1. Changes in methane at the Last Glacial Maximum To what extent have changes in methane sinks influenced its concentration and isotopic composition in the past? J. G. Levine, E. W. Wolff, A. E. Jones, L. C. Sime, P. J. Valdes, G. D. Carver, N. J. Warwick, J. A. Pyle

2. 1.Concentration of methane at the LGM PI = Pre-industrial era (200yr before present) LGM = Last Glacial Maximum (21kyr before present) B/A YD D-O8 PI LGM

3. 1.Concentration of methane at the LGM PI = Pre-industrial era (200yr before present) LGM = Last Glacial Maximum (21kyr before present) B/A YD D-O8 PI 700 ppbv 360 ppbv LGM

4. 1.Concentration of methane at the LGM Bottom-up model studies suggest changes in methane sources can only account for half the change in [CH 4 ] [Chappellaz et al., 1993; Kaplan, 2002; Valdes et al., 2005]  Could the oxidising capacity have changed sufficiently to account for the remainder? B/A YD D-O8 PI 700 ppbv 360 ppbv LGM ?

5. 1.Concentration of methane at the LGM  Sensitivity experiments with the Cambridge p-TOMCAT CTM □ 3D global Eulerian model; 2.8° x 2.8° on 31 levels ≥10hPa □ HO X /NO X chemistry of CH 4 -C 3 H 8 & C 5 H 8 [Pöschl et al., 2000]  PI model run employing emissions of Valdes et al. [2005] □ Variations on this to explore sensitivity of [CH 4 ] to changes in: NMVOC emissions from vegetation and/or physical conditions

6. 1.Concentration of methane at the LGM AntBL = Antarctic boundary layer (all boxes in the lowest level of the model, south of 70°S) 714 360 PI LGM [CH 4 ] AntBL (ppbv)

7. 1.Concentration of methane at the LGM Removing all NMVOC emissions from vegetation leads to a 22% reduction in [CH 4 ] NB It is estimated these emissions were 40-60% lower at the LGM [e.g. Valdes et al., 2005] 714 360 PI LGM 558 E NMVOCs =0 [CH 4 ] AntBL (ppbv)

8. 1.Concentration of methane at the LGM Employing LGM temperatures and humidities leads to an 18% increase in [CH 4 ]; the temperatures and humidities were taken from a simulation with HadAM3 714 360 PI LGM 558 840 E NMVOCs =0 LGM T&H [CH 4 ] AntBL (ppbv)

9. 1.Concentration of methane at the LGM Combining these changes (removing all NMVOC emissions from vegetation and employing LGM temperatures and humidities) leads to an 11% reduction in [CH 4 ] 714 360 PI LGM 558 840 637 E NMVOCs =0 LGM T&H [CH 4 ] AntBL (ppbv)

10. 1.Concentration of methane at the LGM Employing LGM NMVOC emissions, in addition to LGM temperatures and humidities, leads to a 3% reduction in [CH 4 ]; the emissions were simulated by Valdes et al. [2005] LGM E NMVOCs LGM T&H 714 360 PI LGM 558 840 690 637 E NMVOCs =0 LGM T&H [CH 4 ] AntBL (ppbv)

11. 1.Concentration of methane at the LGM Combined with the changes in methane sources, this is far from sufficient to explain the change in [CH 4 ], and this is before we include OH recycling and/or CO 2 suppression LGM E NMVOCs LGM T&H 714 360 PI LGM 558 840 690 637 E NMVOCs =0 LGM T&H ? [CH 4 ] AntBL (ppbv)

12. 1.Concentration of methane at the LGM The change in oxidising capacity at the LGM, as a result of changes in temperature, humidity and NMVOC emissions from vegetation, had negligible influence on the concentration of methane It is likely we have underestimated the changes in methane sources between the LGM and the PI, and we should re-examine the sensitivity natural methane sources show to a warming climate

13. 10kyr BP 15kyr BP 20kyr BP [Fischer et al., 2008] LGM (PI) B/A YD 2. Isotopic composition of methane at the LGM  13 CH 4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005])

14. 2. Isotopic composition of methane at the LGM  13 CH 4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005]) 10kyr BP 15kyr BP 20kyr BP [Fischer et al., 2008] LGM (PI) +3.6‰ B/A YD

15. 2. Isotopic composition of methane at the LGM Fischer et al. [2008] attributed this enrichment to a shutdown of boreal wetland sources of 13 C-poor CH 4, accompanied by little or no change to biomass burning sources of 13 C-rich CH 4 NB Charcoal records show a reduction in biomass burning at the LGM [Power et al., 2008] 10kyr BP 15kyr BP 20kyr BP LGM [Fischer et al., 2008] (PI) +3.6‰ B/A YD

16. 2. Isotopic composition of methane at the LGM But, Fischer et al. [2008] did not consider CH 4 -oxidation by Cl MBL, which is presently responsible for an enrichment of 2.6‰, and could explain spatial and inter-annual variations in present-day  13 CH 4 [Allan et al., 2005, 2007]  If they had, would they have reached the same conclusions? 10kyr BP 15kyr BP 20kyr BP LGM [Fischer et al., 2008] (PI) +3.6‰ B/A YD

