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 the 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 (Tier 2) 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 capitalize on the relative abundance of paleoenvironmental observations and quantitative climate reconstructions already available for the LGM.
Highlights
The Last Glacial Maximum (LGM), dated ∼ 21 000 years BP, is the last period during which the global ice volume was at its maximum, and eustatic sea level at or near to its minimum, ∼ 115 to 130 m below the present sea level (Lambeck et al, 2014; Peltier and Fairbanks, 2006)
This paper provides guidelines on the implementation of the PMIP4 LGM experiment in the CMIP6 climate models
By increasing the number of simulations available, including important regional forcings, and focusing on uncertainties in these forcings, the LGM PMIP4-CMIP6 experiments will provide a much better data set to re-examine climate sensitivity
Summary
The Last Glacial Maximum (LGM), dated ∼ 21 000 years BP, is the last period during which the global ice volume was at its maximum, and eustatic sea level at or near to its minimum, ∼ 115 to 130 m below the present sea level (Lambeck et al, 2014; Peltier and Fairbanks, 2006). Bereiter et al, 2015, for CO2; Loulergue et al, 2008, for CH4) Both the change in ice sheets and in GHG concentrations are negative radiative forcings and contribute, with impacts of similar orders of magnitude, to a climate much colder than today – the inclusion of dust forcing, either by using models in which the dust cycle is interactive or by prescribing atmospheric dust concentrations, so as to consider the interactions between dust and radiation This is expected to cause significant differences in simulated regional climates and to have impacts on ocean biogeochemistry through a more realistic representation of dust input at the ocean surface; and. Kageyama et al (2016) provide an overview of the PMIP4-CMIP6 project
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