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

=== 12.4.3 Australasia ===

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For the purpose of this assessment, Australasia is divided into five sub-regions as defined in [[IPCC:Wg1:Chapter:Chapter-1#1.4.5|Section 1.4.5]] : Northern Australia (NAU), Central Australia (CAU), Eastern Australia (EAU), Southern Australia (SAU) and New Zealand (NZ).

The Fourth and Fifth IPCC Assessment Reports (AR4 and AR5) identify the most damaging historical hazards in this region to be inland flooding, drought, wildfire and episodic coastal erosion due to storms ( [[#Hennessy--2007|Hennessy et al., 2007]] ; [[#Reisinger--2014|Reisinger et al., 2014]] ). The SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) projects ''very likely'' increases in the intensity and frequency of warm days and warm nights and decreases in the intensity and frequency of cold days and cold nights in Australasia. Furthermore, a ''likely'' increase in the frequency and duration of warm spells is also projected for Australia. The SROCC ( [[#IPCC--2019b|IPCC, 2019b]] ) projects a ''likely'' global mean sea level rise (RCP8.5) that is up to 0.1 m higher than corresponding AR5 projections. The SROCC also projects an increase of mean significant wave height across the Southern Ocean ( ''high confidence'' ) and an increase in the occurrence of historically rare (1-in-100-year) extreme sea levels to 1-in-1-year or more frequent events all around the Australasian region by 2100 under RCP8.5.

A detailed national scale climate change assessment of observed and projected climate change, based on over 40 CMIP5 models and high resolution downscaling (CSIRO and BOM, 2015), and biannual short updates thereafter are available for Australia (CSIRO and BOM, 2016, 2018, 2020). Similar national assessments for New Zealand are also available (MfE and Stats NZ, 2017, 2020; [[#MfE--2018|MfE, 2018]] ). The severe extreme events such as heatwaves and river floods that have occurred in Australasia, especially over the last decade, have enabled a number of attribution studies, improving the understanding of regional climate change mechanisms that drive such extreme events (Chapter 11).

Figure 12.7 illustrates projected changes in two selected climatic impact-driver indices for Australasia.

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[[File:7d935506d0aa68d0dcf7f9e1596462d3 IPCC_AR6_WGI_Figure_12_7.png]]

'''Figure 12.7''' '''|''' '''Projected changes in selected climatic impact-driver indices for Australasia. (a)''' Mean change in 1-in-100-year river discharge per unit catchment area (Q100, m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ) from CORDEX-Australasia models for 2041–2060 relative to 1995–2014 for RCP8.5. '''(b)''' Shoreline position change along sandy coasts by the year 2100 relative to 2010 for RCP8.5 (metres; negative values indicate shoreline retreat) from the CMIP5-based dataset presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] . '''(c)''' Bar plots for Q100 (m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ) averaged over land areas for the AR6 WGI Reference Regions (defined in Chapter 1). The left-hand column within each panel (associated with the left-hand y-axis) shows the ‘recent past’ (1995–2014) Q100 absolute values in grey shades. The other columns (associated with the right-hand y-axis) show the Q100 changes relative to the recent past values for two time periods (‘mid’ 2041–2060 and ‘long’ 2081–2100) and for three global warming levels (defined relative to the pre-industrial period 1850–1900): 1.5°C (purple), 2°C (yellow) and 4°C (brown). The bars show the median (dots) and the 10–90th percentile range of model ensemble values across each model ensemble. CMIP6 is shown by the darkest colours, CMIP5 by medium, and CORDEX by light. SSP5-8.5/RCP8.5 is shown in red and SSP1-2.6/RCP2.6 in blue. '''(d)''' Bar plots for shoreline position change show CMIP5-based projections of shoreline position change along sandy coasts for 2050 and 2100 relative to 2010 for RCP8.5 (red) and RCP4.5 (blue) from [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] . Dots indicate regional mean change estimates and bars show the 5–95th percentile range of associated uncertainty. Note that these shoreline position change projections assume that there are no additional sediment sinks/sources or any physical barriers to shoreline retreat. See Technical ( [[IPCC:Wg1:Chapter:Annex-vi|Annex VI]] for details of indices. Further details on data sources and processing are available in the chapter data table (Table 12.SM.1).

