Changes in marine heatwaves globally over the 20th and 21st centuries

Eric Oliver, Dalhousie University, Department of Oceanography, Halifax, NS, Canada, Markus Donat, Barcelona Supercomputing Center, Barcelona, Spain, Michael T Burrows, Scottish Marine Institute, Department of Ecology, Oban, United Kingdom, Pippa J Moore, Aberystwyth University, Institute of Biological, Environmental and Rural Sciences, Aberystwyth, United Kingdom, Dan E Smale, Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, United Kingdom, Lisa Alexander, University of New South Wales, ARC Centre of Excellence for Climate Extremes, Sydney, NSW, Australia, Jessica Benthuysen, Australian Institute of Marine Science, Perth, Western Australia, Australia, Ming Feng, CSIRO, Environment, Crawley, Western Australia, Australia, Alexander Sen Gupta, University of New South Wales, Climate Change Research Centre, Sydney, NSW, Australia, Neil John Holbrook, University of Tasmania, Institute for Marine and Antarctic Studies, Hobart, TAS, Australia, Sarah Perkins-Kirkpatrick, University of New South Wales, Climate Change Research Centre and ARC Centre of Excellence for Climate System Science, Sydney, NSW, Australia, Hillary A Scannell, University of Washington, School of Oceanography, Seattle, WA, United States, Sandra E Straub, The University of Western Australia, UWA Oceans Institute and School of Biological Sciences, Perth, WA, Australia, Mads S Thomsen, School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, New Zealand and Thomas Wernberg, The University of Western Australia, UWA Oceans Institute and School of Biological Sciences, Perth, Western Australia, Australia
Marine heatwaves are important events in oceanic systems that can have devastating consequences for ecosystems, causing ecological changes and socioeconomic losses. Prominent marine heatwaves have occurred recently and attracted scientific and public interest, but comprehensive assessments of how these events have been changing globally is missing. Using daily satellite observations, daily in situ measurements, and gridded monthly in situ-based sea surface temperatures we identify significant increases in marine heatwaves over the past century. We further estimate future changes in marine heatwaves to the end of the 21st century, as simulated by CMIP5 global climate model projections. We find that from 1925 to 2016, global averages of marine heatwave frequency and duration have increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days. Importantly, these trends can largely be explained by the increase in mean ocean temperatures, rather than a change in variability. Future projections show significant, and accelerating, increases in MHWs properties into the 21st century with many parts of the ocean reaching a near-permanent MHW state by the late 21st century, regardless of emissions scenario considered (RCP4.5, 8.5). Comparison with simulations of a natural world, without anthropogenic forcing, indicate that these trends have emerged from the range of natural variability within the first two decades of the 21st century. This implies that the climate system has departed significantly from natural marine heatwave conditions under which ecosystems evolved, and therefore impacts on marine ecosystems can be expected to be widespread, significant and persistent.