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

==== 4.3.1.1 Surface Air Temperature ====

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The AR5 assessed from CMIP5 simulations and other lines of evidence that GSAT will continue to rise over the 21st century if greenhouse gas (GHG) concentrations continue increasing ( [[#Collins--2013|Collins et al., 2013]] ). The AR5 concluded that GSAT for 2081–2100, relative to 1986–2005 will ''likely'' be in the 5–95% range of 0.3°C–1.7°C under RCP2.6 and 2.6°C–4.8°C under RCP8.5. The corresponding ranges for the intermediate emissions scenarios with emissions peaking around 2040 (RCP4.5) and 2060 (RCP6.0) are 1.1°C–2.6°C and 1.4°C–3.1°C, respectively. The AR5 further assessed that GSAT averaged over the period 2081–2100 are projected to ''likely'' exceed 1.5°C above 1850–1900 for RCP4.5, RCP6.0 and RCP8.5 ( ''high confidence'' ) and are ''likely'' to exceed 2°C above 1850–1900 for RCP6.0 and RCP8.5 ( ''high confidence'' ). Global surface temperature changes above 2°C under RCP2.6 were deemed ''unlikely'' ( ''medium confidence'' ).

Here, for continuity’s sake, we assess the CMIP6 simulations of GSAT in a fashion similar to the AR5 assessment of the CMIP5 simulations. From these, we compute anomalies relative to 1995–2014 and display the evolution of ensemble means and 5–95% ranges (Figure 4.2). We also use the ensemble mean GSAT difference between 1850–1900 and 1995–2014, 0.82°C, to provide an estimate of the changes since 1850–1900 (Figure 4.2, right axis). Finally, we tabulate the ensemble mean changes between 1995–2014 and 2021–2040, 2041–2060, and 2081–2100 respectively (Figure 4.2).

The CMIP6 models show a 5–95% range of GSAT change for 2081–2100, relative to 1995–2014, of 0.6°C–2.0°C under SSP1-2.6 where CO <sub>2</sub> concentrations peak between 2040 and 2060 (see Table 4.2). The corresponding range under the highest overall emissions scenario (SSP5-8.5) is 2.7°C–5.7°C. The ranges for the intermediate and high emissions scenarios (SSP2-4.5 and SSP3-7.0), where CO <sub>2</sub> concentrations increase to 2100, but less rapidly than SSP5-8.5, are 1.4°C–3.0°C and 2.2°C–4.7°C, respectively. The range for the lowest emissions scenario (SSP1-1.9) is 0.2°C–1.3°C.

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'''Table''' '''4.2 |''' '''CMIP6 annual mean surface air temperature anomalies (°C).''' Displayed are multi-model averages and, in parentheses, the 5–95% ranges, for selected time periods, regions, and SSPs. The numbers of models used are indicated in Figure 4.2.

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

| '''SSP1-1.9 (°C)'''

| '''SSP1-2.6 (°C)'''

| '''SSP2-4.5 (°C)'''

| '''SSP3-7.0 (°C)'''

| '''SSP5-8.5 (°C)'''

|-
| '''Global: 2021–2040'''

Relative to 1995–2014

Relative to 1850–1900

| 0.7 (0.3, 1.1)

1.5 (1.1, 2.2)

| 0.7 (0.4, 1.1)

1.6 (1.1, 2.2)

| 0.7 (0.4, 1.2)

1.6 (1.0, 2.3)

| 0.7 (0.5, 1.2)

1.6 (1.0, 2.4)

| 0.8 (0.5, 1.3)

1.7 (1.2, 2.4)

|-
| '''Global: 2041–2060'''

Relative to 1995–2014

Relative to 1850–1900

| 0.8 (0.3, 1.5)

1.7 (1.1, 2.4)

| 1.0 (0.6, 1.6)

1.9 (1.2, 2.7)

| 1.3 (0.8, 1.9)

2.1 (1.5, 3.0)

| 1.4 (0.9, 2.3)

