Majority of the ocean surface has seen marine heat since 2014

Extreme marine heat has become the ‘new normal’: More than HALF of the ocean surface has experienced excessively warm temperatures since 2014, study warns

  • Experts from the Monterey Bay Aquarium studied 150 years of ocean records 
  • Using data from 1870–1919, they created a benchmark of surface temperatures
  • This allowed them to define what has historically constituted ‘extreme heat’
  • They found that by 2019, 57 per cent of the ocean experienced such conditions
  • Extreme heat harms crucial marine ecosystems like coral reefs and kelp forests

More than half of the surface of the world’s oceans have experienced excessively warm temperatures since 2014 — with extreme marine heat now the ‘new normal’.

This is the warning of researchers from the Monterey Bay Aquarium, who analysed a century-and-a-half’s worth of global marine temperature data.

Extreme marine temperatures can lead to the collapse of crucial marine ecosystems like coral reefs, kelp forest and seagrass meadows, the team said.

The findings, they added, are further evidence for the urgent need to reduce climate-changing greenhouse emissions from the burning of fossil fuels.

More than half of the surface of the world’s oceans have experienced excessively warm temperatures since 2014 — with extreme marine heat now the ‘new normal’. Pictured: the fraction of the ocean’s surface experiencing extreme heat, divided up by the hemispheres


The study, the team said, developed out of research into the history of kelp forests growing on the Californian coastline over the last century.

As part of this, they realised that they needed to learn the history of local sea surface heat extremes — which are key stressors of canopy kelps.

From here, the experts expanded their focus beyond California’s shores to understand the occurrence of marine heat episodes on a global scale.

The research was conducted by biologists Kyle Van Houtan and Kisei Tanaka of the Monterey Bay Aquarium in California.

‘Climate change is not a future event,’ said Dr Van Houtan, who led the research during his tenure as chief scientist at the aquarium.

‘The reality is that it’s been affecting us for a while. 

‘Our research shows that for the last seven years more than half of the ocean has experienced extreme heat.’

‘These dramatic changes we’ve recorded in the ocean are yet another piece of evidence that should be a wake-up call to act on climate change.

‘We are experiencing it now, and it is speeding up.’

In their study, the team first determined average ocean temperatures — based on historic records — for the period from 1870 to 1919, which provided them with a fixed historical benchmark.

They then used this, by taking the top two per cent of temperature increases, to define what they would regard as extreme heat.

With this established, the team were then able to map the occurrence of extreme heat episodes over time, looking at how frequently they occur and whether such rates have been increasing over time. 

Their analysis revealed that 2014 was the first year in which more than half of the world’s ocean surface experienced what historically would have been regarded as extreme heat — with the percentage seen to increase over time.

In fact, by the most recent year included in the study, 2019, a whopping 57 per cent of the ocean experienced extreme heat.

‘Today, the majority of the ocean’s surface has warmed to temperatures that only a century ago occurred as rare, once-in-50-year extreme warming events,’ Dr Van Houtan explained. 

‘Climate change is not a future event,’ said biologist Kyle Van Houtan. ‘The reality is that it’s been affecting us for a while.’ Pictured: the change in the proportion of extreme ocean surface temperatures over the last four decades

‘When marine ecosystems near the tropics experience intolerably high temperatures, key organisms such as corals, seagrass meadows or kelp forests can collapse,’ explained Dr Van Houtan.

‘Altering ecosystem structure and function threatens their capacity to provide life-sustaining services to human communities,’ he added.

Such roles, he said, include supporting ‘sustainable fisheries, buffering low-lying coastal regions from extreme weather and serving as a carbon sink to store the excess carbon put in the atmosphere from human-driven greenhouse emissions.’

The full findings of the study were published in the journal PLOS Climate.


On land, heatwaves can be deadly for humans and wildlife and can devastate crops and forests.

Unusually warm periods can also occur in the ocean. These can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries.

Yet until recently, the formation, distribution and frequency of marine heatwaves had received little research attention. 

Long-term change 

Climate change is warming ocean waters and causing shifts in the distribution and abundance of seaweeds, corals, fish and other marine species. For example, tropical fish species are now commonly found in Sydney Harbour.

But these changes in ocean temperatures are not steady or even, and scientists have lacked the tools to define, synthesize and understand the global patterns of marine heatwaves and their biological impacts.

