The specter of an environmental problem once thought to have been solved has risen again. A new study published Feb. 6 in the journal Atmospheric Chemistry and Physics reports that the ozone layer might still be thinning, despite efforts to halt the use of the human-made chemicals thought to be responsible.
That’s not to say that the Montreal Protocol hasn’t been effective. The 1989 agreement banned the use of chemicals such as chlorine-containing chlorofluorocarbons and similar compounds used to cool air in refrigerators and air conditioners, and soon after, there was evidence that the ozone layer was on the mend.
“Thanks to the Montreal Protocol, ozone in the upper stratosphere” — above 30 kilometers (19 miles — “has increased significantly since 1998, and the stratosphere is also recovering above the polar regions,” William Ball, an atmospheric researcher at the Swiss Federal Institute of Technology in Zurich (ETH Zurich) and the study’s lead author, said in a statement.
Detail of naturally aborted ovulate cone at end of growth season from control (non-irradiated) healthy pine. Another recent study has found that radiation, allowed into the atmosphere by a thinning ozone layer, may sterilize trees. Photo and caption by Jeff Benca/University of California, Berkeley.
Most scientists figured that the ozone layer would return to its proper form by 2050 or so. But when Ball and his colleagues measured the total amount of ozone in the atmosphere, they found that the overall concentration was about the same as it’s been, despite that measured boost in the upper layer. That discovery led the team to surmise that the lower level of the ozone layer, which lies between 15 and 24 kilometers (9 and 15 miles) above the Earth’s surface, is still getting thinner. The authors note that this part of the layer usually has the highest density of ozone, which shields the planet from ultraviolet rays and other types of radiation.
The researchers acknowledge that several questions remain unanswered. For example, they weren’t able to show definitively that the ozone layer is in fact thinning in those parts of the atmosphere below 15 kilometers.
It could be that the warming climate is forcing ozone to spread out toward Earth’s poles more quickly than before. Or a new class of compounds that are taking the place of CFCs, known as very short-lived substances, could be having a similar effect. They, too, contain chlorine and bromine, and while some of them form naturally in the atmosphere, a few others are used in industrial applications.
“These short-lived substances could be an insufficiently considered factor in the models,” Ball said.
And Ball and his colleagues aren’t sure what the knock-on effects will be or exactly what’s causing it. A related study, published in the journal Science Advances on Feb. 7, puts forth evidence that higher levels of UV radiation could render trees infertile. What’s more, a thinner ozone layer may have played a part in a mass extinction a few hundred million years ago.
What is clear is that the steps the global community took nearly 30 years ago in Montreal are working to some degree, said Thomas Peter, an atmospheric chemist at ETH Zurich and one of the study’s authors.
“The decline now observed is far less pronounced than before the Montreal Protocol,” Peter said in the statement. “The impact of the Protocol is undisputed, as evidenced by the trend reversal in the upper stratosphere and at the poles.”
Still, this study makes it clear that it hasn’t solved the whole problem.
“[We] have to keep an eye on the ozone layer and its function as a UV filter in the heavily populated mid-latitudes and tropics,” he added.
Ball, W. T., Alsing, J., Mortlock, D. J., Staehelin, J., Haigh, J. D., Peter, T., … & Bourassa, A. (2018). Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery. Atmospheric Chemistry and Physics, 18(2), 1379-1394.
Benca, J. P., Duijnstee, I. A. P., & Looy, C. V. (2018). UV-B–induced forest sterility: Implications of ozone shield failure in Earth’s largest extinction. Science Advances, 4(2).