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Asymmetry in the climate–carbon cycle response to positive and negative CO2 emissions

Abstract

Negative CO2 emissions are a key mitigation measure in emission scenarios consistent with temperature limits adopted by the Paris Agreement. It is commonly assumed that the climate–carbon cycle response to a negative CO2 emission is equal in magnitude and opposite in sign to the response to an equivalent positive CO2 emission. Here we test the hypothesis that this response is symmetric by forcing an Earth system model with positive and negative CO2 emission pulses of varying magnitude and applied from different climate states. Results indicate that a CO2 emission into the atmosphere is more effective at raising atmospheric CO2 than an equivalent CO2 removal is at lowering it, with the asymmetry increasing with the magnitude of the emission/removal. The findings of this study imply that offsetting positive CO2 emissions with negative emissions of the same magnitude could result in a different climate outcome than avoiding the CO2 emissions.

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Fig. 1: Simulated carbon cycle and surface air temperature response to positive and negative CO2 emission pulses.
Fig. 2: Changes in carbon stores as a fraction of cumulative CO2 emissions and removals (equivalent to CO2 pulse size) 100 years after pulse release for simulations initialized from different equilibrium states (1×CO2 to 4×CO2).
Fig. 3: Surface air temperature change (ΔT) as a fraction of cumulative CO2 emissions or removals (CE; equivalent to CO2 pulse size) 100 years after the pulse release for simulations initialized from different equilibrium states (1×CO2 to 4×CO2).

Data availability

The UVic ESCM data underlying this study are available at https://doi.org/10.5281/zenodo.4641434 (ref. 39). Source data are provided with this paper.

Code availability

The code for UVic ESCM version 2.9 is available at http://terra.seos.uvic.ca/model/2.9/.

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Acknowledgements

K.Z. and S.M. acknowledge support from the Natural Sciences and Engineering Research Council (NSERC) Discovery Grant Program. D.A was partly supported by the NSERC USRA Program. This research was enabled in part by computing resources provided by Westgrid and Compute Canada.

Author information

Affiliations

Authors

Contributions

K.Z. conceived the study, designed the model experiments, analysed and interpreted the model data and wrote the manuscript. D.A. performed the model simulations and contributed to the model data analysis and interpretation. S.M. provided analyses for the ocean response and contributed to the interpretation of results. H.D.M. provided suggestions for additional analysis and manuscript revisions.

Corresponding author

Correspondence to Kirsten Zickfeld.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Climate Change thanks Jörg Schwinger and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Response of land carbon fluxes to CO2 removal.

Change in net primary productivity (NPP) and soil respiration (Rh) for a 100 GtC removal applied from a state in equilibrium with twice the pre-industrial atmospheric CO2 concentration (2×CO2). Changes are calculated relative to a control simulation with zero CO2 emissions.

Source data

Extended Data Fig. 2 Revelle factor.

Revelle factor as function of surface ocean CO2 partial pressure (pCO2) for simulations with pulses of different magnitude (±100 GtC, ±500 GtC, ±1000 GtC) applied from 2×CO2 (cold colors) and 4×CO2 (warm colors) initial states. The Revelle factor is a measure of the ocean’s buffer capacity. It is inversely related to the buffer capacity; that is, a higher Revelle factor indicates a lower buffer capacity and vice versa.

Source data

Extended Data Fig. 3 Meridional overturning circulation response to CO2 emissions and removals.

Change in maximum meridional overturning streamfunction for pulse emissions/removals of ±100 GtC and ±1000 GtC applied from a 2×CO2 initial state. Changes are calculated relative to a control simulation with zero CO2 emissions. Dashed lines show the response to positive CO2 emissions mirrored about the horizontal zero-axis to illustrate asymmetries in the response.

Source data

Extended Data Fig. 4 Carbon cycle and temperature response to CO2 emissions and removals in biogeochemically coupled simulations.

Carbon cycle and surface air temperature responses to positive and negative CO2 emission pulses as simulated in the biogeochemically coupled simulations (see Methods). Pulse emissions and removals are applied from a 2×CO2 initial state. (a), Atmospheric CO2 concentration anomaly, (b) surface air temperature anomaly, (c) land carbon storage change, (d) ocean carbon storage change. Changes are calculated relative to a control simulation with zero CO2 emissions. Dashed lines show the response to positive CO2 emission pulses mirrored about the horizontal zero-axis to illustrate asymmetries in the responses.

Source data

Extended Data Fig. 5 Carbon cycle and temperature response to CO2 emissions and removals applied from a 4×CO2 state.

