Impact of negative and positive CO 2 emissions on global warming metrics using an ensemble of Earth system model simulations

[摘要] The benefits of implementing negative emission technologies in the globalwarming response to cumulative carbon emissions until the year 2420 are assessedfollowing the shared socioeconomic pathway (SSP) 1-2.6, the sustainabledevelopment scenario, with a comprehensive set of intermediate-complexityEarth system model integrations. Model integrations include 86 differentmodel realisations covering a wide range of plausible climate states. Theglobal warming response is assessed in terms of two key climate metrics: theeffective transient climate response to cumulative CO 2 emissions(eTCRE), measuring the surface warming response to cumulative carbonemissions and associated non-CO 2 forcing, and the effective zeroemissions commitment (eZEC), measuring the extent of any continued warmingafter net-zero CO 2 emissions are reached. The transient climate response to cumulative CO 2 emissions (TCRE) is estimated as 2.2 K EgC −1 (median value) with a 10 %–90 % range of 1.75 to 3.13 K EgC −1 in 2100, approximated from the eTCRE by removing the contribution of non-CO 2 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 −1 due to a weakening in the dependence of radiative forcing on atmospheric carbon, which is partly opposed by an increasing fraction of the radiative forcingwarming the surface as the ocean stratifies. During the net negative andzero emission phases, a progressive reduction in the eTCRE to 2.0 (1.39 to2.96) K EgC −1 is driven by the reducing airborne fraction as atmospheric CO 2 is drawn down mainly by the ocean. The model uncertainty in the slopes ofwarming versus cumulative CO 2 emissions varies from being controlled bythe radiative feedback parameter during positive emissions to being affectedby carbon-cycle parameters during net negative emissions, consistent withthe drivers of uncertainty diagnosed from the coefficient of variation ofthe contributions in the eTCRE framework. The continued warming afterCO 2 emissions cease and remain at zero gives a model mean eZEC of − 0.03 K after 25 years, which decreases in time to − 0.21 K at 90 years after emissionscease. However, there is a spread in the ensemble with a temperatureovershoot occurring in 20 % of the ensemble members at 25 years after cessation of emissions. If netnegative emissions are included, there is a reduction in atmosphericCO 2 and there is a decrease in temperature overshoot so that the eZECis positive in only 5 % of the ensemble members. Hence, incorporatingnegative emissions enhances the ability to meet climate targets and avoidrisk of continued warming after net zero is reached.

[学科分类] 大气科学