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

===== 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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