Marine ‘heatwaves’ on rise

Rising frequency result of oceans being warmed by climate change, say scientists

Marine “heatwaves” like that which stoked New Zealand’s record-hot summer have become longer, stronger and more frequent over the past century — and especially in the past four decades.

And with more than 90 per cent of the heat caused by global warming going into our oceans, the scientists behind a new international study say the trend will only continue.

Marine heatwaves happen when sea surface temperatures (SSTs) rise as a result of stronger-than-normal warm ocean currents, or from being forced by the atmosphere.

In the case of summer’s Tasman Sea marine heatwave, it was a case of the latter.

It was a freak combination of persistent highs, a La Nina climate system in the tropics and a positive Southern Annular Mode pattern to our south, all set against a background of climate change.

Over what was our hottest summer ever observed, SSTs around New Zealand climbed to at least 1.5C above average — and in some spots off the West Coast rose as high as 6C above in the journal Nature Communications, showed the frequency of such events had grown as a direct result of oceans being warmed by climate change.

Worldwide over the past century, the frequency of marine heatwaves had increased on average by a third and the length of each grew by 17 per cent. Combined, it meant the number of marine heatwave days every year had risen by more than half over the period.

There was a noticeable acceleration of the trend from 1982.

Climate scientist Dr Jim Salinger said the study showed incidence of heatwaves had increased slightly around New Zealand — something that was linked to SSTs that had warmed with climate change.

“With further global warming, the likelihood of marine heatwaves is very likely to increase with anthropogenic climate change,” Salinger said.

“This summer’s marine heatwave, as well as having impacts on ocean life, had widespread impacts on terrestrial and managed ecosystems such as New Zealand’s glaciers and farming, as examples.

“Should the frequency of these increase, so will the impacts on New Zealand’s land as well as marine ecosystems.”

Victoria University climate scientist Professor James Renwick agreed.

“Climate change is obvious everywhere we look,” he said.

Many marine species were very sensitive to temperature and such rapid changes in the oceans were bound to be affecting marine food webs, especially when combined with the continued acidification of the oceans, Renwick said.

“The only way to arrest these changes is to drastically reduce global emissions of greenhouse gases.”

Niwa meteorologist Ben Noll said summer’s marine heatwave was “a striking feature” on both a regional and global climate scale, as at one point it represented some of the most unusually warm seas in the world.

But the chance of such a set of climate circumstances coming together like they did in the near future was still low, even under climate change projections.

“In the meantime, a cold southerly wind blows over our scorched seas on Wednesday, bringing Antarctic air that may well end the 2017-18 marine heatwave as we knew it.”