Tonga’s volcanic eruption rapidly depleted ozone layer

Last year’s gargantuan Tonga eruption launched an unprecedented amount of water vapour into the sky — depleting about 5 per cent of the ozone layer in some regions within just a week.

That’s according to a new study describing yet another record observation from an underwater cataclysm that proved the planet’s biggest bang in 140 years.

Hunga Tonga-Hunga Ha’apai’s eruption on January 15, 2022, triggered the fastest underwater flows ever measured, a tsunami that towered tens of metres high and a rare pressure shockwave that travelled the globe multiple times.

Its cascade of lingering effects on the climate might also see Earth cross the global warming landmark of 1.5C sooner than expected.

Now, scientists report the eruption injected more water vapour into the atmosphere than ever observed in the satellite era — and equivalent to about 10 per cent of the total global mean stratospheric burden.

Their study, published this morning in the journal Science, described how material was sent soaring to altitudes of up to 55km, where it had rapid and dramatic effects on stratospheric ozone (O3).

Using scientific balloons launched from Reunion Island in the Indian Ocean, the scientists observed a 5 per cent drop in stratospheric ozone levels above the tropical southwestern Pacific and Indian Ocean region — and within only one week.

That scale of loss was significant — but still not as large as the Antarctic ozone hole, where around 60 per cent of the ozone layer was depleted between September and November each year.

The water vapour’s increased presence also led to higher relative humidity and radiative cooling in the stratosphere, allowing chemical reactions to occur on the surfaces of volcanic aerosols at temperatures warmer than usual.

“Our study sheds light on the complex interactions between a large volcanic eruption and tropical stratospheric O3, bridging a large gap in our knowledge,” said the study’s authors, led by Dr Stephanie Evan of the French National Centre for Scientific Research.

“Beyond its volcanic relevance, our research offers crucial insights into atmospheric chemistry and its implications for climate change.”

Niwa principal scientist Dr Olaf Morgenstern offered more context for the volume of the water vapour described in the paper.

“Usually, there are roughly four million water molecules per million in the stratosphere, and the volcanic plume contained up to 300,” he said.

“The authors show that this massive local enhancement of water caused substantial impacts on atmospheric chemistry, increased ozone loss — both due to gas-phase chemistry and in association with the enhanced volcanic aerosol. “As a result, some highly unusual ozone depletion happened in the tropics in the aftermath of the eruption.” Morgenstern said the scientific community had been expecting some unusual polar ozone depletion to happen after the volcanic material reached the Antarctic. Yet it didn’t appear to have affected the 2022 ozone hole season, as the material failed to make it to the pole in time, while the 2023 season, contrary to expectations, hadn’t been unusual.

Associate Professor Laura Revell, of the University of Canterbury’s School of Physical and Chemical Sciences, said the latest study was a “fascinating snapshot” of what happened in the stratosphere in the aftermath of the eruption.

“However, it’s not over: the increased stratospheric water vapour may linger for several years yet,” Revell said.

Though some have pointed to the atmospheric fall-out as a direct cause of last summer’s extreme rainfall in New Zealand, scientists noted a mix of big-picture drivers — including La Nia patterns, warmer sea temperatures and background climate change — were much more readily ascribable than a singular signal from an eruption.