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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