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

=== Atlas.6.1 Key Features of the Regional Climate and Findings From Previous IPCC Assessments ===

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==== Atlas.6.1.1 Key Features of the Regional Climate ====

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Australasia is divided into five regions for the Atlas (Figure Atlas.21), as follows: New Zealand (NZ), with a varied climate with diverse landscapes, mainly maritime temperate with four distinct seasons; Northern Australia (NAU), which is mainly tropical with monsoonal summer-dominated rainfall (monsoon season December to March, see Annex V), but with a hot, semi-arid climate in the south of the region; Central Australia (CAU) with a predominantly hot, dry desert climate; Eastern Australia (EAU) with a temperate oceanic climate at the coast to semi-arid inland; and Southern Australia (SAU), which ranges from Mediterranean and semi-arid in the west to mainly cool temperate maritime climate in the south-east. Various remote drivers have notable teleconnections to regions within Australasia, including an effect of the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (Table Atlas.1 and Annex IV). Much of southern NZ and SAU are affected by systems within the westerly mid-latitude circulation, in turn affected by the Southern Annular Mode (SAM). The monsoon and the Madden–Julian Oscillation (MJO) affect rainfall variability in northern Australia.

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[[File:c79a9dca5752dd2259cd32629a3a280a IPCC_AR6_WGI_Atlas_Figure_21.png]]

'''Figure Atlas.21''' '''|''' '''Regional changes over land in annual mean surface air temperature and precipitation relative to the 1995–2014 baseline for the reference regions in Australasia (warming since the 1850–1900 pre-industrial baseline is also provided as an offset).''' Bar plots in theleft panel of each region triplet show the median (dots) and 10th–90th percentile range (bars) across each model ensemble for annual mean temperature changes for four datasets (CMIP5 in intermediate colours; a subset of CMIP5 used to drive CORDEX in light colours; CORDEX overlying the CMIP5 subset with dashed bars; and CMIP6 in solid colours); the first six groups of bars represent the regional warming over two time periods (near-term 2021–2040 and long-term 2081–2100) for three scenarios (SSP1-2.6/RCP2.6, SSP2-4.5/RCP4.5 and SSP5-8.5/RCP8.5), and the remaining bars correspond to four global warming levels (GWLs: 1.5°C, 2°C, 3°C and 4°C). The scatter diagrams of temperature against precipitation changes display the median (dots) and 10th–90th percentile ranges for the above four warming levels for December–January–February (DJF; middle panel) and June–July–August (JJA; right panel), respectively; for the CMIP5 subset only the percentile range of temperature is shown, and only for 3°C and 4°C GWLs. Changes are absolute for temperature (in °C) and relative (as %) for precipitation. See [[#Atlas.1.3|Atlas.1.3]] for more details on reference regions ( [[#Iturbide--2020|Iturbide et al., 2020]] ) and [[#Atlas.1.4|Atlas.1.4]] for details on model data selection and processing. The script used to generate this figure is available online ( [[#Iturbide--2021|Iturbide et al., 2021]] ) and similar results can be generated in the Interactive Atlas for flexibly defined seasonal periods. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

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==== Atlas.6.1.2 Findings From Previous IPCC Assessments ====

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The AR5 WGI and WGII reports ( [[#IPCC--2013c|IPCC, 2013c]] ; [[#Stocker--2013|Stocker et al., 2013]] ; [[#Reisinger--2014|Reisinger et al., 2014]] ) give ''very high confidence'' that air and sea temperatures in the region have warmed; cool extremes have become rarer in Australia and New Zealand since 1950, while hot extremes have become more frequent and intense (e.g., it is ''very likely'' that the number of warm days and nights have increased). The AR5 reported that it is ''virtually certain'' that mean air and sea temperatures will continue to increase, with ''very high confidence'' that the greatest increase will be experienced by inland Australia and the smallest increase by coastal areas and New Zealand. The AR5 reported a range of different precipitation trends within the region. For example, while annual rainfall has been significantly increasing in north-western Australia since the 1950s ( ''very high confidence'' ), it has been decreasing in the north-east of the South Island of New Zealand over 1950–2004 ( ''very high confidence'' ) and over the south-west of the state of Western Australia. In line with these trends, AR5 reported it is ''likely'' that drought has decreased in north-west Australia. Future projections for precipitation extremes indicate an increase in most of Australia and New Zealand, in terms of rare daily rainfall extremes (i.e., current 20-year return period events) and of short duration (sub-daily) extremes ( ''medium confidence'' ). Likewise, however, there is a projected increase in the frequency of drought in southern Australia ( ''medium confidence'' ) and in many parts of New Zealand ( ''medium confidence'' ). Owing to hotter and drier conditions there is ''high confidence'' that the occurrence of fire weather will increase in most of southern Australia, and ''medium confidence'' that the fire danger index will increase in many parts of New Zealand.

The AR5 reported mean sea levels have also increased in Australia and New Zealand at average rates of relative sea level rise of 1.4 ± 0.6 mm yr <sup>–1</sup> from 1900 to 2011, and 1.7 ± 0.1 mm yr <sup>–1</sup> from 1900 to 2009, respectively ( ''very high confidence'' ). The assessment found that the volume of ice in New Zealand has declined by 36–61% from the mid- to late 1800s to the late 1900s ( ''high confidence'' ), while late-season significant snow depth has also declined in three out of four Snowy Mountain sites in Australia between 1957 and 2002 ( ''high confidence'' ). As mean sea level rise is projected to continue for at least several more centuries, there is ''very high confidence'' that this will lead to large increases in the frequency of extreme sea level events in Australia and New Zealand. On the other hand, the volume of winter snow and the number of days with low-elevation snow cover in New Zealand are projected to decrease in the future ( ''very high confidence'' ), while both snow depth and area are projected to decline in Australia ( ''very high confidence'' ).

The SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) reports on the observed and projected decline in snow cover in Australasia, as well as the retreat of New Zealand glaciers following an advance in 1983–2008 due to enhanced snowfall. It also reports on the vulnerability of some Australian communities and ecosystems to sea level rise, increases in the intensity and duration of marine heatwaves driven by human influence ( ''high confidence'' ), the decrease in frequency of tropical cyclones’ landfall on eastern Australia since the late 1800s ( ''low confidence'' in an anthropogenic signal), and presents a case study on the multiple hazards, compound risk and cascading impacts from climate extremes in Tasmania in 2015–2016 (including an attributable human influence on some events). The SRCCL ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ) found widespread vegetation ‘greening’ has occurred in parts of Australia, and an increase in the desertification and drought risk in future in southern Australia.

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