Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.7.1.2 Change in Global Climate Indices Beyond 2100 ====

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This subsection assesses changes in global climate indices out to 2300 using extensions of the SSP scenarios ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ) and literature based on extensions to the RCP scenarios from CMIP5 ( [[#Meinshausen--2011|Meinshausen et al., 2011]] ), which differ from the SSPs despite similar labelling of global radiative forcing levels ( [[#4.6.2|Section 4.6.2]] ). [[#Meinshausen--2020|Meinshausen et al. (2020)]] describe the extensions to the SSP scenarios, which differ slightly from the ScenarioMIP documentation ( [[#O’Neill--2016|O’Neill et al., 2016]] ). A simplified approach across scenarios reduces emissions such that after 2100, land-use CO <sub>2</sub> emissions are reduced to zero by 2150; any net negative fossil CO <sub>2</sub> emissions are reduced to zero by 2200, and positive fossil CO <sub>2</sub> emissions are reduced to zero by 2250. Non-CO <sub>2</sub> fossil fuel emissions are also reduced to zero by 2250 while land-use-related non-CO <sub>2</sub> emissions are held constant at 2100 levels. The extensions are created up to the year 2500, but ESM simulations have only been requested, as part of the CMIP6 protocol, to run to 2300. As a result, unlike the RCP8.5 extension, SSP5-8.5 sees a decline in CO <sub>2</sub> concentration after 2250, but the radiative forcing level is similar, reaching approximately 12 W m <sup>–2</sup> during most of the extension. Both SSP1-2.6 and SSP5-3.4-OS decrease radiative forcing after 2100. SSP5-3.4-OS is designed to return to the same level of forcing as SSP1-2.6 during the first half of the 22nd century. Because relatively few CMIP6 ESMs have submitted results beyond 2100, GSAT projections using the MAGICC7 emulator (see [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) are also shown here.

Changes in climate at 2300 have impacts and commitments beyond this timeframe ( ''high confidence'' ). Sea level rise may exceed 2 m on millennial time scales even when warming is limited to 1.5°C–2°C, and tens of metres for higher warming levels (Table 9.10). [[#Randerson--2015|Randerson et al. (2015)]] showed increasing importance on carbon cycle feedbacks of slow ocean processes, [[#Mahowald--2017|Mahowald et al. (2017)]] showed the long-lasting legacy of land-use effects and J.K. [[#Moore--2018|]] [[#Moore--2018|Moore et al. (2018)]] show how changes in Southern Ocean winds affect nutrients and marine productivity well beyond 2300. [[#Clark--2016|Clark et al. (2016)]] show that physical and biogeochemical impacts of 21st century emissions have a potential committed legacy of at least 10,000 years.

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===== 4.7.1.2.1 Global surface air temperature =====

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Both CMIP6 and CMIP5 results show that global temperature beyond 2100 is strongly dependent on scenario, and the difference in GSAT projections between high- and low-emissions scenarios continues to increase ( ''high confidence'' ). Under the extended RCP2.6 ( [[#Caesar--2013|Caesar et al., 2013]] ) and SSP1-2.6 scenarios, where CO <sub>2</sub> concentration and radiative forcing continue to decline beyond 2100, GSAT stabilizes during the 21st century before decreasing and remaining below 2°C until 2300, except in some of the very high climate-sensitivity ESMs, which project GSAT to stay above 2°C by 2300 (Figure 4.40). Under RCP8.5, regional temperature changes above 20°C have been reported in multiple models over high-latitude land areas ( [[#Caesar--2013|Caesar et al., 2013]] ; [[#Randerson--2015|Randerson et al., 2015]] ). Non-CO <sub>2</sub> forcing and feedbacks remain important by 2300 ( ''high confidence'' ). [[#Randerson--2015|Randerson et al. (2015)]] found that 1.6°C of warming by 2300 came from non-CO <sub>2</sub> forcing alone in RCP8.5, and [[#Rind--2018|Rind et al. (2018)]] show that regional forcing from aerosols can have notable effects on ocean circulation on centennial time scales. High latitude warming led to longer growing seasons and increased vegetation growth in the CESM1 model ( [[#Liptak--2017|Liptak et al., 2017]] ), and [[#Burke--2017|Burke et al. (2017)]] found that carbon release from permafrost areas susceptible to this warming may amplify future climate change by up to 17% by 2300.

