Total number of marine heatwave days globally. Globally averaged time series of total marine heatwave (MHW) days from NOAA OI SST over 1982–2016. The black line shows the globally averaged time series of total MHW days from NOAA OI SST over 1982–2016. The red line shows this metric after removing the signature of ENSO. The light red and blue shading indicate El Niño and La Niña periods, respectively, defined by periods exceeding ±1 s.d. of the MEI index for three consecutive months. Graphic: Oliver, et al., 2018 / Nature Communications

11 April 2018 (University of Tasmania) – An international study in Nature Communications co-authored by researchers from the ARC Centre of Excellence for Climate Extremes and the Institute for Marine and Antarctic Studies (IMAS) reveals 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 study found the frequency of marine heatwaves had increased on average by 34% and the length of each heatwave had increased by 17%. Combined, 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 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.”

Some recent examples show just 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 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;
  • More recently, 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 Niño 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.

“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,” Professor Holbrook said.

More information on marine heatwaves can be found at www.marineheatwaves.org.

Hotter, longer, more frequent – global marine heatwaves on the rise


Influence of climate modes on marine heatwaves globally. Globally averaged time series of annual (first row) marine heatwave (MHW) frequency, (second row) duration and (third row) total MHW days based on monthly proxies over 1900–2016. Shown are the original results which include the influence of major global-scale climate modes (i.e., ENSO, PDO, AMO; blue lines) and results after the removal of their global signature (thin coloured lines and thick black lines). The shaded areas show the 95% confidence intervals based on model errors, averaged across proxy data sets (taking into account temporal covariance; after removal of climate modes). Graphic: Oliver, et al., 2018 / Nature Communications

ABSTRACT: Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.

Longer and more frequent marine heatwaves over the past century

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