Editing IPCC:AR6/SROCC/Chapter-6 (section)

==== 6.4.1.3 Future Changes ====

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MHWs will increase in frequency, duration, spatial extent and intensity throughout the ocean under future global warming (Oliver et al., 2017 <sup>[[#fn:r368|368]]</sup> ; Ramírez and Briones, 2017 <sup>[[#fn:r369|369]]</sup> ; Alexander et al., 2018 <sup>[[#fn:r370|370]]</sup> ; Frölicher et al., 2018 <sup>[[#fn:r371|371]]</sup> ; Frölicher and Laufkötter, 2018 <sup>[[#fn:r372|372]]</sup> ; Darmaraki et al., 2019 <sup>[[#fn:r373|373]]</sup> ). Projections based on 12 CMIP5 Earth system models suggest that, on global scale, the probability of MHWs exceeding the pre-industrial (1850–1900) 99th percentile will ''very likely'' increase by a factor of 20–27 by 2031–2050 and ''very likely'' by a factor of 46–55 by 2081–2100 under the RCP8.5 greenhouse gas (GHG) scenario (Figure 6.4a; Frölicher et al. 2018 <sup>[[#fn:r374|374]]</sup> ). In other words, a one-in-hundred-day event at pre-industrial levels is projected to become a one-in-four-day event by 2031–2050 and a one-in-two-day event by 2081–2100. The duration of MHW is projected to ''very likely'' increase from 8–10 days at 1850–1900, to 126–152 days in 2081–2100 under the RCP8.5 scenario (Frölicher et al., 2018 <sup>[[#fn:r375|375]]</sup> ). The maximum intensity (maximum exceedance of the 1850–1900 99th percentile) will ''very likely'' increase from 0.3°C–0.4°C in 1850–1900, to 3.1°C–3.8°C in 2081–2100 under the RCP8.5 scenario. Under the RCP2.6 scenario, the magnitude of changes in the different MHW metrics would be substantially reduced (Frölicher et al., 2018 <sup>[[#fn:r376|376]]</sup> ). For example, the probability ratio would ''very likely'' increase by a factor of 16–24 by 2081–2100 for RCP2.6; less than half of that is projected for the RCP8.5. The magnitude of changes in the probability ratio scales with global mean atmospheric surface temperature and is independent of the warming path (Figure 6.4b), that is, it does not depend on whether a particular warming level is reached sooner (RCP8.5) or later (RCP2.6).

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'''Figure 6.4'''

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'''Figure 6.4 | Global and regional changes in the probability ratio of marine heatwaves (MHWs). The probability ratio is the fraction by which the number of MHW days yr–1 has changed since 1850–1900. (a) Changes in the annual mean probability ratio of MHWs exceeding the 99th percentile of pre-industrial local daily sea surface temperature (SST) […]'''
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Figure 6.4 | Global and regional changes in the probability ratio of marine heatwaves (MHWs). The probability ratio is the fraction by which the number of MHW days yr–1 has changed since 1850–1900. (a) Changes in the annual mean probability ratio of MHWs exceeding the 99th percentile of pre-industrial local daily sea surface temperature (SST) averaged over the ocean. The thick lines represent the multi-model averages of 12 climate models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5) covering the 1861–2100 period for the Representative Concentration Pathway (RCP) 8.5 and RCP2.6 scenarios, respectively. The shaded bands indicate the 90% confidence interval of the standard error of the mean. The black line shows an observational-based estimate. As daily SST data are available only for the 1982–2016 period, we assume that the observed mean temperature change is the main cause of the change in frequency of extremes (Frölicher et al. 2018; Oliver, 2019). We therefore subtracted first the differences between 1854–1900 and 1982–2016 obtained from the extended reconstructed SST Version 4 dataset (ERSSTv4; Huang et al. 2015a) from the daily satellite data before calculating the 99th percentile for the observations. (b) Same as (a), but the probability ratio is plotted for different levels of global surface atmospheric warming and for the individual models. The simulated time series in (b) are smoothed with a 10-year running mean. (c,d) Simulated regional changes in the multi-model mean probability ratio of MHWs exceeding the preindustrial 99th percentile in 2081–2100 for the (c) RCP2.6 scenario and the (d) RCP8.5 scenario. The grey contours in (c,d) highlight the spatial pattern. Figure is modified from Frölicher et al. (2018) <sup>[[#fn:r380|380]]</sup> .

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The changes in MHWs will not be globally uniform. CMIP5 models project that the largest increases in the probability of MHWs will occur in the tropical ocean, especially in the western tropical Pacific, and the Arctic Ocean (Figure 6.4c,d), and that most of the large marine ecosystems will also experience large increases in the number of MHW days (Alexander et al., 2018 <sup>[[#fn:r381|381]]</sup> ; Frölicher et al., 2018 <sup>[[#fn:r382|382]]</sup> ). Smallest increases are projected for the Southern Ocean. In addition, MHW events in the Great Barrier Reef, such as the one associated with the bleaching in 2016, are projected to be at least twice as frequent under 2°C global warming than they are today (King et al., 2017 <sup>[[#fn:r383|383]]</sup> ). The magnitude of projected changes at the local scale is uncertain, partly due to issues of horizontal and vertical resolution of CMIP5-type Earth system models. Only a few studies have used higher resolution oceanic models (eddy-resolving) to assess the local-to-regional changes in MHW characteristics. For example, regional high-resolution coupled climate model simulations suggest that the Mediterranean Sea will experience at least one long lasting MHW every year by the end of the 21st century under the RCP8.5 scenario (Darmaraki et al., 2019 <sup>[[#fn:r384|384]]</sup> ), and eddy-resolving ocean model simulations project a further increase in the likelihood of extreme temperature events in the Tasman Sea (Oliver et al., 2014 <sup>[[#fn:r385|385]]</sup> ; Oliver et al., 2015 <sup>[[#fn:r386|386]]</sup> ; Oliver et al., 2017 <sup>[[#fn:r387|387]]</sup> ).

Most of the global changes in the probability of MHWs, when defined relative to a fixed temperature climatology and using coarse resolution CMIP5-type climate models, are driven by the global-scale shift in the mean ocean temperature (Alexander et al., 2018 <sup>[[#fn:r388|388]]</sup> ; Frölicher et al., 2018 <sup>[[#fn:r389|389]]</sup> ). However, previously ice-covered regions, such as the Arctic Ocean, will exhibit larger SST variability under future global warming. This is because of an enhanced SST increase in summer due to sea ice retreat, but SST remaining near the freezing point in winter (Carton et al., 2015 <sup>[[#fn:r390|390]]</sup> ; Alexander et al., 2018 <sup>[[#fn:r391|391]]</sup> ). When contrasting the changes in the probability of MHWs with land-based heatwaves (Fischer and Knutti, 2015 <sup>[[#fn:r392|392]]</sup> ), it is evident that MHWs are projected to occur more frequently (Frölicher et al., 2018 <sup>[[#fn:r393|393]]</sup> ; Frölicher and Laufkötter, 2018 <sup>[[#fn:r394|394]]</sup> ). This is because the temperature variability is much smaller in ocean surface waters than in the atmosphere (Frölicher and Laufkötter, 2018 <sup>[[#fn:r395|395]]</sup> ) .

We conclude that there is ''very'' ''high confidence'' that MHWs will increase in frequency, duration, spatial extent and intensity in all ocean basins under future global warming, mainly because of an increase in mean ocean temperature. However, higher resolution models are needed to make robust projections at the local-to-regional scale.

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