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==== 12.4.3.1 Heat and Cold ====

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'''Mean air temperature:''' Across Australia mean temperatures have increased by 1.44°C ± 0.24°C during the period 1910–2019, with most of the warming occurring since 1950 (Atlas.6.2; CSIRO and BOM, 2020; [[#Trewin--2020|Trewin et al., 2020]] ). In New Zealand, an increase of 1.1°C has been measured from 1909–2016 (Atlas.6.2; MfE and Stats NZ, 2020). In the period 1980–2014 a rate of increase of 0.1°C–0.3°C per decade has been observed (Atlas.11 and Atlas.20).

Mean temperature in Australasia is projected to continue to rise through the 21st century ( ''virtually certain'' ) (Atlas.6.4). Projections for Australia indicate that the average temperature will increase by +1.1°C (0.84–1.52°C 10–90th percentile range) by 2041–2060 (mid-century), and by +1.9°C (1.29–2.58°C) by 2081–2100 (end-century), relative to the baseline period of 1995–2014, under SSP2-4.5 (Interactive Atlas). For SSP5-8.5, the projected changes are up to +1.5°C (1.17–1.96°C) and +3.7°C (2.75–4.91°C) for mid- and end-century respectively. For SSP1-2.6, mean temperature is projected to rise by +0.9°C (0.55–1.26°C) and +1.0°C (0.55–1.54°C) relative to 1995–2014 by mid- and end-century respectively (Interactive Atlas). In New Zealand, an increase of mean temperature of +1.0°C (0.60–1.32°C) relative to 1995–2014 is projected by mid-century, and an increase of +1.6°C (1.03–2.26°C) by end-century under SSP2-4.5. For SSP5-8.5, the projected increase in mean temperature is +1.3°C (0.91–1.66°C 10–90th percentile range) and +3.1°C (2.20–4.05°C) relative to 1995–2014 by mid- and end-century respectively. For SSP1-2.6, the projected increase in mean temperature is +0.75°C (0.39–1.06°C) and +0.8°C (0.47–1.46°C) relative to 1995–2014 by mid- and end-century, respectively (Interactive Atlas).

'''Extreme heat:''' The region has a ''very likely'' trend of increasing frequency and severity of hot extremes since the 1950s (Table 11.10). Extreme minimum temperatures have increased in all seasons over most of Australia and exceeds the increase in extreme maximum temperatures (X.L. [[#Wang--2013|]] [[#Wang--2013|Wang et al., 2013]] ; [[#Jakob--2016|Jakob and Walland, 2016]] ). Heatwave characteristics and hot extremes have increased across many Australian regions since the mid-20th century (Table 11.10; CSIRO and BOM, 2020). The number of days per year with maximum temperature greater than 35°C has increased over most parts of Australia from 1957–2015, with the largest increasing trends of 0.4–1 days/year occurring in north-western, Northern, north-eastern Australia and parts of Central Australia (CSIRO and BOM, 2016). Long-term changes of hot extremes in Australia have been attributed to anthropogenic influence (Table 11.10). In New Zealand, the number of annual heatwave days increased at 18 of 30 sites during the period 1972–2019 (MfE and Stats NZ, 2020).

More frequent hot extremes and heatwaves are expected over the 21st century in Australia ( ''virtually certain'' ) (Table 11.10). Heat thresholds potentially affecting agriculture and health, such as 35°C or 40°C, are projected to be exceeded more frequently over the 21st century in Australia under all RCPs ( ''high confidence'' ). By 2090 under RCP4.5, the average number of days per year with maximum temperatures above 35°C is highly spatially variable and is expected to increase by 50–100%, while the number of days per year with maximum temperatures above 40°C is expected to increase by 200%, relative to 1985–2005 (CSIRO and BOM, 2015). Under RCP8.5 the corresponding projected increases are even greater, with a greater than 100% increase in most of Australia, and far greater increases in Central and Northern Australia (up to a 20-fold increase in Darwin). Projections for New Zealand indicate more frequent hot extremes ( ''virtually certain'' ) (Table 11.10). Figure 12.4b, c shows CMIP6 projections of mean number of days per year with maximum temperature exceeding 35°C under SSP5-8.5, which are consistent with the above assessed literature and across the two CMIP generations, and indicate a strong difference depending on the mitigation scenario (e.g., over 100 days more per year under SSP5-8.5 in NAU, but, in general, less than 60 days more per year under SSP1-2.6 in NAU; Figure 12.SM.1).

