Climate Change

Does tropical ozone hole exist? Experts are divided

Tropical ozone hole is about seven times larger than Antarctica and it appears across all seasons

By Rohini Krishnamurthy
Published: Friday 15 July 2022

A new ozone hole has been detected over the tropics, at latitudes of 30 degrees South to 30 degrees North, a recent study claimed. But some experts have raised questions about the findings.

According to the study published May 22, 2022, in AIP Advances, the ozone hole is located at altitudes of 10-25 km over the tropics.

The tropical ozone hole is about seven times larger than Antarctica, the study suggested. It also appears across all seasons, unlike that of Antarctica, which is visible only in the spring.

Also read: Climate variability may explain annual spike in ozone-destroying gas

“The hole has become significant since the 1980s. But it was not discovered until this study,” Qing-Bin Lu, Professor at the University of Waterloo and one of the authors of the study, told Down To Earth.

“The tropical ozone hole, which makes up 50 per cent of Earth’s surface, could cause a global concern due to the risks associated with it. It is likely to cause skin cancer, cataracts and other negative effects on the health and ecosystems in tropical regions,” he added.

Lu said his earlier studies suggested another mechanism of ozone depletion in the Antarctic: Cosmic rays.

According to studies, chlorofluorocarbon’s (CFC) role in depleting the ozone layer is well-documented.

Despite bans on such chemicals, evidence suggests ozone depletion continues even after slowing down, according to a press statement from the American Institute of Physics.

Lu also noted that the tropical stratosphere recorded a low temperature of 190-200 Kelvin (K). This can explain why the tropical ozone hole is constantly formed over the seasons, he wrote in his paper.

Taking these factors into account, Lu hypothesised that an ozone hole could have formed over the tropics. 

The researcher used both ground-based and satellite-based datasets of stratospheric ozone, temperature and CFCs to test his hypothesis. He compared the present relative ozone changes in per cent with the ozone value in the undisturbed atmosphere, for instance, in the 1960s.

Lu compared the relative ozone changes in per cent in the 2000s with those in both the 1960s and 1980s. These observed results were then compared to datasets from combined ground-based and satellite measurements used by other researchers.

Lu used a different definition of the ozone hole in his study. He defined it as an area with ozone loss larger than 25 per cent compared to the normal ozone value.

According to NASA, an ozone hole is an area where ozone levels drop below the historical threshold of 220 Dobson Units (DU is the measure of ozone concentrations).

The reason for the changed definition, Lu said, was that the tropical ozone hole is essentially unchanged across seasons and is therefore invisible in original observed data.

This absence, he added, is partially due to the conventional definition of ozone.

Experts unconvinced

Other experts not involved in the study have raised doubts about the study’s claims.

“One of the issues is that the mechanism of ozone loss proposed by this author has been discredited,” Martyn Chipperfield, Professor at the School of Earth and Environment, University of Leeds, told DTE.

The other issue, according to Chipperfield, is that this paper used the 1960s dataset on ozone changes.

The trouble is that there were very few observations in the 1960s, he said, adding that they were based on model reconstructions, which were poor.

“There are other better ozone datasets which are used widely in the community to determine ozone trends from the 1960s. They do not show this large drop in ozone,” he pointed out.

Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at the Massachusetts Institute of Technology, argued that a large proportion of the decrease in tropical ozone is probably just a change in stratospheric winds that bring up ozone-poor tropical air. 

She told DTE that she was not convinced that cosmic rays drove ozone depletion in the poles. Further, she added, no specific chemical calculations are provided to support the putative cosmic ray claim.

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