As bushfires raged across Tasmania, Victoria and New Zealand, and north Queensland faced a massive cleanup after unexpected flooding, a different extreme weather event was silently forming in the Tasman Sea over summer.

For the second year in a row, a stubborn high-pressure system over the Tasman Sea was warming the surface of the ocean to above-average temperatures, forming a marine heatwave, wreaking destruction and providing a glimpse of the new ecological order in the marine Anthropocene. Globally marine heatwaves are becoming more frequent and prolonged and affecting biodiversity, according to new research published in Nature Climate Change this week.

In the summer of 2017-18, the intense marine heatwave was combined with a land-based heatwave, together covering four million sq km. Scientists found the extreme weather event caused unprecedented loss of glacial ice in the New Zealand Southern Alps, changes to wine-grape harvests, and major disruption of marine ecosystems including kelp habitat loss, new species invasions and fisheries season changes.

This year the National Institute of Water and Atmospheric Research in New Zealand reported that sea surface temperatures in the Tasman were again above average.

Like coral reefs and tropical rainforests, the ocean suffers the slow torture of climate change peppered with high-intensity hits from extreme weather.

A window into the future

Marine heatwaves are generally out of sight and out of mind until one gets so bad it becomes impossible to ignore, says CSIRO research scientist Alistair Hobday.

A marine heatwave happens when the ocean temperature is much warmer than usual for the time of year from sunlight heating the surface water or warm water being brought via ocean currents – or both.

Climate change is causing marine heatwaves to happen more frequently and with more intensity. There may not be scorched earth or destroyed homes left in its wake, but a marine heatwave impacts our future in different ways – and serves as a warning.

“Marine heatwaves provide a window into what our oceans will look like in the future, which is why it’s important to keep track of them,” Hobday says.

In 2015, New Zealand experienced its longest and most intense marine heatwave on record. For some it was good news – “It brought down valuable tropical species like kingfish and snapper,” Hobday says.

For the aquaculture industry, however, it brought disease outbreaks in oyster farms, disruptions to salmon farming and abalone deaths along the coast.

Clinging on to kelp

When an extreme marine heatwave lingered over the Shark Bay world heritage area in 2011, seagrass and kelp forests died en masse. Some kelp species became regionally extinct over hundreds of kilometres, says marine ecologist Cayne Layton from the University of Tasmania. “That [kelp loss] was a direct effect of the heat, but also due to herbivorous fish following the warm water and moving in to munch on the kelp.”

Like coral, kelp provides habitat structure, shelter and food for an entire ecosystem – without it the ecosystem would cease to exist. But, also like coral, the planet is losing kelp forests to climate change at an alarming rate.

A healthy kelp forest (left) versus a degraded one (right). Photograph: NOAA / Matthew Doggett

Forests of giant kelp, known as Macrocystis pyrifera, used to dominate Tasmania’s coastline. But in recent years, 95% have been lost and largely replaced by common kelp, says Layton.

“It’s the equivalent of losing a forest on land and having it replaced by shrubbery,” he says. “It might still have some diversity, but you’ve undoubtedly lost something important.”

Although common kelp might take over in the short term, a marine heatwave or a plague of hungry sea urchins could quickly knock that out too, allowing even hardier turf algae to move in.

From the forest to the shrub, now to a lawn – albeit a weedy one.

Species that, like kelp, can’t keep pace with the fast rate of anthropogenic warming, can be quickly be written off by an extreme event such as a marine heatwave or tropical cyclone.

Weeds of the sea

Some more physically and physiologically nimble species are set to thrive in a warmer ocean. “[In a warmer ocean] you might expect to see more ‘weedy species’ that can adapt more quickly to change,” says marine ecologist Zoe Doubleday from the University of South Australia.

Weedy isn’t a technical classification of plants or animals, but rather a type of lifestyle. Animals and plants considered weedy species typically have short lifespans and a flexible lifestyle that allows them to take advantage of sparse or patchy resources, Doubleday says.

“They can change their shape, what they eat and when they mature and reproduce to take advantage of novel conditions created by climate change,” she says.

“For example jellyfish, algae and cephalopods – they’re very different types of organisms, but they have some similar characteristics. They’re short-lived, tolerant, adaptable – just like a terrestrial weed that you might see in your garden.”

Receding kelp forests are already being replaced by seaweed turfs, bigger numbers of some cephalopods and jellyfish blooms. “We already eat weedy species, and cephalopods are becoming increasingly important in our fisheries, and they might be better than fish in terms of sustainability in the future,” Doubleday says.

“Rather than just suppressing the weeds, maybe we could start predicting exactly which species are going to thrive and how we could use them.”

So while jellyfish may not be on the menu now, soon there may be few other options.