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).