Editing IPCC:AR6/WGII/Chapter-11 (section)

=== 11.2.2 Projected Climate Change ===

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There are three main sources of uncertainty in climate projections: emission scenarios, regional climate responses and internal climate variability ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ). Emission scenario uncertainty is captured in Representative Concentration Pathways (RCPs) for greenhouse gases and aerosols. RCP2.6 represents low emissions, RCP4.5 medium emissions and RCP8.5 high emissions. Regional climate response uncertainty and internal climate variability uncertainty are captured in climate model simulations driven by the RCPs.

Further climate change is inevitable, with the rate and magnitude largely dependent on the emission pathway ( ''very high confidence'' ) ( [[#IPCC--2021|IPCC, 2021]] ). Preliminary projections based on Climate Model Intercomparison Project Phase 6 (CMIP6) models are described in the IPCC Working Group I Atlas. For Australia, the CMIP6 projections broadly agree with CMIP5 projections except for a group of CMIP6 models with greater warming and a narrower range of summer rainfall change in the north and winter rainfall change in the south ( [[#Grose--2020|Grose et al., 2020]] ). For New Zealand, the CMIP6 projections are similar to CMIP5, but the CMIP6 models indicate greater warming, a smaller increase in summer precipitation and a larger increase in winter precipitation ( [[#Gutiérrez--2021|Gutiérrez et al., 2021]] ).

Dynamical and statistical downscaling offer the prospect of improved representation of regional climate features and extreme weather events (IPCC 2021: Working Group I [[IPCC:Wg2:Chapter:Chapter-10|Chapter 10]] (Doblas-Reyes et al., 2021)), but the added value of downscaling is complex to evaluate ( [[#Ekström--2015|Ekström et al., 2015]] ; [[#Rummukainen--2015|Rummukainen, 2015]] ; [[#Virgilio--2021|Virgilio et al., 2021]] ). Downscaled simulations are available for New Zealand ( [[#MfE--2018|MfE, 2018]] ) and various Australian regions ( [[#Gutiérrez--2021|Gutiérrez et al., 2021]] ). Further downscaling was recommended by the Royal Commission into National Natural Disaster Arrangements ( [[#CoA--2020e|CoA, 2020e]] ). Projections for rainfall, thunderstorms, hail, lightning and tornadoes have large uncertainties ( [[#Walsh--2016|Walsh et al., 2016]] ; [[#MfE--2018|MfE, 2018]] ).

Future changes in climate variability are affected by the El Niño Southern Oscillation (ENSO), Southern Annular Mode (SAM), Indian Ocean Dipole (IOD) and Interdecadal Pacific Oscillation (IPO). An increase in strong El Niño and La Niña events is projected ( [[#Cai--2015|Cai, 2015]] ), along with more extreme positive phases of the IOD ( [[#Cai--2018|Cai et al., 2018]] ) and a positive trend in SAM ( [[#Lim--2016|Lim et al., 2016]] ), but potential changes in the IPO are unknown ( [[#NESP%20ESCC--2020|NESP ESCC, 2020]] ). There is uncertainty about regional climate responses to projected changes in ENSO ( [[#King--2015|King et al., 2015]] ; [[#Perry--2020|Perry et al., 2020]] ; [[#Virgilio--2021|Virgilio et al., 2021]] ).

Australian climate projections are quantified with references in Table 11.3a. Further warming is projected, with more hot days, fewer cold days, reduced snow cover, ongoing sea level rise (SLR) and ocean acidification ( ''very high confidence'' ). Winter and spring rainfall and soil moisture are projected to decrease, with higher evaporation rates, decreased wind over southern mainland Australia, increased wind over Tasmania, and more extreme fire weather in southern and eastern Australia ( ''high confidence'' ). Heavy rainfall intensity is projected to increase, with more droughts over southern and eastern Australia ( ''medium confidence'' ). Increased winter rainfall is projected over Tasmania, with decreased rainfall in southwestern Victoria in autumn and in western Tasmania in summer, fewer tropical cyclones with a greater proportion of severe cyclones and decreased soil moisture in the north ( ''medium confidence'' ). Hailstorm frequency may increase ( ''low confidence'' ).

'''Table 11.3a |''' Projected climate change for Australia. Projections are given for different RCPs (RCP2.6 is low, RCP4.5 is medium, RCP8.5 is high) and years (e.g., 20-year period centred on 2090). Uncertainty ranges are generally 10th–90th percentile, and median projections are given in square brackets where possible. The four Australian regions are shown in [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] of ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ). Preliminary projections based on CMIP6 models are included for some climate variables from the [[#IPCC--2021|IPCC (2021)]] WGI report.

