Three decades after nations around the world banned environmentally damaging chemicals used in refrigerants and aerosols, the hole in the ozone layer over Antarctica has begun to heal, scientists say.
In a new paper in the journal Science, a MIT-led team of researchers presented evidence that a 1987 international treaty is paying dividends by reducing the chemicals that destroy ozone, a vital layer of the atmosphere that protects life on earth from harmful ultraviolet radiation.
That accord, known as the Montreal Protocol, led to massive reductions of chlorofluorocarbons, which were commonly used in refrigerators, spray cans, and foams.
While previous studies have shown that ozone in the stratosphere has been repairing itself around the world, the new study shows clear signs of improvements over Antarctica, where cold temperatures and other unique atmospheric conditions have caused the worst depletion of the ozone layer.
“We can now be confident that the things we’ve done have put the planet on a path to heal,” said Susan Solomon, lead author of the study and an MIT professor of atmospheric chemistry and climate science. “This shows that international cooperation on environmental problems can work and be effective — that we can make choices, as a planet, that really can make a difference.”
The researchers found that the seasonal ozone hole has shrunk by more than 4 million square kilometers — about half the area of the continental United States — since 2000, when the amount of ozone had fallen to its lowest point.
The authors used what they called “fingerprints” in the ozone to conclude that the reduction of chlorine and bromine — the harmful components of chlorofluorocarbons — in the atmosphere is driving the recovery.
“This is an important, noteworthy paper that shows us that the Montreal Protocol is working,” said Ross Salawitch, professor of atmospheric and oceanic science at the University of Maryland, who was not involved in the study. “It provides a complete, comprehensive analysis of decades worth of data that provides a very compelling scientific argument that the ozone hole is truly healing.”
The paper follows a 2014 study by the World Meteorological Organization and the United Nations that predicted the planet’s overall ozone layer would be restored in the 2050s, returning to the stable levels of the 1980s. It takes decades for chlorofluorocarbons to be removed from the atmosphere.
Ozone protects humans from skin cancer and impairments that can cause blindness. The previous study found that the Montreal Protocol will prevent about 2 million cases of skin cancer annually by 2030.
It also found that the reduction of chlorofluorocarbons, which include heat-trapping gases, is helping reduce the impact of climate change.
Scientists first discovered the phenomenon of ozone depletion in the 1950s, and later linked ozone loss to the surge of chlorine in the atmosphere. Chlorine has a detrimental impact on ozone when it’s cold enough that polar stratospheric clouds can form.
Ozone depletion over Antarctica has started each year in late August as the region emerges from its dark winter, and the hole fully forms by early October.
Solomon’s team looked at ozone levels in September, when winter temperatures prevail and the ozone hole is still forming. They found that the hole has opened up more slowly as chlorine levels have decreased between 2000 and 2015.
“September is a better time to look because chlorine chemistry is firmly in control of the rate at which the hole forms,” Solomon said.
Her team analyzed ozone data from weather balloons and satellites, along with satellite measurements of sulfur dioxide spewed by volcanoes, which contributes to ozone depletion. They then compared ozone measurements with computer models that predict ozone levels based on the amount of chlorine scientists have estimated to be in the atmosphere.
They found that the decline paralleled the model’s predictions, and concluded that more than half of the reduction was caused by falling amounts of chlorine in the atmosphere.
The team also solved a riddle that had perplexed many scientists. In 2015, the ozone hole surprisingly surged to a record size. Some scientists began to question whether the reduction in chlorofluorocarbons was really working.
Solomon’s team was able to attribute the plummeting ozone to the eruption that year of the Chilean volcano Calbuco. The small particles launched into the atmosphere by an erupting volcano increase the amount of polar stratospheric clouds, which causes a chemical reaction with chlorine to reduce ozone.
David W. Fahey, director of the chemical sciences division at the NOAA Earth System Research Laboratory in Boulder, Colo., said the study has shown that the atmosphere is responding in the way that scientists hoped it would.
“Ozone is our life-support system,” he said. “This study draws attention to the need for continued vigilance, but it also shows that the controls on chlorofluorocarbons are working.”