C4MIP: Understanding Climate-Carbon Feedbacks
How does the carbon cycle respond to global warming? The Coupled Climate-Carbon cycle Model Intercomparison Project (C4MIP) tackles this challenge by coordinating multiple Earth System Models to quantify carbon-climate feedbacks, carbon budgets, and assess what happens after reaching net-zero emissions.
An emerging focus is the Zero Emission Commitment (ZEC) — the remaining temperature change that continues even after CO₂ emissions stop. A recent multi-model study led by Laura Gibbs used emission-driven simulations, where models directly simulate how atmospheric CO₂ changes from anthropogenic emissions, and found that ZEC tends to increase with global warming. By decomposing ZEC into constant concentration warming and carbon sink cooling, the study shows how these competing effects shape long-term temperature responses, with important implications for interpreting what “net-zero” means.
Carbon Isotope Forcing Data for CMIP7
The carbon isotopic composition of atmospheric CO₂ is crucial for constraining carbon cycle processes in C4MIP and Ocean Multi-model Intercomparison Project (OMIP) simulations. The development of the CMIP7 carbon isotope forcing dataset, led by Heather Graven, provides historical and future atmospheric fields for both δ¹³CO₂ and Δ14CO₂. The dataset integrates ice-core data, firn measurements, and direct atmospheric observations, extending the historical period from 1700 to 2023.
A key methodological improvement in CMIP7 is the treatment of spatial patterns. While CMIP6 used three zonal bands for Δ14CO₂, CMIP7 uses four bands with a split at the equator to better represent regional variations. Using historical Δ14CO₂ carbon isotope, Graven et al. constrained global fossil fuel emissions from 2015 to 2023 to 80-92 PgC (±7%). Beyond constraining emissions, these isotopes also provide insights on water use efficiency, land carbon turnover, and ocean circulation patterns. The CMIP7 carbon isotope forcing dataset is now available via input4MIP.
Understanding climate effects of wildfire aerosols
Wildfire emissions have significant climate impacts, but quantifying such effects remains challenging. A study led by Rafaila-Nikola Mourgela estimated effective radiative forcing from wildfire emissions using three wildfire datasets and three Earth System Models, comparing pre-industrial and present-day wildfire emissions. While global effective radiative forcing differences were small, regional variations were pronounced, highlighting the importance of spatial variability in wildfire impacts.
Accurately representing these effects is complicated by limitations in biomass burning emission inventories. Major global inventories often underestimate aerosol emissions, introducing biases into climate models. Tom Eames highlighted that some regions are prone to under-detected fires and may exhibit substantial variability in combustion efficiency, which is often assumed constant in emission inventories. This is particularly evident in tropical and temperate savannas, where small, hard-to-detect fires contribute to greater uncertainty in emission estimates.
To address these gaps, the Fire Multi-model Intercomparison Project (FireMIP) coordinates the analysis of CMIP7 historical and future simulations incorporating fire emissions. This effort aims to better constrain uncertainties and improve the representation of fire-related processes in climate models.
Bringing It Together
With CMIP7 on the horizon, advances in model development, observational constraints, and process understanding can be evaluated in an integrated framework. Multi-model initiatives such as C4MIP and FireMIP provide a structure to assess how these developments influence simulations and support coordinated analysis across Earth System Models. CMIP7 offers the opportunity to consolidate these efforts, enabling more robust climate simulations and supporting the next generation of climate research.
