Source vs. Sink Contributions Source vs. Sink Contributions to Atmospheric Methane to Atmospheric Methane

Trends: 1990-2004Trends: 1990-2004

Arlene M. Fiore

Larry Horowitz (NOAA/GFDL)Ed Dlugokencky (NOAA/GMD)

Jason West (Princeton)

Informal Seminar at NOAA GMDJanuary 24, 2006

More than half of global methane emissions are influenced by human activities

~300 Tg CH4 yr-1 Anthropogenic [EDGAR 3.2 Fast-Track 2000; Olivier et al., 2005]

~200 Tg CH4 yr-1 Biogenic sources [Wang et al., 2004]

ANIMALS90

LANDFILLS +WASTEWATER

50GAS + OIL60

COAL30RICE 40TERMITES

20

WETLANDS180

BIOMASS BURNING + BIOFUEL 30

GLOBAL METHANESOURCES

(Tg CH4 yr-1)

Observed trend in Surface CH4 (ppb) 1990-2004

Data from 42 GMD stations with 8-yr minimum record is area-weighted, after averaging in bands

60-90N, 30-60N, 0-30N, 0-30S, 30-90S

GMD Network

Global Mean CH4 (ppb)Hypotheses for leveling offdiscussed in the literature:

1. Approach to steady-state 2. Source Changes Anthropogenic

WetlandsBiomass burning

3. Transport

4. Sink (OH) Humidity Temperature OH precursor emissions overhead O3 columns

How does a 3-D forward model compare with GMD observations?

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1990 1992 1994 1996 1998 2000 2002 2004

Global Mean Surface Methane (ppb)

OBSERVED

Model with constant emissions largely captures

observed trend in CH4 during the 1990s

Captures flattening post-1998 but underestimates abundance

Emissions problem?

MOZART-2 [Horowitz et al., 2003]• NCEP• 1.9°x1.9°• 28 -levels• EDGAR v.2.0 emissions

• CH4 EDGAR

emissions for 1990s

Bias and Correlation vs. GMD Surface CH4: 1990-2004

BASE simulation with constant emissions: Overestimates interhemispheric gradient Correlates poorly at high northern latitudes

BASE

Mean Bias (ppb) r2

Estimates for Changing Methane Sources in the 1990s

547

Tg

CH

4 yr

-1

EDGAR anthropogenic inventory

Biogenic adjusted to maintainconstant total source

BASE ANTH

200

210

220

230

240

250

260

270

1990 1995 2000 2005

Inter-annually varying wetland emissions1990-1998 from Wang et al. [2004](Tg CH4 yr-1); distribution changes

Apply climatological mean (224 Tg yr-1) post-1998

ANTH + BIO

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1790

1990 1992 1994 1996 1998 2000 2002 2004

OBSBASEANTH

Mea

n B

ias

(pp

b)

r2

Bias & Correlation vs. GMD CH4 observations: 1990-2004

ANTH simulation with time-varying EDGAR 3.2 emissions: Improves abundance post-1998 Interhemispheric gradient too high Poor correlation at high N latitudes

Mea

n B

ias

(pp

b)

ANTH+BIO simulation with time-varying EDGAR 3.2 + wetland emissions improves: Global mean surface conc. Interhemispheric gradient Correlation at high N latitudes

Bias & Correlation vs. GMD CH4 observations: 1990-2004

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1990 1995 2000 2005

OBSBASEANTHANTH+BIO

r2

S Latitude N

Met

han

e C

on

cen

trat

ion

(n

mo

l/m

ol

= p

pb

)OBS (GMD) BASE ANTH ANTH+BIO

Alert (82.4N,62.5W)

1900

1850

1800

Mahe Island (4.7S,55.2E)

1800

1750

1700

Midway (28.2N,177.4W)

184018201800178017601740

South Pole (89.9S,24.8W)

1990 1995 2000 2005

174017201700168016601640

Model capturesdistinct seasonalcycles at GMD stations

Model withBIO wetlandsimproves:

1)high N latitudeseasonal cycle

2)trend

3)low biasat S Pole,especiallypost-1998

Time-Varying Emissions: Summary

Next: Focus on Sinks-- Examine with BASE model (constant

emissions)-- Recycle NCEP winds from 2004 “steady-

state”

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1990 1995 2000 2005

OBSBASEANTHANTH+BIO

Annual mean CH4 in the “time-varying ANTH+BIO” simulation best captures observed distribution

Methane rises again when 1990-1997 winds are applied to “steady-state” 2004 concentrations

Recycled NCEP 1990-2004

Meteorological drivers for observed trend Not just simple approach to steady-state

Area-weighted global mean CH4 concentrations in BASE simulation (constant emissions)

9.9

10

10.1

10.2

10.3

10.4

10.5

273.2 273.4 273.6 273.8 274 274.2

Global mean lower trop Temp 1991-2004

Lif

eti

me

ag

ain

st

tro

p O

H

r2 = 0.83

How does meteorology affect the CH4 lifetime?

]CH][OH[

]CH[

4

4

k=

9.9

10

10.1

10.2

10.3

10.4

10.5

10.6

10.7

1990 1995 2000 2005 2010 2015 2020

Recycled NCEP

CH4 Lifetime against Tropospheric OH

105 molecules cm-3

TemperatureHumidityLightning NOx

Photolysis

Rapid transport to sink regions

Candidate Processes:

9.9

10

10.1

10.2

10.3

10.4

10.5

1.31E+05 1.32E+05 1.32E+05 1.33E+05 1.33E+05 1.34E+05 1.34E+05 1.35E+05

Global Mean Lower Trop OH 1991-2004

Life

time

agai

nst T

rop

OH

Lifetime Correlates Strongly With Lower Tropospheric OH and Temperature

r2 = 0.51

r2 = 0.89 vs. OHif 1998 neglected(0.80 for Temp)

Deconstruct (-0.17 years) from 1991-1995 to 2000-2004 into individual contributions by varying OH and temperature

separately

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

climT climOH BASET(+0.3K) OH(+1.4%) BASE

+ =

OH increases in the model by 1.4% due to a 0.3 Tg N yr-1 increase in lightning NOx emissions

Large uncertainties in magnitude (and trend) of lightning NOx

emissions in the real atmosphere

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1990 1995 2000 2005

Methane Distribution and Trends: Conclusions

Global Mean CH4 (ppb)

Potential for strong climate feedbacks

OBSERVED

CH4 decreases by ~2% from 1991-1995 to 2000-2004 due to warmer temperatures (35%) and higher OH (65%), resulting from a 10% increase in lightning NOx emissions

BASE (constant emissions) captures observed rate of increase 1990-1997 and leveling off post-1998

ANTH improves modeled CH4 post-1998

Wetland emissions in ANTH+BIO best match observed seasonality, inter-hemispheric gradient and global trend

Q: How will future global change influence atmospheric CH4?Potential for complex biosphere-atmosphere interactions…

can be studied with GFDL earth system model

CH4 + OH …products

Soil

BVOC NOx

Higher in a warmer climate?