Tonga eruption seen as minor contributor to recent extreme weather

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Opinion

The underwater eruption off Tonga last year has cropped up several times lately as an “explanation” for our recent extreme wet weather.
It was a remarkable event that hurled a lot of water vapour into the stratosphere, increasing the global amount by about 10 percent. The eruption is expected to have caused an overall warming effect — unlike most large volcanic eruptions in the Earth’s past which have resulted in cooling for a year or two, as the sulphate clouds they emit into the atmosphere block sunlight.
However, in a March 1 NZ Herald article, climate scientists say New Zealand’s wet 2022 and record wet summer owe much more to what has been happening in and on tropical waters far to our north, and on our planet’s warming atmosphere, than any one-off effects from the eruption at Hunga Tonga early last year.
Victoria University climate scientist James Renwick said the smaller amount of water vapour released into the lowest layer of the atmosphere will have rained out within weeks of the eruption.
Water vapour is a greenhouse gas so the increased amount in the stratosphere will have a warming effect on Earth, but also a cooling effect up there that will have influenced a ring of climate variability that encircles the South Pole towards a phase that has a tendency for subtropical storms to penetrate more easily down the east coast of the North Island.
However, Dr Renwick said there were two much more obvious contributors to our stormy summer: a third La Niña summer in a row piling up warmer water in the tropical Southwest Pacific, which helps seed bigger storms; and background climate change, which puts more water vapour into the air from evaporation and increases the water-holding capacity of the air.
A study published last week estimated that climate change boosted the rainfall from Cyclone Gabrielle by 20-30 percent.
In further response to the “eruption” letter today, climate has varied widely over millions to billions of years but human civilisation has been able to develop over the last 10,000 years because the climate has been very stable, in the global average. Yes, there have been events such as the “little ice age” though that was largely regional, cooling Europe and North America the most. The global average cooling was about 0.5C, which had largely passed by the mid-1700s. The Intergovernmental Panel on Climate Change uses 1850-1900 as the reference period for pre-industrial temperature.