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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.
The lgm experimental set-up foreseen for PMIP-CMIP5 is as follows (cf. PMIP3 web site, with some modifications):
Insolation (quite similar to pre-industrial)
eccentricity = 0.018994
obliquity = 22.949°
perihelion-180° = 114.42°
solar constant: same as PI
Concentration of atmospheric trace gases:
CO2 = 185 ppm
CH4 = 350 ppb
N2O = 200 ppb
CFC = 0
O3 = same as in CMIP5 PI
Topography/bathymetry/coastlines/ice sheets :
This is one of the major changes compared to pre-industrial. Imposing the lgm ice sheets means imposing new coastlines, new topography/bathymatry and new ice sheet extent.
For those modelling groups running the lgm experiment but not the deglaciation, we recommend to use the PMIP3 ice sheets/coastlines, for simulations for PMIP3-CMIP5 and CMIP6 to be strictly comparable.
For those 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.
Vegetation and land surface:
Computed using a dynamical vegetation module,
Or prescribed as in PI, with phenology computed for models with active carbon cycle or prescribed from data
Interactive, with atmospheric concentration prescribed and ocean and land carbon fluxes diagnosed as recommended in CMIP5
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.
A note on the fresh water budget
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.
Glocal ocean salinity Add 1PSU