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

=== 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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