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Environment
In 2011, a strong La Niña event caused water temperatures off western Australia to soar five to seven degrees higher than normal. The extreme warmth persisted for a month, during which time fish were found out of range, corals colonized further south than usual and kelp forests collapsed and were replaced by sea grass — an entirely different ecosystem that persists to this day.
The event was so notable that research papers published after the fact coined a new term for it: a marine heatwave.
Fluctuations in ocean temperatures are normal and, in the case of El Niño and La Niña events in the eastern-central Pacific, can have a profound effect on global weather patterns. But a new international study lead by Dr. Eric Oliver, a professor of oceanography at Dalhousie University in Halifax, finds that marine heatwaves are occurring more regularly and lasting longer.
“From the early 20th century to now the number of days per year that are part of a heatwave have increased by almost 50 per cent,” says Oliver. “We’re getting more events and each event is getting longer on average.”
The study was based on analysis of surface temperatures due to the lack of below-surface data. However, Oliver notes that what happens in the upper 50 to 100 metres of the ocean can reveal much. “That’s where a lot of marine ecosystems lie and where a lot of our economic and recreational interaction with the ocean occurs, so it’s quite relevant,” he says.
To account for regional variability, Oliver and his colleagues in Australia and the United Kingdom defined a heatwave as when daily sea-surface temperatures peak above the 90th percentile of temperature on a given day for at least five days. In the North Atlantic off Nova Scotia, where chaotic temperature swings are the norm due to the effects of weather, eddies and the Gulf Stream, it takes a greater temperature spike—five degrees or more—to be considered extreme. In the tropics, where variation is weak, one or two degrees of above-average warming in summertime is enough to bleach coral.
Oliver says when discussion turns to big-picture subjects like global warming, climate change and the Paris Accord targets, which focus on average temperature increases of 1.5 to two degrees Celsius, the public are left wondering what that really means. He believes people don’t notice or feel an “average” temperature or climate. To many, even a two-degree difference in the average would seem inconsequential.
“What we do tend to feel are the leading edge of the extremes, whether it’s prolonged heat or cold or heavy rain or drought,” he says. The same goes for ecosystems.
“Marine ecosystems have evolved in a typical range of temperatures from a specific cool level up to a warm level and the naturally occurring marine heatwaves sort of set that upper level. But in a warming climate, if these events are becoming more frequent, that upper level is going to be hit more often and maybe with hotter temperatures, and that’s where the leading edge of change is going to be experienced.”
Oliver’s ongoing research is focused in part on forecasting marine heatwaves, with the ultimate goal being to predict up to a year in advance, if not the exact location, intensity and duration of a marine heatwave, then the likelihood of one, so experts and communities can prepare for any kind of disruption to ecosystems or fisheries. He’s concerned about what current trends suggest about the frequency, duration, intensity and impacts of late 21st century events.
“Regardless of what we do with emissions, there’s so much inertia in the system that, even if we stopped emitting greenhouse gases right now, we know these trends are going to continue,” he says. “The globe is going to continue warming for the near future.”
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