Climate Change Multiplies Harmful Marine Heatwaves

The number of days marked by potentially destructive ocean heatwaves has doubled in 35 years, and will multiply another five-fold at current rates of climate change, scientists warned.

Even if humanity does manage to cap global warming “well below” two degrees Celsius, as called for in the Paris climate treaty, marine heatwaves will sharply increase in frequency, intensity and duration, they reported in the journal Nature.

Compared to hot spells over land, which have claimed tens of thousands of lives since the start of the century, ocean heatwaves have received scant scientific attention.

But sustained spikes in sea-surface temperature - typically to a depth of several metres - can also have devastating consequences.

A 10-week marine heatwave near western Australia in 2011, for example, shattered an entire ecosystem and permanently pushed commercial fish species into colder waters.

Another ocean hot spell off the coast of California warmed waters 6°C and lasted for more than a year. Known at “The Blob”, it generated toxic algae blooms, caused the closure of crab fisheries, and led to the death of sea lions, whales and sea birds.

“Marine heatwaves have already become longer-lasting and more frequent, extensive and intense in the past few decades,” lead author Thomas Frolicher, an environmental physicist at the University of Bern, Switzerland, told AFP.

“This trend will accelerate in the future under further global warming.”

Coral reefs - which cover less than one per cent of the ocean’s surface but support a quarter of marine species are especially vulnerable to warming waters.

Recent spikes in tropical and sub-tropical sea surface temperatures, magnified by an especially potent El Nino, have triggered an unprecedented mass bleaching of corals, affecting 75 per cent of global reefs.