Rise of the marine heatwave

While heatwaves have been striking across the northern hemisphere this summer, new research shows how marine heatwaves have increased since 1850. 

In the northern hemisphere, the summer of 2022 will be one to remember. Japan, the USA, and countries across Europe are just a few that reported heatwaves with record-breaking temperatures. Thanks to climate change, heatwaves are set to increase in intensity and frequency.  

Heatwaves, however, aren’t just an atmospheric phenomenon—they also occur in the ocean. In fact, just as over land, climate change seems to be driving an increase in marine heatwaves. 

The number of marine heatwaves is increasing

With satellite technology measuring sea surface temperature across the globe, identifying a marine heatwave is relatively straightforward. However, satellites have only been providing this data since the 1980s. To understand how marine heatwaves change over time, we must go back further.  

In a recently published study in Environmental Research Letters, student Arno von Kietzell (University of Edinburgh) and supervisors have built up a picture of marine heatwaves as far back as 1850. 

Their analysis was possible thanks to the Met Office Hadley Centre SST data set (HadSST4). This dataset collates sea surface temperature from different sources and different technologies. “Back in the 1850s, people measured sea temperature by taking buckets out of the ocean and putting a thermometer in it,” explains von Kietzell. To account for these discrepancies and ensure comparability, the data are adjusted to correct for systematic errors in reporting. 

Analysing the data, “we found evidence of marine heatwaves as far back as the 1850s, but since 1970, the frequency of heatwaves shoots up,” says von Kietzell. Moreover, “our analysis suggests the spatial extent of marine heatwaves has increased four-fold post-2010 compared to pre-1970.” 

With the climate crisis continuing unabated, marine heatwaves will likely be a part of ‘the new normal.’ Knowing when and where a heatwave is likely to strike can help industries adapt. 

Predicting the onset and persistence of marine heatwaves. Credit: Nature Magazine

Marine heatwaves are forecastable

Earlier this year, Dr Michael Jacox (NOAA Southwest Fisheries Science Center) and colleagues published a paper in the journal Nature demonstrating the feasibility of marine heatwave forecasting up to one year in advance. These forecasts, the researchers say, are a “key advance toward improved climate adaptation and resilience for marine-dependent communities around the globe.” 

Although the forecasting is global, some regions have more reliable forecasts than others. For example, forecasts generally perform well in the tropics but struggle in the Southern Ocean, where current variability can produce “intense but relatively short-lived marine heatwaves.” Forecasts are also more accurate during El Niños and, much like weather forecasts, short-term forecasts (up to two months) are generally more reliable than those with longer lead times. 

Among the industries most likely to benefit from such forecasts are fisheries and aquaculture. 

Helping the seafood industry adapt

Marine heatwaves and warming waters, in general, can profoundly impact where marine species live and how well they do. Research published in Science magazine by Dr Katie Smith (Marine Biological Association) and fellow researchers reports the economic impact of individual marine heatwave events. The most expensive event occurred in 2016 in Chile, with fish farm mortalities worth some U$800 million.  

For aquaculturalists and fishers, forecasts could help with planning. Fishers, for example, may plan for different species that move in and out of their region depending on temperature or reduce the catch of certain species that will be under significant stress when the heat wave arrives. Aquaculturalists may plan for heat-related diseases, adapt harvesting or schedules or, in the case of salmonids, hen juveniles are transferred to net pens in the sea. 

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Dr Sam Andrews is a marine ecologist and science writer