Summer 2023 saw some of the highest ocean temperatures on record, with the North Atlantic experiencing "unprecedented" high temperatures in June and July, including several "extreme" and "beyond extreme" marine heatwaves, as defined by the US National Oceanic and Atmospheric Agency (NOAA).
However, such extreme events could become the "new normal", according to a new research paper published in the journal Nature, with an international team of marine scientists warning we could be heading towards "a state of permanent heatwave".
This means better monitoring is urgently needed to predict such extreme events, the researchers say, enabling the seafood and aquaculture industries, as well as governments and conservation agencies, to prepare and take action.
Marine heatwaves are defined as at least five consecutive days when sea temperatures are in the top 10% of temperatures for that day of the year. According to the NOAA, "Weeks, months, or years of unusually warm waters can cause mass die-offs of fish, marine mammals, and seabirds, disrupt food webs and fisheries, bleach corals, spur harmful algal blooms and wipe out seaweeds. Billions of dollars are lost in such events around the world each year."
Professor Michael Burrows of the Scottish Association for Marine Science (SAMS) is one of research authors, and warns that marine heatwaves are "becoming more common as the years pass; this is not a one-off."
"We are seeing an increase in the occurrence and intensity of marine heatwaves all over the world, not just in the more tropical regions," said Burrows in a press release this week.
"Off northern UK, we had a marine heatwave that lasted 237 days, from August 2022 to April 2023. Then, after a brief period of more normal temperatures, there was a rapid and intense 39-day heatwave in June and July that saw sea surface temperatures nearly three degrees Celsius higher than normal."
"With nearly 80% of the last year as a marine heatwave in the UK, there is now a debate about whether we should shift the baseline from which we detect marine heatwaves. The baseline on which we based the definition of a heatwave was taken from the average temperatures between 1980 and 2013. Just 10 years on from the end of that period, it seems we may have entered a 'new normal' of ocean temperatures."
This "new normal" means the aquaculture and seafood industries will need to adapt to the new conditions, Burrows says.
"Aquaculture may need to change husbandry practices and change harvesting to be in advance of anticipated damaging heatwaves," he explains.
"Fisheries may need to reduce catch limits to protect heat-stressed stocks, and change practices during climate-enforced reduced activity."
"To really understand the impacts of marine heatwaves, we should scale-up monitoring efforts to characterize conditions before, during and after an event, including physical, chemical, and biodiversity measurement at multiple temporal and spatial scales."
Added to the rising baseline temperatures is the looming threat of El NIño, the cyclical climate phenomenon which emerges every few years in the Pacific, further raising ocean temperatures.
Already this year, the El Niño phenomenon has made an impact on pelagic fisheries in the Pacific, with Peru's anchovy season cancelled during the second quarter. In previous years, El Niño has also been associated with habitat changes, severe coral bleaching, and other negative effects on major fisheries.
The paper's lead author, Dr Alistair Hobday of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), argues that government agencies also need to step in to help mitigate the effects of extreme ocean temperatures.
"As a politician, as a researcher, or as an industry manager – if the information is there but you choose not to take steps to prepare, then really you are neglecting to look after your future," he commented.
Taking the example of Australia's work on identifying and preparing for bushfires, Hobday said similar efforts can be made in monitoring and action on marine heatwaves.
"Now we need to see the same level of co-ordination around extreme events in the ocean. With more information available, we have the opportunity to provide much better support for our marine industries and the blue economy."
The Scottish Association for Marine Science (SAMS), based near Oban, is Scotland's largest and oldest independent marine science organisation, dedicated to delivering marine science for a healthy and sustainable marine environment through research, education, enterprise and engagement with society. It is a charitable organisation (009206) and a partner of the University of the Highlands and Islands.
