Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO<sub>2</sub>

Abstract

<jats:p>Abstract. The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. A total of 18 Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO2 concentration. Many models conducted additional second-priority simulations with different cumulative emission totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is −0.36 to 0.29 ∘C, with a model ensemble mean of −0.07 ∘C, median of −0.05 ∘C, and standard deviation of 0.19 ∘C. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both the carbon uptake by the ocean and the terrestrial biosphere are important for counteracting the warming effect from the reduction in ocean heat uptake in the decades after emissions cease. This warming effect is difficult to constrain due to high uncertainty in the efficacy of ocean heat uptake. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory. </jats:p>

An integrated approach to quantifying uncertainties in the remaining carbon budget

“…In our estimate of the RCB and its distribution, we have therefore internalised the sources of geophysical uncertainty which were reported separately by SR1.5: (1) current non-CO 2 forcing uncertainty is now explicitly represented by uncertainty in our parameter f nc ; (2) future non-CO 2 forcing uncertainty is internally consistent with historical uncertainty such that higher and lower values of f nc are paired with correspondingly higher and lower values of future f * nc ; (3) recent emission uncertainty is folded into the uncertainty in E; (4) historical temperature uncertainty is captured by our ΔT anth distribution; (5) our TCRE distribution reflects the distributions of the input parameters and is therefore implicit in the remaining budget distribution; (6) adjustments for under-represented feedbacks in ESMs are embedded in our method, given that our TCRE estimate is derived from observed quantities that include the effect of all relevant processes operating in the Earth system; and (7) we have additionally included uncertainty in ΔT ZEC which was only recently quantified 24 and thus not included as a quantified uncertainty in the SR1.5 (or any other) carbon budget analysis. On near-term decadal time scales relevant to achieving the 1.5°C or 2.0°C target, this ΔT ZEC term also accounts for the additional changes in global mean temperature due to feedbacks, such as permafrost carbon release 24 that are not captured by the TCRE but may contribute to warming on longer time scales.…”

“…22 , based on the average of the three observational temperature datasets with full spatial coverage. The distributions for E and ΔT ZEC are based on Gaussian distributions fitted to cumulative CO 2 emissions from 1870 to 2019 from the Global Carbon Project 23 , and model-simulated Zero Emissions Commitment (ZEC) values from the coordinated model intercomparison project ZECMIP 24 , respectively. We represent the current non-CO 2 forcing fraction as a 30-year average from 1990 to 2019, and computed the uncertainty range for f nc using the FaIR emulator 18 driven by scenarios from SR1.5 database 25,26 .…”

Long-term economic benefits of stabilizing warming without overshoot – the ENGAGE model intercomparison

“…The overall cumulative deployment of net negative CO2 emissions over the 21 st century ranges from a few megatons to about 600 GtCO2 across models in the scenarios that only cap the end-of-century budget, and our scenarios show a techno-economic potential for declining warming after its peak by 0.12 to 0.37˚C until 2100 across models (Figure 1b). This temperature reversal is not only driven by net negative CO2 emissions but can also be partially the result of reductions in non-CO2 forcers after the point when net zero CO2 emissions is reached (known as the Zero Emissions Commitment) 16 . The drawdown due to Non-CO2 emissions in the net-zero budget scenarios is between 0°C-0.14°C by 2100 (see blue dots in Figure 1b).…”

Millennium time-scale experiments on climate-carbon cycle with doubled CO2 concentration

“…Another example of the typical condition that has not been well examined is the period after net zero anthropogenic emission is achieved. Although Rogelj et al (2018) assumes, in the calculation of the total amount of CO 2 that can be emitted to not exceed a specific warming target "remaining carbon budget", that the temperature change after zero emission (the temperature change is called "zero-emission commitment", ZEC) would be almost zero, it has not been systematically examined how models project the ZEC (Jones et al 2019;MacDougall et al 2020).…”

“…In addition, the final goal of our attempts in relation to the Paris agreement, beyond the urgent necessity to curb anthropogenic emissions to avoid exceeding a specific warming target, should be the eventual climate stabilization and concentration stabilization of greenhouse gases at a certain level. The eventual stabilization of CO 2 concentration can be realized by reducing anthropogenic emissions to the same level as the total natural C sink or by net zero emission of anthropogenic C (in this case, the CO 2 concentration would slowly decline over hundreds or thousands of years and then the stabilization of concentration and global temperature will be eventually realized (Eby et al 2009;MacDougall et al 2020)). In both cases, the climate-C system continues to change for a long time to adjust to the new external forcing, likely from century to millennium time-scale (Archer et al 2009;Li et al 2013;Frölicher et al 2013;Joos et al 2013;Yamamoto et al 2018;Rugenstein et al 2019).…”