Marine heat waves have become longer and more frequent over the last century. An international study published recently in Nature Communications, co-authored by researchers from the ARC Centre of Excellence for Climate Extremes (CLEX) and the Institute of Marine and Antarctic Studies (IMAS), revealed that marine heat waves have increased globally over the past century in number, length, and intensity as a direct result of warming oceans. The research shows that annual marine heatwave days have increased by 54% from 1925 to 2016, with an accelerating trend since 1982. The researchers found that from 1925-2016, the frequency of marine heat waves had increased on average by 34% and the length of each heatwave had increased by 17%. Together, this led to a 54% increase in the number of marine heatwave days every year. "Our research also found that from 1982, there was a noticeable acceleration of the trend in marine heatwaves," said lead author Eric Oliver from Dalhousie University in Canada. He added, "while some of us may enjoy the warmer waters when we go swimming, these heat waves have significant impacts on ecosystems, biodiversity, fisheries, tourism, and aquaculture. There are often profound economic consequences that go hand in hand with these events." Some recent examples revealed how significant marine heatwave events can be. In 2011, Western Australia saw a marine heatwave that shifted ecosystems from being dominated by kelp to being dominated by 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, yet a crash in prices that seriously hurt the industry's profits, according to the press release of Nature Communications. Persistent warm water in the north Pacific from 2014-2016 led to fishery closures, mass stranding of marine mammals, and harmful algal blooms along coastlines. That heatwave even changed large-scale weather patterns in the Pacific Northwest. More recently still, 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. Then they 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 heat waves, much the same as we see increases in extreme heat events related to the increase in global average temperatures," said co-author Neil Holbrook from IMAS at the University of Tasmania. He further explained that 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."
