1. Distribution and Trends: Impacts on Climate and Ozone Air Quality
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 Sciences
Department Seminar
Massachusetts Institute of Technology
December 16, 2005

2. IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050
Atmospheric CH4: Past Trends, Future Predictions
IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050
Variations of CH4 Concentration (ppb)
Over the Past 1000 years
[Etheridge et al., 1998]
Scenarios
1600
900
A1B
A1T
A1F1 A2 B1 B2
IS92a
1400
800
1200
700
1000
800
600
1000
1500
2000
2000
2020
2040
Year
Year

3. 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]
BIOMASS BURNING + BIOFUEL 30
ANIMALS 90
WETLANDS
180
LANDFILLS +
WASTEWATER 50
GLOBAL METHANE
SOURCES
(Tg CH4 yr-1)
GAS + OIL 60
COAL 30
TERMITES 20
RICE 40

4. Air quality-Climate Linkage: CH4, O3 are important greenhouse gases CH4 contributes to background O3 in surface air O3
Free Troposphere OH
HO2
VOC, CH4, CO NO
NO2 hn
Global Background O3
Direct Intercontinental Transport
Boundary layer
(0-3 km)
NOx
NMVOCs O3
air pollution (smog)
NOx
NMVOCs O3
air pollution (smog)
CONTINENT 1
CONTINENT 2
OCEAN

5. Attributed mainly to increases in methane and NOx
Observations indicate historical increase in background ozone; IPCC scenarios project future growth
Ozone at European mountain sites [Marenco et al., 1994].
Change in 10-model mean July surface O3 [Prather et al., 2003]
2100 SRES A
Attributed mainly to increases in methane and NOx
[Wang et al., 1998; Prather et al., 2003]
Adapted from J. West

6. Rising background O3 at northern mid-latitudes has implications for attaining air quality standards
new CA
standard
8-hr avg
Europe
seasonal
Pre-industrial
background 20 40 60 80
100
Current background
O3 (ppbv)
WHO/Europe
8-hr average
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 ]

7. Hansen, Scientific American, 2004
Radiative Forcing of Climate from Preindustrial to Present: Important Contributions from Methane and Ozone
Hansen, Scientific American, 2004

8. GEIA/Harvard emissions Uniform, fixed CH4 NCEP meteorology
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
[Horowitz et al., 2003]
GEOS GMAO meteorology
4°x5°; 20 s-levels
GEIA/Harvard emissions
Uniform, fixed CH4
NCEP meteorology
1.9°x1.9°; 28 s-levels
EDGAR v. 2.0 emissions
CH4 EDGAR emissions for 1990s
3-D model structure

9. Double dividend of Methane Controls:
Decreased greenhouse warming and improved air quality
Number of U.S. summer grid-square days with O3 > 80 ppbv
Radiative Forcing (W m-2)
50%
anth.
VOC
50%
anth.
CH4
50%
anth.
NOx
2030 A1
2030 B1
1995
(base)
50%
anth.
VOC
50%
anth.
CH4
50%
anth.
NOx
2030 A1
2030 B1
GEOS-Chem Model Simulations (4°x5°)
IPCC scenario
Anthrop. NOx emissions
(2030 vs. present)
Global U.S.
Methane emissions A1
+80%
-20%
+30% B1
-5%
-50%
+12%
CH4 links air quality
& climate via
background O3
Fiore et al., GRL, 2002

10. Ozone decreases by 1-6 ppb ~3 ppb over land in US summer
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.

11. Impacts of O3 Precursor Reductions on
U.S. Summer Afternoon Surface O3 Frequency Distributions
GEOS-Chem Model Simulations (4°x5°)
West & Fiore, ES&T, 2005

12. Tropospheric ozone response to anthropogenic methane emission changes is fairly linear
MOZART-2 (this work)
TM3 [Dentener et al., ACPD, 2005]
GISS [Shindell et al., GRL, 2005
GEOS-CHEM [Fiore et al., GRL, 2002]
IPCC TAR [Prather et al., 2001] X

13. How Much Methane Can Be Reduced?
0.7
10% of anthrop.
emissions
1.4
20% of anthrop.
emissions
1.9
Methane reduction potential (Mton CH4 yr-1)
IEA [2003] for 5 industrial sectors
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

14. Ozone Abatement Strategies Evolve as our Understanding of the Ozone Problem Advances
O3 smog recognized
as an URBAN problem: Los Angeles,
Haagen-Smit identifies
chemical mechanism
Smog considered
REGIONAL problem;
role of biogenic
VOCs discovered
A GLOBAL perspective:
role of intercontinental
transport, background
1980s
Present
1950s
Abatement Strategy:
NMVOCs
+ NOx
+ CH4??

