Marine heatwaves set to soar

Marine heatwaves are becoming more frequent and intense because of global warming, and the grim trend is likely to continue, according to Swiss researchers. 

Writing in the journal Nature, a team led by Thomas Frölicher from the Oeschger Centre for Climate Change Research at the University of Bern reports that the number of marine heatwave (MHW) days doubled between 1982 and 2016.

Like their terrestrial equivalents, marine heatwaves have significant environmental and economic impacts. However, beyond the acknowledgement that each represents an anomalous event, there is no universally agreed definition of the phenomenon.

In 2016, a large team of scientists, led by oceanographer Alistair Hobday from Australia’s CSIRO research organisation, formulated a well received standard descriptor: “a prolonged discrete anomalously warm water event that can be described by its duration, intensity, rate of evolution, and spatial extent. Specifically, we consider an anomalously warm event to be a MHW if it lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.”{%recommended 717%}

In the latest research, Frölicher and colleagues suggest that if temperatures rise by 3.5 degrees Celsius above preindustrial levels by the end of the century, as predicted, the average probability of marine heatwaves occurring will be 41 times higher than in the period before industrialisation. 

It is the second such paper published in just a few months. In April, also in Nature, an international team reported that between 1925 and 2016 the global average MHW frequency and duration increased by 34% and 17% respectively, resulting in a global 54% increase in annual marine heatwaves.

Neither report is really surprising. Previous research has firmly established that global warming affects ocean as well as atmospheric temperatures.

However, the Swiss team says that until now our knowledge about past occurrences and the likely progression of MHWs has been limited.

“This knowledge gap is of considerable concern given the high vulnerability of marine ecosystems and fisheries, but also human societies, to such events,” the scientists write.

Both reports note several studies that have reported the damaging impact of individual events, among them a heatwave in the Mediterranean Sea in 2003, record-high ocean warming off the coast of Western Australia in early 2011, and the 2012 MHW in the northwest Atlantic.

However, the aim of Frölicher and his colleagues was to take a bigger picture view by analysing daily global sea surface temperature data from 1982 to 2016, and a dozen global Earth system models for 1861 to 2100.

For this study, the researchers took a conservative view and defined an event as an MHW when the surface sea temperature exceeded its local 99th percentile – higher than the CSIRO-derived definition – as determined from data from either the preindustrial model or the satellite observations.

They then quantified the fraction by which the number of MHW days per year has changed, the relative change in the average area of an individual heatwave, by how much it topped the 99th percentile, its annual mean duration and annual cumulative mean intensity.  

The result of all this complex analysis was a suggestion that if the predicted 3.5-degree increase does occur, the spatial extent of MHWs will be 21 times larger. Each one will last on average 112 days, and will reach a maximum sea surface temperature anomaly intensity of 2.5 degrees Celsius.

However, these increases would be reduced if warming was limited to 1.5 or two degrees. Under a scenario in which temperatures rose by 1.5 degrees, the probability of MHWs occurring would be 40% of the risk at 3.5 degrees.

The largest changes are projected to occur in the western tropical Pacific and Arctic oceans.

“Our results suggest that MHWs will become very frequent and extreme under global warming, probably pushing marine organisms and ecosystems to the limits of their resilience and even beyond, which could cause irreversible changes,” the report says.