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

Forest fires fuelled by the El Niño Southern Oscillation (ENSO) phenomenon might be responsible for the short-term spikes in methyl bromide, a known destroyer of the ozone layer. This is according to a new study.

ENSO is a climate pattern that switches between warm and cool phases. These patterns, which trigger changes in temperature and precipitation, can be traced back to the waters of the Pacific Ocean.

Methyl bromide is an odourless, colourless gas that has both natural and industrial sources. It naturally enters the atmosphere from oceans and forest fires.

Human activities can release the gas too. Earlier, farmers relied on methyl bromide to keep a wide variety of pests, including fungi, weeds, insects, roundworms and rodents, in check.

About 50-95 per cent of the substance eventually makes it back into the atmosphere, according to the United States Environment Protection Agency.

However, in 1995, parties to the Montreal Protocol, an international treaty to phase out chemicals destroying the Earth’s natural sunscreen the ozone layer, decided to phase out the gas in stages. It took 10 years to completely eliminate the gas.

Consequently, methyl bromide levels dropped. But Stephen A Montzka, one of the study’s authors, noticed an unusual trend: A mysterious annual short-term spike in the gas.

“I was looking closely at my atmospheric concentration measurement record for methyl bromide, which includes observations at about a dozen sites across the globe back to 1995,” Montzka told Down To Earth.

Those variations could not be explained by the Montreal Protocol-mandated decline in human-related production, he added.

Previous analysis suggested that the variability had something to do with ENSO variations, he said.

Montzka and his team decided to dig deeper for more clues. So, they used surface measurements from the National Oceanic and Atmospheric Administration or NOAA’s global air sampling network. Next, they analysed the various sources of this gas.

The team attributes 46 per cent of the annual variation of atmospheric methyl bromide abundance to global fires.

Their analysis showed that increased forest fires during the warm phase of ENSO (El Niño) could probably explain the inter-annual variations in methyl bromide levels.

“We first found a strong correlation between the growth rate of atmospheric methyl bromide concentrations and ENSO,” lead author Melinda Nicewonger, postdoctoral fellow at Global Monitoring Laboratory at NOAA, said.

“This is an interesting paper that points out a complex interplay between climate variability (ENSO) and ozone depletion,” Martyn Chipperfield, professor of atmospheric chemistry at the University of Leeds, UK, told DTE. The expert was not involved in the study.

He said that the overall situation with methyl bromide is positive, thanks to the Montreal Protocol. “Nevertheless, it is important to understand natural emissions and how they might vary,” he noted.

In the early 1980s, scientists noticed that the ozone layer was dramatically thinning over Antarctica. Since the Montreal Protocol, ozone has been on the road to recovery. The hole is expected to recover by 2050, according to modelling studies.

“We still expect the ozone layer to recover, but the variations in natural emissions of methyl bromide might change that,” Chipperfield said.

Montzka agreed. “If the future ENSOs became stronger or more frequent, then the added methyl bromide could delay ozone hole recovery,” he said.

Subscribe to Daily Newsletter :

SUPPORT US

We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.