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

=== 4.3.4 Synthesis Assessment of Projected Change in Global Surface Air Temperature ===

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GSAT change is assessed using multiple lines of evidence including the CMIP6 projection simulations out to year 2100. The assessment combines CMIP6 projections driven by SSP scenarios with observational constraints on simulated past warming ( [[#box-4.1|Box 4.1]] and Figure 4.11a,b; [[#Brunner--2020|Brunner et al., 2020]] ; [[#Liang--2020|Liang et al., 2020]] ; [[#Nijsse--2020|Nijsse et al., 2020]] ; [[#Tokarska--2020|Tokarska et al., 2020]] ; [[#Ribes--2021|Ribes et al., 2021]] ), as well as the AR6-updated assessment of ECS and TCR in Section 7.5. The approaches of ( [[#Liang--2020|Liang et al., 2020]] ; [[#Tokarska--2020|Tokarska et al., 2020]] ; [[#Ribes--2021|Ribes et al., 2021]] ) have first been extended to all 20-year averaging periods between 2000 and 2100. For each 20-year period, the 5%, 50%, and 95% percentile GSAT values of these three constrained CMIP6 results are averaged percentile by percentile (Figure 4.11c). Then, an emulator based on a two-layer energy balance model (e.g., [[#Held--2010|Held et al., 2010]] ) is driven by the Chapter 7-derived ERF. The emulator parameters are chosen such that the central estimate, lower bound of the ''very likely'' range, and upper bound of the ''very likely'' range of climate feedback parameter and ocean heat uptake coefficient take the values that map onto the corresponding combination of ECS (3°C, 2°C and 5°C, respectively) and TCR (1.8°C, 1.2°C and 2.4°C, respectively) of Section 7.5 (see [[#box-4.1|Box 4.1]] ). As a final step, the constrained-CMIP6 and the emulator-based 5%, 50%, and 95% percentile GSAT values are averaged percentile by percentile (Figure 4.11c,d and Table 4.5). Constrained CMIP6 results and the ECS- and TCR-based emulator thus contribute one-half each to the GSAT assessment. Because the emulator results and ( [[#Ribes--2021|Ribes et al., 2021]] ) represent the forced response only, and averaging over the other two individual estimates ( [[#Liang--2020|Liang et al., 2020]] ; [[#Tokarska--2020|Tokarska et al., 2020]] ) further reduces the contribution from internal variability, the assessed GSAT time series are assumed to represent purely the forced response.

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[[File:5abe3e9913b6dadb1d9a6a4dcb0b2702 IPCC_AR6_WGI_Figure_4_11.png]]

'''Figure 4.1''' '''1 |''' '''Multiple lines of evidence for global surface air temperature (GSAT) changes for the long-term period, 2081–2100, relative to the average over 1995–2014, for all five priority scenarios.''' The unconstrained CMIP6 5–95% ranges (coloured bars) in '''(a)''' differ slightly because different authors used different subsamples of the CMIP6 archive. The constrained CMIP6 5–95% ranges (coloured bars) in '''(b)''' are smaller than the unconstrained ranges in (a) and differ because of different samples from the CMIP6 archive and because different observations and methods are used. In '''(c)''' , the average of the ranges in (b) is formed (grey bars). Green bars in (c) show the emulator ranges, defined such that the best estimate, lower bound of the ''very likely'' range, and upper bound of the ''very likely'' range of climate feedback parameter and ocean heat uptake coefficient take the values that map onto the corresponding values of ECS and TCR of Section 7.5 (see [[#box-4.1|Box 4.1]] ). The time series in '''(d)''' are constructed by taking the average of the constrained CMIP6 ranges and the emulator ranges. The y-axes on the right-hand side are shifted upward by 0.85°C, the central estimate of the observed warming for 1995–2014, relative to 1850–1900 (Cross-Chapter Box 2.3, Table 1). Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Averaged over the period 2081–2100, GSAT is ''very'' ''likely'' to be higher than in the recent past (1995–2014) by 0.3°C–0.9°C in the low-emissions scenario SSP1-1.9 and by 2.6°C–4.7°C in the high-emission scenario SSP5-8.5. For the scenarios SSP1-2.6, SSP2-4.5, and SSP3-7.0, the corresponding ''very'' ''likely'' ranges are 0.6°C–1.4°C, 1.3°C–2.5°C, and 2.0°C–3.8°C, respectively (Figure 4.11 and Table 4.5). Because the different approaches for estimating long-term GSAT change produce consistent results (Figure 4.11), there is ''high confidence'' in this assessment. These ranges of the long-term projected GSAT change generally correspond to AR5 ranges for related scenarios but the likelihood is increased to ''very likely'' ranges, in contrast to the ''likely'' ranges in AR5. Over the mid-term period 2041–2060, the ''very likely'' GSAT ranges of SSP1-1.9 and SSP5-8.5 are almost completely distinct ( ''high confidence'' ) (Table 4.5; see also [[#4.3.1|Section 4.3.1]] ).

