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

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


PI = Pre-industrial era (200yr before present)

LGM = Last Glacial Maximum (21kyr before present)


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


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?


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

?


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


AntBL = Antarctic boundary layer (all boxes in the lowest level of the model, south of 70°S)

714

360

PI

LGM

[CH4]AntBL (ppbv)


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)


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)


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)


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)


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)


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])


13CH4 was approximately -47‰ 1kyr before present [Ferretti et al., 2005])


[Fischer et al., 2008]

LGM

(PI)

+3.6‰

B/A

YD


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]


LGM

[Fischer et al., 2008] (PI)

+3.6‰

B/A

YD


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?


LGM

[Fischer et al., 2008] (PI)

+3.6‰

B/A

YD


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


[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


[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


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


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


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)


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..


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]


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


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


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)

changes in methane at the last glacial maximum

Documents

pyleconcentration of

lgm pibayd2

lgmemploying lgm temperatures

lgm sensitivity experiments

employing emissions

sensitivity of ch4

pi model

kyr bpfischer