At a meeting in early 2015, we convened a group of scientists with expertise in atmospheric climatology, oceanography and ecology to form a marine heatwaves working group to develop a definition for the phenomenon: A prolonged period of unusually warm water at a particular location for that time of the year. Importantly, marine heatwaves can occur at any time of the year, summer or winter.

Unusually warm periods can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries worldwide (pictured)

With the definition in hand, we were finally able to analyse historical data to determine patterns in their occurrence.

Analysis of marine heatwave trends

Over the past century, marine heatwaves have become longer and more frequent around the world. The number of marine heatwave days increased by 54 per cent from 1925 to 2016, with an accelerating trend since 1982.

We collated more than 100 years of sea surface temperature data around the world from ship-based measurements, shore station records and satellite observations, and looked for changes in how often marine heatwaves occurred and how long they lasted.

This graph shows a yearly count of marine heatwave days from 1900 to 2016, as a global average.

We found that from 1925 to 1954 and 1987 to 2016, the frequency of heatwaves increased 34 per cent and their duration grew by 17 per cent.

These long-term trends can be explained by ongoing increases in ocean temperatures. Given the likelihood of continued ocean surface warming throughout the 21st century, we can expect to see more marine heatwaves globally in the future, with implications for marine biodiversity.

 ‘The Blob’ effect

Numbers and statistics are informative, but here’s what that means underwater.

A marine ecosystem that had 30 days of extreme heat in the early 20th century might now experience 45 days of extreme heat. That extra exposure can have detrimental effects on the health of the ecosystem and the economic benefits, such as fisheries and aquaculture, derived from it.

A number of recent marine heatwaves have done just that.

In 2011, a marine heatwave off western Australia killed off a kelp forest and replaced it with turf seaweed. The ecosystem shift remained even after water temperatures returned to normal, signalling a long-lasting or maybe even permanent change.

That same event led to widespread loss of seagrass meadows from the iconic Shark Bay area, with consequences for biodiversity including increased bacterial blooms, declines in blue crabs, scallops and the health of green turtles, and reductions in the long-term carbon storage of these important habitats.

Examples of marine heatwave impacts on ecosystems and species. Coral bleaching and seagrass die-back (top left and right). Mass mortality and changes in patterns of commercially important species s (bottom left and right)

Similarly, a marine heatwave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. The warm water in late spring allowed lobsters to move inshore earlier in the year than usual, which led to early landings, and an unexpected and significant price drop.

More recently, a persistent area of warm water in the North Pacific, nicknamed ‘The Blob’, stayed put for years (2014-2016), and caused fishery closures, mass strandings of marine mammals and harmful algal bloom outbreaks along the coast. It even changed large-scale weather patterns in the Pacific Northwest.

 As global ocean temperatures continue to rise and marine heatwaves become more widespread, the marine ecosystems many rely upon for food, livelihoods and recreation will become increasingly less stable and predictable.

The climate change link 

Anthropogenic, that is human-caused, climate change is linked to some of these recent marine heatwaves.

For example, human emissions of greenhouse gases made the 2016 marine heatwave in tropical Australia, which led to massive bleaching of the Great Barrier Reef, 53 times more likely to occur.

Even more dramatically, the 2015-16 marine heatwave in the Tasman Sea that persisted for more than eight months and disrupted Tasmanian fisheries and aquaculture industries was over 300 times more likely, thanks to anthropogenic climate change.

For scientists, the next step is to quantify future changes under different warming scenarios. How much more often will they occur? How much warmer will they be? And how much longer will they last?

Ultimately, scientists should develop forecasts for policy makers, managers and industry that could predict the future impacts of marine heatwaves for weeks or months ahead. Having that information would help fishery managers know when to open or close a fishery, aquaculture businesses to plan harvest dates and conservation managers to implement additional monitoring efforts.

Forecasts can help manage the risks, but in the end, we still need urgent action to curb greenhouse gas emissions and limit global warming. If not, marine ecosystems are set for an ever-increasing hammering from extreme ocean heat.

Source: Eric Oliver, Assistant Professor, Dalhousie University; Alistair Hobday, Senior Principal Research Scientist – Oceans and Atmosphere, CSIRO; Dan Smale, Research Fellow in Marine Ecology, Marine Biological Association; Neil Holbrook, Professor, University of Tasmania; Thomas Wernberg, ARC Future Fellow in Marine Ecology, University of Western Australia in a piece for The Conversation.

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