Marine Heatwaves: A Guide

We examine the causes, categories and effects of severe ocean heat on shellfish, the marine food web, corals, mangrove forests and other coastal ecosystems. We explain what drives this type of extreme weather event and examine its connection to climate change. We ask: how bad will marine heatwaves get?
Mass mangrove die-off in the Gulf of Carpentaria,
Mass mangrove die-off in the Gulf of Carpentaria, 2016, due to marine heatwave. Photo: © Professor Norm Duke.

We know that heatwaves occur over land. Everyone is familiar with periods of excessively hot weather. Those scorching hot summer days are occurring ever more frequently due to the effects of global warming. What is less well known, is that such extreme weather events also occur in the ocean. We call them marine heatwaves (MHWs).

In this article we discuss how episodes of severe ocean warming can affect coastal biomes and marine ecosystems around the world, as well as the human populations that depend on them for food, fishing and aquaculture. They can lead to mass die-off of marine plants and animals, owing to their sudden and intense onset.

As climate change has taken hold over the past century, occurrences of these events have increased significantly in frequency, size and duration, and are predicted to become much worse. 1

Effects of Global Warming on Oceans
How Do Oceans Influence Climate Change?

What Exactly Is A Marine Heatwave?

A marine heatwave is defined as a string of 5 or more days when sea temperatures in a particular region are in the top 10 percent compared with the previous 3 decades.

The IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate 2 uses the definition of the top 1 percent (99 percentile).

A marine heatwave can:

  • Last for weeks to months and even years.
  • Occur in summer and also in winter where they are known as ‘winter warm-spells’. They are defined simply in relation to normal temperatures for the location and time of year.
  • Be confined to individual bays around small islands or along short sections of coastlines. Or they can be distributed more widely over regional seas, even spanning oceans.

Marine Heatwave Categories

Since marine heatwaves have become more frequent, scientists have decided to rank them like other natural disasters. They are now ranked similar to hurricanes – in categories numbered 1 to 4. Scientists have also proposed naming marine heatwaves based on their location and year. For example, The Blob (more on this below) might have been called Northeast Pacific 2013.

Examples of Marine Heatwaves

Perhaps not surprisingly, studies show there is a strong correlation between marine heatwaves and ocean temperature. One of the effects of global warming on oceans has been to increase occurrences of MHWs by 50 percent over the past several decades, a change that is projected to become much worse as ocean temperatures continue to rise. 1

Despite this, coordinated research into marine heatwaves only received prominence following the extreme event off Western Australia in 2011.

Australia: 2011 to 2017

Different regions of Australia between 2011 and 2017 were impacted by numerous marine heatwaves, many of which coincided with other events – such as intense rainfall from tropical storms, strong currents and drought.

In 2011, seawater along the entire western coast of Western Australia experienced a 60-day marine heatwave involving temperatures of 3°C (5.4°F) above the norm.

This and a subsequent series of events led to abrupt mortality of key marine habitat-forming organisms – coral reefs, mangroves and other coastal vegetation like seagrasses and kelp. As well as destruction of habitats, it also triggered a southwards migration of seaweed, fish and sharks.

The repeated occurrence of these events in Australia is consistent with predictions of increased frequency and intensity of MHWs, and has wider implications as similar trends are predicted globally.

Australian Bushfires and Climate Change.

North Pacific MHW: 2013 to 2016 – The Blob

This was a particularly large heatwave, stretching from Alaska to Mexico across more than 4 million square kilometers of ocean. Nicknamed The Blob, it devastated the Pacific fishing industry and upended the marine food chain. More on The Blob below.

Tasmanian MHW: 2015/16 Tasman Sea

The Tasman Sea warming was the longest and most intense marine heatwave on record to date. Lasting 251 days and involving temperatures of 2.9 ℃ (5.2°F). It had profound impacts on the marine ecosystem and local economy. It impacted pacific oyster, blacklip abalone and farmed Atlantic salmon. Swimmers and surfers noted the unusual warmth in Tasmanian waters at the time. 3

North American MHW: 2019

A marine heatwave occurred in the north-east Pacific off North America in 2019, when temperatures rose 3°C above normal. A return of The Blob?

Seagrasses Ocean Floor
Seagrasses are important carbon sinks and vital to the health of marine ecosystems. Due to their requirements for high light and low nutrient conditions, these marine plants are particularly vulnerable to disturbances such as rising sea levels, strong winds, rising temperatures and ocean acidification. Seagrass meadows are nursery habitats for many important fish species. Photo: Heather Dine/NOAA

Did you know?