2.3 (1.6, 3.2)

| 1.7 (1.2, 2.5)

2.6 (1.8, 3.4)

|-
| '''Global: 2081–2100'''

Relative to 1995–2014

Relative to 1850–1900

| 0.7 (0.2, 1.5)

1.5 (1.0, 2.2)

| 1.2 (0.6, 2.0)

2.0 (1.3, 2.8)

| 2.0 (1.4, 3.0)

2.9 (2.1, 4.0)

| 3.1 (2.2, 4.7)

3.9 (2.8, 5.5)

| 4.0 (2.7, 5.7)

4.8 (3.6, 6.5)

|-
| Land: 2081–2100

Relative to 1995–2014

| 0.9 (0.3, 2.0)

| 1.5 (0.8, 2.6)

| 2.7 (1.7, 4.0)

| 4.1 (3.0, 6.2)

| 5.3 (3.5, 7.6)

|-
| Ocean: 2081–2100

Relative to 1995–2014

| 0.6 (0.1, 1.2)

| 1.0 (0.5, 1.8)

| 1.8 (1.2, 2.7)

| 2.7 (1.8, 4.0)

| 3.4 (2.3, 4.9)

|-
| Tropics: 2081–2100

Relative to 1995–2014

| 0.5 (0.1, 1.1)

| 1.0 (0.5, 1.6)

| 1.8 (1.2, 2.5)

| 2.7 (2.0, 4.0)

| 3.5 (2.4, 4.9)

|-
| Arctic: 2081–2100

Relative to 1995–2014

| 2.4 (0.5, 6.6)

| 3.3 (0.4, 7.5)

| 5.4 (2.8, 10.0)

| 7.7 (4.5, 13.4)

| 10.0 (6.2, 15.2)

|-
| Antarctic: 2081–2100

Relative to 1995–2014

| 0.5 (0.0, 1.1)

| 1.1 (0.1, 2.9)

| 1.9 (0.6, 3.2)

| 2.8 (1.3, 4.5)

| 3.6 (1.7, 5.6)

|}

In summary, the CMIP6 models show a general tendency toward larger long-term globally averaged surface warming than did the CMIP5 models, for nominally comparable scenarios ( ''very high confidence'' ). In SSP1-2.6 and SSP2-4.5, the 5–95% ranges have remained similar to the ranges in RCP2.6 and RCP4.5, respectively, but the distributions have shifted upward by about 0.3°C ( ''high confidence'' ). For SSP5-8.5 compared to RCP8.5, the 5% bound of the distribution has hardly changed, but the 95% bound and the range have increased by about 20% and 40%, respectively ( ''high confidence'' ). About half of the warming increase has occurred because of more models with higher climate sensitivity in CMIP6, compared to CMIP5; the other half of the warming increase arises from higher effective radiative forcing in nominally comparable scenarios ( ''medium confidence,'' see [[#4.6.2|Section 4.6.2]] ).

With regards to global warming levels (GWLs) of 1.5°C, 2.0°C and 3.0°C, we note that there is unanimity across all of the CMIP6 model simulations that GSAT change relative to 1850–1900 will rise above: (i) 1.5°C following SSP2-4.5, SSP3-7.0, or SSP5-8.5 (on average around 2030); (ii) 2.0°C following either SSP3-7.0 or SSP5-8.5 (on average around 2043); and (iii) 3.0°C following SSP5-8.5 (on average around 2062). Under SSP1-1.9, 55% and 36% of the model simulations rise above 1.5°C and 2.0°C, respectively, while for SSP1-2.6 those percentages increase to 87% and 58%, respectively. Here, the time of GSAT exceedance is determined as the first year at which 21-year running averages of GSAT exceed the given GWL. In ( [[#4.3.4|Section 4.3.4]] , these values are reassessed using CMIP6 ensemble in combination with other lines of evidence.

