Changes in methane at the Last Glacial Maximum To what extent have changes in methane sinks influenced its concentration and isotopic composition in the.
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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
1.Concentration of methane at the LGM
PI = Pre-industrial era (200yr before present)
LGM = Last Glacial Maximum (21kyr before present)
Composite CH4 measurements from GRIP and NGRIP(EPICA Community Members 2006, Nature 444, pp 195-198)
300
400
500
600
700
800
0 10,000 20,000 30,000 40,000
Age (GICC05 yrs BP)
[CH
4] (
pp
bv
)
B/A
YD
D-O8
PI
LGM
1.Concentration of methane at the LGM
PI = Pre-industrial era (200yr before present)
LGM = Last Glacial Maximum (21kyr before present)
Composite CH4 measurements from GRIP and NGRIP(EPICA Community Members 2006, Nature 444, pp 195-198)
300
400
500
600
700
800
0 10,000 20,000 30,000 40,000
Age (GICC05 yrs BP)
[CH
4] (
pp
bv
)
B/A
YD
D-O8
PI 700 ppbv
360 ppbv LGM
1.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?
Composite CH4 measurements from GRIP and NGRIP(EPICA Community Members 2006, Nature 444, pp 195-198)
300
400
500
600
700
800
0 10,000 20,000 30,000 40,000
Age (GICC05 yrs BP)
[CH
4] (
pp
bv
)
B/A
YD
D-O8
PI 700 ppbv
360 ppbv LGM
?
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
□ 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
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
[CH4]AntBL (ppbv)
1.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]
714
360
PI
LGM
558ENMVOCs=0
[CH4]AntBL (ppbv)
1.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
714
360
PI
LGM
558
840
ENMVOCs=0
LGM T&H
[CH4]AntBL (ppbv)
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 [CH4]
714
360
PI
LGM
558
840
637
ENMVOCs=0
ENMVOCs=0LGM T&H
LGM T&H
[CH4]AntBL (ppbv)
1.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]
LGM ENMVOCs
LGM T&H
714
360
PI
LGM
558
840
690637
ENMVOCs=0
ENMVOCs=0LGM T&H
LGM T&H
[CH4]AntBL (ppbv)
1.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
LGM ENMVOCs
LGM T&H
714
360
PI
LGM
558
840
690637
ENMVOCs=0
ENMVOCs=0LGM T&H
LGM T&H
?
[CH4]AntBL (ppbv)
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
10kyr BP 15kyr BP 20kyr BP
[Fischer et al., 2008]
LGM
(PI) B/A
YD
2. Isotopic composition of methane at the LGM
13CH4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005])
2. Isotopic composition of methane at the LGM
13CH4 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
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]
10kyr BP 15kyr BP 20kyr BP
LGM
[Fischer et al., 2008] (PI)
+3.6‰
B/A
YD
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
13CH4 [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
2. Isotopic composition of methane at the LGM
Very simple calculations to explore the sensitivity of [ClMBL], and hence 13CH4,
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
2. Isotopic composition of methane at the LGM
[Schaefer and Whiticar, 2008]
[Allan et al., 2007]
X% increase in (Cl-1).FCl 0.026X‰ increase in 13CH4, as FCl « 1-FCl
X% increase in (Cl-1).kCl.[ClMBL] 0.026X‰ increase in 13CH4, provided FCl kCl.[ClMBL]
[Saueressig et al., 1995]
[Sander et al., 2003]
[Allan et al., 2001, 2007] . 1 tanh(3 )sin(2 ( 90) / 365)MBL baseCl Cl t
13
13 14
1
1
.( 1).
i
n
E i ni
j jnj
ii
C ECH F
E
6.455
1.043 TCl e
1360129.6 10 . T
Clk e
( 1). 2.6‰Cl ClF
2. Isotopic composition of methane at the LGM
[Schaefer and Whiticar, 2008]
[Allan et al., 2007]
X% increase in (Cl-1).FCl 0.026X‰ increase in 13CH4, as FCl « 1-FCl
X% increase in (Cl-1).kCl.[ClMBL] 0.026X‰ increase in 13CH4, provided FCl kCl.[ClMBL]
[Saueressig et al., 1995]
[Sander et al., 2003]
Sea salt loading uP [Gong et al., 2002] . 1 tanh(3 )sin(2 ( 90) / 365) . . PMBL base
Cl Cl t N u
13
13 14
1
1
.( 1).
i
n
E i ni
j jnj
ii
C ECH F
E
6.455
1.043 TCl e
1360129.6 10 . T
Clk e
( 1). 2.6‰Cl ClF
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
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
2. Isotopic composition of methane at the LGM
Annual-mean (Cl-1).kCl.[ClMBL] (10-10 molecules-1 cm3 s-1)
PI LGM
(Cl-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 Cl)
2. Isotopic composition of methane at the LGM
Percentage change in (Cl-1).kCl.[ClMBL] at the LGM
Globally, (Cl-1).kCl.[ClMBL] increases by 7% 0.2‰ increase in 13CH4,
which is small compared to the 3.6‰ increase observed [Fischer et al., 2008] but..
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]
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]
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%, (Cl-1).kCl.[ClMBL] increases by 48% 1.3‰ increase in 13CH4
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%, (Cl-1).kCl.[ClMBL] increases by 48% 1.3‰ increase in 13CH4
- over a third of the increase observed
2. Isotopic composition of methane at the LGM
Changes in the strength of the ClMBL sink have the potential to strongly influence 13CH4
An enrichment in 13CH4 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 13CH4 signal
[Gong et al., 2002]
winds P (u≤5ms-1) P (u>5ms-1) 13CH4 (‰)
PILGM 1.39 3.41 +0.2
PILGM 1.66 3.41 +0.2
PILGM(SHW +25%)
1.39 3.41 +1.2
PILGM(SHW +25%)
1.66 3.41 +1.2
Biomass burning Oceans Vegetation Soils Lightning Wetlands Termites Total
NO2 1.4 - - 5.1 4.8 - - 11.3
CH4 11.0 13.0 - - - 147.9 27.0 198.9
CO 100.0 50.0 150.0 (100.5) - - - - 300.0 (250.5)
C2H6 0.7 - 3.5 (2.3) - - - - 4.2 (3.0)
C3H8 0.2 0.5 3.5 (2.3) - - - - 4.2 (3.0)
CH3COCH3 0.1 - 20.0 (13.4) - - - - 20.1 (13.5)
C5H8 - - 673.7 (258.9) - - - - 673.7 (258.9)
C2H4 1.4 - 20.0 (13.4) - - - - 21.4 (14.8)
HCHO 0.3 - - - - - - 0.3
CH3CHO 0.8 - - - - - - 0.8
[CH4]AntBL (ppbv) CH4 burden (Tg) CH4 lifetime (years)
PI 714 1699 8.5
PI-V 558 (-22) 1329 (-22) 6.7 (-22)
PI-H 763 (+7) 1812 (+7) 9.1 (+7)
PI-H-K 840 (+18) 1994 (+17) 10.0 (+17)
PI-H-K-V 637 (-11) 1514 (-11) 7.6 (-11)
PI-H-K-VLGM 690 (-3) 1642 (-3) 8.2 (-3)
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