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New Zealand sea sponges are ‘dying by the millions’ due to climate change

On land, heat waves can be deadly to people 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 kelp forests and corals, and have other significant impacts on marine ecosystems, fisheries and aquaculture.

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

Long term change

Climate change is warming ocean water, causing shifts in the distribution and abundance of seaweed, 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 stable or even, and scientists have not had the tools to define, synthesize and understand the global patterns of marine heat waves and their biological effects.

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

Unusually warm spells can last for weeks or months, killing kelp forests and corals, and other significant impacts on marine ecosystems, fisheries and aquaculture industries around the world (photo)

Unusually warm spells can last for weeks or months, killing kelp forests and corals, and other significant impacts on marine ecosystems, fisheries and aquaculture industries around the world (photo)

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

Analysis of marine heat wave trends

Over the past century, marine heat waves around the world have become longer and more frequent. The number of days with a heat wave at sea increased 54 percent between 1925 and 2016, with an accelerating trend since 1982.

We collected more than 100 years of sea surface temperature data around the world from ship measurements, coast station records and satellite observations, looking for changes in how often marine heat waves occurred and how long they lasted.

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

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

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

These long-term trends can be explained by continued increases in ocean temperatures. Given the likelihood of continued warming of the ocean surface in the 21st century, we can expect more marine heat waves worldwide in the future, impacting marine biodiversity.

‘The Blob’ Effect

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

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

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

In 2011, a heat wave at sea off the coast of Western Australia killed a kelp forest and replaced it with peat seaweed. The ecosystem shift continued even after water temperatures returned to normal, indicating a long-term or perhaps even permanent change.

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

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

Similarly, an offshore heatwave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. The warm waters in late spring allowed lobsters to migrate to shore earlier in the year than usual, leading to early landings and an unexpected and significant drop in prices.

More recently, a persistent area of ​​warm water in the North Pacific, nicknamed “The Blob,” remained dormant for years (2014-2016) causing fisheries closures, mass strandings of marine mammals and damaging outbreaks of algal blooms along the coast. It even changed large-scale weather patterns in the Pacific Northwest.

As global ocean temperatures continue to rise and sea heat waves widen, the marine ecosystems on which many depend for food, livelihood and recreation will become less stable and predictable.

The link about climate change

Anthropogenic, ie human-induced, climate change has been linked to some of these recent marine heatwaves.

For example, human greenhouse gas emissions 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 dramatic, the 2015-16 marine heatwave in the Tasman Sea, which lasted for more than eight months and disrupted Tasmanian fishing and aquaculture industries, was more than 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 long will they last?

Ultimately, scientists need to develop forecasts for policymakers, managers and industry that can predict the future impacts of heat waves at sea over the coming weeks or months. Having that information would help fisheries managers know when to open or close a fishery, aquaculture companies to plan harvest dates, and conservation managers to implement additional monitoring efforts.

Forecasting can help manage the risks, but ultimately we still need urgent action to curb greenhouse gas emissions and limit global warming. If not, marine ecosystems are poised 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, researcher 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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