The PMIP4 contribution to CMIP6 – Part 4: Scientific objectives and experimental design of the PMIP4-CMIP6 Last Glacial Maximum experiments and PMIP4 sensitivity experiments

Abstract

Abstract. The Last Glacial Maximum (LGM, 21,000 years ago) is one of the suite of paleoclimate simulations included in the current phase of the Coupled Model Intercomparison Project (CMIP6). It is an interval when insolation was similar to present, but global ice volume was at a maximum, eustatic sea level was at or close to a minimum, greenhouse gas concentrations were lower, atmospheric aerosol loadings were higher than today, and vegetation and land-surface characteristics were different from today. The LGM has been a focus for the Paleoclimate Modelling Intercomparison Project (PMIP) since its inception, and thus many of the problems that might be associated with simulating such a radically different climate are well documented. The LGM state provides an ideal case study for evaluating climate model performance because the changes in forcing and temperature between the LGM and pre-industrial are of the same order of magnitude as those projected for the end of the 21st century. Thus, the CMIP6 LGM experiment could provide additional information that can be used to constrain estimates of climate sensitivity. The design of the Tier 1 LGM experiment (lgm) includes an assessment of uncertainties in boundary conditions, in particular through the use of different reconstructions of the ice sheets and of the change in dust forcing. Additional sensitivity experiments have been designed to quantify feedbacks associated with land-surface changes and aerosol loadings, and to isolate the role of individual forcings. Model analysis and evaluation will capitalise on the relative abundance of palaeoenvironmental observations and quantitative climate reconstructions already available for the LGM.

Implementation of the CMIP6 Forcing Data in the IPSL‐CM6A‐LR Model

“…The changes imposed for the last glacial maximum are very small in comparison and small compared to the ice sheet forcing. As expected, these figures realized from the output or the different simulations are similar to the one of the corresponding protocol papers (see Otto‐Bliesner et al, , Figure 3; Kageyama et al, , Figure 2), when the modern calendar is used to compute monthly values. Note however that the orbital parameters in piControl are set to 1990 and not 1850 as recommended in CMIP6 (Eyring et al, ).…”

“…In all experiments aiming at representing past equilibrium climate states, that is, for the mid‐Holocene ( midHolocene ), Last Interglacial ( lig127k ), mid‐Pliocene Warm Pariod ( midPliocene‐eoi400 ), and Last Glacial Maximum ( lgm ), long‐lived greenhouse gases are set to a constant value for the duration of the simulation, as recommended by the PMIP4‐CMIP6 protocols (Kageyama et al, , ; Haywood et al, ; Otto‐Bliesner et al, ). Those values are particularly low for the lgm experiment, representing a glacial climate, and CO 2 is significantly higher than the piControl value for the midPliocene‐eoi400 experiment, which represents a climate warmer than present (Figure ).…”

“…They mostly modulated the annual mean solar forcing between the equator and the poles and the seasonal distribution (Figure ). For each simulation we prescribed the orbital parameters as requested in the corresponding protocol (i.e., Haywood et al, , for midPliocene‐eoi400 ; Otto‐Bliesner et al, , for lig127k and midHolocene ; Kageyama et al, , for lgm ; and ; Jungclaus et al, , for past1000 ). The changes in the Earth's orbit are associated with different lengths of the seasons, or time spent between equinoxes and solstices (Berger & Loutre, , ; Joussaume & Braconnot, ).…”

“…For the lgm simulation, the North American and European ice sheets represent changes in altitude up to 3 km higher than the present configuration. Two reconstructions are being tested, from W. R. Peltier (ICE‐6G_C) and L. Tarasov (GLAC‐1D), following the PMIP4 protocol (Kageyama et al, ). In practice, we compute a topography at the same resolution as used to define the piControl altitude‐related parameters, that is, the mean altitude, and also the second moment characteristics needed for the orographic drag parameterization (Lott & Miller, ), by adding the reconstructed LGM—present anomaly to the present altitude.…”

Peatland area and carbon over the past 21 000 years – a global process based model investigation

“…A growing database of buried peat and knowledge emerging from the growing literature on anthroprogenically drained peatlands might shed more light on the fate of old peat carbon and inform future modeling studies. New timeslice and transient climate model simulations under PMIP4 (Ivanovic et al, 2016;Kageyama et al, 2017) together with an increased effort to fill in gaps in sample coverage both for today's peatlands and buried peat layers might help to constrain past peat dynamics further and to test the robustness of the results presented here. https://doi.org/10.5194/bg-2020-110 Preprint.…”