Coordinated experimental design and implemen-
tation has become a cornerstone of global climate modelling.
Model Intercomparison Projects (MIPs) enable systematic
and robust analysis of results across many models, by reduc-
ing the influence of ad hoc differences in model set-up or ex-
perimental boundary conditions. As it enters its 6th phase,
the Coupled Model Intercomparison Project (CMIP6) has
grown significantly in scope with the design and documenta-
tion of individual simulations delegated to individual climate
science communities.
The Coupled Climate–Carbon Cycle Model Intercompar-
ison Project (C4MIP) takes responsibility for design, docu-
mentation, and analysis of carbon cycle feedbacks and in-
teractions in climate simulations. These feedbacks are poten-
tially large and play a leading-order contribution in determin-
ing the atmospheric composition in response to human emis-
sions of CO
2
and in the setting of emissions targets to sta-
bilize 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 experimen-
tal design, the simulations also fit within the wider CMIP ac-
tivity, conform to some common standards including docu-
mentation and diagnostic requests, and are designed to com-
plement the CMIP core experiments known as the Diagnos-
tic, 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 sim-
ulations with 1 % per year increases in CO
2
to enable di-
agnosis 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 be-
yond.
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 diag-
nostics. It is important that modelling groups participating
in C4MIP adhere as closely as possible to this experimental
design.