region as it modulates marine life and global climate through important oceanic connections between the Pacific, Atlantic and Southern Oceans (Stellema et al. 2019). A weakening of the Leeuwin Current and Undercurrent off the west coast of Australia is projected due to reduced onshore flow and downwelling; the reduced flow is related to changes in the alongshore pressure gradient which is consistent with the projected weakening of the Indonesian Throughflow. A strong weakening in the SW Indian Ocean of the NE and SE Madagascar Currents, Agulhas Current and flow through the Mozambique Channel is predicted, as this reduced western boundary flow is partly associated with a weaker Indonesian Throughflow (Stellema et al. 2019).
2.7.5 Sea‐Level Rise (SLR)
Global mean sea‐level is rising and accelerating. Data from tide gauges and altimetry observations indicate that global mean sea‐level increased from 1.4 mm a−1 over the period 1901–1190 to 2.1 mm a−1 over the period 1970–2015 to 3.2 mm a−1 over the period 1993–2015 to 3.6 mm a−1 over the period 2006–2015 (Oppenheimer et al. 2019). SLR is projected to rise between 0.43 m (likely range: 0.29–0.59 m) and 0.84 m (likely range: 0.61–1.10 m) by 2100 relative to 1986–2005 (Oppenheimer et al. 2019). Sea‐level is projected to continue to rise for centuries beyond 2100 due to continuing deep ocean heat uptake and mass loss of the Greenland and Antarctic ice sheets and will remain elevated for thousands of years. Under the ‘business‐as‐usual’ scenario (RCP8.5), the rate of SLR will be 15 mm a−1 (likely range: 10–20 mm a−1) in 2100 and could exceed several cm a−1 in the twenty‐second century.
SLR involves a significant anthropogenic component, mainly induced by global ocean thermal expansion and the melting of land ice. SLR patterns relative to land are also influenced by geological processes such as glacial isostatic adjustment. In response to a changing climate, SLR will not be spatially uniform but show complex patterns. As a result, some regions could experience local SLRs considerably greater and larger than the global average, whereas the local SLR elsewhere may be well below the global mean or even negative.
Multiple factors impact global and regional SLR: oceanic net mass change related to an increase/decrease of ice sheets and glaciers, groundwater mining and dam building, glacial isostatic adjustment (vertical movement of the earth's crust due to ice sheet mass changes), changes in ocean water temperature and salinity, changes of the earth's gravitational field related to the melting of ice sheets and ocean dynamics associated with variations in wind‐driven or buoyancy‐driven ocean circulation (Webster 2020). There is significant regional heterogeneity, as SLR and its long‐term trends due to steric and dynamic components (steric sea‐level is rising and falling of sea level as the temperature and salinity of the water column varies; dynamic sea‐level is sea‐level changing as water mass is redistributed within the ocean or is added or removed) deviate significantly from the global mean and between different ensemble members.
FIGURE 2.11 Model ratios of ensemble averaged 20 year mean sea‐level rise and the decadal trend of sea‐level rise and the global mean. Mean sea‐level rise over 2021–2040 under a (a) ‘business‐as‐usual’ emission scenario (RCP8.5) and (b) a ‘lower emission’ scenario (RCP4.5), mean sea‐level rise over 2061–2080 under a (c) ‘business‐as‐usual’ scenario and (d) under a ‘lower emission’ scenario relative to mean sea‐level over 1986–2005. The decadal sea‐level rise trend, the average 10 year trend over the period 2006–2080, under a (e) ‘business‐as‐usual’ scenario and (f) a ‘lower emissions’ scenario.
Source: Hu and Bates (2018), figure 1, p.3. Licensed under CC BY 4.0. © Springer Nature Switzerland AG.
Model projections of regional and global SLR based on the RCP4.5 (lower emission scenario) and the RCP8.5 (the ‘business‐as‐usual’ scenario) assumptions (Figure 2.11) show that the pattern of the ensemble mean mid‐century (Figure 2.11a and b) and late‐century (Figure 2.11c and d) regional SLR and the pattern of long‐term (Figure 2.11e and f) trends are similar between the two scenarios (Hu and Bates 2018). Higher than average SLR is forecast for the subtropical Pacific, South Atlantic, Arctic, part of the subpolar North Atlantic, equatorial Pacific, southeast part of the South Pacific and subpolar North Pacific in both scenarios. The similarity of both forecasts suggests that the underlying internal processes are similar for both scenarios and scaled by the strength of forcing by greenhouse gases (Hu and Bates 2018). There is less certainty in the SLR projections in the regions that might experience the largest SLR. The steric and dynamic variability of SLR in the twenty‐first century increases in most regions for both scenarios compared with in the late twentieth century, with SLR variance in the long term showing an increase in the Indian Ocean, west of Australia, the west and east coastal regions of the Pacific, west coast of Europe and the Atlantic sector of the Southern Oceans; decreases in sea level are forecast to occur in many other regions for both scenarios.
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