Last updated September 3, 2018 at 2:38 pm
Studies predict a dramatic impact if global warming continues.
Marine heatwaves (MHW) are becoming more frequent and intense as a result of global warming and that trend is likely to continue, according to Swiss researchers. And the figures are pretty grim.
In a paper published in Nature, they report that the number of MHW days doubled between 1982 and 2016 and suggest that if temperatures rise by 3.5°C relative to preindustrial levels by the end of the century, as has been predicted, the average probability of these events occurring will be 41 times higher than in preindustrial times.
It is the second such paper to be published in just a few months. An international team reported that between 1925 and 2016 the global average MHW frequency and duration increased by 34 per cent and 17 per cent respectively, resulting in a 54 per cent increase in annual MHW days globally.
Neither report is really surprising. It stands to reason that global warming will heat the sea as well as the land. However, the Swiss team, led by Thomas Frölicher from the Oeschger Centre for Climate Change Research at the University of Bern, says that until now our knowledge about past occurrences and the likely progression of MHWs has been limited.
Ecosystems and fisheries vulnerable
“This knowledge gap is of considerable concern given the high vulnerability of marine ecosystems and fisheries, but also human societies, to such events,” they write.
Both reports note a number of 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 1982-2016 and 12 global Earth system models for 1861-2100.
MHWs are prolonged periods of anomalously high ocean surface temperatures. For this study the researchers defined an event as an MHW when the surface sea temperature (SST) exceeded its local 99th percentile, as determined from data from either the preindustrial model or the satellite observations.
They then quantified the annual mean probability ratio (the fraction by which the number of MHW days per year has changed), relative change in the annual spatial extent (average area of an individual heatwave), maximum annual intensity (maximum exceedance of the 99th percentile), annual mean duration (number of days of exceedance) and annual cumulative mean intensity (the product of the duration and the mean intensity of exceedance).
Spatial extent 21 times larger
The result of all this complex analysis was a suggestion that if the predicted 3.5°C increase does occur, the spatial extent of MHWs will be 21 times larger, they will last on average 112 days and they will reach maximum sea surface temperature anomaly intensities of 2.5°C.
However, these increases would be reduced if warming was limited to 1.5 or 2.0°C. Under a scenario in which temperatures rose by 1.5°C, the probability of MHWs occurring would be 40 per cent of that at 3.5°C.
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.