The mid-Holocene (~6000 years ago) is often described as a warm state, or the most recent “climate optimum”, in which dominant features of the global hydrological cycle, such as the North African and Asian monsoon, were amplified. As such, the mid-Holocene has been the focus for paleo-data syntheses since the beginning of the PMIP project and therefore are well documented in terms of temperature, hydrological cycle and land surface type. Some long standing model-data disagreement are echoing preoccupations for future climate change, such as the systematic underestimation of the northward penetration of the African monsoon rainfall onto the continent compared to available records for the Mid-Holocene.

The reference experiment to which the mid-Holocene is compared to is the pre-industrial control, which is part of the DECK.

For the mid-Holocene, our proposed experimental set-up for PMIP-CMIP6 is unchanged compared to the PMIP3-CMIP5 set-up:

The main forcing is the insolation , and the astronomical parameters must be set to:

eccentricity = 0.018682

obliquity = 24.105°

perihelion-180° = 0.87°

Date of vernal equinox = March 21 at noon

solar constant = same as in pre-industrial

CO₂ = 280 ppm

CH₄ = 650 ppb

N₂O = 270 ppb

CFC = 0

O₃ = same as in CMIP5 PI

Unchanged compared to pre-industrial

• Computed using a dynamical vegetation module,
• or prescribed as in PI, with phenology computed for models with active carbon cycle
• or prescribed from data

No change in ice sheet extent

Interactive, with atmospheric concentration prescribed and ocean and land carbon fluxes diagnosed as recommended in CMIP5

We invite the modelling groups planning to run simulations with an interactive dust cycle to comment on the most appropriate methodology for their model, in particular as to their ability to use an interactive vegetation description.

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