Atmospheric Methane Distribution and Trends:

Impacts on Climate and Ozone Air Quality

Arlene M. Fiore Larry Horowitz (NOAA/GFDL)

Jason West (Princeton)Ed Dlugokencky (NOAA/GMD)

Earth, Atmospheric, and Planetary SciencesDepartment Seminar

Massachusetts Institute of TechnologyDecember 16, 2005

Year

600

800

700

Scenarios

A1BA1TA1F1A2B1B2IS92a

900

Variations of CH4 Concentration (ppb)Over the Past 1000 years

[Etheridge et al., 1998]

Year20001000

800

IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050

1200

1600

1400

1000

1500 2000 2020 2040

Atmospheric CH4: Past Trends, Future Predictions

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)

CONTINENT 2 OCEAN

Boundary layer

(0-3 km)

Free Troposphere

CONTINENT 1

Air quality-Climate Linkage:CH4, O3 are important greenhouse gases

CH4 contributes to background O3 in surface air

OH HO2

VOC, CH4, CO

NONO2

hνO3

Direct Intercontinental Transport

Global Background O3

NOxNMVOCs

O3air pollution (smog)

NOxNMVOCs O3

air pollution (smog)

Change in 10-model mean July surface O3 [Prather et al., 2003]

Adapted from J. West

Attributed mainly to increases in methane and NOx[Wang et al., 1998; Prather et al., 2003]

Observations indicate historical increase in background ozone; IPCC scenarios project future growth

Ozone at European mountain sites 1870-1990 [Marenco et al., 1994].

2100 SRES A2 - 2000

Rising background O3 at northern mid-latitudes has implications for attaining air quality standards

20 40 60 80 100

O3 (ppbv)U.S. 8-hr average

Analyses of surface O3 from North American and European monitoring sites indicate increasing background[Lin et al., 2000; Jaffe et al., 2003,2005; Vingarzen et al., 2004; EMEP/CCC-Report 1/2005 ]

Current background

Pre-industrialbackground

Europeseasonal

WHO/Europe8-hr average

new CAstandard8-hr avg

Radiative Forcing of Climate from Preindustrial to Present:

Important Contributions from Methane and Ozone

Hansen, Scientific American, 2004

Approach: Use 3-D Models of Atmospheric Chemistry to examine climate and air quality response to emission changes

GEOS-CHEM[Bey et al., 2001]

MOZART-2[Horow itz et al., 2003]

3-D model structure

• GEOS GMAO meteorology• 4°x5°; 20 σ-levels• GEIA/Harvard emissions• Uniform, fixed CH4

• NCEP meteorology• 1.9°x1.9°; 28 σ-levels• EDGAR v. 2.0 emissions• CH4 EDGAR emissions for

1990s

50% anth.NOx

2030 A1

50% anth.CH4

50% anth.VOC

2030 B1

1995(base)

50% anth.VOC

50% anth.CH4

50% anth.NOx

2030 A1

2030 B1

IPCC scenario

Anthrop. NOx emissions(2030 vs. present)

Global U.S.

Methane emissions

(2030 vs. present)

A1 +80% -20% +30%B1 -5% -50% +12%

Number of U.S. summer grid-square days with O3 > 80 ppbv

Rad

iativ

e Fo

rcin

g (W

m-2

)

CH4 links air quality & climate via

background O3

Fiore et al., GRL, 2002

Double dividend of Methane Controls:Decreased greenhouse warming and improved air quality

GEOS-Chem Model Simulations (4°x5°)

Response of Global Surface Ozone to 50% decrease in global methane emissions (actually changing uniform concentration from 1700 to 1000 ppbv)

• Ozone decreases by 1-6 ppb• ~3 ppb over land in US summer** ~60% of reduction in 10 yr; ~80% in 20 yr.

Impacts of O3 Precursor Reductions on U.S. Summer Afternoon Surface O3 Frequency Distributions

West & Fiore, ES&T, 2005

GEOS-Chem Model Simulations (4°x5°)

Tropospheric ozone response to anthropogenic methane emission changes is fairly linear

X

MOZART-2 (this work)TM3 [Dentener et al., ACPD, 2005]GISS [Shindell et al., GRL, 2005GEOS-CHEM [Fiore et al., GRL, 2002]IPCC TAR [Prather et al., 2001]

How Much Methane Can Be Reduced?

Comparison: Clean Air Interstate Rule (proposed NOx control) reduces 0.86 ppb over the eastern US, at $0.88 billion yr-1

West & Fiore, ES&T, 2005

0 20 40 60 80 100 120Methane reduction potential (Mton CH4 yr-1)

North AmericaRest of Annex IRest of World

Ozone reduction (ppb)

Cost-saving reductions

<$10 / ton CO2 eq.

All identifiedreductions

0.7

1.4

1.9

IEA [2003] for 5 industrial sectors

10% of anthrop. emissions

20% of anthrop. emissions

0 20 40 60 80 100 120Methane reduction potential (Mton CH4 yr-1)

1950s Present1980s

NMVOCs + NOx + CH4??

