Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.3.5 Coastal and Oceanic ====

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'''Relative sea level:''' Around Australasia, from 1900–2018, a new tide gauge-based reconstruction finds a regional mean RSL change of 1.33 [0.80 to 1.86] mm yr <sup>–1</sup> in the Indian Ocean–South Pacific region ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of around 1.7 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5). For the period 1993–2018, the RSLR rates, based on satellite altimetry, increased to 3.65 [3.23 to 4.08] mm yr <sup>–1</sup> ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of 3.25 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5).

Relative sea level is ''virtually certain'' to increase throughout the region over the 21st century ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3|Section 9.6.3]] , Figure 9.28). Regional mean RSLR projections for the oceans around Australasia range from 0.4–0.5 m under SSP1-2.6 to 0.7–0.9 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which means local RSL change falls within the range of mean projected GMSL change ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.1|Section 9.6.3.1]] ). However these RSLR projections may be underestimated due to potential partial representation of land subsidence ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.2|Section 9.6.3.2]] ).

'''Coastal flood:''' The most commonly used index for episodic coastal inundation in Australia is the summation of a high end SLR and the 1-in-100-year storm tide level (the combined sea level due to storm surge and tide) (CSIRO and BOM, 2016; [[#McInnes--2016|McInnes et al., 2016]] ). However, episodic coastal flooding is caused by extreme total water levels (ETWL), which is the combination of SLR, tides, surge and wave setup ( [[#12.3.5.2|Section 12.3.5.2]] ). The present-day 1-in-100-year ETWL is between 0.5–2.5 m around most of Australia, except the north-western coast where 1-in-100-year ETWL can be as large as 6–7 m ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#O’Grady--2019|O’Grady et al., 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ).

Extreme total water level magnitude and occurrence frequency are expected to increase throughout the region ( ''high confidence'' ) (Figure 12.4p–r and Figure 12.SM.6). Across the region, the 5–95th percentile range of the 1-in-100-year ETWL is projected increase (relative to 1980–2014) by 5–35 cm and by 10–40 cm by 2050 under RCP4.5 and RCP8.5 respectively (Figure 12.4q). By 2100 (Figure 12.4p,r), this range is projected to be 25–80 cm and 50–190 cm under RCP4.5 and RCP8.5 respectively ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Furthermore, the present-day 1-in-100-year ETWL is projected to have median return periods of around 1-in-20-years by 2050 and 1-in-1-year by 2100 in SAU and NZ and return periods of around 1-in-50-years by 2050 and 1-in-20-years by 2100 in NAU under RCP4.5 ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ), while the present-day 1-in-50-year ETWL is projected to occur around three times a year by 2100 with a SLR of 1 m around Australasia ( [[#Vitousek--2017|Vitousek et al., 2017]] ).

'''Coastal erosion:''' Satellite derived shoreline retreat rates for the period between 1984–2015 show retreat rates between 0.5 and 1 m yr <sup>–1</sup> around the region, except in SAU where a shoreline progradation rate of 0.1 m yr <sup>–1</sup> has been observed ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). [[#Mentaschi--2018|Mentaschi et al. (2018)]] report a coastal area loss of 350 km <sup>2</sup> over the same period in Western Australia from satellite observations.

Projections indicate that a majority of sandy coasts in the region will experience shoreline retreat, throughout the 21st century ( ''high confidence'' ) (Figure 12.7b,d). Median shoreline change projections (CMIP5) under both RCP4.5 and RCP8.5 presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] show that, by mid-century, sandy shorelines will retreat (relative to 2010) by between 50 and 80 m all around Australasia, except in SAU and NZ where the projected retreat (relative to 2010) is between 35 and 50 m. By 2100, median shoreline retreats exceeding 100 m (relative to 2010) are projected along the sandy coasts of NAU (about 150 m), CAU (about 160 m), and EAU (about 110 m) under RCP4.5m, while projections for SAU and NZ are around 80–90 m. Under RCP8.5, shoreline retreat exceeding 100 m is projected all around the region by 2100 (relative to 2010) with retreats as high as 220 m in NAU and CAU (about 170 m in EAU and about 130 m in SAU and NZ; Figure 12.7b,d). The total length of sandy coasts in Australasia that is projected to retreat by more than a median of 100 m by 2100 under RCP4.5 and RCP8.5 is about 12,500 and 16,000 km respectively, an increase of approximately 30%.

Distinct from long-term coastline recession, storms and storm surges also result in episodic coastal erosion. In general, the historically measured maximum episodic coastal erosion (either eroded volume or coastline retreat distance) or that due to a 1-in-100-year return period storm wave height is used as a design criterion for coastal zone management and planning in Australia ( [[#Wainwright--2014|Wainwright et al., 2014]] ; [[#Mortlock--2017|Mortlock et al., 2017]] ).

