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Ocean Heat Waves | World Environment Day

Ocean 'heat waves' on the rise, destroying entire ecosystems

Dr. Mario Picazo
Meteorologist, PhD

Tuesday, June 5, 2018, 11:24 - When we hear the words "global warming", the first thing that usually comes to mind is air temperatures being higher than average. Often times, most people don't associate these terms with rising ocean water temperatures, although these play a crucial role in the overall global temperature rise scenario we hear about almost every day.

The importance of rising ocean temperatures around the planet has led a group of scientists directed by Eric Oliver of Dalhousie University in Canada, to publish a study in Nature Communications showing how between 1925 and 2016 the number of "marine heat wave days", as they call them, have increased 54%. Marine heat waves are like earth air heat waves but they occur in the water. An ocean heat wave is one in which sea surface temperatures for a given area are usually warm for at least five consecutive days.

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Total number of marine heat waves from Eric Oliver's study

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This value obtained by the Canadian research team reflects the total number of days each year that a marine heat wave is occurring somewhere around the world. They have also concluded that these heat waves are about 17% longer than in the past, and that their frequency have increased by about a third.

Graph average global sea surface temperature change 1880-2015

Behind this escalating ocean heat, Oliver's team points towards human driven climate change as the main cause. There have always been natural variation signals like El Niño, which do affect the severity and frequency of these heat episodes, although here, the increases are traced back to be mainly driven by long-term sea surface temperature changes.  

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According to the study, in the coming decades these intense heat periods will occur more often and last longer. This pattern would imply major impacts for marine biodiversity and the goods and services ocean ecosystems provide. 

Sea surface temperature change 1901-2015

Specific data from the study also shows how increases in global ocean temperatures are accelerating, and how the greatest changes have been recorded in the last few decades. Past studies had already made clear that oceans are steadily accumulating energy since the 1980s, with a warm-up that is no longer found only on the surface, but also in the deep water layers below.

Image courtesy of NOAA. Coral bleaching through the years


Regarding the impact of ocean heat waves on earth's climate and ecosystems, some worrying discoveries have already been made. In 2016, a large marine heat wave led to severe bleaching of the Great Barrier Reef in Australia. Actually, coral reefs around the world, have been the most impacted marine ecosystems, as water temperatures have been soaring well above normal values for quite some time. Scientists however, point in other directions other than just coral reef ecosystem impacts.


In other non-tropical regions of the world, marine heat waves have been responsible for widespread loss of habitat-forming species, their distribution and the structure of their communities. This has a tremendous economic impact on seafood industries that have to deal with the decline of essential fishery species. 

It is likely that over the past century, the impacts on ecological chains have been more frequent as ocean temperatures have escalated decade after decade. This is the case in Nova Scotia, where kelp forests are literally being wiped-out by water which is much warmer than usual. In this corner of Canada, the ocean is not just a form of recreation, it also means a way of life for many that rely on fisheries and aquaculture as an important part of their economy.

Nova Scotia Warm SST

Far from eastern Canada, on the other side of North America, everyone remembers "The Blob", one of the largest marine heat waves in both duration and extension. It developed in 2014 and lasted 2 years, extending from the west coast of Mexico to the Bearing Sea off the coast of Alaska. This warm water super pool was responsible for a constant dance of species from the tropics northward away from their common habitat zones. Tropical fish arrived as far north as the Pacific Northwest and toxic algae bloomed along some areas of the U.S. West Coast.  

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Blob Impact on Alaska


Oliver and his team do not foresee a cooling trend occurring in the coming years. It is actually more likely that marine heat waves will continue to intensify and accelerate with time. Despite the willingness of most nations around the globe to reduce emissions and slow down the accumulation of excess energy caused by greenhouse gas emissions, the oceans have so much inertia, that the heat they are trapping today will remain in the system for decades to come. 


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