Ozone Abatement Strategies Evolve as our Understanding of the Ozone Problem Advances

Abatement Strategy:

O3 smog recognizedas an URBAN problem:Los Angeles,Haagen-Smit identifieschemical mechanism

Smog consideredREGIONAL problem; role of biogenic VOCs discovered A GLOBAL perspective:

role of intercontinentaltransport, background

Addressing the CH4-O3 air quality-climate linkage

1. Does CH4 source location influence the O3 response?

2. What is driving recent trends in atmospheric CH4 ? Sources? Sinks?

Methane controls are receiving attention as a means to simultaneously address climate and global air pollution

[EMEP/CCC report 1/2005]

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1992 1994 1996 1998 2000 2002 2004

Observed Global Mean CH4 (ppb)

1300

1350

1400

1450

1500

1550

1600

1650

1700

1750

1800

0 5 10 15 20 25 30

Year

Surf

ace

Met

hane

(ppb

)Methane Control Simulations in MOZART-2:

30% Decrease in Global Anthropogenic CH4 Emissions

Global surface CH4 conc. (ppb)

-400

-350

-300

-250

-200

-150

-100

-50

00 5 10 15 20 25 30

Year

Change in global surface CH4 conc. from 30% decreasein anthrop. emis.

-12

-10

-8

-6

-4

-2

01 6 11 16 21 26

Year

Decrease in Tropospheric O3 Burden

ppb

Approaching steady-state after 30 years Does O3 impact depend on source location?

(1) global -30% anthrop. emissions (2) zero Asian emissions (=30% global)

BASE CASE

-30% anthrop. emis.

Tg

CLIMATE IMPACTS: Change in July 2000 Trop. O3 Columns (to 200 hPa)

30% decrease in global anthrop.CH4 emissions

Zero CH4 emissions from Asia(= 30% decrease in global anthrop.)

Dobson Units-34 -27 -20 -14 -7 mW m-2 (Radiative Forcing)

No Asia – (30% global decrease)

Tropospheric O3 column response is independent of CH4 emission location except for small (~10%) local changes

DU-5.1 -3.4 -1.7 -0.7 mW m-2+0.7

U.S. Surface Afternoon Ozone Response in Summer also independent of methane emission location

MEAN DIFFERENCE MAX DIFFERENCE(Composite max daily afternoon mean JJA) NO ASIAN ANTHROP. CH4

GLOBAL 30% DECREASE IN ANTHROP. CH4

Stronger sensitivity in NOx-saturated regions (Los Angeles),partially due to local ozone production from methane

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

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1992 1994 1996 1998 2000 2002 2004

GMD Network

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

1. Approach to steady-state

2. Source Changes AnthropogenicWetlandsBiomass burning

3. Transport

4. Sink (OH)HumidityTemperatureOH precursor emissionsoverhead O3 columns

How does BASE CASE Model compare with GMD observations?

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1992 1994 1996 1998 2000 2002 2004

Model with constant emissions largely captures observed trend in CH4 during the 1990s G

loba

l Mea

n S

urfa

ce M

etha

ne (p

pb)

OBSERVED

BASE CASE MODEL

Captures flattening post-1998 but underestimates abundance

Emissions problem?

Possible explanations for observed behavior:(1) Source changes (2) Meteorologically-driven changes in CH4 lifetime(3) Approach to steady-state with constant lifetime

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

0

100

200

300

400

500

1990 v2.0 1990 v3.2 1995 v3.2 2000 v3.2

WastewaterLandfillsEnergy

547

Tg C

H4

yr-1

EDGAR anthropogenic inventory

BiogenicBiomass BurningRuminantsRice Biogenic adjusted to maintain

constant 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

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1992 1994 1996 1998 2000 2002 2004

OBSBASEANTH

Mea

n B

ias

(ppb

)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

(ppb

)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

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1995 2000 2005

OBSBASEANTHANTH+BIO

r2

S Latitude N

Met

hane

Con

cent

ratio

n (n

mol

/mol

= 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”

1710

1720

1730

1740

1750

1760

1770

1780

1790

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

1710

1720

1730

1740

1750

1760

1770

1780

1990 1995 2000 2005 2010 2015 2020

Year (NCEP winds recycled such that 2005 = 1990 met fields)

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

10.6

10.7

1.28 1.30 1.32 1.34 1.36 1.38

How does meteorology affect the CH4 lifetime?

Lifetime Correlates Strongly With Lower TroposphericOH and Temperature

]CH][OH[]CH[

4

4

kτ =

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 vs. Tropospheric OH

9.910

10.110.210.310.410.510.610.7

273.2 273.4 273.6 273.8 274 274.2

r2 = 0.65

105 molecules cm-3

r2 = 0.69

K

TemperatureHumidityLightning NOxPhotolysis

Rapid transport to sink regions

Candidate Processes:

Methane Distribution and Trends: Climate and Air Quality Impacts

• 20% anthrop. CH4 emissions can be reduced at low cost

• Ozone response largely independent of CH4 source location

• 30% decreases in anthrop. CH4 reduces radiative forcing by 0.2 Wm-2 and JJA U.S. surface O3 by 1-4 ppbv

1710

1720

1730

1740

1750

1760

1770

1780

1790

1990 1992 1994 1996 1998 2000 2002 2004

Global Mean CH4 (ppb) Hypotheses for leveling off:

1. Approach to steady-state not the whole story

2. Source Changes improve simulated abundances

but not driving trend3. Transport

4. Sink (OH)

Meteorology major driver;further work needed to isolate cause

Potential for strong climate feedbacks

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

CH4 + OH …products

Soil

BVOC NOx