On the morning of Tuesday 10th March 2026 at CMIP26, held at the International Conference Centre in Kyoto, Japan, session PLE1.1: CMIP7 Frontiers: Forcings, Datasets and Model Responses explored the inputs to Earth System Models (ESMs) and their responses, including under future emissions scenarios.
Because Integrated Assessment Models (IAMs) – used to project future emissions under certain assumptions – aren’t designed to exactly follow from the historical emissions data, a process of harmonisation must be undertaken to ensure consistency across historical and future simulations. This session began with Jarmo Kikstra (IIASA) presenting the CMIP7 version of this complex – and time-consuming – process, which is nearly complete across scenarios, allowing ESM teams to begin their dependent experiments. In CMIP7, the future scenarios begin in 2025, with their trajectories differing from those in CMIP6 due to the observed emissions trends since that generation was produced. Emissions of CO2 in the intervening period have approximately tracked the medium-emissions SSP245 scenario, with sulfur emissions slightly lower. On the high end, this trend has caused the highest emissions scenarios to be lower than their equivalent versions in the previous generation, with trajectories approximately flat as the world has seemingly avoided the worst-case emissions scenarios. On the lower end, however, the strongest mitigation scenarios consistent with the lower Paris agreement goals are steeper than their prior counterparts, due to the accumulated emissions since 2015.
Moving to the model response to these emissions, Shiv Priyam Raghuraman (University of Illinois) pointed out that the Instantaneous Radiative Forcing (IRF) due to changes in CO2 should be similar across ESMs, with the spread in Effective Radiative Forcings (ERFs) therefore much larger and overwhelmingly caused by feedback differences. However, around half the ERF uncertainty is actually due to variations in IRF, which is inconsistent with the much higher level of certainty around these instantaneous effects. ESM forcing parameterisations should therefore be investigated; though it should be noted that non-CO2 forcings play a significant role.
The varied and complex effect of these non-CO2 factors was the focus of the other talks in the session. Firstly, Robert Allan (UC Riverside) presented the Regional Aerosol MIP (RAMIP), which alters regional aerosol emissions between scenarios to study their local and remote effects under emissions scenarios. The first round of RAMIP, using CMIP6 models and scenarios, has been used to suggest a key role of China’s emissions reductions in recent warming trends, with many other studies underway. Round 1 data should be available shortly on ESGF, and the team are preparing a plan for round 2 for CMIP7.
Glen Chua (NASA GISS) discussed the relative roles of climate and chemistry on the atmospheric CH4 concentration, in the context of its recent rapid increase. A key determinant of CH4 lifetime is the OH radical, which reacts with CH4, removing it from the atmosphere. Since this is the main CH4 sink, rising CH4 emissions deplete OH, in turn increasing the CH4 lifetime – a self-driven feedback effect. OH production and its reactions are also temperature-dependent, indicating a role for climate feedbacks. Using CMIP6 scenario-based experiments in AerChemMIP, these complex effects can be isolated; but key processes are missing or under-represented in CMIP6 models, such as fires, lightning, and wetland emissions. Addressing these issues is key to understanding the changing role of methane in the earth system, and CMIP7 is well-placed to make advances on this front.
These climate-methane feedbacks were the subject of the final talk, as Takashi Sekiya (Japan Agency for Marine-Earth Science and Technology) presented analysis of them in the MIROC-ES2L-CHEM ESM. Positive feedbacks from terrestrial emissions (e.g. a warming climate causing increased wetland emissions) compete with negative ones from atmospheric chemistry due to changes in OH. Overall, anthropogenic methane emissions still dominate the cycle, with positive feedbacks important in high-emissions scenarios. This emphasises the need to look at the methane cycle across modelling scales in a consistent framework.
Overall, ESM forcings generate complex changes in earth system processes, with significant inter-model uncertainty. The CMIP scenario and experiment framework is ideally placed to investigate these effects, with extensive analyses undertaken in the CMIP6 era, and the imminent CMIP7 scenarios set to help explore these further.
