Last Deglaciation Greenhouse Gases

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Please use the Discussion section below to specifically comment on the choice of greenhouse gases for the core experiment.

The composite record (EPICA Dome C, Vostok and Taylor Dome) from Lüthi et al. (2008)^[1] provides CO₂ data spanning 800 ka to present. The data will be provided on the AICC2012 timescale of Veres et al. (2013)^[2].

Comparison of CO2 records and the PMIP3 LGM: ¹⁾
In particular, note how the new AICC2012^[2] age model affects the timing of the record and the 'LGM' (21 ka; vertical dashed line) CO₂ value; now 188 ppm compared to 185 ppm in PMIP3. There are also differences in timing and in the LGM value compared to the higher resolution Marcott et al. (2014)^[3] record.

We are proposing to use the composite record of Lüthi et al. (2008)^[1]; as per the magenta and red lines.

The EPICA Dome C record from Loulergue et al. (2008)^[4] provides CH₄ data also spanning 800 ka to present. The data will be provided on the AICC2012 timescale of Veres et al. (2013)^[2].

EPICA Dome C and the PMIP3 LGM ²⁾

Also note how the new AICC2012^[2] age model affects the 'LGM' (21 ka; vertical dashed line) CH₄ value.

The TALDICE record from Schilt et al. (2010)^[5] provides N₂O data spanning 140 ka to present. The data will be provided on the AICC2012 timescale of Veres et al. (2013)^[2].

TALDICE and the PMIP3 LGM ³⁾

The old PMIP3 'LGM' (21 ka; vertical dashed line) CH₄ value, probably remains representative of the glacial period for the model spin-up, although the 21 ka value for the transient run will be lower, as per the record above.

Please think about the following points and add any comments on these or any other aspects of the experiment design to the discussion section below: [Topics will be added here as they are raised below or by email.]

• RESOLVED With the new AICC2012^[2] age model, the LGM values for CO₂^[1] and CH₄^[4] are inaccurate, as can be seen from the plots above. Do we need a new PMIP LGM definition for these greenhouse gases in accordance with the records? DECISION (Oct 2015): We have used the new, up to date CO₂ (188 ppm) and CH₄ (375 ppb) values for our LGM equilibrium-type 21 ka spinup. We do not know what the LGM group's decision will be.
  
  
• RESOLVED N₂O^[1] values take a dive in the lead up to 21 ka. The PMIP3 value of 200 ppb looks fairly representative of the LGM state, even though this is a bit higher than the actual 21 ka value. We recommend that groups spin-up with 200 ppb N₂O, and begin the transient simulation from 21 ka in the record (as shown in the plot above). DECISION (Oct 2015): We have stuck with the old, more representative N₂O value of 200 ppb. However, the transient 'spinup' 26-21 ka and the transient run (21-9 ka) should use the chronologically accurate values, as plotted above. We do not know what the LGM group's decision will be.
  
  
• RESOLVED Should the core experiment use the Marcott et al. (2014)^[3] CO₂ records?
  For example, these newer data are higher resolution than Lüthi et al. (2008)^[1], but are restricted to 23-9 ka. Therefore more consistency may be be gained from using the older Lüthi et al. (2008)^[1] CO₂ records, if longer simulations are planned by groups. Also, CH₄ and N₂O data are not yet available in such high resolution. DECISION (Oct 2015): We have chosen to use the older data from Lüthi et al. (2008)^[1].
  
  

1. Lüthi, D. et al. High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature 453, 379–382 (2008).
2. Veres, D. et al. The Antarctic ice core chronology (AICC2012): an optimized multi-parameter and multi-site dating approach for the last 120 thousand years. Clim Past 9, 1733–1748 (2013). 10, 297–317 (1991).
3. Marcott, S. A. et al. Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514, 616–619 (2014).
4. Loulergue, L. et al. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature 453, 383–386 (2008).
5. Schilt, A. et al. Atmospheric nitrous oxide during the last 140,000 years. Earth Planet. Sci. Lett. 300, 33–43 (2010).
   
   

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