CMIP6 models project a wider range of GSAT change than the assessed range ( ''high confidence'' ) ( [[#4.3.1|Section 4.3.1]] ). The CMIP6 models with a higher climate sensitivity simulate warming rates higher than assessed ''very likely'' here ( [[#4.3.1|Section 4.3.1]] ); these rates are ''very'' ''unlikely'' but not impossible to occur and hence cannot be excluded. The implications of these ''very'' ''unlikely'' warming rates for patterns of surface temperature and precipitation change are assessed in [[#4.8|Section 4.8]] .

For the near term, initialized decadal forecasts constitute another line of evidence over the period 2019–2028 ( [[#box-4.1|Box 4.1]] ). The forecasts are consistent with the assessed GSAT ''very likely'' range (Box 4.1, Figure 1), strengthening the confidence in the near-term assessment.

The assessed ranges of GSAT change can be converted to change relative to mean GSAT over the period 1850–1900 for a consistent comparison with AR5 ( [[#IPCC--2013|IPCC, 2013]] ) and SR1.5 ( [[#IPCC--2018a|IPCC, 2018a]] ). GSAT was warmer in 1995–2014 (recent past) than 1850–1900 by 0.85 [0.67 to 0.98] °C. GSAT diagnosed for 1986–2005 (AR5 recent past) relative to 1850–1900 is 0.08°C higher than was diagnosed in AR5, due to methodological and dataset updates (Cross-Chapter Box 2.3, Table 1).

The uncertainty in GSAT relative to 1850–1900 includes the ''very likely'' ranges of assessed GSAT change relative to 1995–2014 (depending on scenario and period, between 0.5°C and 2.4°C; Figure 4.11d and Table 4.5), the uncertainty in historical GSAT change from the mean over 1850–1900 to 1995–2014 (about 0.3°C; Cross-Chapter Box 2.3), and the estimate of internal variability in 20-year GSAT averages (5–95% range about 0.15°C, Box 4.1; [[#Maher--2019|Maher et al., 2019]] ). These uncertainties are assumed to be independent and are added in quadrature, meaning that the total uncertainty is only slightly larger than the dominating contribution by the GSAT change relative to 1995–2014 (Table 4.5). The addition is done by numerically sampling a normal distribution fitted to the 5%, 50% and 95% percentiles of the internal variability, as well as sampling skew-normal distributions (e.g., [[#O’Hagan--1976|O’Hagan and Leonard, 1976]] ) fitted to the 5%, 50% and 95% percentiles of both historical warming and GSAT relative to 1995–2014. The result is a joint probability distribution of GSAT change and 20-year period.

Averaged over the period 2081–2100, GSAT is ''very'' ''likely'' to be higher than in the period 1850–1900 by 1.0°C–1.8°C in the low-emissions scenarios SSP1-1.9 and by 3.3°C–5.7°C in the high-emissions scenario SSP5-8.5. For the scenarios SSP1-2.6, SSP2-4.5, and SSP3-7.0, the corresponding ''very'' ''likely'' ranges are 1.3°C–2.4°C, 2.1°C–3.5°C, and 2.8°C–4.6°C, respectively (Table 4.5).

Time series of assessed GSAT change are now used to assess the time when certain thresholds of GSAT increases are crossed (Table 4.5). The threshold-crossing time is defined as the midpoint of the first 20-year period during which the average GSAT exceeds the threshold. During the near term (2021–2040), a 1.5°C increase in the 20-year average of GSAT, relative to the average over the period 1850–1900, is ''very likely'' to occur in scenario SSP5-8.5, ''likely'' to occur in scenarios SSP2-4.5 and SSP3-7.0, and ''more likely than not'' to occur in scenarios SSP1-1.9 and SSP1-2.6. In all scenarios assessed here except SSP5-8.5, the central estimate of crossing the 1.5°C threshold lies in the early 2030s, in the early part of the ''likely'' range (2030 '''–''' 2052) assessed in SR1.5, which assumed continuation of the then-current warming rate. Roughly half of this difference arises from a larger historical warming diagnosed in AR6, while the other half arises because for central estimates of climate sensitivity, most scenarios show stronger warming over the near term than was estimated as ‘current’ in SR1.5 ( ''medium confidence'' ). The SR1.5 estimate with a median of 0.2°C per decade has been confirmed in AR6 ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1|Section 3.3.1]] ); by contrast, the assessed GSAT change shows central-estimate rates over the period 2010 to 2035 that range from 0.21°C per decade under SSP1-1.9 to 0.30°C per decade under SSP5-8.5. When considering scenarios similar to SSP1-1.9 instead of linear extrapolation, the SR1.5 estimate of when 1.5°C global warming is crossed is close to the central estimate reported here (SR1.5, Table 2.SM. 12). If ECS and TCR lie near the lower end of the assessed ''very likely'' range, crossing the 1.5°C warming threshold is avoided in scenarios SSP1-1.9 and SSP1-2.6 ( ''medium confidence'' ). It is ''more likely than not'' that under SSP1-1.9, GSAT relative to 1850–1900 will remain below 1.6°C throughout the 21st century, implying a potential temporary overshoot above 1.5°C of no more than 0.1°C. All statements about crossing the 1.5°C threshold assume that no major volcanic eruption occurs during the near term.

