~~DISCUSSION~~

====== PMIP-CMIP6 Experimental Design Discussion: Last Glacial Maximum ======


<note warning>The PMIP4 experimental design available on the PMIP3 wiki has been **superseded** by the experimental design detailed on the [[https://pmip4.lsce.ipsl.fr/doku.php/exp_design|PMIP4 web site]]</note>

<note tip>\\ Please submit your comments in the Discussion section at the end of this page</note>

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:design:21k:final|PMIP3-CMIP5 set-up]]:

===== Orbital parameters =====

**eccentricity** = 0.018994 

**obliquity** = 22.949° 

**perihelion-180°** = 114.42°

**Date of vernal equinox** = March 21 at noon 

**solar constant** = same as PI


===== Concentration of atmospheric trace gases =====

**CO<sub>2</sub>** = 185 ppm

**CH<sub>4</sub>** = 350 ppb

**N<sub>2</sub>O** = 200 ppb

**CFC** = 0

**O<sub>3</sub>** = same as in CMIP5 PI 


===== Topography/bathymetry/coastlines/ice sheets =====

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.

  * Modelling groups planning to run the lgm experiment but **not the deglaciation** are advised to use the [[pmip3:design:21k:final#ice-sheets_and_related_changes|PMIP3 ice sheets/coastlines]], so that PMIP3-CMIP5 and CMIP6 can be rigorously compared.

  * Modelling groups planning to **run lgm as the start of the the deglaciation experiment** (cf [[pmip3:wg:degla:index|PMIP3 deglaciation working group]]) can use the new boundary conditions provided by Dick Peltier and Lev Tarasov.


===== 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


===== Carbon cycle =====

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.


===== Dust cycle =====

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.


===== 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.


===== Global ocean salinity =====

Add **1 PSU**.


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