Marine Heatwaves Increasing in Intensity
An international study has demonstrated that marine heatwaves have increased globally over the past century in number, length and intensity as a direct result of warming oceans.
From 1925-2016, the researchers found the frequency of marine heatwaves had increased on average by 34 percent and the length of each heatwave had increased by 17 percent. Combined, this led to a 54 percent increase in the number of marine heatwave days every year. Heatwave frequency has increased for 97 percent of the global ocean surface, with the North Atlantic the only exception.
The study was co-authored by researchers from Australia's Centre of Excellence for Climate Extremes and Institute for Marine and Antarctic Studies (IMAS). “Our research also found that from 1982 there was a noticeable acceleration of the trend in marine heatwaves,” said lead author Dr. Eric Oliver from Dalhousie University, Canada. “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 researchers uses a definition of a marine heatwave that was recently agreed by an international working group. In short, says Oliver, when daily ocean temperatures exceed a threshold for what is considered very warm at that time of year – technically, the 90th percentile, meaning it is only exceeded with a 10 percent chance – for at least five days, we call it a marine heatwave.
Marine heatwaves can be caused by a range of factors. The most common drivers include ocean currents which can build up areas of warm water and air-sea heat flux, or warming through the ocean surface from the atmosphere. Winds can enhance or suppress the warming in a marine heatwave, and climate modes like El Niño can change the likelihood of events occurring in certain regions.
Some recent examples of marine heatwaves include:
• In 2011, Western Australia saw a marine heatwave that shifted ecosystems from being dominated by kelp to being dominated by other seaweed. That shift remained even after water temperatures returned to normal;
• In 2012, a marine heatwave in the Gulf of Maine led to an increase in lobsters but a crash in prices that seriously hurt the industry’s profits;
• Persistent warm water in the north Pacific from 2014-2016 led to fishery closures, mass strandings of marine mammals and harmful algal blooms along coastlines. That heatwave even changed large-scale weather patterns in the Pacific Northwest;
• Tasmania’s intense marine heatwave in 2016 led to disease outbreaks and slowing in growth rates across aquaculture industries.
The researchers used a variety of observational datasets to reveal the trend of increasing marine heatwaves, combining satellite data with a range of century-long datasets taken from ships and various land-based measuring stations. They then removed the influences of natural variability caused by the El Nino Southern Oscillation, the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation to find the underlying trend.
“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,” said IMAS co-author Professor Neil Holbrook.
The Intergovernmental Panel on Climate Change's fifth assessment report projects that the global ocean will continue to warm well into the 21st century. The warming in the upper ocean is projected to be between 0.6°C and 2°C.
“With more than 90 percent of the heat from human caused global warming going into our oceans, it is likely marine heatwaves will continue to increase," says Holbrook. "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.”