the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Nov 2025
FRIDA-Clim v1.0.0: a Simple Climate Model with Process-Based Carbon Cycle used in the FRIDAv2.1 IAM
Abstract. The new global Feedback-based knowledge Repository for IntegrateD Assessments version 2.1 (FRIDAv2.1) Integrated Assessment Model (IAM) seeks to study the dynamics of the coupled human-Earth system. Connecting anthropogenic emissions to the resultant climate response is one part of this two-way feedback. This paper documents the Climate Module within FRIDAv2.1, of which a modified version is separately simulated as a standalone simple climate model termed FRIDA-Clim version 1.0. This approach, based loosely on the existing FaIR simple climate model, simulates the key radiative forcings and the resultant temperature response, with process-based representations of the carbon cycle across the ocean, land, and atmosphere. When connected within the FRIDA IAM, it features deep connections to the other modules, being affected by processes such as water use for irrigation and land use change. In both uses, coupled and uncoupled, its climate drivers are simplified as compared to FaIR, to allow for this reduced set of key drivers to be interactively simulated within FRIDA, tightly coupling the evolution of the social and climate systems within the full model. Both the Climate Module and FRIDA-Clim are fully calibrated to accurately reproduce observations of key climate variables, with a systematic exploration of the uncertainty in the climate response. Together with the rest of the FRIDA model, this module is used to incorporate climate change systematically in the FRIDA System Dynamics IAM. As a standalone climate model, FRIDA-Clim comprises a simple climate model, enabling fast calculation of the global climate response to forcing; to explore this, the response of the model to both idealised CO2 emissions experiments and plausible future scenarios is also presented here. This setup will allow FRIDA-Clim to contribute to inter-model simple climate modelling initiatives, helping to explore the structural uncertainty in this modelling domain.
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Status: open (until 30 Dec 2025)
- RC1: 'Comment on egusphere-2025-4766', Anonymous Referee #1, 09 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-4766', Anonymous Referee #2, 12 Dec 2025
reply
This manuscript introduces FRIDA-Clim v1.0.0, a Simple Climate Model (SCM) designed for use both as a standalone tool and as the integrated Climate Module within the FRIDAv2.1 Integrated Assessment Model (IAM). The model differentiates itself from standard impulse response SCMs (such as FaIR) by incorporating a process-based carbon cycle for the ocean and land. The authors detail the model structure, the coupling between human and natural systems, and a rigorous two-stage calibration process involving a 30,000-member prior ensemble constrained against historical observations. The model is evaluated against idealized CO2 experiments (flat10MIP) and standard SSP scenarios, demonstrating general consistency with observational records but notable divergence from the FaIR model in high-emission future scenarios.
I suggest the following issues be addressed by authors before publication.
1- The model simulates emissions of NOx, VOCs, CO, and Black Carbon via linear regression against other drivers (such as N2O and SO2). While historically valid, these correlations are structural weaknesses in an IAM context, as future policies may decouple these relationships. The authors must perform a sensitivity analysis to test the model's robustness in scenarios where these historical correlations break down.
2- Unexplained Temperature Divergence from FaIR In high-emission scenarios (SSP5-8.5), FRIDA-Clim projects long-term temperatures roughly 0.5 K lower than the FaIR model. The authors attribute this to a “more responsive” process-based carbon cycle, but lack a mechanistic explanation. The authors should isolate whether this cooling is driven by the carbon cycle or the EBM parameterization to validate the model's physical plausibility.
3- In Section 2.4.3, the definition of "FLU emissions" includes the realized soil carbon changes from land use transitions. Please clarify if this accounts for the committed emissions instantly or if it tracks the legacy flux over time. The distinction is important for annual budget accounting in the IAM.
Minor note: The use of the "IAM" abbreviation in the title may be obscure to readers outside the immediate integrated assessment community. I suggest spelling it out to improve accessibility and discoverability.
Citation: https://doi.org/10.5194/egusphere-2025-4766-RC2
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- 1
1. General comments
The authors present relevant extensions to the FRIDA framework. The climate impact of Green House Gases such as CO2, CH4, N2O, aerosols, stratospheric water vapor can be simulated, and together with the FRIDA IAM, the dynamics of the coupled human-Earth system can be modeled. Idealized CO2 emission experiments are performed and detailed calibration routines and explained in this work. The paper is very comprehensive and detailed which probably is due to the complexity of the involved models and quantity of implemented functionalities.
The authors try to describe the uniqueness of FRIDA-Clim, however, these arguments should be further elaborated: In which key aspects differs FRIDA-Clim from other/previous models? What are the unique selling points? Furthermore, the philosophy of the model should be better motivated: Why do the authors choose a "minimal required climate" approach?
