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Anniversaries of volcanic eruptions

On this day 2006 years ago, Mount Vesuvius famously blew its top.

The major stratovolcano in southern Italy violently spewed a cloud of super-heated tephra and gases to a height of 33km, ejecting molten rock, pulverized pumice, and hot ash at 1.5 million tons per second.

Ultimately, the eruption released 100,000 times the thermal energy of the atomic bombings of Hiroshima and Nagasaki.

Among the collateral damage were several Roman towns and settlements in the vicinity, most memorably Pompeii, its preserved remains providing rich archaeological insights into the lives of its unfortunate inhabitants.

Producing “just” 3-4km³ of rock and ash, the eruption of Vesuvius was a minnow compared to, for example, the repeated “super eruptions” that rocked eastern Australia between 256 and 252 million years ago, vomiting at least 150,000 km³ of material during that period – including massive amounts of greenhouse gasses high into the atmosphere that are believed to have triggered global climate change.

Meanwhile, tomorrow marks the 140th anniversary of the eruption of Krakatoa in Indonesia reaching its climatic phase.

Considered one of the deadliest and most destructive volcanic events in recorded history, the explosion was heard 3110 kilometres away in Perth, and the resulting acoustic pressure wave circled the globe more than three times.

Among its other effects was causing a volcanic winter, with average Northern Hemisphere summer temperatures falling by 0.4 °C the year after the eruption and record rainfall hitting Southern California between July 1883 to June 1884.

Today is also 587 days since the 2022 eruption of the submarine Tongan volcano of Hunga Tonga–Hunga Ha‘apai reached its peak.

The resulting eruption column reached elevations of 57 kilometres, making it highest known eruption column in history.

The column injected a large quantity of water into the stratosphere, where it disturbed the local temperature balance and caused the formation of anomalous winds.

It’s known that large volcanic eruptions can inject large amounts of sulfur dioxide into the stratosphere, causing the formation of aerosol layers that reflect sunlight and can cause a cooling of the climate [as per Krakatoa].

However, during the Hunga Tonga–Hunga Ha‘apai eruption this sulfur was accompanied by large amounts of water vapour, which, by acting as a greenhouse gas, overrode the aerosol effect and caused a net warming of the climate system.

One study estimated a 7 per cent increase in the probability that global warming will exceed 1.5°C in at least one of the next five years as a consequence.

Despite that, there appears to be a curious lack of acknowledgement of how this recent eruption may have contributed to – and even largely driven – the recent spate of extreme weather events we’ve been experiencing locally and globally in the months since [it doesn’t feature at all in a just-announced package of new extreme weather-related research by Niwa, for example].

Even mentioning this will no doubt provoke animated accusations of “climate denialism” in the letters column.

But surely, even as recent events are cited as evidence of accelerating climate change – and fuel increasingly radical calls to action to mitigate it – it behoves us to be precise in our prognosis, lest the prescription misses the mark.

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