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The last glacial maximum (lgm, ~21000 years ago) is the last global cold extreme in which greenhouse gas concentrations were at their minimum and continental ice-sheet at their maximum size, covering large areas of northern North America and northwestern Eurasia. Like the mid-Holocene, the lgm has been a key PMIP experiment since PMIP started, and as such has been the focus for paleo-data syntheses.
The reference experiment to which the lgm is compared to is the pre-industrial control, which is part of the DECK.
For the LGM, our proposed experimental set-up for PMIP-CMIP6 is based on the PMIP3-CMIP5 set-up:
eccentricity = 0.018994
obliquity = 22.949°
perihelion-180° = 114.42°
Date of vernal equinox = March 21 at noon
solar constant = same as PI
CO2 = 185 ppm
CH4 = 350 ppb
N2O = 200 ppb
CFC = 0
O3 = same as in CMIP5 PI
This is one of the major changes compared to pre-industrial. Prescribing the lgm ice sheets means implementing new coastlines, a new topography/bathymetry and a new ice sheet extent.
For the modelling groups running the lgm experiment but not the deglaciation, we recommend using the PMIP3 ice sheets/coastlines, in order for simulations for PMIP3-CMIP5 and CMIP6 to be strictly comparable.
For the modelling groups who plan to run the deglaciation experiment (cf PMIP3 deglaciation working group), the new boundary conditions provided by Dick Peltier and Lev Tarasov can be used as well.
Interactive, with atmospheric concentration prescribed and ocean and land carbon fluxes diagnosed as recommended in CMIP5.
For PCMIP: fully interactive with atmospheric concentration computed by the model.
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 a interactive vegetation description.
Modelling groups are advised to carefully check the fresh water budget in their lgm experiments in order to avoid unnecessary drifts of the ocean salinity. It can be necessary to route the snow which has fallen in excess on the ice sheets to the ocean. Given the change in coastlines, it is also sometimes necessary relocate the large rivers' estuaries on the coast.