Carbon cycle and temperature responses to positive and negative CO2 emission pulses of different magnitude applied from a state at equilibrium with four times the pre-industrial atmospheric CO2 concentration (4×CO2). (a), Atmospheric CO2 concentration anomaly, (b) surface air temperature anomaly, (c) land carbon storage change, (d) ocean carbon storage change. Changes are calculated relative to a control simulation with zero CO2 emissions initialized from the 4×CO2 equilibrium state. Dashed lines show the response to positive CO2 emission pulses mirrored about the horizontal zero-axis to illustrate asymmetries in the responses.

Source data

Extended Data Fig. 6 Changes in net primary productivity 100 years after CO2 pulse release.

(a), 1000 GtC pulse applied from a 2×CO2 initial state, (b) 1000 GtC pulse applied from a 4×CO2 initial state, (c) -1000 GtC pulse applied from a 2×CO2 initial state, (d) -1000 GtC pulse applied from a 4×CO2 initial state. Changes are calculated relative to a control simulation with zero CO2 emissions initialized from a 2×CO2 (panels a, c) or 4×CO2 equilibrium state (panels b, d).

Extended Data Fig. 7 Spatial changes in vegetation carbon 100 years after CO2 pulse release.

Changes in vegetation carbon 100 years after CO2 pulse release. (a), 1000 GtC pulse applied from a 2×CO2 initial state, (b) 1000 GtC pulse applied from a 4×CO2 initial state, (c) -1000 GtC pulse applied from a 2×CO2 initial state, (d) -1000 GtC pulse applied from a 4×CO2 initial state. Changes are calculated relative to a control simulation with zero CO2 emissions initialized from a 2×CO2 (panels a, c) or 4×CO2 equilibrium state (panels b, d).

Extended Data Fig. 8 Spatial changes in soil respiration 100 years after CO2 pulse release.

Changes in soil respiration 100 years after CO2 pulse release. (a) 1000 GtC pulse applied from a 2×CO2 initial state, (b) 1000 GtC pulse applied from a 4×CO2 initial state, (c) -1000 GtC pulse applied from a 2×CO2 initial state, (d) -1000 GtC pulse applied from a 4×CO2 initial state. Changes are calculated relative to a control simulation with zero CO2 emissions initialized from a 2×CO2 (panels a, c) or 4×CO2 equilibrium state (panels b, d).

Extended Data Fig. 9 Spatial changes in soil carbon 100 years after CO2 pulse release.

Changes in soil carbon 100 years after CO2 pulse release. (a), 1000 GtC pulse applied from a 2×CO2 initial state, (b) 1000 GtC pulse applied from a 4×CO2 initial state, (c) -1000 GtC pulse applied from a 2×CO2 initial state, (d) -1000 GtC pulse applied from a 4×CO2 initial state. Changes are calculated relative to a control simulation with zero CO2 emissions initialized from a 2×CO2 (panels a, c) or 4×CO2 equilibrium state (panels b, d).

Extended Data Fig. 10 Variability in surface air temperature response.

Variability in the surface air temperature response to a positive and negative 100 GtC CO2 emission pulse applied from a state in equilibrium with the pre-industrial atmospheric CO2 concentration (1×CO2). The dashed line shows the response to the positive CO2 emission pulse mirrored about the horizontal zero-axis to illustrate the asymmetry in the response. Temperature variability in the model arises due to variability in the ocean-sea ice system.

Source data

Supplementary information

Supplementary Information

Supplementary Text, Table 1 and Figs. 1–7.

Source data

Source Data Fig. 1

Numerical model data used to generate figure.

Source Data Fig. 2

Numerical model data used to generate figure.

Source Data Fig. 3

Numerical model data used to generate figure..

Source Data Extended Data Fig. 1

Numerical model data used to generate figure

Source Data Extended Data Fig. 2

Numerical model data used to generate figure.

Source Data Extended Data Fig. 3

Numerical model data used to generate figure.

Source Data Extended Data Fig. 4

Numerical model data used to generate figure.

Source Data Extended Data Fig. 5

Numerical model data used to generate figure.

Source Data Extended Data Fig. 10

Numerical model data used to generate figure.

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Zickfeld, K., Azevedo, D., Mathesius, S. et al. Asymmetry in the climate–carbon cycle response to positive and negative CO2 emissions. Nat. Clim. Chang. 11, 613–617 (2021). https://doi.org/10.1038/s41558-021-01061-2

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