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Abstract:
The pre-industrial millennium is among the periods selected by the
Paleoclimate Model Intercomparison Project (PMIP) for experiments
contributing to the sixth phase of the Coupled Model Intercomparison
Project (CMIP6) and the fourth phase of the PMIP (PMIP4). The past1000
transient simulations serve to investigate the response to (mainly)
natural forcing under background conditions not too different from
today, and to discriminate between forced and internally generated
variability on interannual to centennial timescales. This paper
describes the motivation and the experimental set-ups for the
PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents
orbital, solar, volcanic, and land use/land cover changes, and
variations in greenhouse gas concentrations. The past1000 simulations
covering the pre-industrial millennium from 850 Common Era (CE) to 1849
CE have to be complemented by historical simulations (1850 to 2014 CE)
following the CMIP6 protocol. The external forcings for the past1000
experiments have been adapted to provide a seamless transition across
these time periods. Protocols for the past1000 simulations have been
divided into three tiers. A default forcing data set has been defined
for the Tier 1 (the CMIP6 past1000) experiment. However, the PMIP
community has maintained the flexibility to conduct coordinated
sensitivity experiments to explore uncertainty in forcing
reconstructions as well as parameter uncertainty in dedicated Tier 2
simulations. Additional experiments (Tier 3) are defined to foster
collaborative model experiments focusing on the early instrumental
period and to extend the temporal range and the scope of the
simulations. This paper outlines current and future research foci and
common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data).
