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Reliable estimates of soil carbon change are required to determine the carbon budgets consistent with the Paris climate targets. This study evaluates projections of soil carbon during the 21st century in CMIP6 Earth System Models (ESMs) under a range of atmospheric composition scenarios. In general, we find a reduced spread of changes in global soil carbon (ΔCs) in CMIP6 compared to the previous CMIP5 model generation. However, similar reductions were not seen in the derived contributions to ΔCs due to both increases in plant Net Primary Productivity (NPP, named ΔCs,NPP) and reductions in the effective soil carbon turnover time (τs, named ΔCs,τ). Instead, we find a strong relationship across the CMIP6 models between these NPP and τs components of ΔCs, with more positive values of ΔCs,NPP being correlated with more negative values of ΔCs,τ. We show that this emergent relationship is the result of 'false priming', which leads to a decrease in the effective soil carbon turnover time as a direct result of NPP increase and occurs when the rate of increase of NPP is relatively fast compared to the slower timescales of a multipool soil carbon model. The inclusion of more soil carbon models with multiple pools in CMIP6 compared to CMIP5, therefore seems to have contributed towards the reduction in the overall model spread in future soil carbon projections. [ABSTRACT FROM AUTHOR]

Scenarios avoiding global warming greater than 1.5 or 2 ∘ C, as stipulated in the Paris Agreement, may require the combined mitigation of anthropogenic greenhouse gas emissions alongside enhancing negative emissions through approaches such as afforestation–reforestation (AR) and biomass energy with carbon capture and storage (BECCS). We use the JULES land surface model coupled to an inverted form of the IMOGEN climate emulator to investigate mitigation scenarios that achieve the 1.5 or 2 ∘ C warming targets of the Paris Agreement. Specifically, within this IMOGEN-JULES framework, we focus on and characterise the global and regional effectiveness of land-based (BECCS and/or AR) and anthropogenic methane (CH4) emission mitigation, separately and in combination, on the anthropogenic fossil fuel carbon dioxide (CO2) emission budgets (AFFEBs) to 2100. We use consistent data and socio-economic assumptions from the IMAGE integrated assessment model for the second Shared Socioeconomic Pathway (SSP2). The analysis includes the effects of the methane and carbon–climate feedbacks from wetlands and permafrost thaw, which we have shown previously to be significant constraints on the AFFEBs. Globally, mitigation of anthropogenic CH4 emissions has large impacts on the anthropogenic fossil fuel emission budgets, potentially offsetting (i.e. allowing extra) carbon dioxide emissions of 188–212 Gt C. This is because of (a) the reduction in the direct and indirect radiative forcing of methane in response to the lower emissions and hence atmospheric concentration of methane and (b) carbon-cycle changes leading to increased uptake by the land and ocean by CO2 -based fertilisation. Methane mitigation is beneficial everywhere, particularly for the major CH4 -emitting regions of India, the USA, and China. Land-based mitigation has the potential to offset 51–100 Gt C globally, the large range reflecting assumptions and uncertainties associated with BECCS. The ranges for CH4 reduction and BECCS implementation are valid for both the 1.5 and 2 ∘ C warming targets. That is the mitigation potential of the CH4 and of the land-based scenarios is similar for regardless of which of the final stabilised warming levels society aims for. Further, both the effectiveness and the preferred land management strategy (i.e. AR or BECCS) have strong regional dependencies. Additional analysis shows extensive BECCS could adversely affect water security for several regions. Although the primary requirement remains mitigation of fossil fuel emissions, our results highlight the potential for the mitigation of CH4 emissions to make the Paris climate targets more achievable. [ABSTRACT FROM AUTHOR]

Climate engineering arises as one of the potential methods that could contribute to meeting the 1.5 °C global warming target agreed under the Paris Agreement. We examine how permafrost and high‐latitude vegetation respond to the large‐scale implementation of climate engineering. Specifically, we explore the impacts of applying the solar radiation management method of stratospheric aerosol injections (SAI) on permafrost temperature and the global extent of near‐surface permafrost area. We compare the RCP8.5 and RCP4.5 scenarios to several SAI deployment scenarios using the Norwegian Earth System Model (CE1 = moderate SAI scenario to bring down the global mean warming in RCP8.5 to the RCP4.5 level, CE2 = aggresive SAI scenario to maintain the global mean temperature toward the preindustrial level). We show that large‐scale application of SAI may help slow down the current rate of permafrost degradation for a wide range of emission scenarios. Between the RCP4.5 and CE1 simulations, the differences in the permafrost degradation may be attributed to the spatial variations in surface air temperature, rainfall, and snowfall, which lead to the differences in the timing of permafrost degradation up to 40 years. Although atmospheric temperatures in CE1 and RCP4.5 simulations are similar, net primary production is higher in CE1 due to CO2 fertilization. Our investigation of permafrost extent under large‐scale SAI application scenarios suggests that circum‐Arctic permafrost area and extent is rather sensitive to temperature changes created under such SAI application. Our results highlight the importance of investigating the regional effects of climate engineering, particularly in high‐latitude ecosystems. Key Points: Stratospheric aerosol injection may help slow down the current rate of permafrost degradationRegional differences in temperature and precipitation led to differences in the timing of permafrost degradation up to 40 yearsIt is important to investigate the regional effects of climate engineering, particularly in high‐latitude ecosystems [ABSTRACT FROM AUTHOR]