The authors try to explain the interplay and interfaces of the different modules. A major issue is that the used terminology is not always well defined nor used within the text. Important key words such as external, internal, endogenous, coupled, uncoupled, integrated should be well defined and not mixed within the text. This would better describe the boundaries and interfaces of the different modules and the reader would better understand which component is meant.
2. Specific comments
### Title
- Use of abbreviation "IAM" in the title not recommended, better write the full wording "Integrated Assessment Model".
- In the beginning, it is not clear to the reader, that these are two software modules: FRIDA-Clim and FRIDA IAM. This is later explained in the abstract, but the title should be more self-explanatory, otherwise it is confusing to the reader having two version numbers.
- Maybe a different terminology would help, such as FRIDA framework or FRIDA "core" for the overall framework, and FRIDA-Clim as extension/module to that framework?
### Abstract
- What is the other part of the *two-way feedback*?
- Add definition of *coupled* and *uncoupled* . Otherwise, it is unclear to the reader which software module couples to what other module.
### Introduction
- Both FRIDA-Clim and the integrated Climate Module are documented here
- What it meant by the *integrated* Climate Module? Is it related to *coupled* mode as stated in the abstract?
### Model description
- In the text, the terms *external*, *internal*, *endogenous* should be better explained. How do these modes relate to coupled/uncoupled/integrated ?
- Philosophy should be better motivated
minimum level of detail in the climate system required in order to adequately reproduce historical and future expected global climate dynamics should be represented.
- Why should climate functionality minimized? e.g. Model performance reasons?
- Table 1 give a very good overview of anthropogenic climate drivers
- Section 2.2 Effective Radiative Forcing: Motivate why ERF concept is used instead of RF concept.
- Section 2.2.4 Aerosols: FRIDA or FRIDA IAM?
- Section 2.2.5 Ozone: Is FRIDA and FRIDA-Clim focusing on effects within troposphere/stratosphere or both? Which of the anthropocentric emissions have the largest contribution to ozone increase or decrease?
- Section 2.2.9 No reference given for stated approach of linear relation between CH4 and stratospheric water forcing
- Section 2.2.11 Volcanic: Which are the concrete emissions linked to volcanic activity driving the resulting forcing?
- Section 2.4: between FRIDA-Clim and the Climate Module
- Are these different modules? To my understanding, FRIDA-Clim is the climate module. Do you mean the "core" climate module of FRIDA IAM?
- Section 2.4.1 Units GtC Mha-1 yr-1
- Hectar is a non-SI unit. Unless this a very common unit in this research field or context, I would suggest to convert into SI-units, e.g. km² or m²
- Section 2.4.2 Soil Carbon, equation (9) mix of units °C and K in same equation should be avoided!
### Calibration and Initialization
Section 3.1 FRIDA-Clim calibration
- Missing motivation for relevance of ensemble sizes, members. see also given numbers in Figure 4. What does it mean to have an ensemble size of 30,000? Is this a large or small number? How does this number compare to?
### Experiments using FRIDA-Clim
- Section 4.1 Idealized CO2 Experiments
- Figure 5. panels i,j,k,l missing description/units of y-axis
- Better explain and motivate used metrics ZEC, TNZ, T0 etc. Consider putting definitions of and motivations for used metrics into a table for a better overview.
- Last paragraph concludes: *FRIDA-Clim thus simulates approximate linearity in the land carbon response under idealised removal, with hysteresis in the ocean sink and consequent over-compensation in the atmospheric response.... Generally, these results are consistent with the findings across flat10MIP (Sanderson et al., 2025)*
- Explain better in the individual graphs, how authors came to these conclusions.
3. Technical corrections
- Consider introducing abbreviations of other models as well: FaIR, DICE, LPJmL, LOSCAR, iLOSCAR, CICERO, ...
- Introduce abbreviation AFOLU
- Section 2.2.7 Introduce abbreviation GHG
- Section 2.3: Introduce abbreviation GMST
- Figure 1 gives a very good overview of the dependencies of the different software modules. However, the chosen colors of the lines are not ideal for people with red-green deficit. Consider use of different line styles, e.g. continuous and dashed. Abbreviation *EBM* in the figure caption is not introduced.
- Figure 2: revise colors of lines, especially orange/red-green lines
- Figure 3: revise orange/red - green colors. Colors might be hard to distinguish for some people. The arrows under the "fast soil" and "slow soil" boxes are confusing. Is there a way to summarize/aggregate/abstract the exchanges between the different components for a clearer visual representation?
- What is meant by (t-PW; k) Is this a reference to one of the figure panels? Then, it is better to explicitly write "see Fig. 5, panel (k) ...", same comment applies to similar used abbreviations in this section.