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

==== 4.7.1.1 Climate Change Following Zero Emissions ====

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The zero emissions commitment (ZEC) is the climate change commitment that would result, in terms of projected GSAT, from setting carbon dioxide (CO <sub>2</sub> ) emissions to zero. It is determined by both inertia in physical climate system components (ocean, cryosphere, land surface) and carbon cycle inertia (see Annex VII). In its widest sense it refers to emissions of all compounds including greenhouses gases, aerosols and their pre-cursors. A specific sub-category of zero emissions commitment is the zero CO <sub>2</sub> emissions commitment, which refers to the climate system response to a cessation of anthropogenic CO <sub>2</sub> emissions excluding the impact of non-CO <sub>2</sub> forcers. Assessment of remaining carbon budgets requires an assessment of zero CO <sub>2</sub> emissions commitment as well as of the transient climate response to cumulative carbon emissions (TCRE; Section 5.5.2).

There is an offset of continued warming following cessation of emissions by continued CO <sub>2</sub> removal by natural sinks ( ''high confidence'' ) (e.g., Matthews and Caldeira, 2008; [[#Solomon--2009|Solomon et al., 2009]] ; [[#Joos--2013|Joos et al., 2013]] ; [[#Ricke--2014|Ricke and Caldeira, 2014]] ). Some models continue warming by up to 0.5°C after emissions cease at 2°C of warming ( [[#Frölicher--2014|Frölicher et al., 2014]] ; [[#Frölicher--2015|Frölicher and Paynter, 2015]] ; [[#Williams--2017|Williams et al., 2017]] ), while others simulate little to no additional warming ( [[#Nohara--2015|Nohara et al., 2015]] ). In SR1.5, the available evidence indicated that past CO <sub>2</sub> emissions do not commit to substantial further warming ( [[#Allen--2018|Allen et al., 2018]] ). A ZEC close to zero was thus applied for the computation of the remaining carbon budget ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). However, the available literature consisted of simulations from a small number of models using a variety of experimental designs, with some simulations showing a complex evolution of temperature following cessation of emissions (e.g., [[#Frölicher--2014|Frölicher et al., 2014]] ; [[#Frölicher--2015|Frölicher and Paynter, 2015]] ; [[#Williams--2017|Williams et al., 2017]] ).

Here we draw on new simulations to provide an assessment of ZEC using multiple ESMs ( [[#Jones--2019|Jones et al., 2019]] ) and EMICs ( [[#MacDougall--2020|MacDougall et al., 2020]] ). Figure 4.39 shows results from 20 models that simulate the evolution of CO <sub>2</sub> and the GSAT response following cessation of CO <sub>2</sub> emissions for an experiment where 1000 PgC is emitted during a 1% per year CO <sub>2</sub> increase. All simulations show a strong reduction in atmospheric CO <sub>2</sub> concentration following cessation of CO <sub>2</sub> emissions in agreement with previous studies and basic theory that natural carbon sinks will persist. Therefore, there is ''very high confidence'' that atmospheric CO <sub>2</sub> concentrations would decline for decades if CO <sub>2</sub> emissions cease. Temperature evolution in the 100 years following cessation of emissions varies by model and across time scales, with some models showing declining temperature, others having ZEC close to zero, and others showing continued warming following cessation of emissions (Figure 4.39). The GSAT response depends on the balance of carbon sinks and ocean heat uptake ( [[#MacDougall--2020|MacDougall et al., 2020]] ). The 20-year average GSAT change 50 years after the cessation of emissions (ZEC <sub>50</sub> ) is summarized in Table 4.8. The mean value of ZEC <sub>50</sub> is –0.079°C, with 5–95% range –0.34°C–0.28°C. There is no strong relationship between ZEC <sub>50</sub> and modelled climate sensitivity (neither ECS nor TCR; [[#MacDougall--2020|MacDougall et al., 2020]] ). It is therefore ''likely'' that the absolute magnitude of ZEC <sub>50</sub> is less than 0.3°C, but we assess ''low'' ''confidence'' in the sign of ZEC on 50-year time scales. This is small compared with natural variability in GSAT.

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[[File:db2b2f90b934309946649a02c9646f1f IPCC_AR6_WGI_Figure_4_39.png]]

'''Figure 4.39''' '''|''' '''Zero emissions commitment (ZEC).''' Changes in '''(a)''' atmospheric CO <sub>2</sub> concentration and '''(b)''' evolution of global surface air temperature (GSAT) following cessation of CO <sub>2</sub> emissions branched from the 1% per year experiment after emissions of 1000 Pg C ( [[#Jones--2019|Jones et al., 2019]] ; [[#MacDougall--2020|MacDougall et al., 2020]] ). ZEC is the temperature anomaly relative to the estimated temperature at the year of cessation. ZEC <sub>50</sub> is the 20-year mean GSAT change centred on 50 years after the time of cessation (see Table 4.8) – this period is marked with the vertical dotted lines. Multi-model mean is shown as thick black line, individual model simulations are in grey. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

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'''Table 4.8''' '''|''' '''The 20-year average GSAT change 50 years after the cessation of emissions (ZEC50).''' Displayed are ZEC50 estimated from eleven ESMs (top) and nine EMICs (bottom).

{| class="wikitable"
|-
| Model

| ZEC <sub>50</sub> (°C)

|-
| ACCESS-ESM1.5

| 0.01

|-
| CanESM5

| –0.14

|-
| CESM2

| –0.31

|-
| CNRM-ESM2-1

| 0.06

|-
| GFDL-ESM2M

| –0.27

|-
| GFDL-ESM4

| –0.21

|-
| GISS-E2-1-G

| –0.15

|-
| MIROC-ES2L

| –0.08

|-
| MPI-ESM1.2-LR

| –0.27

|-
| NorESM2-LM

| –0.33

|-
| UKESM1-0-LL

| 0.28

|-
| Bern3D-LPX

| 0.01

|-
| DCESS1.0

| 0.06

|-
| CLIMBER-2

| –0.07

|-
| IAPRAS

| 0.28

|-
| LOVECLIM 1.2

| –0.04

|-
| MESM

| 0.01

|-
| MIROC-lite

| –0.06

|-
| PLASM-GENIE

| –0.36

|-
| UVic ESCM 2.10

| 0.03

|}

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