Global oceans feel the heat: Warming trend worries scientists

Ocean heatwaves are becoming alarmingly more common as a direct result of human-induced climate change, scientists have warned.

A new study reveals that globally between 1925 and 2016 the frequency of marine heatwaves increased by more than a third.

During this time the length of each recorded heatwave event rose by 17%. Overall, there had been a 54% increase in the number of global "marine heatwave days" occurring each year.

In addition researchers detected a significant acceleration of the trend since 1982.

Lead scientist Dr Eric Oliver, from Dalhousie University in Canada, said: "While some of us may enjoy the warmer waters when we go swimming, these heatwaves have significant impacts on ecosystems, biodiversity, fisheries, tourism and aquaculture. There are often profound economic consequences that go hand in hand with these events."

The team, whose findings appear in the journal Nature Communications, combined satellite data with ship records and land-based observations dating back 100 years.

For the study, a marine heatwave was defined by a complex process that took account of sea surface temperatures rising above a seasonally variable threshold for at least five days.

Globally the frequency of ocean heatwaves ranged between one and three events per year.

The scientists cited examples of the ecological and economic harm caused by marine heatwaves in recent years.

In 2011, Western Australia experienced an ocean heatwave that caused ecosystems dominated by kelp to be taken over by seaweed. The new order remained even after water temperatures returned to normal.

The following year, a marine heatwave in the Gulf of Maine, US, led to a lobster population explosion but also a price crash that seriously damaged the local crustacean industry.

Persistent warm water in the north Pacific from 2014 to 2016 had also led to fishery closures, mass strandings of marine mammals, and the spread of toxic algal blooms along coastlines.

The changes can firmly be attributed to man-made global warming pouring heat into the oceans, say the researchers.

Co-author Professor Neil Holbrook, from the University of Tasmania, said: "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.

"With more than 90% of the heat from human caused global warming going into our oceans, it is likely marine heatwaves will continue to increase. The next key stage for our research is to quantify exactly how much they may change.

"The results of these projections are likely to have significant implications for how our environment and economies adapt to this changing world."

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