Claims of new 'tropical ozone hole' raise controversy

A scientist recently claimed that he'd discovered a gargantuan hole in the ozone layer of Earth's atmosphere that first appeared above the tropics in the 1980s but went unacknowledged until now. However, upon his research being published, the scientist received swift criticism from experts who flagged his study as deeply flawed.

"I am surprised that this study was published at all in its current form," Martyn Chipperfield, a professor of atmospheric chemistry at the University of Leeds in England, told the Science Media Centre, an independent U.K.-based press office that works with researchers, journalists and policymakers to disseminate accurate scientific information.

"The claim in this research of such large ozone changes in the tropics have not been apparent in other studies, which makes me very suspicious," Chipperfield said. "Science should never depend on just one study, and this new work needs careful verification before it can be accepted as fact."

The author of the new study, Qing-Bin Lu, a professor in the Department of Physics and Astronomy at the University of Waterloo in Ontario, said that he disagrees with Chipperfield's and others' critiques. "In my opinion, these criticisms are groundless and cannot stand from the review of scientific literature," he told Live Science in an email. 

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The controversial report was published July 5 in the journal AIP Advances. The study went through the journal's standard peer-review process, through which an independent reviewer determined it was suitable for publication, A.T. Charlie Johnson, Jr., deputy editor of AIP Advances, told Live Science in an email. The journal's editors then determined that the work was newsworthy enough to be highlighted as a featured article on their website. 

"To our knowledge, we have not received any communication from the outside community questioning its validity," Johnson said. "We encourage readers and researchers to contact the authors whenever possible to discuss potential technical deficiencies, so they may be addressed in corrections in the literature or in comments and responses." Or, alternatively, readers may contact the journal directly, he said. At that time, the journal would work to validate any claims made about the work, request an explanation or response from the author and correct the literature, if necessary.

A new definition for 'ozone hole'? 

Ozone — a gas made up of three oxygen atoms bound together — forms in Earth's upper atmosphere. Most ozone sits in the stratosphere, the atmospheric layer that lies 6 to 31 miles (10 to 50 kilometers) above the planet's surface. There, the gas acts as a kind of sunscreen, shielding Earth from the sun's powerful ultraviolet (UV) rays.

In the 1980s, scientists found that long-lived atmospheric pollutants called chlorofluorocarbons (CFCs) break down into chlorine and bromine when exposed to UV rays beyond the ozone layer, according to NASA's Earth Observatory. These reactive elements tear O3 molecules apart and thus thin out regions of the ozone layer, creating "holes," primarily over Antarctica, where the frigid atmospheric conditions allow ozone-shredding reactions to unfold very efficiently. 

Conventionally, an ozone hole is defined as a region where the ozone concentration dips below 220 "Dobson Units" — a measure of the number of ozone molecules in a given column of air that stretches from the planet's surface to space. The discovery of ozone holes prompted the passage of the 1987 Montreal Protocol, an international treaty aimed at phasing out the production of ozone-depleting chemicals such as CFCs, and now, the ozone layer is on the road to recovery, according to the World Meteorological Organization (WMO). 

However, in Lu's new study, he warned that a newfound ozone hole may be threatening the lives of billions of people living in the tropics. 

Specifically, Lu reported discovering a "large and all-season ozone hole" in the lower stratosphere over the tropics, 6.2 to 15.5 miles (10-25 km) above Earth's surface. This hole is similar in "depth" to the seasonal ozone hole that opens up over Antarctica in late winter and early spring, but covers an area seven times larger than that of the springtime Antarctic hole, he reported. 

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"The whole-year large tropical O3 hole could cause a great global concern as it can lead to increases in ground-level ultraviolet radiation and affect 50% of the Earth's surface area, which is home to approximately 50% of the world’s population," Lu wrote in the AIP report. "Exposure to enhanced UV-B levels could increase the incidence of skin cancer and cataracts in humans, weaken human immune systems, decrease agricultural productivity, and negatively affect sensitive aquatic organisms and ecosystems."

Rather than using the conventional definition of an ozone hole, Lu defined a hole as "an area of O3 loss larger than 25% compared with the undisturbed atmosphere." Ozone holes observed over the North Pole were marked by a roughly 25% drop in ozone, so this new definition is justified, he told Live Science. It's key to note that "no ozone hole over the tropics would be observed by the conventional definition of an ozone hole," because the total ozone levels over the tropics fall above the 220 Dobson Unit threshold, Lu noted in his report.      

Soon after Lu's study was published, Chipperfield and several other experts shared their critiques of the study with the Science Media Centre.

"There is no 'tropical ozone hole,'" said Paul Young, an atmospheric scientist at Lancaster University in England and co-lead author of the 2022 Scientific Assessment of Ozone Depletion, a report prepared by the WMO and the United Nations.

"The author’s identification of a 'tropical ozone hole' is down to him looking at percentage changes in ozone, rather than absolute changes, with the latter being much more relevant for damaging UV reaching the surface," Young said. "Interestingly, his article also does not draw from the vast literature that explores and documents ozone trends in all regions of the atmosphere."

One huge factor that influences ozone concentrations in the tropical stratosphere is a phenomenon called the Brewer-Dobson circulation, a global pattern of air circulation that pushes ozone out of the tropics and toward the poles, said Marta Ábalos Álvarez, a researcher in the Department of Earth Physics and Astrophysics at the Complutense University of Madrid. This circulation has sped up in recent years due to climate change, and this acceleration explains the long-term patterns of ozone depletion observed in the tropics, she said.

"In my opinion, [Lu's] article lacks the scientific rigour necessary to be a reliable contribution," Ábalos Álvarez said. "It contains a lot of reasoning with serious errors and unsubstantiated assertions, contradicting previous results that are substantiated."

Originally published on Live Science.