Reply on RC1

<strong class="journal-contentHeaderColor">Abstract.</strong> The benefits of implementing negative emission technologies in the global warming response to cumulative carbon emissions until the year 2420 are assessed following the shared socioeconomic pathway (SSP) 1-2.6, the sustainable development scenario, with a comprehensive set of intermediate-complexity Earth system model integrations. Model integrations include 86 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 CO<span class="inline-formula"><sub>2</sub></span> emissions (eTCRE), measuring the surface warming response to cumulative carbon emissions and associated non-CO<span class="inline-formula"><sub>2</sub></span> forcing, and the effective zero emissions commitment (eZEC), measuring the extent of any continued warming after net-zero CO<span class="inline-formula"><sub>2</sub></span> emissions are reached. The transient climate response to cumulative CO<span class="inline-formula"><sub>2</sub></span> emissions (TCRE) is estimated as 2.2 K EgC<span class="inline-formula"><sup>−1</sup></span> (median value) with a 10 %–90 % range of 1.75 to 3.13 K EgC<span class="inline-formula"><sup>−1</sup></span> in 2100, approximated from the eTCRE by removing the contribution of non-CO<span class="inline-formula"><sub>2</sub></span> forcing. During the positive emission phase, the eTCRE decreases from 2.71 (2.0 to 3.65) to 2.61 (1.91 to 3.62) K EgC<span class="inline-formula"><sup>−1</sup></span> due to a weakening in the dependence of radiative forcing on atmospheric carbon, which is partly opposed by an increasing fraction of the radiative forcing warming the surface as the ocean stratifies. During the net negative and zero emission phases, a progressive reduction in the eTCRE to 2.0 (1.39 to 2.96) K EgC<span class="inline-formula"><sup>−1</sup></span> is driven by the reducing airborne fraction as atmospheric CO<span class="inline-formula"><sub>2</sub></span> is drawn down mainly by the ocean. The model uncertainty in the slopes of warming versus cumulative CO<span class="inline-formula"><sub>2</sub></span> emissions varies from being controlled by the radiative feedback parameter during positive emissions to being affected by carbon-cycle parameters during net negative emissions, consistent with the drivers of uncertainty diagnosed from the coefficient of variation of the contributions in the eTCRE framework. The continued warming after CO<span class="inline-formula"><sub>2</sub></span> emissions cease and remain at zero gives a model mean eZEC of <span class="inline-formula">−</span>0.03 K after 25 years, which decreases in time to <span class="inline-formula">−</span>0.21 K at 90 years after emissions cease. However, there is a spread in the ensemble with a temperature overshoot occurring in 20 % of the ensemble members at 25 years after cessation of emissions. If net negative emissions are included, there is a reduction in atmospheric CO<span class="inline-formula"><sub>2</sub></span> and there is a decrease in temperature overshoot so that the eZEC is positive in only 5 % of the ensemble members. Hence, incorporating negative emissions enhances the ability to meet climate targets and avoid risk of continued warming after net zero is reached.

Topic: Atmospheric and Environmental Gas Dynamics
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences