Here, we report on local and global changes in MHW characteristics over time as recorded by satellite and in situ measurements of sea surface temperature (SST) and defined using a quantitative MHW framework, which allows for comparisons across regions and events 1. While centennial increases in ocean surface temperatures have been extensively reported 35, 36, global trends in ocean temperature extremes remain largely unexplored.
For example, the 2003 European heatwave caused tens of thousands of deaths 33 and was later superseded in intensity by the 2010 European heatwave, events which can be expected to increase in probability over the 2010–2050 period 34.
Atmospheric heatwaves can have significant impacts on human health 31 and attribution studies have shown that these events, and atmospheric heatwaves in general, have become much more likely as a result of anthropogenic warming 32. While marine heatwaves have only started to draw attention in recent years, the increasing intensity, frequency and duration of atmospheric heatwaves have been extensively documented 30. Marine heatwaves have been accompanied by large-scale shifts in marine species location, phenological changes, changes in ecosystem structure and in some cases high levels of mortality, often with socioeconomic consequences 24, 25, 26, 27 and widespread and devastating ecological and socioeconomic consequences 28, 29. In a warming climate some of the most dramatic ecosystem changes have been associated with extreme heatwaves 23. Species are often more strongly impacted by environmental extremes than by slow changes in mean conditions 22. Upper ocean temperatures have warmed significantly in most regions of the world over recent decades, with anthropogenic greenhouse gas forcing very likely being the main contributor 21. Given the expected intensification in extreme temperature events due to anthropogenic climate change 20 and the potential for profound ecological and socioeconomic impacts, quantifying trends and patterns of MHWs is a pressing issue. Such impacts demonstrate the damaging consequences of MHWs and their influence on the structure and sustainability of marine communities and ecosystems 4, 8, 10, 12, 13, 16, 17, 18, 19. These events resulted in substantial ecological and economic impacts, including sustained loss of kelp forests 10, coral bleaching 11, reduced surface chlorophyll levels due to increased surface layer stratification 6, mass mortality of marine invertebrates due to heat stress 8, 12, rapid long-distance species’ range shifts and associated reshaping of community structure 8, 10, 13, fishery closures or quota changes 8, 13, 14 and even intensified economic tensions between nations 15. Notable events occurred in the northern Mediterranean Sea in 2003 2, 3, along the Western Australian coast in 2011 4, the northwest Atlantic in 2012 5, the northeast Pacific over 2013–2015 6, 7, off southeastern Australia in 2015/16 8 and across northern Australia in 2016 9. Several prominent marine heatwaves (MHWs)-prolonged periods of anomalously high sea surface temperatures 1-have had severe impacts on marine ecosystems in recent years. 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. 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. 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. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences.