They’ve been playing a hidden role in summer heatwaves, big glacier melts and extreme deluges.

Now, scientists report that higher sea temperatures - currently taking a bite out of winter’s chill - is something our part of the planet can especially expect as the world continues to warm.

New Zealand’s waters have been experiencing relentless marine heatwaves over recent years, causing cascading losses in mussel and kelp beds and driving tropical fish into normally colder climes.

Even through wintry lows and bracing south-westerly winds, surface temperatures around most of our coasts have been hovering well above average, keeping last month among our five warmest Julys on record.

And if our past proves a window to our future, the trend is one that’ll only worsen over coming decades and centuries.

In a new study, a team led by GNS Science and Niwa reached back three million years ago, to the last time the world’s carbon dioxide levels were as high as today, to find that ocean surface temperatures across the Southwest Pacific were nearly double the global average.

Because of its similar CO2 concentrations – and the fact its global average temperatures were in line with what scientists project for 2100 - the mid-Pliocene warm period is often used as an analogue for our near-future climate.

New Zealand’s waters have been experiencing relentless marine heatwaves over recent years. Photo / Sylvie Whinray

Drawing on marine sediment cores drilled from several sites, the scientists found the ocean around New Zealand during the mid-Pliocene was, on average, about 4.2C warmer than pre-industrial times, and nearly twice as warm as the global average at the time.

The results also proved consistent with high-resolution climate model projections that put our seas several degrees warmer by this century’s end.

“These results are consistent with warming observed currently, expressed by the marine heatwaves recently experienced around New Zealand,” said the study’s lead author, GNS Science sedimentologist Dr Georgia Grant.

“It’s important for New Zealanders to be aware that with 2C global warming we should expect higher ocean temperatures here.

“As an island nation, the ocean dictates much of our weather, and increasing ocean temperatures are one of the factors to why storms like Cyclone Gabrielle are expected to increase in severity under climate change.”

Widespread marine heatwave conditions persisted around New Zealand's coasts last month. Image / Niwa

The UN’s latest scientific stocktake found that, if the world failed to rein emissions into the Paris agreement’s targets, the planet’s average temperature was very likely to rise to between 2.1C to 3.5C above pre-industrial times.

“To be truly resilient we need to plan for greater than 2C, accounting for ocean warming and tipping points that are not included in climate projections.”

The findings come after recent studies have pointed to marine heatwaves growing longer, stronger and more frequent in New Zealand.

Our seas have been warming by an average 0.2C per decade - and that pace of heating is quickening.

On top of what we’re already witnessing, scientists have warned that average sea temperatures could rise by 1.4C within four decades – and almost 3C by the century’s end.

That would mean that, by mid-century, we could be facing 260 days of marine heatwaves per year – and 350 days by 2100 – compared with the 40-odd days we see now.

For some regions such as the southern tip of the South Island, recent Niwa-led research found, there was a high chance that marine heatwaves could start to last more than a year.

A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University

Those warmer seas meant warmer temperatures on land, and shrinking snowlines – but also more energy for the subtropical-flavoured storm systems we’ve just seen over an extreme summer and last year’s record-wet winter.

The study also comes as marine heatwaves have been engulfing coastal waters across the Northern Hemisphere, over a July that proved the planet’s hottest month in 120,000 years.

As temperatures soared over 40C in Italy, and wildfires raged across Sicily, the Mediterranean Sea’s surface temperature reached a record 28.71C on July 24: its highest reading in four decades.

Right now, one of the most intense heatwaves on Earth is playing out off the West Coast of the US, with water temperatures peaking at nearly 5C above normal.

Similar anomalies were observed last summer around Fiordland, where scientists were alarmed to discover millions of sea sponges there had turned from velvet-brown to bone-white – making for one of the worst bleaching events ever documented among sponge species anywhere.

Niwa meteorologist Ben Noll said it was surprising to have seen the marine heatwave trend persist through winter, even with an El Niño forming.

By the end of last month, sea surface temperatures were sitting 1.8C, 1.2C and 0.9C above average in the east, north and west of the South Island respectively.

“The north of the North Island, at 0.2C above average, is the only region at this point where things have eased pretty well.”

Noll added that big anomalies had also been observed in the sub-surface of our coastal seas, meaning it could take longer for water closer to the surface to cool off with ocean churning.