Marine scientists are warning that large parts of the ocean are heading towards a state of permanent heatwave, as average global sea temperatures rise.
In a new paper published in the journal Nature, an international team of researchers, including Prof Michael Burrows of the Scottish Association for Marine Science (SAMS) in Oban, have called for increased monitoring of ocean temperatures in order to improve marine heatwave forecasting.
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. They can have devastating effects on ocean life, particularly corals and other species that are fixed in one location. Heatwaves also have an impact on aquaculture and fishing industries, which would benefit from accurate forecasting.
Prof Burrows said: “We are seeing an increase in the occurrence and intensity of marine heatwaves all over the world, not just in the more tropical regions.
“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.
“We have seen marine heatwaves becoming more common as the years pass; this is not a one-off.
“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.”
The warning comes as the planet looks likely to enter a period of El Niño, a climate phenomenon seen every few years, further increasing ocean temperatures. It occurs when winds over the tropical Pacific falter and the warmest waters in the western Pacific flood eastwards, disrupting atmospheric circulation.
Marine heatwaves are more common in El Niño years. On previous occasions this has led to detrimental impacts on major fisheries, changes to the habitat of certain marine species, and severe coral bleaching.
Prof Burrows added: “Aquaculture may need to change husbandry practices and change harvesting to be in advance of anticipated damaging heatwaves. 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.”
The paper’s lead author, Dr Alistair Hobday of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), said: “If we’re coming up to a period of time where we are forecasting there are going to be impacts on the Great Barrier Reef, for example, that gives you the opportunity to ask yourself what you might do about it.
“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.
“Australia’s efforts around identifying bushfire risks - preparing for those events and planning for the recovery stage – have improved greatly in recent years,” Dr Hobday continues. “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 Nature paper ‘With the arrival of El Niño, prepare for stronger marine heatwaves’ is available here: https://www.nature.com/articles/d41586-023-02730-2