All humans depend directly or indirectly on the ocean and the planetary hydrosphere. We rely on these systems to help combat our climate crisis by absorbing the carbon dioxide emissions that cause global warming. They also play a crucial role in the water cycle, and thus our climate system.

Unfortunately, damaging processes caused by global warming – such as marine heatwaves, ocean deoxygenation and acidification, melting ice sheets and rising sea levels – are causing problems which will be irreversible on a human time scale. 4

What Causes Marine Heatwaves?

The science behind marine heatwaves is still in its infancy, but evidence suggests that they can be caused by a whole range of factors – and not all factors are present for each event.

Each heatwave has its own constellation of causes. However, there is one common and increasingly potent factor, and that is global warming. As our oceans soak up more and more heat from the atmosphere, heatwaves both on land and sea are becoming ever more frequent.

Research shows that the frequency of marine heatwaves has increased ten-fold due to man-made global warming and the emission of greenhouse gases such as carbon dioxide (CO2) and methane (CH4). 5

It is a perfect illustration of the Anthropocene epoch at work. MHWs typically used to occur once in hundreds to thousands of years in pre-industrial times. They are likely to occur between once a year to every decade if global temperatures rise by 3°C.

For more, see Rising Temperatures On Earth: What To Expect.

Other drivers of marine heatwaves include:

Rising sea surface temperatures – which have increased at a rate of nearly 0.6°C (1.1°F) per century since 1880. This warming in turn increases the likelihood of MHWs occuring. Recent research shows that the ocean absorbs 93 percent of the heat produced by global warming. By the end of the century, under a comparatively low-emissions scenario, the ocean is likely be two to four times hotter than it was in 1970. Under a high-emissions scenario, the ocean is likely to be five to seven times hotter.

Regional weather systems – like El Nino-Southern Oscillation (ENSO), its neighbor the Indian Ocean Dipole (IOD), the travelling Madden-Julian Oscillation (MJO), and the Southern Annular Mode (SAM) – can generate climate conditions that act to increase or decrease the occurrence of MHWs. For example, ENSO is predicted to occur about twice as often by the end of the century.

Changes in ocean currents – research shows that the deep water thermohaline circulation currents which distribute heat and cold around the world, may be weakening.

Clearly Mother Nature’s air conditioning system is breaking down.

What Is The Blob?

The Blob 6 is a large mass of warm water first detected on the Northeast Pacific off the Alaska coast in 2013. It continued to spread throughout 2014 and 2015 stretching from Alaska to Mexico across more than 4 million square km (2.5 million sq miles) of ocean. Sea temperature anomalies reached over 6 °C (10.8 °F) in some places.

The heatwave was nicknamed The Blob by a climatologist, because it had echos of a 1950s horror film about an alien life form that keeps growing and eating everything in its path.

The Blob Spreads

By 2016, the heatwave was wreaking havoc on marine life and the fishing industry. It triggered a underwater famine, stressing marine microbes at the base of the food chain, small creatures like krill, and even big guys like seals and whales.

Over 100 million Pacific cod suddenly vanished, essentially erasing a fishing industry worth $100 million a year. 7 As the base of the marine food chain crumbled, starving seabirds washed up on shore and many were found staggering along highways like zombies. It is estimated as many as half a million died.

A Baby Blob?

Although The Blob has dissipated, recovery of the marine life population is slow. Moreover, in 2019, a new baby blob appeared in the area. Researchers are applying what they’ve learned from its parent to help guide predictions of how future marine heatwaves might unfold.

Pacific Ocean Map Ocean Temperatures
Map of sea surface temperatures across the northeastern Pacific in August 2019. Red colors show areas that were hotter than average for the same month from 1985-2012 and blues were colder than average. Image Credit: NASA.

What Problems Do Marine Heatwaves Cause?

1. Collapse of The Marine Food Chain

MHCs attack marine ecosystems at the base of the food chain – stressing tiny phytoplankton and zooplankton, for example, upon which everything in the ocean depends. This fall out spills into higher trophic levels. MHWs have affected lobsters, crabs, scallops and pollock, and many other species.

MHWs tend to enhance stratification of the upper ocean. This typically leads to a decrease in nutrient supply and sometimes mass starvation. 8 Mostly, the severity of the impact comes from the suddenness and intensity of the heatwave. Marine wildlife simply cannot react quickly enough.

2. Ecological Impacts

MHWs upset the delicate balance of ocean life – suddenly and harshly. They trigger biodiversity loss and changes in species behaviour or performance, loss of adaptive capacity and genetic diversity. They can change the habitat ranges of certain species, such as the spiny sea urchin off southeastern Australia. This creature has been pushing into Tasmania at the expense of kelp forests upon which it feeds.

