The world’s oceans are suffocating. Over the last century, marine heat waves have become more common and last longer. New research carried out by scientists from Canada, the US, the UK and Australia and published in the journal Nature , suggests that the annual number of days that part of the ocean is experiencing a heat wave has increased by 54 % from 1925 to 2016.
Scientists generally define a marine heat wave as at least five consecutive days of unusually high temperatures for a particular ocean region or season. These extreme temperatures can be deadly for marine species such as corals, seaweed and oysters, and can wreak havoc on fisheries and aquaculture.
In the new study, the researchers looked for such recorded events in sea surface temperature data recorded as far back as 1900 and in satellite data from 1982. Not only are heat waves 34% more common on average, but they also last an average of 17% more.
This trend is mainly influenced by climate change that causes the warming of surface ocean waters , rather than by large atmospheric-ocean weather patterns, such as periodic warming and cooling of waters in the equatorial Pacific called the Southern Oscillation or the ‘El Niño’ phenomenon.
Eric Oliver, assistant professor of oceanography at Dalhousie University and lead author of the study, said that in the early 20th century, there were an average of two marine heat waves per year worldwide, but now there are three or four. While they used to last 10 days on average, now they last an average of 13 or 14 days.
The number of marine heat wave days worldwide increased by 54% between 1925 and 2016, according to the study.
The findings are concerning because marine heat waves can have long-lasting effects, Oliver clarifies. “I’m not surprised, but it is consistent with what we know about climate change and therefore alarming.”
“The whole package of global warming that we are experiencing is generally alarming because we are seeing impacts in our natural environment that in some cases we will not be able to undo, or it will take much longer to undo what it took to cause them,” explains the scientist.
The focus of the problem: climate change
In 2016, a marine heat wave caused severe bleaching of the Great Barrier Reef in Australia.
One of the largest marine heat waves in history, dubbed “The Blob,” unfolded in 2014 and lasted until 2016, from Mexico to the Bering Sea. The patch brought sea creatures that generally live in warm waters to northern locations, and some scientists believe it may have been responsible for the toxic algal blooms off the west coast.
In 2011, a marine heat wave off Australia persisted for about a month, resulting in fish kills and the shutdown of the lucrative abalone fishery. The hotspot also caused the area’s kelp forests to collapse, which were permanently replaced by seagrasses.
“So we have this event that comes and goes and then the environment goes back to normal, but the ecosystem has actually changed from one state to another ,” Oliver said.
Marine heat waves are part of the natural weather system and can be caused by ocean currents or high atmospheric temperatures, but scientists found that the trend for more frequent and longer heat waves has intensified in recent decades.
The trend is unlikely to abate anytime soon. In fact, researchers predict even more frequent marine heat waves in the coming decades.
“The probability that it will continue is, I would say, almost certain, because the trend is basically explained by the increase in ocean temperatures and we know that this is not going to stop rising. Even if we stop emitting greenhouse gases now, there is so much inertia in the system that is not going to stop heating for a while, “concludes Oliver.
Reference: ECJ Oliver et al . Longer and more frequent marine heatwaves over the past century. Nature Communications . Published online April 10, 2018. doi: 10.1038 / s41467-018-03732-9.
