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Onshore, heat waves can kill humans and wildlife and can destroy crops and forests.

Unusually warm periods can also occur in the ocean. These can take weeks or months, kill kelp forests and corals, and have other significant effects on marine, fishing and aquaculture ecosystems.

But until recently, the formation, distribution and frequency of marine heat waves had received little attention from the research.

Long term change

Climate change is heating ocean water and causing shifts in the spread and abundance of seaweeds, corals, fish and other marine species. For example, tropical fish species are now commonly found in Sydney Harbor.

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

During a meeting at the beginning of 2015, we convened a group of scientists with expertise in atmospheric climatology, oceanography and ecology to form a working group for offshore heat waves to develop a definition for the phenomenon: a prolonged period of unusually warm water on a certain location for that time of the year. It is important that marine heat waves can occur at any time of the year, summer or winter.

Unusually warm periods can last for weeks or months, kill kelp forests and corals and have other significant consequences for the marine, fishing and aquaculture worldwide (photo)

Unusually warm periods can last for weeks or months, kill kelp forests and corals and have other significant consequences for the marine, fishing and aquaculture worldwide (photo)

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

Analysis of marine heat waves

In the past century, marine heat waves around the world have become longer and more frequent. The number of marine heat wave days rose by 54 percent from 1925 to 2016, with a rapidly rising trend since 1982.

We collected more than 100 years of data on seawater temperature around the world from ship-based measurements, shore station records and satellite observations, and looked for changes in how often marine heat waves occurred and how long they lasted.

This graph shows an annual number of marine heat wave days from 1900 to 2016, as a global average

This graph shows an annual number of marine heat wave days from 1900 to 2016, as a global average

This graph shows an annual number of marine heat wave days from 1900 to 2016, as a global average.

We discovered that from 1925 to 1954 and from 1987 to 2016, the frequency of heat waves increased by 34 percent and their duration increased by 17 percent.

These long-term trends can be explained by the continuing rise in ocean temperatures. Given the likelihood of continuing ocean surface heat in the 21st century, we can expect that more marine heat waves will occur worldwide in the future, with consequences for marine biodiversity.

& # 39; The Blob & # 39; effect

Numbers and statistics are informative, but this is what that means under water.

A marine ecosystem that had 30 days of extreme heat in the early 20th century can now experience 45 days of extreme heat. This additional exposure can have an adverse impact on the health of the ecosystem and the resulting economic benefits such as fishing and aquaculture.

A number of recent marine heat waves have done exactly that.

In 2011, a sea heat wave from western Australia killed a kelp forest and replaced it with peat seaweed. The ecosystem shift remained even after the water temperature had returned to normal, indicating a prolonged or perhaps even permanent change.

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

Examples of marine heat wave effects on ecosystems and species. Coral bleaching and seagrass die back (top left and right). Mass mortality and changes in patterns of commercially important species (bottom left and right)

Examples of marine heat wave effects on ecosystems and species. Coral bleaching and seagrass die back (top left and right). Mass mortality and changes in patterns of commercially important species (bottom left and right)

Examples of marine heat wave effects on ecosystems and species. Coral bleaching and seagrass die back (top left and right). Mass mortality and changes in patterns of commercially important species (bottom left and right)

Similarly, a heat wave in the Gulf of Maine disrupted lucrative lobster fishing in 2012. Thanks to the warm water in late spring, lobsters were able to go ashore earlier in the year than usual, leading to early landings and an unexpected and significant fall in prices.

More recently, a persistent area of ​​warm water in the North Pacific, nicknamed & # 39; The Blob & # 39 ;, remained locked up for years (2014-2016) and caused fishing closures, massive strandings of marine mammals and harmful outbreaks of algae blooms along the coast. It even changed large-scale weather patterns in the Pacific Northwest.

As global ocean temperatures continue to rise and marine heat waves occur more generally, marine ecosystems, many of which depend on food, livelihood and recreation, will become less stable and predictable.

The climate change link

Anthropogen, which is man-made, climate change is linked to some of these recent marine heat waves.

For example, human emissions of greenhouse gases caused the heat wave of 2016 in tropical Australia, which led to massive bleaching of the Great Barrier Reef, which was 53 times more likely.

Even more dramatic was the 2015 2015 heat wave in the Tasman Sea, which lasted more than eight months and disrupted the Tasmanian fishing and aquaculture sector, more than 300 times more likely due to anthropogenic climate change.

For scientists, the next step is to quantify future changes in different heating scenarios. How often will they occur? How much warmer will they be? And how long do they last?

Ultimately, scientists should develop forecasts for policy makers, managers, and companies that can predict the future effects of marine heat waves over weeks or months. With that information, fishing managers could know when to open or close a fishing or aquaculture business to schedule harvest dates and conservation managers to make additional monitoring efforts.

Forecasting can help control risks, but in the end we still need urgent action to reduce greenhouse gas emissions and mitigate global warming. If that is not the case, marine ecosystems are set for an ever-increasing sound of extreme ocean heat.

Source: Eric Oliver, assistant professor, University of Dalhousie; 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 one piece The conversation.

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