While there is wide recognition in Australia that the combined effect of SLR, changing storm surge and wave climates will directly affect future episodic coastal erosion ( [[#McInnes--2016|McInnes et al., 2016]] ; [[#Ranasinghe--2016|Ranasinghe, 2016]] ; [[#Harley--2017|Harley et al., 2017]] ) only a few projections of how this hazard may evolve are available for Australia. In one such study, [[#Jongejan--2016|Jongejan et al. (2016)]] provide projections of how the full exceedance probability curve of the maximum erosion per year may evolve over the 21st century (due to the combined action of SLR, storm surge and storm waves). Their results show that, for example, the 0.01 exceedance probability maximum coastline retreat in 2025 will have an exceedance probability of 0.015 by 2050 and 0.07 by 2100.

'''Marine heatwave:''' The mean SST of the ocean around Australia and east of New Zealand has warmed at a rate of about 0.22°C per decade between 1992 and 2016 ( [[#Wijffels--2018|Wijffels et al., 2018]] ), which is higher than the global average SST increase of 0.16°C per decade ( [[#Oliver--2018|Oliver et al., 2018]] ). This mean ocean surface warming is connected to longer and more frequent marine heatwaves in the region ( [[#Oliver--2018|Oliver et al., 2018]] ). Over the period 1982–2016, the coastal ocean of Australia experienced on average more than 1.5 marine heatwaves (MHWs) per year, with the north coast of Western Australia and the Tasman Sea experiencing on average 2.5–3 MHWs per year. The average duration was between 10 and 15 days, with somewhat longer and hotter MHWs in the Tasman Sea. In New Zealand, the south-east coast of South Island experiences the most MHWs (2.5–3 per year). The duration of MHW in New Zealand is on average 10–15 days ( [[#Oliver--2018|Oliver et al., 2018]] ). Changes around Australasia over the 20th century, derived from MHW proxies, show an increase in frequency between 0.3 and 1.5 MHW per decade, except along the south-east coast of New Zealand (Box 9.2); an increase in duration per event; and the total number of MHW days per decade, with the change being stronger in the Tasman Sea than elsewhere ( [[#Oliver--2018|Oliver et al., 2018]] ).

There is ''high confidence'' that MHWs will increase around most of Australasia. Under RCP4.5 and RCP8.5 respectively, mean SST is projected to increase by 1°C and 2°C around Australia by 2100, with a hotspot of around 2°C for RCP4.5 and of 4°C for RCP8.5 along the south-east coast between Sydney and Tasmania (Interactive Atlas). Under all RCPs, the mean SST around Australia is expected to increase in the future, with median values of around 0.4°C–1.0°C by 2030 under RCP4.5, and 2°C–4°C by 2090 under RCP8.5 (CSIRO and BOM, 2015). Warming is expected to be largest along the north-west coast of Australia, southern Western Australia, and along the east coast of Tasmania (CSIRO and BOM, 2018). More frequent, extensive, intense and longer lasting MHWs are projected around Australia and New Zealand for GWLs of 1.5°C, 2°C and 3.5°C relative to the modelled reference value for 1861–1880 ( [[#Frölicher--2018|Frölicher et al., 2018]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around Australasia by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''In general, there is''' high confidence '''that most coastal/ocean-related hazards in Australasia will increase over the 21st century. Relative sea level rise is''' virtually certain '''to continue in the oceans around Australasia, contributing to increased coastal flooding in low-lying areas''' ( high confidence ''') and shoreline retreat along most sandy coasts''' ( high confidence '''). Marine heatwaves are also expected to increase around the region over the 21st century''' ( high confidence ''').'''

The assessed direction of change in climatic impact-drivers for Australasia and associated confidence levels are illustrated in Table 12.5, together with emergence time information ( [[#12.5.2|Section 12.5.2]] ). No assessable literature could be found for hail and snow avalanches, although these phenomena may be relevant in parts of the region.

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'''Table 12.5''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in Australasia, representing their aggregate characteristic changes for mid-century for scenarios RCP4.5, SSP2-4.5, SRES A1B, or above within each AR6 region (defined in Chapter 1), approximately corresponding (for CIDs that are independent of sea level rise) to global warming levels between 2°C and 2.4°C (see [[#12.4|Section 12.4]] for more details of the assessment method).''' The table also includes the assessment of observed or projected time-of-emergence of the CID change signal from the natural interannual variability if found with at least ''medium confidence'' in [[#12.5.2|Section 12.5.2]] .

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