suggests we may be seeing the back of one of the worst environmental threats humanity has faced, and a vindication of the international cooperation that addressed the problem.
Many of the gasses humanity once used to propel our aerosol cans, refrigerate our food and even clean electronic equipment can, upon reaching the stratosphere, catalyze the destruction of ozone molecules at high altitude. Fears that this would expose the planet to lethal doses of ultraviolet radiation led to only limited action when they were raised in the 1970s. However, when it emerged that atmospheric conditions were enhancing this effect over Antarctica every spring, the world moved with a speed that's sadly lacking on more recent challenges.
Penguins' food supply remains threatened by ozone depletion, but the worst may be over. Polina Melnyk/Shutterstock
Nevertheless, 29 years after the signing of the Montreal Protocol to address the problem, scientists are still unsure if things are getting better. According to a team led by Professor Susan Solomon of Massachusetts Institute of Technology, signs that the hole has started shrinking are there if you know where to look.
Although the Montreal Protocol is a superb example of swift international action to tackle a global problem, the gasses it covered were only phased out (rather than eliminated immediately). Moreover, many take years to reach the heights where they do the most damage. Consequently, the ozone hole over Antarctica went on getting worse for many years.
There is also considerable year-to-year variation in the extent of depletion, driven by climatic effects and volcanic activity. All this makes it hard to work out exactly when the hole reached its peak and started to shrink, if it yet has.
The hole is at its largest in the Southern Hemisphere spring (September to November) and, as the paper observes, “most analyses of Antarctic ozone recovery to date consider October or Sep-Oct-Nov averages. The historic discovery of the Antarctic ozone hole was based on observations taken in October, and healing cannot be considered complete until the ozone hole ceases to occur in that month, around mid-century. However, October need not be the month when the onset of the healing process emerges.”
Even though October depletion is, on average, the worst, the authors note that there is more variation between years in October than in other months, including a record expanse in 2015.
Solomon's wisdom lay in realizing that annual variation is smaller earlier in the season, making it easier to detect a signal in the noise. She examined balloon and satellite observations of September's ozone at high southern latitudes and found with 90 percent confidence that recovery is occurring. The size of the hole in September has shrunk by an estimated 4.5 million square kilometers (1.7 million square miles) – an area twice the size of Greenland – since 2000.
Eventually, September's progress will flow into October, and we can all sigh with relief.