Figure 5. Mean projected changes of annual peak HS (HSpeak, °C) by the end of 21st century (2071-2100) relative to the historical period (1985-2014). The mean states from the first to last columns are simulations by GFDL-ESM4, CESM2-WACCM and MRI-ESM2 and those from the first to last rows refer to the simulations under SSP1-2.6, SSP2-4.5, SSP3-7.0 scenarios, respectively.
The duration (Dc), peak intensity (HSpeak) and accumulated heat stress (Ac) of warm-season MHWs are projected to increase across most of ocean, except the high latitudes of the North Atlantic, over the course of the 21st century according to all three models (Figure 3-5). The heating rate (HRc) and priming period (Dp) are projected to decrease in some parts of ocean, especially with GFDL-ESM4 (Figure 6-7). The magnitude of the projected changes in these thermal properties is shaped by the models’ climate sensitivities, with the highest projected changes in CESM2-WACCM (4.8K) followed by MRI-ESM2 (3.4K) and GFDL-ESM4 (2.7 K). The magnitude of these projected changes track that of surface ocean warming under different SSP scenarios; the changes are similar across all scenarios by mid-century, and greatest in the scenario with highest radiative forcing (SSP3-7.0) by end-century (also shown in Oliver et al., 2019; Figure 3-7, S6-10).
The largest increases of Dc occur in the tropical regions and the high latitudes of the Southern Hemisphere in all three models under SSP3-7.0 (Figure 3). For example, Dc is projected to increase by >300 day in GFDL-ESM4 in the western equatorial Pacific and south of southern Africa, implying a near-permanent warm-season MHW state. The limited seasonality in these regions implies that even a small increase in SST trend can lead to a sharp increase in the duration of heat stress, as SST is close to the climatological maximum throughout the year. The greatest model disagreement in duration occurs in the regions where there is also large disagreement in the mean values during the historical period (Figure 3, S1a-c). For example, the increases projected by CESM2-WACCM is up to 240 day longer than those projected by GFDL-ESM4 in the eastern tropical Pacific (Figure 3).
The accumulated heat stress shows similar global pattern to that of the equivalent duration variable (Figure 3, 4). The largest increases of Ac in GFDL-ESM4 are up to 320, 480 and 640 °C·day under SSP1-2.6, 2-4.5 and 3-7.0, respectively, which are substantially greater than what are currently classified as severe warm-season MHWs for coral reefs (e.g., a Level 2 Bleaching Alert from NOAA Coral Reef Watch occurs at the equivalent of 56 °C·day). As with Dc, there are disagreements among the models across the tropical Pacific with the highest climate sensitivity model CESM2-WACCM projecting increases of 480 °C·day greater than GFDL-ESM4 under SSP3-7.0 (Figure 3, 4). The largest increases of HSpeak with changes >5 °C are projected to occur in the high latitudes of the Northern Hemisphere in all three models, particularly in the subpolar gyre in Pacific (Figure 5). The projected changes of HSpeak in the tropical eastern Pacific are distinct among the models; for example, HSpeak is projected to increase by 2 °C more in CESM2-WACCM than in the other two models under SSP3-7.0 (Figure 5).