The projected frequency of exceeding dangerous humid heat thresholds is increasing in Australia, with a strong increase in Northern Australia for RCP8.5 ( ''high confidence'' ) ( [[#Zhao--2015|Zhao et al., 2015]] ; [[#Mora--2017|Mora et al., 2017]] ; [[#Brouillet--2019|Brouillet and Joussaume, 2019]] ), consistently across CMIP5, CMIP6 and CORDEX simulations (Figure 12.4d–f and Figure 12.SM.2). Using the HI index, by end-century, the average number of days exceeding 41°C is projected to increase in NAU by about 100 days and by about 25 days under SSP5-8.5 and SSP1-2.6, respectively. The projections for New Zealand indicate no appreciable increase in the number of days with HI &gt; 41°C across SSPs, time periods and CMIP generations (Figure 12.4d–f and Figure 12.SM.2).

'''Cold spell and frost:''' Excepting parts of Southern Australia, the Australasian region has a significant trend of decreasing frequency in cold extremes since the 1950s ( ''high confidence'' ) (Table 11.10) and there is ''high confidence'' that such trends are attributable to anthropogenic influence (Table 11.10). The number of frost days per year in Australia has on average declined at a rate of 0.15 days/decade in the past century ( [[#Alexander--2017|Alexander and Arblaster, 2017]] ), except in some regions of Southern Australia, where an increase in both number and season length has been reported ( [[#Dittus--2014|Dittus et al., 2014]] ; [[#Crimp--2016b|Crimp et al., 2016b]] ). The number of frost days has decreased at 12 of 30 monitoring sites around New Zealand over the period 1972–2019 (MfE and Stats NZ, 2020).

Less frequent cold extremes are ''virtually certain'' in Australasia (Table 11.10) while a decrease of frost days is projected with ''high confidence'' for the region. Projections, relative to 1986–2005, for the number of frost days per year in Australia indicate declines of 0.9 days by mid-century and 1.1 days by end-century for RCP4.5, while for RCP8.5, the projected declines are 1.0 days and 1.3 days by mid- and end-century respectively ( [[#Alexander--2017|Alexander and Arblaster, 2017]] ; [[#Herold--2018|Herold et al., 2018]] ). Projections for New Zealand indicate that the number of frost days will decrease by 30% (RCP2.6) to 50% (RCP8.5) by 2040, relative to 1986–2005. By 2090, the decrease ranges from 30% (RCP2.6) to 90% (RCP8.5) (MfE and Stats NZ, 2017).

'''In general, there is''' high confidence '''that most heat hazards in Australasia will increase and that cold hazards will decrease over the 21st century. The mean temperature in Australasia is''' virtually certain '''to continue to rise through the 21st century, accompanied by less frequent cold extremes''' ( virtually certain ''') and frost days''' ( high confidence '''), and more frequent hot extremes''' ( virtually certain '''). Heat stress is projected to increase in Australia''' ( high confidence ''').'''

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==== 12.4.3.2 Wet and Dry ====

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'''Mean precipitation:''' Here, only increases in precipitation (under ‘Wet’) are addressed, with decreases (under ‘Dry’) addressed in ‘Aridity’ below.

In terms of wet climatic impact-drivers, detectable anthropogenic increases in precipitation in Australia have been reported particularly for north-central Australia for the period 1901–2010 ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). Figure Atlas.11 indicates no significant trend in precipitation over the region during the baseline period 1960–2015, except for the Global Precipitation Climatology Project (GPCP) dataset, which shows an increasing trend in north-central Australia. In New Zealand, increases in annual rainfall have been observed between 1960–2019 in the south and west of the South Island and east of the North Island. Note however, for the most part, the above reported trends in New Zealand have been classified as statistically not significant (Figure Atlas.20).

Annual mean precipitation is projected to increase in Central and north-east Australia ( ''low confidence'' ) and in the south and west of New Zealand ( ''medium confidence'' ) (Atlas.6.4). [[#Liu--2018a|Liu et al. (2018a)]] show that under 1.5°C warming, Central and north-east Australia will become wetter. Projected patterns in annual precipitation exhibit increases in the west and south of New Zealand (Atlas.6.4; [[#Liu--2018a|Liu et al., 2018a]] ) and project that the South Island will be wetter under both 1.5°C and 2°C warming. However, there is limited model agreement for projected rainfall changes in Australasia as shown in the Atlas.

