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.

Highlights

  • 50 There is an increasing need to adopt negative emission technologies (Luderer et al, 2013; Rogelj et al, 2015; Beerling et al, 2020) to enhance the chance of meeting the Paris climate agreement targets of global warming of 1.5 °C or less than 2 °C given the ongoing growth in greenhouse gas concentrations (Boucher et al 2012; JeltschThömmes et al, 2020)

  • The global warming response is assessed in terms of two key climate metrics: the effective transient climate response to cumulative CO2 emissions, measuring the surface warming response to cumulative carbon emissions and associated non-CO2 (RCP4.5) 30 forcing, and the zero emissions commitment (ZEC), measuring the extent of any continued warming after net zero is reached

  • 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 35 warming the surface as the ocean stratifies

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Summary

Introduction

50 There is an increasing need to adopt negative emission technologies (Luderer et al, 2013; Rogelj et al, 2015; Beerling et al, 2020) to enhance the chance of meeting the Paris climate agreement targets of global warming of 1.5 °C or less than 2 °C given the ongoing growth in greenhouse gas concentrations (Boucher et al 2012; JeltschThömmes et al, 2020). An effective TCRE (eTCRE) may be defined to include non-CO2 warming and the non-CO2 radiative forcing (Williams et al, 2016; Williams et al, 2017). The TCRE may be viewed as a product of two terms, the change in global mean air temperature divided by the change in the atmospheric carbon inventory, ∆T(t)/∆I (t), and the airborne fraction, ∆I (t)/. In the more realistic framework we apply here, the warming response includes contributions from non-CO2 forcing In such experiments, Matthews et al (2021) advocate estimating the TCRE. In order to allow for possible changes in the thermal and carbon responses from the non-CO2 forcing, we prefer to define an effective TCRE (eTCRE) including the effect of the radiative forcing from non-CO2 and CO2 radiative components using a series of mathematical identities (Williams et al, 2016 and 2017), where. Our subsequent model diagnostics focus on evaluating the eTCRE and the thermal, radiative and carbon dependences using Eq 9

Methods
Carbon feedback
Thermal feedback
Carbon dependence for the effective TCRE
Radiative forcing dependence on atmospheric CO2 for the effective TCRE
Thermal dependence for the effective TCRE
The asymmetry of the
The Zero Emissions Commitment
Findings
Conclusions
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