Rather than a precursor, the number of heatwave days may even be an underestimate of what is to come as the planet warms, Professor Alexander said. “We could see it accelerated even more, given what we’ve seen recently," she said. Episodes of extreme heat over land have been studied more closely than those beneath the waves. Oceans, though, not only absorb about 93 per cent of the additional heat being trapped by rising greenhouse gas levels, they are also the main driver of the Earth's climate. Loading “Thank goodness we have the oceans as this massive sink [for both heat and carbon dioxide] but they are also changing too, and we tend to forget that," said Sarah Perkins-Kirkpatrick, an author of the paper and also a researcher at the UNSW CCRC. Dr Perkins-Kirkpatrick likened the oceans to the tropics, where temperatures typically move within a narrow band. Even moderate increases can have big impacts on humans and ecosystems alike.

The paper, which defined heatwaves as at least five consecutive days with sea-surface temperatures in the top 10 per cent of warmth over a 30-year period, found such events were on the increase in most parts of the world. Global hot spots Australia was home, along with the north Pacific and north Atlantic, of some of the global ocean hot spots. While coral bleaching from extended heat over the Great Barrier Reef and elsewhere in recent years had drawn international attention, many other regions had seen "substantial ecological and economic impacts", as fishing and tourism industries they support were hit, the paper said. For instance, an extreme event off the Western Australia coast in 2011 led to large-scale effects in the Ningaloo region. Kelp forests south of Ningaloo were hammered and are yet to recover.

“You only need to have that one event to have this complete shift in the ecological environments," Dr Perkins-Kirkpatrick said, noting such changes have tended to be less dramatic on land. "Will it ever change back? Have we reached the point of no return for certain marine environments?" she said. "There are a lot of unknowns there, but it’s quite concerning.” Coral bleaching events have garnered much of the attention but many other marine species, including kelp forests off Tasmania, can be vulnerable to changing conditions. "[Corals] are the sort of poster child for ecological change, and other systems aren’t maybe as pretty to look at," Professor Alexander said. "But [others] are equally as important in the ecosystems and food chains”.

Tasman Sea heat The westward boundaries of the continents tend to be where oceans are warming fastest, including off the east Australian coast. The Tasman Sea had experienced an increase in heatwave events even before this past summer's record burst, that fell outside the researchers' period of study.

In a special climate statement released last month by the Bureau of Meteorology and New Zealand's National Institute of Water and Atmospheric Research, the agencies found the south Tasman Sea recorded sea-surface anomalies of as much as 2.12 degrees last December and 1.96 degrees in January. Those readings were compared with a 1981-2010 baseline - and broke the record for those months by about a degree - an unusual departure from the norm for ocean readings. Giant kelp forests off Tasmania's east coast have been hard hit by warming waters. Credit:Craig Sanderson Not surprisingly, the high sea temperatures for an extended period affected nearby land masses. "That New Zealand had its warmest summer on record had a lot to do with how warm it was in the Tasman Sea,” Dr Perkins-Kirkpatrick said.