'''River flood:''' Streamflow observations in Australia have shown that negative trends dominate in annual maximum flow and that stations with significant negative trends were mostly located in the south-east and south-west ( [[#Gu--2020|Gu et al., 2020]] ). The observed peak flow trend in Southern Australia is attributed to the decrease of soil moisture, although an increase of flood magnitude is possible for very rare events. For the more frequent flood events, the increase of extreme precipitation is balanced by the decrease of soil moisture. ( [[#Wasko--2019|Wasko and Nathan, 2019]] ).

While median annual runoff is projected to decrease in most of Australia ( [[#Chiew--2017|Chiew et al., 2017]] ), consistent with projected decreases in average rainfall (CSIRO and BOM, 2015; [[#Alexander--2017|Alexander and Arblaster, 2017]] ), river floods are projected to increase due to more intense extreme rainfall events and associated increase in runoff ( ''medium confidence'' ). [[#Asadieh--2017|Asadieh and Krakauer (2017)]] found a decrease in the value of the 95% percentile of mean streamflow with RCP8.5 by the end of the century in all of Australia, except in a small part in centre of the country. In terms of relative increases, flooding is expected to increase more in Northern Australia (driven by convective rainfall systems) than in Southern Australia (where more intense extreme rainfall may be compensated by drier antecedent moisture conditions; [[#Alexander--2017|Alexander and Arblaster, 2017]] ; [[#Dey--2019|Dey et al., 2019]] ) with flood frequency increasing in Northern Australia and along parts of the east coast and decreasing in south-western Western Australia ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ). [[#Gu--2020|Gu et al. (2020)]] project larger flood magnitude and volumes under both RCP2.6 and RCP8.5 in Northern Australia, and smaller flood magnitudes and volumes in Southern Australia under the same RCPs. These findings are in general agreement with the patterns in peak flow, corresponding to the 1-in-100-year return period streamflow, shown in Figure 12.7a,c for mid-21st century under RCP8.5.

There is ''medium confidence'' that river flooding will increase in New Zealand. Projections for New Zealand indicate that the 1-in-50-year and 1-in-100-year flood peaks for rivers in many parts of the country may increase by 5 to 10% by 2050 and more by 2100 (with large variation between models and emissions scenarios), with a corresponding decrease in return periods for specific flood levels ( [[#Gray--2005|Gray et al., 2005]] ; [[#Carey-Smith--2010|Carey-Smith et al., 2010]] ; [[#McMillan--2010|McMillan et al., 2010]] , 2012; [[#Ballinger--2011|Ballinger et al., 2011]] ).

'''Heavy precipitation and pluvial flood:''' Rainfall extremes have been detected to increase in Australasia, with ''low confidence'' (Table 11.10). There is ''high confidence'' that R × 1 day and R × 5 day precipitation extremes will increase for 2°C or lower warming for the region as a whole, but on a sub-regional basis there is only ''medium confidence'' of increases in NAU and CAU and ''low confidence'' of increases on EAU, SAU and NZ. For warming levels exceeding 2°C, these extremes are ''very likely'' to increase in NAU and CAU and they are ''likely'' to increase elsewhere in the region ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ).

'''Landslide:''' Based on local slope characteristics, lithology and seismic activity, the South Island and the eastern half of the North Island of New Zealand are vulnerable to landslide occurrence ( [[#Broeckx--2020|Broeckx et al., 2020]] ). The potential for land and rockslides increases with, amongst other factors, total precipitation rates, precipitation intensity, mountain permafrost thaw rates, glacier retreat and air temperature ( [[#Crozier--2010|Crozier, 2010]] ; [[#Allen--2013|Allen and Huggel, 2013]] ; [[#Gariano--2016|Gariano and Guzzetti, 2016]] ; [[#IPCC--2019a|IPCC, 2019a]] ). Given the increase of the magnitude of these physical variables in areas that are already highly susceptible to mass movements ( [[#MfE--2018|MfE, 2018]] ), there is ''low confidence'' that the occurrence of landslides will increase under future climate conditions.

