Thanks to warmer conditions in the stratosphere over Antarctica, 2017's ozone hole is the smallest it's been since 1988.
The ozone layer first garnered attention in 1974, when Paul Crutzen, Mario Molina and F. Sherwood Rowland each earned a Nobel Prize for their research on ozone depletion caused by chlorofluorocarbons (CFCs), a common ingredient used in spray cans and refrigerants at the time.
According to Smithsonian magazine, the discovery prompted scientists to examine the health of the ultraviolet-blocking ozone layer. Then, 11 years later, the problems were more dire than imagined.
Richard Farman, an experienced Antarctic atmospheric scientist, initially thought his instruments were broken when he detected a dramatic drop in ozone levels all over the frozen continent. His instruments were operating fine -- he'd just discovered the annual event known as the ozone hole.
Each year in spring (fall in the Northern Hemisphere), cold air over Antarctica forms polar stratospheric clouds, which cause CFCs and other pollutants such as bromine to catalyze a reaction that destroys ozone. The hole shrinks as the season progresses, and the process repeats itself the next year.
The discovery of the hole prompted widespread panic in the 1980s, with one environmentalist referring to it as "AIDS in the sky," Smithsonian magazine reports.
Stratospheric ozone shields the earth from ultraviolet rays; without it, humans would be more likely to develop skin cancer. Vital crops and forests would also suffer damage.
Residents of the Southern Hemisphere feared for their health, prompting world leaders to ban CFCs under 1987's Montreal Protocol. According to CBS, the United Nations determined that had the Montreal Protocol not been adopted, two million more people would have developed skin cancer by 2030.
Scientists at NASA and the National Oceanic and Atmospheric Administration (NOAA) say that stratospheric ozone has been declining since the Montreal Protocol, although it is still not entirely healed. They do not expect the hole to return to 1980 levels until 2070, mostly because CFCs remain the atmosphere for a long time.
The average area of the maximum ozone hole size since 1991 is about 10 million square miles. The largest maximum was 11.5 million square in 2000, CBS reports.
The ozone hole's maximum size in 2016 was 8.9 million square miles, a 2 million square mile reduction from the previous year. In 2017, the ozone hole reached a peak of 7.6 million square miles on Sept. 11. That's about two and a half times as large as the United States, The Washington Post reports.
While the improvement in stratospheric ozone is a good thing, NASA and NOAA scientists say that the hole's smaller size is more indicative of annual variation in weather patterns than a spontaneous change in ozone levels. Even with recent recovery of the Antarctic ozone layer, it's still nowhere near what it used to be.
Ozone concentration in the stratosphere is measured in Dobson Units (DUs). 2017's minimum ozone concentration was 136 DUs. In the 1960s, the annual minimum was about 250 to 350 DUs.
Globally, the average ozone concentration in the stratosphere ranges from 300 to 500 DUs. This might seem good compared to the polar regions where stratospheric ozone is thinner, but we have another place with ozone to worry about: the ground.
When referring to ozone, the adage is "Good up high, bad nearby." Unlike stratospheric ozone, ground-level, or tropospheric ozone, can be produced by humans.
E360, a publication run by Yale's School of Forestry & Environmental Studies, reports that common creators ground-level ozone are carbon monoxide, nitrogen oxides, and hydrocarbons found in vehicle exhaust and oil fields. Those compounds react with sunlight to form ozone, which acts as a greenhouse gas and causes health problems in people, animals and plants. Researchers at the Massachusetts Institute of Technology estimate that 4,700 to 19,000 U.S. citizens die of ozone-related heart and lung illnesses per year.
Ambient ozone levels have steadily increased over the past few decades. Some scientists fear that global temperature increases could cause ozone to form more rapidly, which could further increase the rate of warming by affecting the ability of plants to take in carbon dioxide through photosynthesis.
"There should still be a global alert about ozone," said plant ecophysiologist Howard Neufield, who works at Appalachian State University, E360 reports. "In fact, more than ever."
If the world were to have a "global alert" to ground ozone in the same way that it did for stratospheric ozone, the effects may be felt sooner. Ground-level ozone lasts a few months, as opposed to a century for CFCs.
Unlike CFCs, pollutants related to ozone formation are more difficult to phase out, and so the solution may be a long time in the works.