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Abstract

Coordinated experimental design and implementation has become a cornerstone of global climate modelling.
Model Intercomparison Projects (MIPs) enable systematic
and robust analysis of results across many models, by reducing
the influence of ad hoc differences in model set-up or experimental
boundary conditions. As it enters its 6th phase,
the Coupled Model Intercomparison Project (CMIP6) has
grown significantly in scope with the design and documentation
of individual simulations delegated to individual climate
science communities.
The Coupled Climate–Carbon Cycle Model Intercomparison
Project (C4MIP) takes responsibility for design, documentation,
and analysis of carbon cycle feedbacks and interactions
in climate simulations. These feedbacks are potentially
large and play a leading-order contribution in determining
the atmospheric composition in response to human emissions
of CO2 and in the setting of emissions targets to stabilize
climate or avoid dangerous climate change. For over
a decade, C4MIP has coordinated coupled climate–carbon
cycle simulations, and in this paper we describe the C4MIP
simulations that will be formally part of CMIP6. While the
climate–carbon cycle community has created this experimental
design, the simulations also fit within the wider CMIP activity,
conform to some common standards including documentation
and diagnostic requests, and are designed to complement
the CMIP core experiments known as the Diagnostic,
Evaluation and Characterization of Klima (DECK).
C4MIP has three key strands of scientific motivation and
the requested simulations are designed to satisfy their needs:
(1) pre-industrial and historical simulations (formally part
of the common set of CMIP6 experiments) to enable model
evaluation, (2) idealized coupled and partially coupled simulations
with 1% per year increases in CO2 to enable diagnosis
of feedback strength and its components, (3) future scenario simulations to project how the Earth system will respond to anthropogenic activity over the 21st century and beyond.
This paper documents in detail these simulations, explains
their rationale and planned analysis, and describes how to
set up and run the simulations. Particular attention is paid to
boundary conditions, input data, and requested output diagnostics.
It is important that modelling groups participating in C4MIP adhere as closely as possible to this experimental design.