'''Aridity:''' In terms of dry climatic impact-drivers, a substantial decrease in precipitation has been observed across Southern Australia during the cool season (April–October) ( ''medium confidence'' ). The drying trend has been particularly strong over south-west Western Australia between May and July, with rainfall since 1970 being around 20% less than the 1900–1969 average (CSIRO and BOM, 2020). Detectable decreases in mean precipitation, attributable at least in part to anthropogenic forcing, have been reported for parts of south-west Australia ( [[#Delworth--2014|Delworth and Zeng, 2014]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ), south-east Australia, and Tasmania ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). In New Zealand, the north-east of the South Island and western and the northern parts of the North Island show decreasing precipitation trends during 1960–2019 (MfE and Stats NZ, 2020).

Aridity is projected to increase, especially during winter and spring, with ''medium confidence'' in SAU but with ''high confidence'' in south-west Western Australia (Table 11.11 and Atlas.6.4). In EAU and in the north and east of NZ, aridity is projected to increase with ''medium confidence'' , while a decrease is projected with ''medium confidence'' in the south and west of NZ (Atlas.6.4). Although there is only ''low confidence'' in the projected decrease of mean annual precipitation in south-western and eastern Australia and the north and east of New Zealand, there is ''high confidence'' of reduced winter and spring precipitation in Australia in future, mostly in south-western and eastern Australia (Atlas.6.4). [[#Liu--2018b|Liu et al. (2018b)]] show that under 2°C warming, most of Australia is projected to become drier based on the Palmer Drought Severity Index (PDSI), with the exception of the tropical north-east. [[#Ferguson--2018|Ferguson et al. (2018)]] project that between 1976–2005 and 2070–2099, winters will become drier (mainly in Southern Australia) under RCP8.5. [[#Liu--2018b|Liu et al. (2018b)]] project that the North Island of New Zealand will be drier under both 1.5°C and 2°C warming.

'''Hydrological drought:''' There is ''low confidence'' of observed changes in hydrological droughts in Australasia, except in SAU where there is ''medium confidence'' of an observed increase in the south-east and south-west. Future projections indicate ''medium confidence'' in further hydrological drought increases for Southern Australia for warming levels of 2°C or higher ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). Mean annual runoff in far south-east and far south-west Australia are projected to decline by median values of 20 and 50% respectively, by mid-century under RCP8.5 ( [[#Chiew--2017|Chiew et al., 2017]] ). [[#Prudhomme--2014|Prudhomme et al. (2014)]] assess changes in the Drought Index (DI), defined as areal runoff less than the 10th percentile over the reference period 1976–2005, and project DI increases for both Australia and New Zealand by 10–20% by 2070–2099 under RCP8.5, with the greatest effects being in the southern parts of the Australian continent. These projections are consistent with the trends shown in Figure 12.4g–i (Figure 12.SM.3). The SPI drought frequency is projected to increase in SAU and particularly in south-west Western Australia by mid-century, while by the end of the century SPI drought frequency is projected to increase all over Australia, and particularly strongly in south-west Western Australia as well as southern Victoria (see Figure 12.4g–i). For the Murray–Darling basin, [[#Ferguson--2018|Ferguson et al. (2018)]] project effectively no change (–1%) in mean precipitation, a 27% decrease in P–E, and 30% increase in runoff in 2070–2099 relative to 1976–2005 with RCP8.5.

'''Agricultural and ecological drought:''' There is ''medium confidence'' in observations of agricultural and ecological droughts increasing in SAU and decreasing in NAU, while there is ''low confidence'' of changes elsewhere in the region ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). More regional studies have observed an increase in agricultural and ecological drought intensity in south-west Australia and an increase in drought intensity in parts of south-east Australia, while the length of droughts therein has increased ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). In New Zealand, since 1972–73, soils at 7 of 30 monitored sites became drier, while the 2012–13 drought was one of the most extreme in the previous 41 years (MfE and Stats NZ, 2017). Future evaporative demand is projected to lead to ''medium confidence'' increases in agricultural and ecological droughts for 2°C of global warming in SAU and EAU and ''low confidence'' for changes in CAU, NAU and NZ, although there is ''medium confidence'' of increases in CAU with 4°C of global warming ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). There is ''medium confidence'' for more time in agricultural and ecological drought in SAU by mid-21st century ( [[#Coppola--2021b|Coppola et al., 2021b]] ) as well as by the end of the 21st century ( [[#Herold--2018|Herold et al., 2018]] ). The Standardized Precipitation Evapotranspiration Index (SPEI) shows a springtime intensification in SAU with moderate and severe droughts in the south-west and moderate droughts in the south-east ( [[#Herold--2018|Herold et al., 2018]] ). There is consensus among the different model ensembles (CORDEX-CORE, CMIP5 and CMIP6) that the drought frequency (DF), one of several proxies for agricultural and ecological drought, will increase in all four Australian regions for both mid-century (NAU 0.2–2 DF increase, CAU 0.5–2 DF increase, SAU 1–3 DF increase and EAU 0.8–3 DF increase) and end-century (0.8–2.7 DF increase for NAU, 1.2–2 DF increase for CAU, 2.2–3.8 for SAU and 0.2–3 for EAU) for both RCP8.5 and SSP5-8.5, with CMIP6 showing the lowest increase (Figure 12.4g–l and Figure 12.SM.4; [[#Coppola--2021b|Coppola et al., 2021b]] ).

