Salmon being ensiled in Chile following a mass mortality event caused by an algal bloom. Die-offs can be one of many consequences of marine heatwaves which scientists say are increasing in frequency and intensity.

Marine heatwaves becoming more frequent and longer

Marine heatwaves that have widespread socioeconomic impacts – including farmed fish deaths - are becoming more widespread and frequent, with eight of the 10 most severe recorded events taking place in the past decade, a new study by scientists shows.

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The paper into the effects of anthropogenic (man-made) climate change points out that the global annual number of marine heatwave (MHW) days has risen by 54% over the past century. Impacts of MHWs can include loss of fisheries income, erosion of essential ecosystem services, mass mortalities of iconic species, and conflict.

The marine scientists from the UK, Australia, New Zealand, Denmark and Norway evaluated the social and economic consequences of 34 marine heatwaves in different parts of the world over the past 25 years.

Thomas Wernberg: Each of 34 marine heatwaves could be linked to several socioeconomic effects. Photo: IMR.

Significant disruption

“We found that each of these marine heatwaves could be linked to several socioeconomic effects,” said one of the authors, Thomas Wernberg, a researcher at Norway’s Institute of Marine Research (IMR) and a professor at the University of Western Australia.

“In total, we documented more than 150 individual impacts that span all large sea areas. In short, marine heatwaves have occurred everywhere, with significant disruption of human-ocean interactions.”

The ecosystems in the sea provide a number of services that humans enjoy - for example in the form of food, jobs, nature experiences and cultural values, IMR explained in an article on its website.

Researchers point out that marine heatwaves affect the ability of ecosystems to provide these services. This may be due to the fact that populations move or that many individuals die when the sea suddenly becomes too hot for them. In the extreme, ecosystems can collapse.

Loss of ecosystem services can in turn hit the bottom line for marine industries such as aquaculture, fisheries and tourism.

A recent heatwave mentioned in the study occurred in the Gulf of Alaska and caused a decline in the Pacific cod stock. As a result, commercial cod fishing, valued at over $100 million per year, was closed in 2020.

Another heatwave in the south-eastern Pacific Ocean led to mass mortality of farmed salmon in southern Chile due to harmful algae blooms. The estimated export loss was $800m.

Impacts mostly negative

Marine heat waves have also led to bleaching and mass mortality of corals in Australia’s Great Barrier Reef and other coral reefs in Southeast Asia.

Some heatwaves also have positive effects. A heatwave in the Canadian Arctic, for example, led to increased cod reproduction in 2009.

However, the research of Wernberg and colleagues clearly shows that the overall effect of marine heat waves on ecosystems and economies is negative.

Rise in global average of number of days with marine heat waves since 1900. Source: Holbrook et al. (2020). Click on image to enlarge.

Worse to come

Marine heatwaves have occurred more and more often in the last century. Between 1982 and 2016, their frequency increased by 50%.

An increasing number of studies indicate that the probability of marine heatwaves has increased as a result of man-made climate change. One study estimated that the probability of seven recent, strong heatwaves was seven times more likely than they would otherwise be due to climate change.

In step with increasing climate change, it is estimated that the frequency of marine heatwaves will continue to increase throughout the century. In addition, the heatwaves are expected to be stronger and longer lasting.

“The findings in our study clearly indicate that a future with more and stronger marine heatwaves will also have many negative effects on the ecosystem services we currently enjoy from the sea and coast,” said Wernberg.

Healthy and damaged kelp forest. Warmer water stresses the kelp and provides competition from small, fast-growing algae. Photo: Hartvig Christie / NIVA.

Loss of kelp forests

The consequences of several heatwaves are also felt on the Norwegian coast, and researchers at the IMR are working to map these consequences. Among other things, they have documented a connection between marine heatwaves and the loss of kelp forests in southern Norway.

Sugar kelp is stressed by higher water temperatures and displaced by small, fast-growing algae.

Seaweed forests are considered some of the richest ecosystems in the world. Researchers are also working on solutions to preserve and rebuild ecosystems.

“For example, we are working to develop new methods for large-scale restoration of kelp forests and to identify new, more hardy kelp varieties,” said Wernberg.

The study, Socioeconomic impacts of marine heatwaves: global issues and opportunities, is published in the journal Science. The lead author is Kathryn E Smith from the Marine Biological Association of the United Kingdom in Plymouth and the work was funded by UK Research and Innovation and the Australian Research Council.