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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)

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

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

Until recently, the formation, distribution and frequency of heat waves at sea had received little attention.

Long term change

Climate change heats ocean water and causes shifts in the distribution and abundance of seaweeds, corals, fish and other marine species. For example, tropical fish species are now often found in Sydney Harbor.

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

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 on marine heat waves to develop a definition for the phenomenon: a prolonged period of unusually hot water in a location for that time of the year. It is important that heat waves at sea can occur at any time of the year, summer or winter.

Unusually warm periods can last for weeks or months, killing kelp forests and corals and causing other significant effects on marine ecosystems, fishing and aquaculture industries worldwide (photo)

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Unusually warm periods can last for weeks or months, killing kelp forests and corals and causing other significant effects on marine ecosystems, fishing and aquaculture industries worldwide (photo)

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

Analysis of heat waves at sea

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

We have collected more than 100 years of sea surface temperature data from around the world based on measurements on ships, coastal station records and satellite observations, and searched for changes in how often heat waves occurred at sea and how long they lasted.

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

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

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

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

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These long-term trends can be explained by sustained increases in ocean temperatures. Given the likelihood of continuous ocean surface warming during the 21st century, we can expect more marine heat waves in the future, with implications for marine biodiversity.

& # 39; The Blob & # 39; effect

Figures 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 could now experience 45 days of extreme heat. This additional exposure can have harmful effects on the health of the ecosystem and the economic benefits such as fishing and aquaculture that are derived from it.

A number of recent heat waves at sea have done exactly that.

In 2011, a marine heat wave from Western Australia killed a kelp forest and replaced it with peat seaweed. The change in the ecosystem continued even after the water temperatures had returned to normal, indicating a prolonged or perhaps even permanent change.

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The same event led to widespread loss of seagrass meadows from the iconic Shark Bay area, with an impact on biodiversity, including increased bacterial bloom, deterioration of blue crabs, scallops and the health of green turtles, and reductions in long-term carbon storage of these important habitats.

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

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

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

Similarly, a heat wave in the Gulf of Maine disrupted lucrative lobster fishing in 2012. The warm water in late spring allowed lobsters to enter the coast earlier in the year than normal, leading to early landings and an unexpected and significant fall in prices.

More recently, a persistent hot water area in the North Pacific, nicknamed & # 39; The Blob & # 39 ;, remained built for years and caused the closure of fishing, massive stranding of marine mammals, and harmful outbreaks of algal blooms along the coast. It even changed large-scale weather patterns in the Pacific Northwest.

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As global ocean temperatures continue to rise and marine heat waves become wider, the marine ecosystems that many rely on for food, livelihood and recreation will become less stable and predictable.

The link with climate change

Anthropogenic, which is man-made, climate change has been linked to some of these recent heat waves in the sea.

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

Even more dramatic was the 2015-16 sea heat wave in the Tasmanian Sea, which persisted for more than eight months and disrupted the Tasmanian fishing and aquaculture industry, more than 300 times more likely due to anthropogenic climate change.

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

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Ultimately, scientists need to develop forecasts for policymakers, managers, and industry that can predict the future impact of marine heat waves for weeks or months ahead. With this information, fishermen managers can know when to open or close a fishery, aquaculture companies to plan harvest dates and nature managers to implement additional monitoring efforts.

Predictions can help control risks, but in the end we still need urgent measures to limit greenhouse gas emissions and to limit global warming. If not, marine ecosystems are ready for ever-increasing hammering due to 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 one piece The conversation.

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