'''Fire weather:''' [[#Dowdy--2018|Dowdy and Pepler (2018)]] examined atmospheric conditions conducive to pyroconvection in the period 1979–2016, and found an increased risk in south-east Australia during spring and summer, due to changes in vertical atmospheric stability and humidity, in combination with adverse near-surface fire weather conditions. CSIRO and BOM (2018) and [[#Dowdy--2018|Dowdy (2018)]] found that the annual 90th percentile daily Forest Fire Danger Index (FFDI) has increased from 1950–2016 in parts of Australia, especially in Southern Australia (1–2.5 per decade) and in spring and summer. These studies indicate an increase in the frequency and magnitude of FFDI extreme quantiles, as well as a shift of the fire season start towards spring, lengthening the fire season. The unprecedented large fires of austral spring and summer of 2019 in south-east Australia were a result of extreme hot and dry weather in significantly drier than average conditions that had persisted since 2017, in combination with consistently stronger than average winds, resulting in above average to highest on record FFDI values in much of the country ( [[#Abram--2021|Abram et al., 2021]] ). These fires have been attributed to climate change through the temperature component of fire weather indices ( [[#van%20Oldenborgh--2021|van Oldenborgh et al., 2021]] ). In New Zealand, days with very high and extreme fire weather increased in 12 out of 28 monitored sites, and decreased in 8, in the period 1997–2019 (MfE and Stats NZ, 2020). Attribution studies indicate that there is ''medium confidence'' of an anthropogenically driven past increase in fire weather conditions, essentially due to increase in frequency of extreme heat waves. ( [[#Hope--2019|Hope et al., 2019]] ; [[#Lewis--2020|Lewis et al., 2020]] ; [[#van%20Oldenborgh--2021|van Oldenborgh et al., 2021]] ).

Fire weather indices are projected to increase in most of Australia ( ''high confidence'' ) and many parts of New Zealand ( ''medium confidence'' ), in particular with respect to extreme fire and induced pyroconvection ( [[#Dowdy--2019b|Dowdy et al., 2019b]] ). Increasing mean temperature, cool season rainfall decline, and changes in tropical climate variability all contribute to a future increase in extreme fire risk in Australia ( [[#Abram--2021|Abram et al., 2021]] ). Projections indicate that the annual cumulative FFDI will increase by 31–33% in Southern and Eastern Australia, and by 17–25% in Northern Australia and the Rangelands by 2090 (relative to 1995) under RCP8.5 (CSIRO and BOM, 2015). Using a CMIP5 ensemble of 17 models, [[#Abatzoglou--2019|Abatzoglou et al. (2019)]] found a statistically significant positive trend for fire weather intensity and fire season length for future mid-century conditions under RCP8.5, including a detectable anthropogenic influence on fire risk magnitude and fire season length by 2040 in Western Australia and along the Queensland coastline. Using the C-Haines and FFDI indices with A2 and RCP8.5 respectively, [[#Di%20Virgilio--2019|Di Virgilio et al. (2019)]] and [[#Clarke--2019|Clarke et al. (2019)]] have shown that extreme fire weather frequency will increase in south-eastern Australia by the end of the 21st century. Most of these projections indicate that the biggest increases in fire weather conditions will be in late spring, effectively resulting in longer (stronger) fire seasons in areas where spring is the shoulder (peak) season. In New Zealand, [[#Watt--2019|Watt et al. (2019)]] projected that the number of days with very high to extreme fire risk will increase by 71% by 2040, and by a further 12% by 2090, for the A1B scenario, with fire risk increase all along the east coast. The most marked relative changes by 2090 were projected for Wellington and Dunedin, where very high to extreme fire risk is projected to increase by, respectively, 89% to 32 days and 207% to 18 days, compared to the baseline period 1970–1999.