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[[File:1f36a4626186c03d88c5908762289143 IPCC_AR6_WGI_Figure_4_40.png]]

'''Figure 4.40''' '''|''' '''Simulated climate changes up to 2300 under the extended SSP scenarios.''' Displayed are '''(a)''' projected global surface air temperature (GSAT) change, relative to 1850–1900, from CMIP6 models (individual lines) and MAGICC7 (shaded plumes); '''(b)''' as (a) but zoomed in to show low-emissions scenarios; '''(c)''' global land precipitation change; and '''(d)''' September Arctic sea ice area. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Too few CMIP6 models performed the extension simulations to allow a robust assessment of GSAT projection, and some of those which did had higher than average climate sensitivity values. Therefore, we base our assessment of GSAT projections (Table 4.9) on the MAGICC7 emulator calibrated against assessed GSAT to 2100 ( [[#4.3.4|Section 4.3.4]] , [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ). Because the emulator approach has not been evaluated in depth up to 2300 in the same way as it has up to 2100 ( [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) we account for possible additional uncertainty by assessing the 5–95% range from MAGICC as ''likely'' instead of ''very likely'' . It is therefore ''likely'' that GSAT will exceed 2°C above that of the period 1850–1900 at the year 2300 in the extended SSP scenarios SSP2-4.5, SSP3-7.0 and SSP5-8.5 (Figure 4.40). For SSP1-2.6 and SSP1-1.9, mean warming at 2300 is 1.5°C and 0.9°C respectively. GSAT differences between SSP5-3.4-overshoot and SSP1-2.6 peak during the 21st century but decline to less than about 0.25°C after 2150 ( ''medium confidence'' ).

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'''Table 4.9''' '''|''' '''Change of global surface air temperature at 2300.''' Displayed are the median and 5–95% range of GSAT change at 2300 relative to 1850–1900 for the six scenarios used with MAGICC7.

{| class="wikitable"
|-
| Scenario

| Median (°C)

| 5–95% Range (°C)

|-
| SSP5-8.5

| 9.6

| 6.6–14.1

|-
| SSP3-7.0

| 8.2

| 5.7–11.8

|-
| SSP2-4.5

| 3.3

| 2.3–4.6

|-
| SSP5-3.4-OS

| 1.6

| 1.1–2.2

|-
| SSP1-2.6

| 1.5

| 1.0–2.2

|-
| SSP1-1.9

| 0.9

| 0.6–1.4

|}

To place the temperature projections for the end of the 23rd century into the context of paleo temperatures, GSAT under SSP2-4.5 ( ''likely'' 2.3°C–4.6°C higher than over the period 1850–1900) has not been experienced since the Mid Pliocene, about three million years ago. GSAT projected for the end of the 23rd century under SSP5-8.5 ( ''likely'' 6.6°C–14.1°C higher than over the period 1850–1900) overlaps with the range estimated for the Miocene Climatic Optimum (5°C–10°C higher) and Early Eocene Climatic Optimum (10°C–18°C higher), about 15 and 50 million years ago, respectively ( ''medium confidence'' ) (Chapter 2).

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===== 4.7.1.2.2 Global land precipitation =====

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Global land precipitation will continue to increase in line with GSAT under high emissions scenarios ( ''medium confidence'' ). Precipitation changes over land show larger variability and a less clear signal than global total precipitation. [[#Caesar--2013|Caesar et al. (2013)]] showed that under the CMIP5 extension simulations, HadGEM2-ES projected global land precipitation to remain roughly the same in RCP2.6, to increase by about 4% in RCP4.5 and to increase by about 7% in RCP8.5. Their results showed global precipitation increasing linearly with temperature while radiative forcing increases, but then more quickly if forcing is stabilized or reduced. This backs up findings of an intensification of the hydrological cycle following CO <sub>2</sub> decrease which has been attributed to a build-up of ocean heat ( [[#Wu--2010|Wu et al., 2010]] ), and to a fast atmospheric adjustment to CO <sub>2</sub> radiative forcing ( [[#Cao--2011|Cao et al., 2011]] ). Figure 4.40 shows that global land precipitation increases in CMIP6 models until 2300 for SSP5-8.5 but stabilizes in SSP1-2.6 and SSP5-3.4-OS. SSP1-2.6 and SSP5-3.4-OS are not distinguishable in behaviour of projected global land precipitation after 2100.

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===== 4.7.1.2.3 Arctic sea ice =====

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[[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] assesses with ''high confidence'' that on decadal and longer time scales, Arctic summer sea ice area will remain highly correlated with global mean temperature until the summer sea ice has vanished (Section 9.3.1.1). This means that Arctic sea ice will continue to decline in scenarios of continued warming but will begin to recover in scenarios where GSAT begins to decrease. Under the CMIP5 extension simulations, minimum (September) Arctic sea ice area began to recover for most models under RCP2.6 out to 2300, while RCP4.5 and RCP8.5 extensions became ice-free in September ( [[#Hezel--2014|Hezel et al., 2014]] ; [[#Bathiany--2016|Bathiany et al., 2016]] ). They also found increasingly strong winter responses under continued warming such that under the RCP8.5 extension, the Arctic became ice-free nearly year-round by 2300. Consistent with the assessment in Section 9.3.1.1 that Arctic sea ice area is correlated with GSAT, CMIP6 projections to 2300 show partial sea ice recovery by 2300 in SSP1-2.6 in line with GSAT (Figure 4.40), with one model (MRI-ESM2-0) showing near complete recovery to present-day values. SSP1-2.6 and SSP5-3.4-OS are not distinguishable in behaviour of Arctic sea ice in these models after 2100. SSP5-8.5 remains ice-free in September up to 2300.

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