CMIP6 models project increases in area-weighted land, ocean, tropical (30°S–30°N), Arctic (67.7°N–90°N), and Antarctic (90°S–55°S) surface air temperature (Table 4.2). Consistent with AR5, and earlier assessments, CMIP6 models project that annual average surface air temperature will warm about 50% more over land than over the ocean, and that the Arctic will warm about more than 2.5 times the global average ( [[#4.5.1|Section 4.5.1]] ). For 2081–2100, relative to 1995–2014, the CMIP6 models show 5–95% ranges of warming over land of 0.3°C–2.0°C and 3.5°C–7.6°C following SSP1-1.9 and SSP5-8.5, respectively. The corresponding ranges for Arctic surface air temperature change are 0.5°C–6.6°C and 6.2°C–15.2°C, respectively.

The concentration-driven simulations presented above use a prescribed CO <sub>2</sub> pathway calculated by the MAGICC7.0 model using the CMIP6 emissions ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ). This is compared here with the CO <sub>2</sub> concentration simulated by CMIP6 ESMs in response to the SSP5-8.5 emissions (Figure 4.3). The 1995–2014 mean simulated CO <sub>2</sub> level is 375 ppm, very similar to the prescribed 378 ppm, but the ESM 5–95% range is 357–391 ppm. By the end of the 21st century (2081–2100), the ESM mean is 953 ppm – below the prescribed CO <sub>2</sub> pathway (1004 ppm), but with a large 5–95% range of 848–1045 ppm, which spans the prescribed concentration level. This result differs from CMIP5, which showed that ESMs typically simulated CO <sub>2</sub> concentrations higher than the prescribed concentration-driven RCP pathways. Reduced spread in CMIP6 carbon cycle feedbacks compared to CMIP5 has been postulated to be due to the inclusion of nitrogen cycle processes in about half of CMIP6 ESMs ( [[#Arora--2020|Arora et al., 2020]] ). This means that the CMIP6 spread in GSAT response to CO <sub>2</sub> emissions is dominated by climate sensitivity differences between ESMs more than by carbon cycle differences ( ''high confidence'' ) ( [[#Jones--2020|Jones and Friedlingstein, 2020]] ; [[#Williams--2020|Williams et al., 2020]] ).

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[[File:4150ef8436b9e8c198170bcd4080d80f IPCC_AR6_WGI_Figure_4_3.png]]

'''Figure''' '''4.3 |''' '''Comparison ofconcentration-driven and emissions-driven simulation. (a)''' Atmospheric CO <sub>2</sub> concentration; '''(b)''' global surface air temperature from models which performed SSP5-8.5 scenario simulations in both emissions-driven (blue) and concentration-driven (red) configurations. For concentration driven simulations, CO <sub>2</sub> concentration is prescribed, and follows the red line in panel (a) in all models. For emissions-driven simulations, CO <sub>2</sub> concentration is simulated and can therefore differ for each model, blue lines in panel (a). Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Simulated GSAT over 1995–2014, relative to 1850–1900 period, warms by very similar amounts in the two sets of simulations: 0.82°C (0.45–1.31) in emissions-driven compared with 0.75°C (0.53–1.09) in concentration-driven simulations. By the end of the 21st century, warming in emissions-driven simulations is very similar: 4.58°C (3.53–6.70), reflecting the slightly lower CO <sub>2</sub> concentration simulated by the ESMs compared with warming under the prescribed CO <sub>2</sub> pathway of 4.69°C (3.70–6.77). This difference in model-mean response is more than an order of magnitude smaller than the 5–95% spread across model projections. The spread in CO <sub>2</sub> concentration, compared with the prescribed default concentration, leads to a very small increase by about 0.1°C in the spread of GSAT projections, but it is not possible to tell if this is a direct consequence of the simulation configuration or internal variability of the model simulations. These differences due to experimental configuration would be smaller still under scenarios with lower CO <sub>2</sub> levels, and so we assess that results from concentration-driven and emissions-driven configurations do not affect the assessment of GSAT projections ( ''high confidence'' ).

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