​In addition, marine heatwaves can affect the structure of an ecosystem, by suppressing some species and supporting others. For example, after the 2011 marine heatwave in Western Australia the fish communities had a much more tropical feel than previously, and switched from kelp forests to seaweed turfs. 9 As yet, we don’t know what long-term consequences this will have.

3. Loss of Biodiversity

Extreme climate events in Australia between 2011 to 2017 led to an abrupt and extensive mortality in key marine habitat-forming organisms – coral reefs, seagrasses, kelps and mangroves – along 45 percent of the continental coastline of Australia. 10

In 2015-16, heatwaves off Australia’s northern and eastern coasts caused severe bleaching of the Great Barrier Reef. In 2019, a major die-off of mangroves was discovered along the northern coast of Australia, affecting about 8,000 hectares (20,000 acres) of mangrove forest. Scientists believe it was caused by a combination of marine heatwave, sea level rise, and back-to-back tropical storms. Mangroves and seagrass meadows – both of which constitute important reserves of blue carbon – seem to be especially vulnerable to marine heatwaves.

10 Birds Threatened by Climate Change.
Effects of Climate Change On Animals.

4. Financial Losses

MHWs are capable of causing substantial economic loss, causing painful job losses in both fisheries and aquaculture. The 2011 Western Australia MHW, for example, forced scallop fisheries in the region to close for 3-5 years, while the Shark Bay crab fishery closed for 18 months. The same event also led to outbreaks of Pacific oyster mortality syndrome (which affected the pacific oyster aquaculture industry) and also caused underperformance in Atlantic salmon aquaculture.

The Future of Marine Management

Marine managers monitor fishing quotas and help to protect biodiversity. It is critical for marine managers to understand what changes are occuring in the ocean in order to plan ahead. The good news is, Western Australia has introduced a management system which is beginning to cope better, even under persistent impacts. 11

However, managers will still benefit greatly from more research into effective climate change adaptation policies. Learning from previous heatwave stress tests will help oceanographers and others involved in the marine industry to prepare for a future in which extreme ocean temperatures will be the ‘new normal’.

The Future – Just How Bad Will Marine Heatwaves Get?

In short. Very bad. By 2050, if greenhouse gas emissions continue largely unchecked, ocean temperatures in areas like the northeast Pacific where The Blob formed, will become common occurrences, placing us in dangerous uncharted territory.

Even as researchers study the fall-out from The Blob, they are keeping a close watch on other developing hot-spots, like the Bering Sea off Alaska. In the winter of 2017-18, the Bering Sea was ice free for the first time on record. And sea temperatures hit 4°C (7.2°F) above normal. This is not good news – for animals or humans. This region is home to the Alaskan pollock, which supports one of the world’s biggest fisheries. Researchers found that numbers of copepods – the favourite food of the pollock – were 90 percent below average.

The cataclysmic changes occurring in the ocean right now – have gone largely unnoticed by the public. Yet, the devastation is happening on our watch. Sea creatures are choking on microplastics, running out of oxygen because of eutrophication, unable to reproduce because of acidification and going hungry because the water is too hot.

By the time we wake up, it may be too late.

There are plenty of fish in the sea, my grandmother used to say. Perhaps not for much longer.


  1. “Longer and more frequent marine heatwaves over the past century.” Eric C. J. Oliver et al. Nature, 2018. [][]
  2. Special Report on the Ocean and Cryosphere in a Changing Climate []
  3. “Emerging risks from marine heat waves.” Thomas L. Frolicher & Charlotte Laufkotter. Nature 2018. []
  4. IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate []
  5. “Emerging risks from marine heat waves.” Thomas L. Frölicher & Charlotte Laufkötter. Nature 2018. []
  6. The Blob – NASA []
  7. “Ocean heat waves like the Pacific’s deadly ‘Blob’ could become the new normal” – 2019. Science Magazine. []
  8. “Marine heatwaves exacerbate climate change impacts for fisheries in the northeast Pacific.” Nature 2020. William W.L. Cheung. []
  9. “Climate-driven regime shift of a temperate marine ecosystem.” Thomas Wernberg. 2016. []
  10. “Severe Continental-Scale Impacts of Climate Change Are Happening Now: Extreme Climate Events Impact Marine Habitat Forming Communities Along 45% of Australia’s Coast.” 2019. Russell C Babcock et al. []
  11. “Keeping pace with marine heatwaves.” Neil J. Holbrook, Eric C.J. Oliver, Alistair J. Hobday. Nature 2020. []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email