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

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

<div id="h3-45-siblings" class="h3-siblings"></div>

'''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 ''').'''

<div id="12.4.3.2" class="h3-container"></div>

<span id="wet-and-dry-3"></span>