A hole in the ozone layer, seven times larger than the Antarctic ozone hole, is currently sitting over tropical regions and has been since the 1980s, according to a Canadian researcher.
Unlike the Antarctic hole, which has mostly been fixed but still persists at springtime, the tropic hole is all-season, said Qing-Bin Lu, a scientist from the University of Waterloo.
His findings, published in the journal AIP Advances, are based on a different analysis method to typical measures for ozone loss, which he said is more useful for spotting problems.
“The tropics constitute half the planet's surface area and are home to about half the world's population,” said Dr Lu. “The existence of the tropical ozone hole may cause a great global concern.
“The depletion of the ozone layer can lead to increased ground-level UV radiation, which can increase risk of skin cancer and cataracts in humans, as well as weaken human immune systems, decrease agricultural productivity and negatively affect sensitive aquatic organisms and ecosystems.”
The present discovery calls for further careful studies of ozone depletion over the tropical regions, said Dr Lu.
Dr Lu's observation of the ozone hole comes as a surprise to his peers in the scientific community, since it was not predicted by conventional photochemical models.
In the mid-1970s, atmospheric research suggested the ozone layer, which absorbs most of the sun's ultraviolet radiation, might be depleted because of industrial chemicals, primarily chlorofluorocarbons (CFCs).
The 1985 discovery of the Antarctic ozone hole confirmed CFC-caused ozone depletion. Although bans on such chemicals have helped to slow ozone depletion, evidence suggests the problem persisted.
Dr Lu said the scale of the Antarctic hole, which was still observed in the late 2000s and between 2020 and 2021, had not been expected according to climate modelling based on the reduction of CFCs.
Therefore, he asserts that ozone holes are caused not only by damaging substances such as CFCs — strictly regulated under the Montreal Protocol — but by charged particles from space known as cosmic rays. He and his co-workers first proposed the theory 20 years ago.
“CFCs are undoubtedly the main ozone-depleting gases but cosmic rays play a major triggering role in causing both polar and tropical ozone holes,” Dr Lu told The National.
“We cannot control the natural cosmic ray effect. But we must continue to strictly execute the Montreal Protocol and its amendments for phasing out the production and use of halogenated ozone-depleting substances … worldwide,” he said.
He explained his surprise findings were due to using a different analysis model to conventional methods.
“It sounds unbelievable that the large tropical ozone hole was not noticed previously. But there exist some intrinsic challenges in making this discovery,” Dr Lu said.
“First, no tropical ozone hole was expected to exist from the mainstream photochemical theory. Second, unlike the Antarctic/Arctic ozone holes that are seasonal and mainly appear in spring, the tropical ozone hole is essentially unchanged across the seasons and is, therefore, invisible in original observed data.
“Third, the ‘absence’ of the tropical ozone hole is partially due to the conventional definition of an ozone hole.”
The conventional definition is an area in which total ozone values drop below the historical threshold of 220 Dobson Units — the total ozone value first measured by the Nasa Toms satellite in 1979.
With the old definition, no tropical ozone holes could be directly observed in satellite images of the ozone layer.
In this study, an ozone hole is newly defined as an area with ozone loss larger than 25 per cent with respect to the normal ozone value. With this new definition, the tropical ozone hole is clearly visible in the data.