Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6)
V. Eyring, et al.
Observation and Context
Coordinated global climate model simulations are foundational elements of modern climate science, but past phases of the Coupled Model Intercomparison Project (CMIP) faced severe organizational pressures. Over its previous five phases, CMIP’s centralized structure grew increasingly complex. The project struggled to address an ever-expanding range of scientific questions from diverse research communities while managing a massive explosion in the variety and volume of requested model data. This punctuated, centralized schedule severely taxed the physical and technical computing infrastructure of global modeling centers and often distorted their internal, basic research and model development timelines.
Hypothesis
If the organization of CMIP is restructured into a federated design built around three core components — a small set of standardized, continuous baseline experiments (the “DECK”), common infrastructure and data-formatting standards, and a collection of specialized, peer-reviewed Model Intercomparison Projects (MIPs) — then the international modeling community can maintain long-term model continuity, reduce fragmented participation, decouple development cycles from rigid timelines, and systematically address the key grand scientific challenges of climate science.
Experiment and Methodology
To test this federated design, the CMIP Panel conducted a 2-year community consultation to construct the CMIP6 framework. Models seeking to participate in CMIP6 had to complete a standardized “entry card” consisting of the DECK and CMIP6 historical simulations to document their basic climate characteristics:
- The DECK Experiments: This suite consists of four baseline simulations: (1) a historical Atmospheric Model Intercomparison Project (AMIP) run using observed sea surface temperatures and sea ice from 1979 to 2014; (2) a multi-century pre-industrial control run (piControl) representative of 1850 conditions; (3) an abrupt quadrupling of CO₂ simulation (abrupt-4×CO2) to diagnose radiative forcing and climate sensitivity; and (4) a 1% per year CO₂ concentration increase simulation (1pctCO2) to serve as a transient climate response benchmark.
- CMIP6 Historical Simulations: These runs reconstruct historical climates from 1850 to 2014 using time-varying, observation-based natural and anthropogenic forcings (such as solar cycle variations, volcanic aerosols, and human-induced land-use changes).
- CMIP6-Endorsed MIPs: The panel evaluated and selected 21 autonomous, specialized MIPs (such as ScenarioMIP, AerChemMIP, and HighResMIP) based on strict criteria. To be endorsed, each MIP had to align with the World Climate Research Programme’s (WCRP) Grand Science Challenges, utilize CMIP standards, and secure commitments from at least eight modeling centers.
Results and Data
The federated restructuring successfully established a highly organized multi-model database for CMIP6, with model output expected to reach between 20 and 40 petabytes of data. Under the new protocol, the DECK experiments successfully provided a robust baseline that decoupled the model development cycle from specific CMIP scheduling, allowing modeling centers to finalize and submit results sooner on their own strategic timelines.
Common infrastructure standards were successfully enforced. The Earth System Grid Federation (ESGF) and the international ES-DOC activity managed data distribution using standardized metadata and common Controlled Vocabularies (CVs). Furthermore, community-based diagnostic tools, such as the ESMValTool and the PCMDI metrics software, were integrated directly alongside the ESGF. This setup allowed for more routine, rapid benchmarking of new model submissions against satellite and reanalysis observations (such as those from obs4MIPs and ana4MIPs).
Conclusion and Climate Impact
Restructuring CMIP6 into a federated architecture proved that global climate research can be successfully coordinated across decentralized groups. The DECK experiments successfully established a permanent, continuous baseline to track performance improvements across past and future model generations, while the 21 endorsed MIPs provided targeted analyses for highly specialized scientific questions.
The climate impact of the CMIP6 framework is profound. By standardizing and distributing multi-model outputs, CMIP6 provides the essential scientific foundation to answer critical questions regarding Earth’s response to radiative forcing, the origin of systematic model biases, and future climate predictability. Ultimately, these simulations directly inform major national and international assessments, including the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6), directly shaping global climate mitigation, adaptation, and vulnerability strategies.
Citation
Eyring, V., Bony, S., Meehl, G. A., Senior, C. A., Stevens, B., Stouffer, R. J., & Taylor, K. E. (2016). Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization. Geoscientific Model Development, 9(5), 1937-1958.