Atmospheric Methane Distribution and Trends: Impacts on Climate and Ozone Air Quality

Transcription

1 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 Atmospheric CH 4 : Past Trends, Future Predictions Variations of CH 4 Concentration (ppb) Over the Past 1000 years [Etheridge et al., 1998] IPCC [2001] Projections of Future CH 4 Emissions (Tg CH 4 ) to Scenarios A1B A1T A1F1 A2 B1 B2 IS92a 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] GLOBAL METHANE SOURCES (Tg CH 4 yr -1 ) WETLANDS 180 BIOMASS BURNING + BIOFUEL 30 TERMITES 20 RICE 40 ANIMALS 90 COAL 30 LANDFILLS + WASTEWATER 50 GAS + OIL 60

4 Air quality-climate Linkage: CH 4, O 3 are important greenhouse gases CH 4 contributes to background O 3 in surface air NO 2 hν NO O 3 Free Troposphere Global Background O 3 OH HO 2 Direct Intercontinental Transport Boundary layer VOC, CH 4, CO NO x NMVOCs O 3 air pollution (smog) (0-3 km) air pollution (smog) NO x NMVOCs O 3 CONTINENT 1 OCEAN CONTINENT 2

5 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 O 3 [Prather et al., 2003] 2100 SRES A Attributed mainly to increases in methane and NO x [Wang et al., 1998; Prather et al., 2003] Adapted from J. West

6 Rising background O 3 at northern mid-latitudes has implications for attaining air quality standards Pre-industrial background Europe seasonal new CA standard 8-hr avg Current background WHO/Europe 8-hr average U.S. 8-hr average O 3 (ppbv) Analyses of surface O 3 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 Radiative Forcing of Climate from Preindustrial to Present: Important Contributions from Methane and Ozone Hansen, Scientific American, 2004

8 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 σ-levels GEIA/Harvard emissions Uniform, fixed CH 4 NCEP meteorology 1.9 x1.9 ; 28 σ-levels EDGAR v. 2.0 emissions CH 4 EDGAR emissions for 1990s 3-D model structure

9 Double dividend of Methane Controls: Decreased greenhouse warming and improved air quality Radiative Forcing (W m -2 ) 50% anth. VOC 50% anth. CH 4 Number of U.S. summer gridsquare days with O 3 > 80 ppbv 50% % 50% 50% 2030 anth. A1 B1 (base) anth. anth. anth. A1 NO x VOC CH 4 NO x GEOS-Chem Model Simulations (4 x5 ) 2030 B1 IPCC scenario Anthrop. NO x emissions (2030 vs. present) Global U.S. Methane emissions (2030 vs. present) A1 +80% -20% +30% B1-5% -50% +12% CH 4 links air quality & climate via background O 3 Fiore et al., GRL, 2002

10 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 O 3 Precursor Reductions on U.S. Summer Afternoon Surface O 3 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 X 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]

13 How Much Methane Can Be Reduced? Ozone reduction (ppb) Cost-saving reductions <$10 / ton CO 2 eq. All identified reductions % of anthrop. emissions North America Rest of Annex I Rest of World % of anthrop. emissions Methane reduction potential (Mton CH CH 4 yr -1 ) 4 yr -1 ) IEA [2003] for 5 industrial sectors Comparison: Clean Air Interstate Rule (proposed NO x 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 O 3 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 1950s 1980s Present Abatement Strategy: NMVOCs + NO x + CH 4??

15 Addressing the CH 4 -O 3 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] 1. Does CH 4 source location influence the O 3 response? What is driving recent trends in atmospheric CH 4? Sources? Sinks? Observed Global Mean CH 4 (ppb)

16 Methane Control Simulations in MOZART-2: 30% Decrease in Global Anthropogenic CH 4 Emissions Surface Methane (ppb) Global surface CH 4 conc. (ppb) Year BASE CASE -30% anthrop. emis. Approaching steady-state after 30 years Does O 3 impact depend on source location? (1) global -30% anthrop. emissions (2) zero Asian emissions (=30% global) ppb Tg Change in global surface CH 4 conc. from 30% decrease in anthrop. emis. Year Decrease in Tropospheric O 3 Burden -12 Year

