Earth’s fourth global coral bleaching event is CONFIRMED: Scientists warn once brightly-coloured reefs across the Atlantic, Pacific, and Indian Oceans have been bleached white by record water temperatures

On land, heat waves can be fatal to people and wildlife and destroy 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 causing other significant impacts on marine ecosystems, fisheries and the aquaculture sector.

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

Long-term change

Climate change is warming ocean waters and 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 uniform, and scientists have lacked the tools to define, synthesize and understand the global patterns of marine heat waves and their biological consequences.

At a meeting in early 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 warm water at a specific location. time of the year. Importantly, heat waves at sea can occur at any time of the year, summer or winter.

With the definition in hand, we could finally analyze historical data to determine patterns in its occurrence.

Analysis of trends in maritime heat waves

Over the past century, marine heat waves have become longer and more frequent around the world. The number of heat wave days at sea increased by 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-based measurements, coastal station data and satellite observations, and looked for changes in how often marine heat waves occurred and how long they lasted.

We found that between 1925 and 1954 and between 1987 and 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 continued increase in ocean temperatures. Given the likelihood of continued ocean surface warming throughout the 21st century, we can expect more marine heat waves worldwide in the future, with implications for marine biodiversity.

‘The Blob’ effect

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

A marine ecosystem that experienced 30 days of extreme heat at the turn of the 20th century can now experience 45 days of extreme heat. That additional exposure can have detrimental effects on the health of the ecosystem and the resulting economic benefits, such as fisheries and aquaculture.

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

In 2011, a marine heat wave off the coast of Western Australia killed a kelp forest and replaced it with peat seaweed. The ecosystem shift persisted 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 in the iconic Shark Bay area, with impacts on biodiversity including increased bacterial blooms, declines in blue crab, scallop and green turtle health, and reductions in carbon storage of these species in the long term. important habitats.

Similarly, a marine heat wave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. Warm waters in late spring allowed lobsters to move 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 in place for years (2014-2016) causing fishery 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 marine heat waves become more widespread, the marine ecosystems on which many depend for food, livelihoods and recreation will become less and less stable and predictable.

The climate change link

Anthropogenic, that is, human-induced, climate change is linked to some of these recent marine heat waves.

For example, human emissions of greenhouse gases made the 2016 marine heat wave in tropical Australia that led to the mass bleaching of the Great Barrier Reef 53 times more likely to occur.

Even more dramatically, the 2015-2016 marine heatwave in the Tasman Sea, which lasted more than eight months and disrupted the 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 common will they occur? How much warmer will they be? And how long will they last?

Ultimately, scientists must develop forecasts for policymakers, managers and industry that can predict the future impacts of marine heat waves for 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 risks, but ultimately we still need urgent action to curb greenhouse gas emissions and limit global warming. If not, marine ecosystems will increasingly suffer from the extreme heat of the oceans.

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.