15. Addressing the CH4-O3 air quality-climate linkage
Methane controls are receiving attention as a means to simultaneously address climate and global air pollution
[EMEP/CCC report 1/2005]
Does CH4 source location influence the O3 response?
What is driving recent trends in atmospheric CH4 ?
Sources?
Sinks?
Observed Global Mean CH4 (ppb)

16. Methane Control Simulations in MOZART-2: 30% Decrease in Global Anthropogenic CH4 Emissions
Global surface CH4 conc. (ppb)
Change in global
surface CH4 conc.
from 30% decrease
in anthrop. emis.
BASE CASE
ppb
-30% anthrop. emis.
Decrease in
Tropospheric
O3 Burden Tg
Approaching steady-state after 30 years
Does O3 impact depend on source location?
(1) global -30% anthrop. emissions
(2) zero Asian emissions (=30% global)

17. 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

18. U.S. Surface Afternoon Ozone Response in Summer also independent of methane emission location
MAX DIFFERENCE
(Composite max daily
afternoon mean JJA)
MEAN DIFFERENCE
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

19. Observed trend in Surface CH4 (ppb) 1990-2004
Global Mean CH4 (ppb)
Hypotheses for leveling off
discussed in the literature:
1. Approach to steady-state
2. Source Changes
Anthropogenic
Wetlands
Biomass burning
3. Transport
4. Sink (OH)
Humidity
Temperature
OH precursor emissions
overhead O3 columns
GMD Network
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
How does BASE CASE Model compare with GMD observations?

20. Captures flattening post-1998 but underestimates abundance
Model with constant emissions largely captures observed trend in CH4 during the 1990s
OBSERVED
BASE CASE MODEL
Captures flattening post-1998
but underestimates abundance
Emissions problem?
Global Mean Surface Methane (ppb)
Possible explanations for observed behavior:
Source changes
Meteorologically-driven changes in CH4 lifetime
Approach to steady-state with constant lifetime

21. Bias and Correlation vs. GMD Surface CH4: 1990-2004
Mean Bias (ppb) r2
BASE
BASE simulation with constant emissions:
Overestimates interhemispheric gradient
Correlates poorly at high northern latitudes

22. Estimates for Changing Methane Sources in the 1990s
Inter-annually varying wetland emissions
from Wang et al. [2004]
(Tg CH4 yr-1); distribution changes
Biogenic adjusted to maintain
constant total source
547
Apply climatological mean
(224 Tg yr-1) post-1998
Tg CH4 yr-1
ANTH
BASE
ANTH + BIO
EDGAR anthropogenic inventory

23. Bias & Correlation vs. GMD CH4 observations: 1990-2004
Mean Bias (ppb)
OBS
BASE
ANTH
ANTH simulation with time-varying EDGAR 3.2 emissions:
 Improves abundance post-1998
Interhemispheric gradient too high
Poor correlation at high N latitudes r2

24. Bias & Correlation vs. GMD CH4 observations: 1990-2004
Mean Bias (ppb)
OBS
BASE
ANTH
ANTH+BIO
ANTH+BIO simulation with time-varying EDGAR wetland emissions improves:
 Global mean surface conc.
 Interhemispheric gradient
Correlation at high N latitudes r2
S Latitude N

25. OBS (GMD) BASE ANTH ANTH+BIO
Alert (82.4N,62.5W)
1900
1850
1800
Model with
BIO wetlands
improves:
1)high N latitude
seasonal cycle
Midway (28.2N,177.4W)
1840
1820
1800
1780
1760
1740
2)trend
Methane Concentration (nmol/mol = ppb)
3)low bias
at S Pole,
especially
post-1998
Mahe Island (4.7S,55.2E)
1800
1750
1700
Model
captures
distinct
seasonal
cycles at
GMD
stations
South Pole (89.9S,24.8W)
1990
1995
2000
2005
1740
1720
1700
1680
1660
1640

26. Time-Varying Emissions: Summary
Annual mean CH4 in the
“time-varying ANTH+BIO”
simulation best captures
observed distribution
OBS
BASE
ANTH
ANTH+BIO
Next: Focus on Sinks
-- Examine with BASE model (constant emissions)
-- Recycle NCEP winds from 2004 “steady-state”

27. Not just simple approach to steady-state
Methane rises again when winds are applied to “steady-state” 2004 concentrations
Area-weighted global mean CH4 concentrations in
BASE simulation (constant emissions)
 Recycled NCEP
 Meteorological drivers for observed trend
Not just simple approach to steady-state

28. How does meteorology affect the CH4 lifetime?
 Recycled NCEP
CH4 Lifetime vs. Tropospheric OH
Rapid transport to sink regions
Candidate Processes:
t =
Temperature
Humidity
Lightning NOx
Photolysis
Lifetime Correlates Strongly With Lower Tropospheric
OH and Temperature
r2 = 0.69
r2 = 0.65
105 molecules cm-3 K

29. 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
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 trend
3. Transport
4. Sink (OH)
Meteorology major driver;
further work needed to
isolate cause
Potential for strong climate feedbacks

30. Q: How will future global change influence atmospheric CH4
Q: How will future global change influence atmospheric CH4? Potential for complex biosphere-atmosphere interactions
CH4 + OH …products
BVOC
NOx
Soil