{| class="wikitable"
|-
! Climate variable

! Projected change (year, RCP) relative to 1986–2005

! References

|-
| Air temperature

| Annual mean temperature

* +0.5–1.5°C (2050, RCP2.6), +1.5–2.5°C (2050, RCP8.5), +0.5–1.5°C (2090, RCP2.6), +2.5–5.0°C (2090, RCP8.5)
* Weaker increase in the south, stronger increase in the centre
* Preliminary CMIP6 projections: +0.6°C–1.3°C (2050, SSP1-RCP2.6), +1.2°C–2.0°C (2050, SSP5-RCP8.5), +0.6°C–1.5°C (2090, SSP1-RCP2.6), +2.8°C–4.9°C (2090, SSP5-RCP8.5) relative to 1995–2014

| ( [[#NESP%20ESCC--2020|NESP ESCC, 2020]] ; [[#IPCC--2021|IPCC, 2021]] )

|-
| Sea surface temperature

| * + 0.4–1.0°C (2030, RCP8.5)
* +2–4°C (2090, RCP8.5)

| ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] )

|-
| Air temperature extremes

| * Annual frequency of days over 35°C may increase 20–70% by 2030 (RCP4.5) and 25–85% (RCP2.6) to 80–350% (RCP8.5) by 2090
* Heatwave frequency may rise by 85% if global warming increases from 1.5°C to 2.0°C, and it may rise by four times for xxxx 3°C warming
* Annual frequency of frost days may decrease by 10–40% (2030, RCP4.5), 10–40% (2090, RCP2.6) and 50–100% (2090, RCP8.5)

| ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ; [[#Trancoso--2020|Trancoso et al., 2020]] )

|-
| Rainfall

| Annual mean rainfall

* South: −15 to +2% (2050, RCP2.6), −14 to +3% (2050, RCP8.5), −15 to +3% (2090, RCP2.6), −26 to +4% (2090, RCP8.5)
* East: −13 to +7% (2050, RCP2.6), −17 to +8% (2050, RCP8.5), −19 to +6% (2090, RCP2.6), −25 to +12% (2090, RCP8.5)
* North: −12 to +5% (2050, RCP2.6), −8 to +11% (2050, RCP8.5), −12 to +3% (2090, RCP2.6), −26 to +23% (2090, RCP8.5)
* Rangelands: −18 to +3% (2050, RCP2.6), −15 to +8% (2050, RCP8.5), −21 to +3% (2090, RCP2.6), −32 to +18% (2090, RCP8.5)

| ( [[#Liu--2018|Liu et al., 2018]] ; [[#NESP%20ESCC--2020|NESP ESCC, 2020]] )

|-
| Rainfall extremes

| Intensity of daily total rain with 20-year recurrence interval

* +4 to +10% (2050, RCP2.6)
* +8 to +20% (2050, RCP8.5)
* +4 to +10% (2090, RCP2.6)
* +15 to +35% (2090, RCP8.5)

| ( [[#NESP%20ESCC--2020|NESP ESCC, 2020]] )

|-
| Drought

| Time in drought (Standardised Precipitation Index below −1)

* Southern Australia: 32–46% [39%] (1995), 38–68% [54%] (2050, RCP8.5), 41–81% [60%] (2090, RCP8.5)
* Eastern Australia: 25–46% [37%] (1995), 24–67% [47%] (2050, RCP8.5), 19–76% [56%] (2090, RCP8.5)
* Northern Australia: 26–44% [34%] (1995), 18–54% [40%] (2050, RCP8.5), 9–81% [39%] (2090, RCP8.5)
* Australian Rangelands: 29–43% [34%] (1995), 26–58% [42%] (2050, RCP8.5), 23–70% [46%] (2090, RCP8.5)

| ( [[#Kirono--2020|Kirono et al., 2020]] )

|-
| Wind speed

| 0–5% decrease over southern mainland Australia and 0–5% increase over Tasmania (2090, RCP8.5)

| ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] )

|-
| Sea level rise

| * South (Port Adelaide): 13–29 cm [21 cm] (2050, RCP2.6), 16–33 cm [25 cm] (2050, RCP8.5), 23–55 cm [39 cm] (2090, RCP2.6), 40–84 cm [61 cm] (2090, RCP8.5)
* East (Newcastle): 14–30 cm [22 cm] (2050, RCP2.6), 19–36 cm [27 cm] (2050, RCP8.5), 22–54 cm [38 cm] (2090, RCP2.6), 46–88 cm [66 cm] (2090, RCP8.5)
* North ( [[#Darwin%20City%20Council--2011|Darwin City Council, 2011]] ): 13–28 cm [21 cm] (2050, RCP2.6), 17–33 cm [25 cm] (2050, RCP8.5), 22–55 cm [38 cm] (2090, RCP2.6), 41–85 cm [62 cm] (2090, RCP8.5)
* West (Port Hedland): 13–28 cm [20 cm] (2050, RCP2.6), 16–33 cm [24 cm] (2050, RCP8.5), 22–55 cm [38 cm] (2090, RCP2.6), 40–84 cm [61 cm] (2090, RCP8.5)

These projections have not been updated to include an Antarctic dynamic ice sheet factor which increased global sea level projections for RCP8.5 by approx. 10 cm. Preliminary CMIP6 projections indicate +40–50 cm (2090, SSP1-RCP2.6) and +70–90 cm (2090, SSP5-RCP8.5)

| ( [[#McInnes--2015|McInnes et al., 2015]] ; [[#Zhang--2017|Zhang et al., 2017]] ; [[#IPCC--2019b|IPCC, 2019b]] )

