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On land, heat waves can be deadly to humans and wildlife and can devastate crops and forests.
Unusually warm periods may also occur in the ocean. These can last weeks or months, wiping out kelp forests and corals, and producing other significant impacts on marine ecosystems and the fishing and aquaculture industries.
However, until recently, the formation, distribution and frequency of marine heat waves have received little research attention.
Long term change
Climate change is warming ocean waters and causing changes in the distribution and abundance of algae, 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 neither constant nor uniform, and scientists lack the tools to define, synthesize and understand the global patterns of marine heat waves and their biological impacts.
At a meeting in early 2015, we convened a group of scientists with expertise in atmospheric climatology, oceanography, and ecology to form a marine heat wave working group to develop a definition of the phenomenon: a prolonged period of unusually warm water in a particular place for that time of year. It is important to note that marine heat waves can occur at any time of the year, summer or winter.
Unusually warm periods can last weeks or months, killing kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries around the world (pictured).
With the definition in hand, we were finally able to analyze historical data to determine patterns in their appearance.
Analysis of marine heat wave trends
Over the past century, marine heat waves have become longer and more frequent around the world. The number of marine heat wave days 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 records and satellite observations, and looked for changes in the frequency with which marine heat waves occurred and its duration.
This graph shows an annual count of marine heat wave days 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 their duration increased by 17 percent.
These long-term trends can be explained by continued increases in ocean temperatures. Given the likelihood of continued ocean surface warming throughout the 21st century, we can expect to see more global marine heatwaves in the future, with implications for marine biodiversity.
The ‘La Blob’ effect
Numbers and statistics are informative, but here’s what that means underwater.
A marine ecosystem that had 30 days of extreme heat in the early 20th century could now experience 45 days of extreme heat. That additional exposure can have detrimental effects on the health of the ecosystem and the economic benefits, such as fishing and aquaculture, that derive from it.
Several recent marine heat waves have done just that.
In 2011, a marine heatwave off the western coast of Australia wiped out a kelp forest and replaced it with kelp. The ecosystem change remained even after the water temperature returned to normal, indicating a lasting or even permanent change.
That same event caused widespread loss of seagrass beds in the iconic Shark Bay area, with consequences for biodiversity including increased bacterial growth, a decline in the population of blue crabs, scallops, and the health of green turtles, and reductions in the long-term carbon storage of these species. important habitats.
Examples of impacts of marine heat waves on ecosystems and species. Coral bleaching and seagrass dieback (top left and right). Mass mortality and changing patterns of commercially important species (bottom left and right)
Similarly, a marine heat wave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. Warm water in late spring allowed lobsters to come ashore earlier than usual during the year, causing 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) and led to fishery closures, mass strandings of marine mammals, and outbreaks of harmful 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 heatwaves become more widespread, the marine ecosystems that many depend on for food, sustenance and recreation will become less stable and predictable.
The link with climate change
Anthropogenic, that is, human-caused, climate change is linked to some of these recent marine heat waves.
For example, human greenhouse gas emissions made the 2016 marine heatwave in tropical Australia, which caused mass bleaching of the Great Barrier Reef, 53 times more likely.
Even more dramatically, the 2015-16 marine heatwave in the Tasman Sea, which persisted for more than eight months and disrupted Tasmania’s fishing and aquaculture industries, was 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 frequently will they occur? How much warmer will they be? And how much longer will they last?
Ultimately, scientists should 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 fishery managers know when to open or close a fishery, aquaculture companies plan harvest dates, and conservation managers implement additional monitoring efforts.
Forecasts can help manage risks, but ultimately we still need urgent action to curb greenhouse gas emissions and limit global warming. Otherwise, marine ecosystems will be increasingly affected by extreme ocean heat.
Source: Eric Oliver, assistant professor, Dalhousie University; Alistair Hobday, Senior Principal Research Scientist – Oceans and Atmosphere, CSIRO; Dan Smale, marine ecology researcher, Marine Biological Association; Neil Holbrook, professor at the University of Tasmania; Thomas Wernberg, Future ARC Fellow in Marine Ecology, University of Western Australia in an article for The conversation.