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

== Atlas.6 Australasia ==

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The assessment in this section focuses on changes in average temperature and precipitation (rainfall and snow), including the most recent years of observations, updates to observed datasets, the consideration of recent studies using CMIP5 and those using CMIP6 and CORDEX simulations. Assessment of changes in extremes is in [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] (Tables 11.10–12) and climatic impact-drivers in [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] (Table 12.5).

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=== 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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=== Atlas.6.2 Assessment and Synthesis of Observations, Trends and Attribution ===

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Reliable station observations are available from around 1900 in Australasia, but in some regions the coverage was and remains poor. Australia and New Zealand have continued to warm, and many rainfall trends have continued since AR5. Changes and trends in temperature and precipitation from 1961 to 2015 from three different global datasets are displayed in Figure Atlas.11 and the Interactive Atlas, and show significant (at 0.1 significance level) warming trends over southern and eastern Australia. Most of the observed changes in precipitation over the region are not significant over this period. Although observed datasets (e.g., GPCC and GPCP) generally agree on a significant drying trend in the southern regions of New Zealand during the shorter 1980–2015 period, this is in fact the reverse of the longer-term trends in 1961–2015 (Interactive Atlas).

For a longer-term perspective based on high-quality regional datasets, Figure Atlas.20 shows Australasia has warmed over the last century ( ''very high confidence'' ). Australian mean temperature has increased by 1.44°C ± 0.24°C during the period 1910–2019 using the updated observed temperature dataset ACORN-SATv2.1, with 2019 Australia’s hottest year on record and nine out of 10 of the warmest years on record occurring since 2005 ( [[#Trewin--2020|Trewin et al., 2020]] ). Much of the warming has occurred since 1960, there is clear anthropogenic attribution of this change and emergence of the signal from the1850–1900 climate ( [[#BOM%20and%20CSIRO--2020|BOM and CSIRO, 2020]] ; [[#Hawkins--2020|Hawkins et al., 2020]] ). Warming has been more rapid than the national average in central and eastern Australia, with a warming minimum and non-significant trends since the 1960s in the north-west ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ; [[#BOM%20and%20CSIRO--2020|BOM and CSIRO, 2020]] ). The National Institute of Water and Atmospheric Research temperature record, NIWA NZ, shows a warming of 1.13°C ± 0.27°C during the period 1909–2019, although several stations show non-significant trends since 1960 (Figure Atlas.20), including a warming minimum in the south-east at least partly due to a persistent shift in atmospheric circulation ( [[#Sturman--2013|Sturman and Quénol, 2013]] ; [[#MfE%20and%20Stats%20NZ--2017|MfE and Stats NZ, 2017]] , 2020).

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[[File:1613874b91602f0ca427f4419cd0d8b5 IPCC_AR6_WGI_Atlas_Figure_20.png]]

'''Figure Atlas.20''' '''|''' '''Observed trends in mean annual temperature (a, b) and summer (December–January–February, DJF) and winter (June–July–August, JJA) precipitation (c, d) for Australia and New Zealand from high-quality regional datasets.''' Time series show anomalies from the 1961–1990 average and 10-year running mean; maps show annual linear trends for 1960–2019; rainfall trends are shown in % per decade, crosses show areas and stations with a lack of significant trend and regions of seasonally dry conditions (&lt;0.25 mm day–1) are masked and outlined in red. Datasets are Australian Climate Observation Reference Network – Surface Air Temperature version 2.1 (ACORN-SATv2.1) for Australian temperature, the Australian Gridded Climate Data (AGCD) for Australian rainfall ( [[#Evans--2020|Evans et al., 2020]] ), and the 30-station high-quality network for New Zealand temperature and rainfall. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

Since 1960, precipitation has increased in much of mainland Australia in austral summer and decreased in many regions of southern and eastern Australia in austral winter (Figure Atlas.20). A detectable anthropogenic signal of increases in precipitation in Australia has been reported particularly for north central Australia and for a few regions along the south-central coast for the period 1901–2010 ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). Seasonally, there is a significant decline in winter rainfall in the south-west of the state of Western Australia (Figure Atlas.20), with an attributable human influence ( ''high confidence,'' ''robust evidence'' , ''medium agreement'' ) ( [[IPCC:Wg1:Chapter:Chapter-10#10.4|Section 10.4]] and references therein, e.g., [[#Delworth--2014|Delworth and Zeng, 2014]] ). Rainfall trends in the south-east are not significant since 1960 but have shown a notable reduction since the 1990s, and there is ''limited evidence'' for the attribution of this change to human influence (e.g., [[#Rauniyar--2020|Rauniyar and Power, 2020]] ). In New Zealand between 1960 and 2019 in both summer and winter, rainfall increased in some stations in the South Island and decreased at many stations in the North Island, however most station trends are not statistically significant (Figure Atlas.20; [[#MfE%20and%20Stats%20NZ--2020|MfE and Stats NZ, 2020]] ). In JJA, Milford Sound (increasing) and Whangaparaoa (decreasing) trends are significant.

