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

==== 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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