( [[#IPCC--2021|IPCC, 2021]] )

|-
| Sea level extremes

| Increase in the allowance for a storm tide event with 1% annual exceedance probability (100-year return period)

* South (Port Adelaide): 21 cm (2050, RCP2.6), 25 cm (2050, RCP8.5), 41 cm (2090, RCP2.6), 66 cm (2090, RCP8.5)
* East (Newcastle): 24 cm (2050, RCP2.6), 30 cm (2050, RCP8.5), 49 cm (2090, RCP2.6), 86 cm (2090, RCP8.5)
* North (Darwin): 21 cm (2050, RCP2.6), 26 cm (2050, RCP8.5), 43 cm (2090, RCP2.6), 71 cm (2090, RCP8.5)
* West (Port Hedland): 21 cm (2050, RCP2.6), 26 cm (2050, RCP8.5), 43 cm (2090, RCP2.6), 70 cm (2090, RCP8.5)

| ( [[#McInnes--2015|McInnes et al., 2015]] )

|-
| Fire

| * East: annual number of severe fire weather days 0 to +30% (2050, RCP2.6), 0 to +60% (2050, RCP8.5), 0 to +30% (2090, RCP2.6), 0 to +110% (2090, RCP8.5)
* Elsewhere: number of severe fire weather days +5 to +35% (2050, RCP2.6), +10 to +70% (2050, RCP8.5), +5 to +35% (2090, RCP2.6) +20 to +130% (2090, RCP8.5)

| ( [[#Clarke--2019|Clarke and Evans, 2019]] ; [[#Dowdy--2019|Dowdy et al., 2019]] ; [[#Virgilio--2019|Virgilio et al., 2019]] ; [[#Clarke--2020|Clarke et al., 2020]] ; [[#NESP%20ESCC--2020|NESP ESCC, 2020]] ; [[#Clark--2021|Clark et al., 2021]] )

|-
| Tropical cyclones and other storms

| * Eastern region tropical cyclones: −8 to +1% (2050, RCP2.6), −15 to +2% (2050, RCP8.5), −8 to +1% (2090, RCP2.6), −25 to +5% (2090, RCP8.5)
* Western region tropical cyclones: −10 to −2% (2050, RCP2.6), −20 to −4% (2050, RCP8.5), −10 to −2% (2090, RCP2.6), −30 to −10% (2090, RCP8.5)
* East coast lows: −15 to −5% (2050, RCP2.6), −30 to −10% (2050, RCP8.5), −15 to −5% (2090, RCP2.6), −50 to −20% (2090, RCP8.5)
* Hailstorm frequency may increase, but there are large uncertainties

| ( [[#NESP%20ESCC--2020|NESP ESCC, 2020]] ; [[#Raupach--2021|Raupach et al., 2021]] )

|-
| Snow and ice

| * Maximum snow depth at Falls Creek and Mt Hotham may decline 30–70% (2050, B1) and 45–90% (2050, A1FI) relative to 1990
* Maximum snow depth at Mt Buller and Mt Buffalo may decline 40–80% (2050, B1) and 50–100% (2050, A1FI) relative to 1990
* Length of Victorian ski season may contract 65–90% and mean annual snowfall may decline 60–85% (2070–2099, RCP8.5) relative to 2000–2010.
* The snowpack may decrease by about 15% (2030, A2) to 60% (2070, A2)

| ( [[#Bhend--2012|Bhend et al., 2012]] ; [[#Harris--2016|Harris et al., 2016]] ; [[#Di%20Luca--2018|Di Luca et al., 2018]] )

|-
| Ocean acidification

| pH is projected to drop by about 0.1 (2090, RCP2.6) to 0.3 (2090, RCP8.5)

| ( [[#CSIRO%20and%20BOM--2015|CSIRO and BOM, 2015]] ; [[#Hurd--2018|Hurd et al., 2018]] )

|}

New Zealand climate projections are quantified with references in Table 11.3b. Further warming is projected, with more hot days, fewer cold days, less snow and glacial ice, ongoing sea level rise (SLR) and ocean acidification ( ''very high confidence'' ). Increases in winter and spring rainfall are projected in the west of the North and South Islands, with drier conditions in the east and north, caused by stronger westerly winds ( ''medium confidence'' ). In summer, wetter conditions are projected in the east of both islands, with drier conditions in the west and central North Island ( ''medium confidence'' ). Fire weather indices are projected to increase over northern and eastern New Zealand ( ''medium confidence'' ). Heavy rainfall intensity is projected to increase over most regions, with increased extreme wind speeds in eastern regions, especially in Marlborough and Canterbury, and reduced relative humidity almost everywhere, except for the west coast in winter ( ''medium confidence'' ). Drought frequency may increase in the north ( ''medium confidence'' ).

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