17 CLIMATE IMPACTS: Change in July 2000 Trop. O 3 Columns (to 200 hpa) 30% decrease in global anthrop. CH 4 emissions Zero CH 4 emissions from Asia (= 30% decrease in global anthrop.) Dobson Units mw m -2 (Radiative Forcing) No Asia (30% global decrease) Tropospheric O 3 column response is independent of CH 4 emission location except for small (~10%) local changes DU mw m -2

18 U.S. Surface Afternoon Ozone Response in Summer also independent of methane emission location MEAN DIFFERENCE NO ASIAN ANTHROP. CH 4 MAX DIFFERENCE (Composite max daily afternoon mean JJA) GLOBAL 30% DECREASE IN ANTHROP. CH 4 Stronger sensitivity in NO x -saturated regions (Los Angeles), partially due to local ozone production from methane

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

20 Global Mean Surface Methane (ppb) Model with constant emissions largely captures observed trend in CH 4 during the 1990s 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 CH 4 lifetime (3) Approach to steady-state with constant lifetime

21 Bias and Correlation vs. GMD Surface CH 4 : BASE Mean Bias (ppb) r 2 BASE simulation with constant emissions: Overestimates interhemispheric gradient Correlates poorly at high northern latitudes

22 Estimates for Changing Methane Sources in the 1990s Tg CH 4 yr -1 Biogenic Wastewater Biomass Burning Landfills Ruminants Energy Rice Biogenic adjusted to maintain constant total source v v v v3.2 BASE ANTH EDGAR anthropogenic inventory Inter-annually varying wetland emissions from Wang et al. [2004] (Tg CH 4 yr -1 ); distribution changes ANTH + BIO Apply climatological mean (224 Tg yr -1 ) post-1998

23 Bias & Correlation vs. GMD CH 4 observations: OBS BASE ANTH Mean Bias (ppb) ANTH simulation with time-varying EDGAR 3.2 emissions: Improves abundance post-1998 Interhemispheric gradient too high Poor correlation at high N latitudes r 2

24 Bias & Correlation vs. GMD CH 4 observations: OBS BASE ANTH ANTH+BIO Mean Bias (ppb) ANTH+BIO simulation with timevarying EDGAR wetland emissions improves: Global mean surface conc. Interhemispheric gradient Correlation at high N latitudes r 2 S Latitude N

25 OBS (GMD) BASE ANTH ANTH+BIO Methane Concentration (nmol/mol = ppb) Alert (82.4N,62.5W) Midway (28.2N,177.4W) Mahe Island (4.7S,55.2E) South Pole (89.9S,24.8W) Model with BIO wetlands improves: 1)high N latitude seasonal cycle 2)trend 3)low bias at S Pole, especially post-1998 Model captures distinct seasonal cycles at GMD stations

26 Time-Varying Emissions: Summary OBS BASE ANTH ANTH+BIO Annual mean CH 4 in the time-varying ANTH+BIO simulation best captures observed distribution Next: Focus on Sinks -- Examine with BASE model (constant emissions) -- Recycle NCEP winds from 2004 steady-state

27 Methane rises again when winds are applied to steady-state 2004 concentrations Area-weighted global mean CH 4 concentrations in BASE simulation (constant emissions) 1730 Recycled NCEP Year (NCEP winds recycled such that 2005 = 1990 met fields) Meteorological drivers for observed trend Not just simple approach to steady-state

28 How does meteorology affect the CH 4 lifetime? CH 4 Lifetime vs. Tropospheric OH Recycled NCEP τ = Lifetime Correlates Strongly With Lower Tropospheric OH and Temperature r 2 = r 2 = molecules cm -3 K Candidate Processes: Rapid transport to sink regions Temperature [CH 4] k[oh][ch 4 ] Humidity Lightning NO x Photolysis

29 Methane Distribution and Trends: Climate and Air Quality Impacts 20% anthrop. CH 4 emissions can be reduced at low cost Ozone response largely independent of CH 4 source location 30% decreases in anthrop. CH 4 reduces radiative forcing by 0.2 Wm -2 and JJA U.S. surface O 3 by 1-4 ppbv Global Mean CH 4 (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 Meteorology major driver; further work needed to 4. Sink (OH) isolate cause Potential for strong climate feedbacks

30 Q: How will future global change influence atmospheric CH 4? Potential for complex biosphere-atmosphere interactions CH 4 + OH products BVOC NO x Soil