In Australia, there has been a decrease in snow depth and area since the late 1950s, especially in spring ( [[#BOM%20and%20CSIRO--2018|BOM and CSIRO, 2018]] ). Based on a reconstructed snow cover record, the recent rapid decrease in the past five decades has been shown to be larger by more than an order of magnitude than the maximum loss for any five-decade period over the past 2000 years ( [[#McGowan--2018|McGowan et al., 2018]] ). In New Zealand, from 1977 to 2018, glacier ice volume decreased from 26.6 km <sup>3</sup> to 17.9 km <sup>3</sup> (a loss of 33%; [[#Salinger--2019|Salinger et al., 2019]] ).

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=== Atlas.6.3 Assessment of Climate Model Performance ===

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Most studies assessed in AR5 WGII were based on Coupled Model Intercomparison Project Phase 3 (CMIP3) models and Special Report on Emissions Scenarios (SRES) scenarios and CMIP5 models whenever available. The AR5 WGI reported that model biases in annual temperature and rainfall are similar to or lower than other continental regions outside the tropics, with temperature biases generally &lt;1°C in the multi-model mean and &lt;2°C in most models over Australia compared to reanalysis, and with a wet bias over the Australian inland region but a dry bias near coasts and mountain regions of both Australia and New Zealand.

Early results from CMIP6 suggest incremental improvements compared to CMIP5 in the simulation of the mean annual climatology of temperature and precipitation of the Indo-Pacific region surrounding Australasia, the teleconnection between ENSO and IOD and Australian rainfall and other relevant climate features ( [[#Grose--2020|Grose et al., 2020]] ). These assessments suggest that confidence in projections is similar to AR5 or incrementally improved. The CORDEX Australasia simulations are found to have cold biases in daily maximum temperature and an overestimation of precipitation but overall showed added value in the simulation of the current climate ( [[#Di%20Virgilio--2019|Di Virgilio et al., 2019]] ; [[#Evans--2021|Evans et al., 2021]] ).

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=== Atlas.6.4 Assessment and Synthesis of Projections ===

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Similar to the global average (Chapter 4), mean temperature in Australasia is projected to continue to rise through the 21st century at a magnitude proportional to the cumulative greenhouse gas emissions ( ''virtually certain, very high confidence, robust evidence'' ), CMIP5 and CMIP6 results are shown in Figure Atlas.21. A higher end to the range of temperature projections is found in CMIP6 compared to CMIP5 ( [[#Grose--2020|Grose et al., 2020]] ), produced by a group of models with high climate sensitivity ( [[#Forster--2020|Forster et al., 2020]] ), and this creates a higher multi-model-mean change. For example, projections for Australasia including ocean between 1995–2014 and 2081–2100 are 1.4°C (1.1°C–1.8°C, 10th–90th percentile range) in CMIP5 under RCP4.5, but 1.8°C (1.3°C–2.5°C) in CMIP6 under SSP2-4.5.

Using warming levels, the results can be directly compared, accounting for the different distribution of climate sensitivities in the two ensembles. In this framework, Australasia (land only) is projected to warm by a similar amount to the global average: 1.4°C–1.8°C for the 1.5°C warming level, through to 3.9°C–4.8°C for the 4°C warming level from the 1850–1900 baseline in CMIP6 using SSP5-8.5 (results using other SSPs and from CMIP5 are similar). Projected warming is greater over land than ocean, greater in Australia than in New Zealand, and greater over inland Australia than in coastal regions. Due to historical warming, projected temperature change from the AR6 baseline of 1995–2014 is lower: 0.3°C–1.0°C for the 1.5°C warming level, through to 2.9°C–4.0°C for the 4°C warming level. Changes for other warming levels, sub-regions and emissions pathways are shown in Figure Atlas.21 and can be explored in the Interactive Atlas. Regional modelling suggests projected temperature increase is higher in mountainous areas than surrounding low-elevation areas in New Zealand and Australia ( [[#Olson--2016|Olson et al., 2016]] ; [[#MfE--2018|MfE, 2018]] ).

In line with recent trends, a significant reduction in annual mean rainfall in south-west Australia is projected, with the greatest reduction in winter and spring ( ''very likely'' , ''high confidence'' ). There is more than 80% model agreement for projected mean annual rainfall decrease in the south-west of the state of Western Australia for both the mid- (2041–2060) and far (2081–2100) future, and for all warming levels (Interactive Atlas). Rainfall decreases, mainly in winter and spring, are also projected for other regions within southern Australia with only ''medium confidence'' ( ''medium evidence'' , ''medium agreement'' ). Almost all models project continued drying in SAU in winter (JJA) and also in spring (SON), but a few models show little change. CMIP5 and CMIP6 results are similar or with a slightly narrower range in the latter (Figure Atlas.21). CORDEX produces a similar range of change in winter rainfall change for SAU as a whole. Circulation change is the dominant driver of these projected reductions, explaining the range of model results for southern Australia ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ; [[#Mindlin--2020|Mindlin et al., 2020]] ). Studies of winter rainfall change and circulation in southern Australia suggest the wettest changes in winter rainfall change may possibly be rejected ( [[#Grose--2017|Grose et al., 2017]] , [[#Grose--2019a|Grose et al., 2019a]] ).