'''Annual mean precipitation is projected to increase in Central and north-east Australia''' ( low confidence ''') and in the south and west of New Zealand''' ( medium confidence '''), while it is projected to decrease in Southern Australia''' ( medium confidence '''), albeit with''' high confidence '''in south-west Western Australia, in Eastern Australia''' ( medium confidence '''), and in the north and east of New Zealand''' ( medium confidence '''). Heavy precipitation and pluvial flooding are projected to increase with''' medium confidence '''in Northern Australia and Central Australia. There is''' medium confidence '''that river flooding will increase in New Zealand and Australia, with higher increases in Northern Australia. Aridity is projected to increase with''' medium confidence '''in Southern Australia''' ( high confidence '''in south-west Western Australia), Eastern Australia''' ( medium confidence '''), and in the north and east of New Zealand''' ( medium confidence '''). Hydrological droughts are projected to increase in Southern Australia''' ( medium confidence '''), while agricultural and ecological droughts are projected to increase with''' medium confidence '''in Southern Australia and Eastern Australia. Fire weather is projected to increase throughout Australia''' ( high confidence ''') and New Zealand''' ( medium confidence ''').'''

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==== 12.4.3.3 Wind ====

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'''Mean wind speed:''' There is ''low confidence'' of a mean wind speed trend in the last decades ( ''low agreement'' ) ( [[#McVicar--2012|McVicar et al., 2012]] ; [[#Troccoli--2012|Troccoli et al., 2012]] ; [[#Azorin-Molina--2018|Azorin-Molina et al., 2018]] ; J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ), as long-term measurements are not homogeneous.

In future climate scenarios wind speed trends in Australia exhibit generally weak amplitudes with ''low agreement'' among models (Figure 12.4m–o and Figure 12.SM.5) with uncertain consequences on wind power potential (CSIRO and BOM, 2015; [[#Karnauskas--2018a|Karnauskas et al., 2018a]] ; [[#Jung--2019|Jung and Schindler, 2019]] ). However, there is ''medium confidence'' that, by the end of the century, annual mean wind power will significantly increase in north-eastern Australia under RCP8.5, but there is ''low confidence'' of an increase by end-century under RCP4.5, and for any scenario by mid-century ( [[#Karnauskas--2018a|Karnauskas et al., 2018a]] ). In New Zealand, mean wind patterns are projected to become more north-easterly in summer, and westerlies to become more intense in winter ( ''low confidence'' ), in agreement with the strengthening of the Southern Hemisphere storm tracks ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1|Section 4.5.1]] ).

'''Severe wind storm:''' There is generally ''low confidence'' in observed changes in extreme winds and extratropical storms in Australasia ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.2|Section 11.7.2]] ). CMIP5 projections of severe winds indicate a general increase in north-eastern Australia, and decreases in some parts in Southern and Central Australia ( ''medium confidence'' ) by the end of the century under RCP8.5 (CSIRO and BOM, 2015; [[#Kumar--2015|Kumar et al., 2015]] ; [[#Jung--2019|Jung and Schindler, 2019]] ). Elsewhere trends are diverse and vary across simulations with ''low agreement'' . Projections of changes in the 1-in-25-year return period winds (based on annual maxima) for 2074–2100 relative to 1979–2005 for RCP8.5 show an increase in tropical areas of Northern Australia ( [[#Kumar--2015|Kumar et al., 2015]] ).

In New Zealand, the frequency and magnitude of extreme winds have decreased (from 1980–2019) at 12 of 14 monitored sites and increased at two monitored sites (MfE and Stats NZ, 2020). Due to the intensification and the shift of the austral storm track by the end of the century ( [[#Yin--2005|Yin, 2005]] ), increases in extreme wind speed in New Zealand are projected over the South Island and the southern part of the North Island by mid- and end-century for all RCPs ( ''low confidence'' ) ( [[#MfE--2018|MfE, 2018]] ).

