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Changes in methane at the Last Glacial Maximum

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.

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Changes in methane at the Last Glacial Maximum

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

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

  4. Concentration of methane at the LGM • Bottom-up model studies suggest changes in methane sources can only account for • half the change in [CH4] [Chappellaz et al., 1993; Kaplan, 2002; Valdes et al., 2005] •  Could the oxidising capacity have changed sufficiently to account for the remainder? PI 700 ppbv B/A D-O8 ? YD 360 ppbv LGM

  5. 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 • HOX/NOX chemistry of CH4-C3H8 & C5H8 [Pöschl et al., 2000] • PI model run employing emissions of Valdes et al. [2005] • Variations on this to explore sensitivity of [CH4] to changes in: • NMVOC emissions from vegetation and/or physical conditions

  6. Concentration of methane at the LGM • AntBL = Antarctic boundary layer (all boxes in the lowest level of the model, south of 70°S) [CH4]AntBL (ppbv) 714 PI 360 LGM

  7. Concentration of methane at the LGM • Removing all NMVOC emissions from vegetation leads to a 22% reduction in [CH4] • NB It is estimated these emissions were 40-60% lower at the LGM [e.g. Valdes et al., 2005] [CH4]AntBL (ppbv) 714 PI 558 ENMVOCs=0 360 LGM

  8. Concentration of methane at the LGM • Employing LGM temperatures and humidities leads to an 18% increase in [CH4]; • the temperatures and humidities were taken from a simulation with HadAM3 [CH4]AntBL (ppbv) LGM T&H 840 714 PI 558 ENMVOCs=0 360 LGM

  9. 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 [CH4] [CH4]AntBL (ppbv) LGM T&H 840 714 PI 637 ENMVOCs=0 LGM T&H 558 ENMVOCs=0 360 LGM

  10. Concentration of methane at the LGM • Employing LGM NMVOC emissions, in addition to LGM temperatures and humidities, • leads to a 3% reduction in [CH4]; the emissions were simulated by Valdes et al. [2005] [CH4]AntBL (ppbv) LGM T&H 840 714 PI 690 LGM ENMVOCs LGM T&H 637 ENMVOCs=0 LGM T&H 558 ENMVOCs=0 360 LGM

  11. Concentration of methane at the LGM • Combined with the changes in methane sources, this is far from sufficient to explain • the change in [CH4], and this is before we include OH recycling and/or CO2 suppression [CH4]AntBL (ppbv) LGM T&H 840 714 PI 690 LGM ENMVOCs LGM T&H 637 ENMVOCs=0 LGM T&H 558 ENMVOCs=0 ? 360 LGM

  12. 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. 2. Isotopic composition of methane at the LGM (d13CH4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005]) (PI) B/A [Fischer et al., 2008] YD LGM 10kyr BP 15kyr BP 20kyr BP

  14. 2. Isotopic composition of methane at the LGM (d13CH4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005]) (PI) B/A [Fischer et al., 2008] YD +3.6‰ LGM 10kyr BP 15kyr BP 20kyr BP

  15. 2. Isotopic composition of methane at the LGM Fischer et al. [2008] attributed this enrichment to a shutdown of boreal wetland sources of 13C-poor CH4, accompanied by little or no change to biomass burning sources of 13C-rich CH4 NB Charcoal records show a reduction in biomass burning at the LGM [Power et al., 2008] (PI) B/A [Fischer et al., 2008] YD +3.6‰ LGM 10kyr BP 15kyr BP 20kyr BP

  16. 2. Isotopic composition of methane at the LGM But, Fischer et al. [2008] did not consider CH4-oxidation by ClMBL, which is presently responsible for an enrichment of 2.6‰, and could explain spatial and inter-annual variations in present-day d13CH4 [Allan et al., 2005, 2007]  If they had, would they have reached the same conclusions? (PI) B/A [Fischer et al., 2008] YD +3.6‰ LGM 10kyr BP 15kyr BP 20kyr BP

  17. 2. Isotopic composition of methane at the LGM • Very simple calculations to explore the sensitivity of [ClMBL], and hence d13CH4, • to changes in horizontal wind speeds at the sea surface • ClMBL 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 (aCl-1).FCl 0.026X‰ increase in d13CH4, as FCl « 1-FCl • X% increase in (aCl-1).kCl.[ClMBL] 0.026X‰ increase in d13CH4, provided FCl kCl.[ClMBL] • [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 (aCl-1).FCl 0.026X‰ increase in d13CH4, as FCl « 1-FCl • X% increase in (aCl-1).kCl.[ClMBL] 0.026X‰ increase in d13CH4, provided FCl kCl.[ClMBL] • [Saueressig et al., 1995] • [Sander et al., 2003] • Sea salt loading  uP [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 [ClMBL] (molecules cm-3); normalised to 1.8x104 molecules cm-3 globally PI LGM [ClMBL] is similarly distributed to u, owing to the wind-speed dependence we have invoked; we see qualitatively similar changes in [ClMBL], as in u, between the PI and the LGM

  22. 2. Isotopic composition of methane at the LGM Annual-mean (aCl-1).kCl.[ClMBL] (10-10 molecules-1 cm3 s-1) PI LGM (aCl-1).kCl.[ClMBL] is similarly distributed to [ClMBL] and u, though slightly modified by the temperature-dependence of kCl (which more than compensates for that of aCl)

  23. 2. Isotopic composition of methane at the LGM Percentage change in (aCl-1).kCl.[ClMBL] at the LGM Globally, (aCl-1).kCl.[ClMBL] increases by 7%  0.2‰ increase in d13CH4, 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 [ClMBL] at the LGM In our calculations, [ClMBL] 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 [ClMBL] at the LGM In our calculations, [ClMBL] 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 [ClMBL] at the LGM – take 2! When we artificially increase [ClMBL] in the Southern Ocean by 50-200%, by increasing u between 35 and 65°S by 25%, (aCl-1).kCl.[ClMBL] increases by 48% 1.3‰ increase in d13CH4

  27. 2. Isotopic composition of methane at the LGM Percentage change in [ClMBL] at the LGM – take 2! When we artificially increase [ClMBL] in the Southern Ocean by 50-200%, by increasing u between 35 and 65°S by 25%, (aCl-1).kCl.[ClMBL] increases by 48% 1.3‰ increase in d13CH4 - over a third of the increase observed

  28. 2. Isotopic composition of methane at the LGM Changes in the strength of the ClMBL sink have the potential to strongly influence d13CH4 An enrichment in d13CH4 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 ClMBL sink must be considered when interpreting the glacial-interglacial d13CH4 signal

  29. [Gong et al., 2002]

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