The model mean projection of northern Australian wet-season precipitation (a period including DJF) is for little change under all SSPs and warming levels, with ''low confidence'' in the direction of change as the projections include both large and significant decrease and increases (Figure Atlas.21 and Interactive Atlas). Evidence from warming patterns suggests a constraint on the dry end of projections ( [[#Brown--2016|Brown et al., 2016]] ), and the CMIP6 ensemble suggests that the projection follows the zonally averaged rainfall response in the Southern Hemisphere rather than changes in the western Pacific ( [[#Narsey--2020|Narsey et al., 2020]] ). There is also evidence for a projected increase in rainfall variability in northern Australia in scales from days to decades ( [[#Brown--2017|Brown et al., 2017]] ). [[#Liu--2018|Liu et al. (2018)]] find that under 1.5°C warming, central and north-east Australia are projected to become wetter, however this projection has ''low confidence'' . There are similar projections from CMIP5 and CMIP6 (Figure Atlas.21).

Projections for EAU vary by season, with moderate model agreement on a decrease in rainfall in winter and spring, but with lower agreement in CMIP6 compared to CMIP5, and low model agreement on the direction of change in summer (Figure Atlas.21). CAU shows a similar range of change as EAU, with low model agreement on the direction of change in DJF, moderate agreement on direction of change in JJA, but significant changes are projected by some models. Other seasonal and regional rainfall changes in Australia are reviewed in [[#Dey--2019|Dey et al. (2019)]] .

For the NZ reference region, precipitation is projected to increase in winter and annual rainfall, with some differences in magnitude between CMIP5, CMIP6 and CORDEX (Figure Atlas.21). This projection of rainfall increase is a function of changes in the southern extent of the region, and notable regional differences are expected. Regional modelling suggests precipitation increases in the west and south of New Zealand and decreases in the north and east ( [[#MfE--2018|MfE, 2018]] ), with ''medium confidence'' and notable differences by season. [[#Liu--2018|Liu et al. (2018)]] project that the North Island will be drier, while the South Island will be wetter under both 1.5°C and 2°C warming levels. The projected increase in precipitation in the far future (2081–2100) for the southern regions of NZ has ''high agreement'' (Interactive Atlas). Other seasonal and regional rainfall changes in Australia can be explored in the Interactive Atlas.

The CORDEX Australasia simulations produce some regional detail in projected precipitation change associated with important features such as orography. Areas where there is coincident ‘added value’ in the simulation of the current climate and ‘potential added value’ as new information in the projected climate change signal (collectively termed ‘realized added value’) in Australia include the Australian Alps, Tasmania and parts of northern Australia ( [[#Di%20Virgilio--2020|Di Virgilio et al., 2020]] ). There have been several studies of regional climate change for New Zealand and states within Australia at fine resolution (5–12 km) that have produced important insights. One is enhanced drying in cool seasons on the windward slopes of the southern Australian Alps (decreases of 20–30% compared to 10–15% in the driving models), and conversely a chance of enhanced rainfall increase on the peaks of mountains in summer ( [[#Grose--2019b|Grose et al., 2019b]] ), with the summer finding in line with those for the European Alps ( [[#Giorgi--2016|Giorgi et al., 2016]] ).

Under future warming, the snowpack in Australia is projected to decrease by approximately 15% and 60% by 2030 and 2070 respectively under the SRES A2 scenario ( [[#Di%20Luca--2018|Di Luca et al., 2018]] ), while in New Zealand the number of annual snow days is projected to decrease by 30 days or more by 2090 under RCP8.5 ( [[#MfE--2018|MfE, 2018]] ). New Zealand is also projected to lose up to 88 ± 5% of its glacier volume by the end of the 21st century ( [[#Chinn--2012|Chinn et al., 2012]] ; [[#Hock--2019a|Hock et al., 2019a]] ).

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=== Atlas.6.5 Summary ===

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There is ''very high confidence'' that the climate of Australia warmed by around 1.4°C and New Zealand by around 1.1°C since reliable records began in 1910 and 1909 respectively, with human influence the dominant driver. Warming is ''virtually certain'' to continue, with a magnitude roughly equal to the global average temperature. A significant decrease in April to October rainfall in the south-west of the state of Western Australia observed from 1910 to 2019 is attributable to human influence with ''high confidence'' and is ''very likely'' to continue in future, noting consistent projections in CMIP5 and CMIP6. Other observed and projected rainfall trends are less significant or less certain. Model representation of the climatology of Australasian temperature and rainfall has improved since AR5, through an incremental improvement between CMIP5 and CMIP6, and the development of coordinated regional modelling through CORDEX-Australasia. Snow cover is ''likely'' to decrease throughout the region at high altitudes in both Australia and New Zealand ( ''high confidence'' ).

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