The global frequency of marine heatwaves has increased 34 per cent since 1925, presenting a major threat to coral reefs and other ocean ecosystems, according to a study published this week in the journal Nature Communications.
Researchers from the ARC Centre of Excellence for Climate Extremes (CLEX) and the Institute of Marine and Antarctic Studies (IMAS) examined satellite data, as well as a range of century-long datasets from ship and land-based measuring stations, to uncover the uptick in marine heatwaves.
They then removed the influences of natural variability caused by weather patterns such as the El Niño–Southern Oscillation to identify the underlying trend. Analysis further revealed that the average length of each heatwave had increased by 17 percent, leading to a 54 per cent total increase in the number of marine heatwave days each year.
“There was a clear relationship between the rise in global average sea-surface temperatures and the increase in marine heatwaves, much the same as we see increases in extreme heat events related to the increase in global average temperatures,” says IMAS co-author Professor Neil Holbrook.
Scientists define a marine heatwave as a period in which the sea surface temperature in a given area becomes unusually warm for at least five days.
Given that over 90 per cent of the heat generated by man-made global warming is being absorbed by the oceans, the study’s authors anticipate a continued increase in marine heatwaves.
When ocean water is too warm, corals are known to expel the colourful algae that live in their tissues. This leads to the coral “bleaching” and increased mortality events that have been observed across reefs in recent years.
The heightened length and frequency of marine heatwaves also has profound consequences for fisheries, tourism and aquaculture. For instance, an intense heatwave off Tasmania in 2016 led to an outbreak of Pacific Oyster Mortality Syndrome, poor performance of salmon aquaculture and invasions by fish normally seen in warmer, northerly waters.