17. 2. Isotopic composition of methane at the LGM  Very simple calculations to explore the sensitivity of [Cl MBL ], and hence  13 CH 4, to changes in horizontal wind speeds at the sea surface □ Cl MBL comes mainly from sea salt aerosol, the production of which strongly depends on the wind speed [Monahan et al., 1986; Andreas, 1998] □ Paleodata, e.g. polar-ice records of dust [Fischer et al., 2007] and sea salt [e.g. Röthlisberger et al., 2002], may indicate changes in the circulation at the LGM

18. 2. Isotopic composition of methane at the LGM [Schaefer and Whiticar, 2008] [Allan et al., 2007] X% increase in (  Cl -1).F Cl   0.026X‰ increase in  13 CH 4, as F Cl « 1-F Cl X% increase in (  Cl -1).k Cl. [Cl MBL ]   0.026X‰ increase in  13 CH 4, provided F Cl  k Cl.[Cl MBL ] [Saueressig et al., 1995] [Sander et al., 2003] [Allan et al., 2001, 2007]

19. 2. Isotopic composition of methane at the LGM [Schaefer and Whiticar, 2008] [Allan et al., 2007] X% increase in (  Cl -1).F Cl   0.026X‰ increase in  13 CH 4, as F Cl « 1-F Cl X% increase in (  Cl -1).k Cl. [Cl MBL ]   0.026X‰ increase in  13 CH 4, provided F Cl  k Cl.[Cl MBL ] [Saueressig et al., 1995] [Sander et al., 2003] Sea salt loading  u P [Gong et al., 2002]

20. 2. Isotopic composition of methane at the LGM Annual-mean u (ms -1 ); simulated using HadAM3 PI LGM Global picture of u in the PI is dominated by the southern hemisphere westerlies (between 35 and 65°S); at the LGM, u increases in the North Pacific but shows only small changes in the Southern Ocean

21. 2. Isotopic composition of methane at the LGM Annual-mean [Cl MBL ] (molecules cm -3 ); normalised to 1.8x10 4 molecules cm -3 globally PI LGM [Cl MBL ] is similarly distributed to u, owing to the wind-speed dependence we have invoked; we see qualitatively similar changes in [Cl MBL ], as in u, between the PI and the LGM

22. 2. Isotopic composition of methane at the LGM Annual-mean (  Cl -1).k Cl.[Cl MBL ] (10 -10 molecules -1 cm 3 s -1 ) PI LGM (  Cl -1).k Cl.[Cl MBL ] is similarly distributed to [Cl MBL ] and u, though slightly modified by the temperature-dependence of k Cl (which more than compensates for that of  Cl )

23. 2. Isotopic composition of methane at the LGM Percentage change in (  Cl -1).k Cl.[Cl MBL ] at the LGM Globally, (  Cl -1).k Cl.[Cl MBL ] increases by 7%   0.2‰ increase in  13 CH 4, which is small compared to the 3.6‰ increase observed [Fischer et al., 2008] but..

24. 2. Isotopic composition of methane at the LGM Percentage change in [Cl MBL ] at the LGM In our calculations, [Cl MBL ] integrated over the whole of the Southern Ocean hardly changes, yet the Antarctic-ice record shows a 2-3 fold increase in sea salt concentration [Fischer et al., 2007]

25. 2. Isotopic composition of methane at the LGM Percentage change in [Cl MBL ] at the LGM In our calculations, [Cl MBL ] integrated over the whole of the Southern Ocean hardly changes, yet the Antarctic-ice record shows a 2-3 fold increase in sea salt concentration [Fischer et al., 2007]

26. 2. Isotopic composition of methane at the LGM Percentage change in [Cl MBL ] at the LGM – take 2! When we artificially increase [Cl MBL ] in the Southern Ocean by 50-200%, by increasing u between 35 and 65°S by 25%, (  Cl -1).k Cl.[Cl MBL ] increases by 48%   1.3‰ increase in  13 CH 4

27. 2. Isotopic composition of methane at the LGM Percentage change in [Cl MBL ] at the LGM – take 2! When we artificially increase [Cl MBL ] in the Southern Ocean by 50-200%, by increasing u between 35 and 65°S by 25%, (  Cl -1).k Cl.[Cl MBL ] increases by 48%   1.3‰ increase in  13 CH 4 - over a third of the increase observed

28. 2. Isotopic composition of methane at the LGM Changes in the strength of the Cl MBL sink have the potential to strongly influence  13 CH 4 An enrichment in  13 CH 4 at the LGM, as a result of a strengthening of this sink, would allow for a reduction in biomass burning consistent with charcoal records [Power et al., 2008] Further work is needed to constrain the cause of the 2-3 fold increase in sea salt concentration recorded in Antarctic ice: stronger winds, longer lifetime and/or an additional source? The Cl MBL sink must be considered when interpreting the glacial-interglacial  13 CH 4 signal

29. [Gong et al., 2002]

30.  winds P (u≤5ms -1 )P (u>5ms -1 )   13 CH 4 (‰) PI  LGM 1.393.41+0.2 PI  LGM 1.663.41+0.2 PI  LGM (SHW +25%) 1.393.41+1.2 PI  LGM (SHW +25%) 1.663.41+1.2