'''Tropical cyclone:''' In Australia, the number of TCs has generally declined since 1982, and the frequency of intense TCs that make landfall in north-eastern Australia has declined significantly since the 19th century ( ''medium confidence'' ) ( [[#Kuleshov--2010|Kuleshov et al., 2010]] ; [[#Callaghan--2011|Callaghan and Power, 2011]] ; [[#Holland--2014|Holland and Bruyère, 2014]] ; [[#Knutson--2019|Knutson et al., 2019]] ; CSIRO and BOM, 2020). There is ''high confidence'' that cyclones making landfall along north-eastern and northern Australian coastlines will decrease in number and ''low confidence'' of an increase in their intensities for 2°C of global warming as well as for the mid-century period with scenarios RCP4.5 and above ( [[#Roberts--2015|Roberts et al., 2015]] , 2020; [[#Bacmeister--2018|Bacmeister et al., 2018]] ; [[#Knutson--2020|Knutson et al., 2020]] ), with the amplitude of changes increasing from RCP4.5 to RCP8.5 ( [[#Bacmeister--2018|Bacmeister et al., 2018]] ). Decreases in frequency are projected for ‘east coast lows’ ( [[#Walsh--2016b|Walsh et al., 2016b]] ; [[#Dowdy--2019a|Dowdy et al., 2019a]] ).

'''Sand and dust storm:''' Australia is recognized to be the largest dust source in the Southern Hemisphere ( [[#Zheng--2016|Zheng et al., 2016]] ). Land-use and land-cover change have increased dust emissions in Australia in the past 200 years ( [[#Marx--2014|Marx et al., 2014]] ). While projections suggest a decrease in severe winds in Central and Southern Australia, changes in vegetation due to increased aridity and hydrological drought could be expected to result in increased wind erosion and dust emission across the country ( ''medium confidence'' ) ( [[#Webb--2020|Webb et al., 2020]] ).

'''In Australasia, there is''' low confidence '''in projected mean wind speeds and wind power potential, with a''' medium confidence '''increase projected only in north-eastern Australia under high emissions scenarios and by the end of the 21st century. Tropical cyclones in north-eastern and North Australia are projected to decrease in number''' ( high confidence ''') while their intensity is projected to increase''' ( low confidence ''').'''

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==== 12.4.3.4 Snow and Ice ====

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'''Snow:''' The snow season length in Australia has decreased by 5% during 2000–2013 relative to 1954–1999, especially in spring ( [[#Pepler--2015|Pepler et al., 2015]] ). A shift in the date of peak snowfall has also been observed with an 11-day advance over the same period ( [[#Pepler--2015|Pepler et al., 2015]] ). A decreasing trend in maximum snow depth has been observed for Australian alpine regions since the late 1950s, with the largest declines during spring and at lower altitudes. Maximum snow depth is highly variable and is strongly influenced by rare heavy snowfall days, which have no observed trends in frequency (CSIRO and BOM, 2020).

Projections for Southern Australia and New Zealand show a continuing reduction in snowfall during the 21st century ( ''high confidence'' ). The magnitude of decrease varies with the altitude of the region and the emissions scenario. At elevations lower than 1500 m, years without snowfall are projected from 2030 in some models. By 2090, and under RCP8.5, such years are projected to become common (CSIRO and BOM, 2015). The number of annual snow days in New Zealand is projected to decrease under all RCPs, by up to 30 days or more by 2090 under RCP8.5, relative to 1986–2005 ( [[#MfE--2018|MfE, 2018]] ).

'''Glacier:''' Glacier mass and areal extent in New Zealand is projected to continue to decease over the 21st century ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1.3|Section 9.5.1.3]] ). Glacier ice volume from 1977–2018 in New Zealand has decreased from 26.6 to 17.9 km <sup>3</sup> (a loss of 33%; [[#Salinger--2019|Salinger et al., 2019]] ). Relative to 2015, glaciers in New Zealand are projected to lose 36 ± 44%, 53 ± 33% and 77 ± 27% of their mass by the end of the century under RCP2.6, RCP4.5 and RCP8.5 respectively, with the loss rates decreasing over time under RCP2.6 and increasing under RCP8.5 ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

'''In summary, snowfall is expected to decrease throughout the region at high altitudes in both Australia''' ( high confidence ''') and New Zealand''' ( medium confidence '''). In New Zealand, glacier ice mass and extent are expected to decrease over the 21st century for all scenarios''' ( high confidence ''').'''

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==== 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]] .

[[File:dbce92f0fefa1d58b3e5175c9a9c6586 IPCC_AR6_WGI_Chapter12_Table_12_5.jpg]]

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