A warming level of 2°C in GSAT, relative to the period 1850–1900, is ''very likely'' to be crossed in the mid-term period 2041–2060 under SSP5-8.5, ''likely'' to be crossed in the mid-term period under SSP3-7.0, and ''more likely than not'' to be crossed during the mid-term period under SSP2-4.5. During the entire 21st century, a warming level of 2°C in GSAT, relative to the period 1850–1900, will be crossed under SSP5-8.5 and SSP3-7.0, will ''extremely likely'' be crossed under SSP2-4.5, will ''unlikely'' be crossed under SSP1-2.6, and will ''extremely unlikely'' be crossed under SSP1-1.9.

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'''Table 4.''' '''5 |''' '''Assessment results for 20-year averaged GSAT change, based on multiple lines of evidence.''' Thechange is displayed in °C relative to the 1995–2014 and 1850–1900 reference periods for selected time periods (near term 2021–2040, mid-term 2041–2060, and long term 2081–2100), and as the time when certain temperature thresholds are crossed, relative to the period 1850–1900. The recent reference period 1995–2014 was higher in GSAT than the period 1850–1900 by 0.85 [0.67 to 0.98] °C, (Cross-Chapter Box 2.3). The entries give both the central estimate and, in parentheses, the ''very likely'' (5–95%) range. An entry of ‘n.c.’ means that the global warming threshold is ‘not crossed’ during the period 2021–2100.

{| class="wikitable"
|-
| '''Time Period'''

| '''SSP1-1.9 (°C)'''

| '''SSP1-2.6 (°C)'''

| '''SSP2-4.5 (°C)'''

| '''SSP3-7.0 (°C)'''

| '''SSP5-8.5 (°C)'''

|-
| '''Near Term: 2021–2040'''

Relative to 1995–2014

Relative to 1850–1900

| 0.6 [0.4 to 0.9]

1.5 [1.2 to 1.7]

| 0.6 [0.4 to 0.9]

1.5 [1.2 to 1.8]

| 0.7 [0.4 to 0.9]

1.5 [1.2 to 1.8]

| 0.7 [0.4 to 0.9]

1.5 [1.2 to 1.8]

| 0.8 [0.5 to 1.0]

1.6 [1.3 to 1.9]

|-
| '''Mid-term: 2041–2060'''

Relative to 1995–2014

Relative to 1850–1900

| 0.7 [0.4 to 1.1]

1.6 [1.2 to 2.0]

| 0.9 [0.5 to 1.3]

1.7 [1.3 to 2.2]

| 1.1 [0.8 to 1.6]

2.0 [1.6 to 2.5]

| 1.3 [0.9 to 1.7]

2.1 [1.7 to 2.6]

| 1.5 [1.1 to 2.1]

2.4 [1.9 to 3.0]

|-
| '''Long Term: 2081–2100'''

Relative to 1995–2014 Relative to 1850–1900

| 0.6 [0.2 to 1.0]

1.4 [1.0 to 1.8]

| 0.9 [0.5 to 1.5]

1.8 [1.3 to 2.4]

| 1.8 [1.2 to 2.6]

2.7 [2.1 to 3.5]

| 2.8 [2.0 to 3.7]

3.6 [2.8 to 4.6]

| 3.5 [2.4 to 4.8]

4.4 [3.3 to 5.7]

|-
| 1.5°C

Relative to 1850–1900

| 2025–2044

[2013–2032 to n.c.]

| 2023–2042

[2012–2031 to n.c.]

| 2021–2040

[2012–2031 to 2037–2056]

| 2021–2040

[2013–2032 to 2033–2052]

| 2018–2037

[2011–2030 to 2029–2048]

|-
| 2°C

Relative to 1850–1900

| n.c.

[n.c. to n.c.]

| n.c.

[2031–2050 to n.c.]

| 2043–2062

[2028–2047 to 2075–2094]

| 2037–2056

[2026–2045 to 2053–2072]

| 2032–2051

[2023–2042 to 2044–2063]

|-
| 3°C

Relative to 1850–1900

| n.c.

[n.c. to n.c.]

| n.c.

[n.c. to n.c.]

| n.c.

[2061–2080 to n.c.]

| 2066–2085

[2050–2069 to n.c.]

| 2055–2074

[2042–2061 to 2074–2093]

|-
| 4°C

Relative to 1850–1900

| n.c.

[n.c. to n.c.]

| n.c.

[n.c. to n.c.]

| n.c.

[n.c. to n.c.]

| n.c.

[2070–2089 to n.c.]

| 2075–2094

[2058–2077 to n.c.]

|}

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