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The late Miocene palaeorecord provides evidence for a warmer and wetter climate than that of today, and there is uncertainty in the palaeo-CO2 record of at least 200 ppm. We present results from fully coupled atmosphere-ocean-vegetation simulations for the late Miocene that examine the relative roles of palaeogeography (topography and ice sheet geometry) and CO2 concentration in the determination of late Miocene climate through comprehensive terrestrial model-data comparisons. Assuming that these data accurately reflect the late Miocene climate, and that the late Miocene palaeogeographic reconstruction used in the model is robust, then results indicate that: 1. Both palaeogeography and atmospheric CO2 contribute to the proxy-derived precipitation differences between the late Miocene and modern reference climates. However these contributions exibit synergy and so do not add linearly. 2. The vast majority of the proxy-derived temperature differences between the late Miocene and modern reference climates can only be accounted for if we assume a palaeo-CO2 concentration towards the higher end of the range of estimates.

Orbital forcing is a key climate driver over multi-millennial timescales. In particular, monsoon systems are thought to be driven by orbital cyclicity, especially by precession. Here, we analyse the impact of orbital forcing on global climate with a particular focus on the North African monsoon, by carrying out an ensemble of 22 equally spaced (one every 1000 years) atmosphere-ocean-vegetation simulations using the HadCM3L model, covering one full late Miocene precession-driven insolation cycle with varying obliquity (between 6.568 and 6.589 Ma). The simulations only differ in their prescribed orbital parameters, which vary realistically for the selected time period. We have also carried out two modern-orbit control experiments, one with late Miocene and one with present-day palaeogeography, and two additional sensitivity experiments for the orbital extremes with varying CO2 forcing. Our results highlight the high sensitivity of the North African summer monsoon to orbital forcing, with strongly intensified precipitation during the precession minimum, leading to a northward penetration of vegetation up to ∼21° N. The modelled summer monsoon is also moderately sensitive to palaeogeography changes, but it has a low sensitivity to atmospheric CO2 concentration between 280 and 400 ppm. Our simulations allow us to explore the climatic response to orbital forcing not only for the precession extremes but also on sub-precessional timescales. We demonstrate the importance of including orbital variability in model-data comparison studies, because doing so partially reduces the mismatch between the late Miocene terrestrial proxy record and model results. Failure to include orbital variability could also lead to significant miscorrelations in temperature-based proxy reconstructions for this time period, because of the asynchronicity between maximum (minimum) surface air temperatures and minimum (maximum) precession in several areas around the globe. This is of particular relevance for the North African regions, which have previously been identified as optimal areas to target for late Miocene palaeodata acquisition.

Supporting Information forSimulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1) Table S1: New synthesis of global terrestrial MATs generated for the Middle Miocene. Table S2: Miocene SST reconstructions. Sheet 1, contains a basic description of published Miocene ocean surface temperature records. Sites are identified by their site numbers (Fig. 2), which are primarily from the Deep Sea Drilling Program, Ocean Drilling Program, or Integrated Ocean Discovery Program. The column on the far right contains hyperlinks to where the data for each site can be accessed electronically. Different colors in the proxy column are used to facilitate the ability to rapidly distinguish between records that utilize different SST proxies (Uk’37 = black, Mg/Ca = green, TEX86= purple). Note that all records are on their original age models using their original calibration. Sheet 2 lists the Late Miocene, Middle Miocene and MCO mean SST values and uncertainty bounds calculated from these records and used in the model-data comparison. Dataset S1: Updated Miocene boundary conditions used within the new CESM simulationsand described in Text S1. NetCDF file "miocene_topo_pollard_antscape_dolan_0.5x0.5.nc" contains theMiddle Miocene paleogeography and "surfdata_0096x0144_desert_BAM.nc" theland surface datasets. Dataset S2: "MioMIP1.nc" NetCDF file containing the MioMIP1 variables used to make all the figures.

Supporting Information for Simulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1) Table S1: New synthesis of global terrestrial MATs generated for the Middle Miocene. Table S2: Miocene SST reconstructions. Sheet 1, contains a basic description of published Miocene ocean surface temperature records. Sites are identified by their site numbers (Fig. 2), which are primarily from the Deep Sea Drilling Program, Ocean Drilling Program, or Integrated Ocean Discovery Program. The column on the far right contains hyperlinks to where the data for each site can be accessed electronically. Different colors in the proxy column are used to facilitate the ability to rapidly distinguish between records that utilize different SST proxies (Uk’37 = black, Mg/Ca = green, TEX86= purple). Note that all records are on their original age models using their original calibration. Sheet 2 lists the Late Miocene, Middle Miocene and MCO mean SST values and uncertainty bounds calculated from these records and used in the model-data comparison. Dataset S1: Updated Miocene boundary conditions used within the new CESM simulations and described in Text S1. NetCDF file "miocene_topo_pollard_antscape_dolan_0.5x0.5.nc" contains the Middle Miocene paleogeography and "surfdata_0096x0144_desert_BAM.nc" the land surface datasets. Dataset S2: "MioMIP1.nc" NetCDF file containing the MioMIP1 variables used to make all the figures.