Marine heatwaves (MHWs) are periods of extreme warm sea surface temperature that persist for days to months1 and can extend up to thousands of kilometres2. Some of the recently observed marine heatwaves revealed the high vulnerability of marine ecosystems3,4,5,6,7,8,9,10,11 and fisheries12,13,14 to such extreme climate events. Yet our knowledge about past occurrences15 and the future progression of MHWs is very limited. Here we use satellite observations and a suite of Earth system model simulations to show that MHWs have already become longer-lasting and more frequent, extensive and intense in the past few decades, and that this trend will accelerate under further global warming. Between 1982 and 2016, we detect a doubling in the number of MHW days, and this number is projected to further increase on average by a factor of 16 for global warming of 1.5 degrees Celsius relative to preindustrial levels and by a factor of 23 for global warming of 2.0 degrees Celsius. However, current national policies for the reduction of global carbon emissions are predicted to result in global warming of about 3.5 degrees Celsius by the end of the twenty-first century16, for which models project an average increase in the probability of MHWs by a factor of 41. At this level of warming, MHWs have an average spatial extent that is 21 times bigger than in preindustrial times, last on average 112 days and reach maximum sea surface temperature anomaly intensities of 2.5 degrees Celsius. The largest changes are projected to occur in the western tropical Pacific and Arctic oceans. Today, 87 per cent of MHWs are attributable to human-induced warming, with this ratio increasing to nearly 100 per cent under any global warming scenario exceeding 2 degrees Celsius. Our results suggest that MHWs will become very frequent and extreme under global warming, probably pushing marine organisms and ecosystems to the limits of their resilience and even beyond, which could cause irreversible changes.
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T.L.F. acknowledges support from the Swiss National Science Foundation under grant PP00P2_170687 and N.G. under grant XEBUS 200020_175787. E.M.F and N.G. acknowledge support from ETH Zürich. This work is part of the Nippon Foundation Nereus Program, a collaborative initiative by the Nippon Foundation and partners, including ETH Zürich. We thank the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP5, and the climate modelling groups (listed in Methods) for producing and making available their model output. We thank T. Stocker, F. Joos and C. Raible for discussions, U. Beyerle for downloading most of the CMIP5 data, and D. Kessler and M. Aschwanden for their initial analyses. The maps in Fig. 3 and Extended Data Figs. 1– 3 were created using the NCAR Command Language (https://www.ncl.ucar.edu).
Nature thanks R. Asch, E. Oliver and the other anonymous reviewer(s) for their contribution to the peer review of this work.