07 Feb 2022
07 Feb 2022
Status: this preprint is currently under review for the journal BG.

Assessment of negative and positive CO2 emissions on global warming metrics using large ensemble Earth system model simulations

Negar Vakilifard1, Richard G. Williams2, Philip B. Holden1, Katherine Turner2,3, Neil R. Edwards1,4, and David J. Beerling5 Negar Vakilifard et al.
  • 1Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK
  • 2Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University of Liverpool, Liverpool, United Kingdom
  • 3Leverhulme Research Centre for Functional Materials Design, Liverpool, United Kingdom
  • 4Cambridge Centre for Energy, Environment and Natural Resource Governance, University of Cambridge, Cambridge, UK
  • 5Leverhulme Centre for Climate Change Mitigation, School of Biosciences, University of Sheffield, Sheffield, UK

Abstract. The benefits of implementing negative emission technologies for a century (years 2070–2170) on the global warming response to cumulative carbon emissions until year 2420 are assessed with a comprehensive set of intermediate complexity Earth system model integrations. Model integrations include 82 different model realisations covering a wide range of plausible climate states. The global warming response is assessed in terms of two key climate metrics: the effective transient climate response to cumulative CO2 emissions (eTCRE), measuring the surface warming response to cumulative carbon emissions and associated non-CO2 (RCP4.5) forcing, and the zero emissions commitment (ZEC), measuring the extent of any continued warming after net zero is reached. The TCRE is approximated from eTCRE by removing the contributions of non-CO2 forcing as 2.15 °C EgC−1 (with a 10–90 % range of 1.6 to 2.8 °C EgC−1). During the net positive emission phases, the eTCRE decreases from 2.62 (1.90 to 3.65) to 2.30 (1.73 to 3.23) °C EgC−1 due to a weakening in the increase in radiative forcing with an increase in atmospheric carbon, which is partly opposed by an increasing fraction of the radiative forcing warming the surface as the ocean stratifies. During the negative and zero emission phases, a progressive reduction of the eTCRE to 2.0 (1.4 to 2.8) °C EgC−1 is driven by the reducing airborne fraction as CO2 is drawn down by the ocean. The model uncertainty in the slopes of warming versus cumulative CO2 emissions varies from being controlled by the radiative feedback parameter during positive emissions to also being affected by ocean circulation and carbon-cycle parameters during zero or net-negative emissions. There is hysteresis in atmospheric CO2 and surface warming, where atmospheric CO2 and surface temperature are higher after peak emissions compared with before peak emissions. The continued warming after emissions cease defining the ZEC for the model mean without carbon capture is −0.01 °C at 25 years and decreases in time to −0.15 °C at 90 years after emissions cease. However, there is a spread in the ensemble with a temperature overshoot occurring in 50 % of the ensemble members at year 25. The ZEC only becomes negative in all ensemble members if modest carbon capture is included. Hence, incorporating negative emissions enhances the ability to meet climate targets and avoid risk of continued warming after net zero is reached.

Negar Vakilifard et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-38', Anonymous Referee #1, 21 Feb 2022
  • RC2: 'Comment on bg-2022-38', Anonymous Referee #2, 28 Mar 2022

Negar Vakilifard et al.

Negar Vakilifard et al.


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Short summary
To remain within the Paris climate agreement, there is an increasing need to develop and implement carbon capture and sequestration techniques. The global climate benefits of implementing negative emission technologies over the next century are assessed using an Earth system model covering a wide range of plausible climate states. In some model realisations, there is continued warming after emissions cease. This continued warming is only avoided if modest negative emissions are incorporated.