The Paris Agreement necessitates substantial and rapid emission mitigation across various climate forcers. However, climate system inertia and internal variability will delay the observable benefits of mitigation efforts, making it crucial to manage expectations and clearly communicate the timeframe for discernible impacts on global warming. Current climate change is a net result of anthropogenic emissions, resulting in a GMST increase of approximately 0.2°C per decade since the 1970s. Future projections indicate this trend will likely continue for several decades regardless of mitigation scenarios. To maintain public support for mitigation efforts, demonstrating tangible benefits is essential. While changes in atmospheric greenhouse gas concentrations may be readily observable, a reduction in the rate of surface warming compared to a baseline emission scenario serves as a key indicator of progress towards the Paris Agreement goals. The challenge lies in distinguishing the effects of mitigation from natural climate variability, a task complicated by the inherent uncertainties in comparing to a counterfactual, unmitigated scenario. Previous studies by Tebaldi and Friedlingstein (TF13) and Marotzke (M18) have highlighted the significant delays in detecting mitigation benefits due to climate inertia and variability. TF13 projected that emergence would occur roughly 25-30 years after a heavily mitigated emission pathway deviates from higher emission scenarios. M18 further emphasized the possibility of a 'hiatus debate in reverse,' where GMST continues to rise despite substantial mitigation efforts, posing challenges for communication and policy interactions. Both studies primarily focused on CO2 mitigation or simultaneous mitigation of various emissions. This research expands upon these studies by investigating the emergence time of detectable changes in GMST following mitigation of individual climate forcers, addressing the different costs and potential for separate mitigation of various greenhouse gases and aerosols.

Tebaldi and Friedlingstein (2013) quantified the delayed detection of climate mitigation benefits due to climate inertia and variability, finding emergence occurring approximately 25-30 years after a heavily mitigated emission pathway (RCP2.6) departs from higher emission pathways. Marotzke (2018) investigated near-term warming rates under strong mitigation (RCP2.6), finding that in over a third of simulations, warming would continue faster than in the preceding two decades, potentially leading to a 'hiatus debate in reverse'. Both studies primarily focused on CO2 mitigation or scenarios involving simultaneous mitigation of multiple forcers. This study builds on this work by analyzing the emergence time of a detectable signal for individual climate forcers, accounting for the potential for independent mitigation strategies and their varying costs.

This study combines reduced-complexity modeling (MAGICC6) with a large Earth System Model (CESM1 Large Ensemble) to simulate GMST evolution under various emission scenarios and account for internal variability. Emergence is defined as the time when the temperature difference between a mitigation scenario and a baseline (RCP4.5) becomes statistically significant (p<0.05) using a Student's t-test, considering at least 66% of the ensemble members. Three idealized mitigation scenarios are investigated for each climate forcer: zero emissions, a 5% annual emission reduction, and transitioning to RCP2.6 emissions in 2020. The analysis considers both cumulative temperature differences and changes in the rate of GMST warming. Consistency checks with trend-based approaches from previous studies (TF13 and M18) were performed. The CESM1 Large Ensemble provided 32 ensemble members to represent internal variability under RCP8.5. MAGICC6, a reduced complexity climate model, was used to simulate GMST under the various scenarios, employing a default configuration and equilibrium climate sensitivity of 3°C. The internal variability from the CESM1 LENS was added to the MAGICC6 outputs, creating 32 time series for each scenario and forcer combination. To avoid comparing identical evolutions, one time series was shifted by five years when comparing scenarios. The time of emergence was determined by the first year when at least 66% of ensemble members exhibited a statistically significant difference from the baseline. Additional analyses assessed the impact on the rate of GMST change over coming decades and explored the trade-off between mitigation potential and the emission mass required to achieve discernible effects.

The study's key finding is that for most anthropogenic climate forcers, including CO2, CH4, N2O, aerosols, and other gases, a significant change in GMST evolution will not be observable until several decades after implementing strong mitigation policies. Even fully removing anthropogenic emissions of short-lived warming forcers like black carbon (BC) might not yield a statistically significant impact for a decade. While combined mitigation of multiple components could hasten emergence, it would also involve offsetting warming and cooling effects. Analysis of RCP pathways showed that moving from RCP8.5 to RCP2.6 would only become visible around 2046. Idealized single-forcer mitigation scenarios revealed that CO2 mitigation is the most effective in the long term, but requires significant reductions for rapid discernible effects. BC mitigation offers the most rapid near-term discernible effect but has a low long-term payoff. Methane mitigation shows a relatively rapid near-term impact and substantial long-term benefits. Other gases and aerosols, even with complete emission removal, are unlikely to have discernible impacts before mid-century. Analysis of the rate of surface warming showed that even under strong single-component mitigation, there remains a chance that warming rates might not fall below current levels before mid-century, except for complete CO2 removal. The analysis also identified a trade-off between mitigation potential and the emission mass required for emergence. BC mitigation is most efficient in the short term, but CO2 mitigation, although requiring greater mass reduction, yields the highest long-term payoff. Methane is identified as a middle-ground, with relatively rapid emergence and notable avoided warming potential.

The findings confirm that despite the relative ease of calculating the temperature response to mitigation using emission metrics and simplified models, internal variability significantly impedes the rapid emergence of a detectable signal for plausible mitigation pathways. The results highlight the significant influence of internal variability and emphasize the limitations of using GMST alone as an indicator of mitigation success in the near term. Other factors, including model uncertainties, feedbacks, and geophysical processes, could further complicate detection. The study relies on year-to-year variability from CESM1 LENS under RCP8.5, acknowledging potential differences with other models and the model’s limitations in representing regionally resolved aerosol-cloud interactions. The reliance on MAGICC6, with its assumed equilibrium climate sensitivity, and its default parameterization is also a limitation, although sensitivity analyses suggest that the overall conclusions are not significantly altered. The study’s idealized scenarios, starting in 2020, and the use of RCP4.5 baseline might not precisely reflect real-world emissions. However, these factors are not expected to alter the overarching conclusions. While individual component emergence times aren't directly additive, the underlying framework could readily be extended to study multi-component mitigation and sector-wide policies. Sensitivity tests indicate near additivity, but realistic scenarios require comprehensive Earth system models to capture biogeophysical interactions and regional patterns. The study underscores the need for clear communication of expected timelines for observable impacts of mitigation efforts, highlighting the importance of using other indicators such as emissions and concentrations of greenhouse gases, along with economic factors, to track progress toward climate goals.

Even strong mitigation efforts may not yield readily observable impacts on global mean surface temperature for several decades due to climate system inertia and internal variability. CO2, CH4, and BC mitigation offer the most potential for early, discernible impacts, but even these changes may not be visible before mid-century. Other forcers, such as NOx and organic carbon, will likely show limited effects until the second half of the century. Multi-component mitigation strategies show potential for earlier detectability. This research emphasizes the necessity of clear communication to policymakers and the public about the expected timelines for observable impacts to avoid hindering climate mitigation efforts. In the near term, other indicators of progress towards the Paris Agreement, such as greenhouse gas concentrations and emissions, remain crucial.

The study's reliance on a specific Earth System Model (CESM1 LENS) and a reduced-complexity climate model (MAGICC6) introduces model-specific uncertainties. The idealized mitigation scenarios, while useful for exploring fundamental dynamics, may not perfectly reflect real-world emission pathways and policy interventions. The analysis focuses primarily on global mean temperature changes and might not capture the full spectrum of regional variations in climate response to mitigation. The assumed equilibrium climate sensitivity also introduces uncertainty, affecting, particularly, the analysis of warming rates.