Recent and future trends in atmospheric methane: Connecting global chemistry, climate and ozone pollution

Transcription

1 Recent and future trends in atmospheric methane: Connecting global chemistry, climate and ozone pollution Arlene M. Fiore Acknowledgments: Larry Horowitz, Chip Levy (NOAA/GFDL) Jason West, Vaishali Naik (Princeton University) Ellen Baum, Joe Chaisson (Clean Air Task Force) Funding from Luce Foundation via Clean Air Task Force Berkeley Atmospheric Sciences Symposium, September 29, 2006

2 Historical increase in atmospheric methane and ozone Variations of CH 4 Concentration (ppb) Over the Past 1000 years [Etheridge et al., 1998] 1600 Ozone at European mountain sites [Marenco et al., 1994] Year 2000

3 Radiative forcing of climate, 1750-Present: Important contributions from methane and ozone IPCC [2001] Level of scientific understanding

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 IPCC [2001] scenarios project future growth Projections of future CH 4 emissions (Tg CH 4 ) to 2100 Change in 10-model mean surface O SRES A Attributed mainly to increases in methane and NO x [Prather et al., 2003]

6 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 as REGIONAL problem; role of NO x and biogenic VOCs recognized A GLOBAL perspective: role of intercontinental transport, background 1950s 1980s Present Abatement Strategy: NMVOCs + NO x + CH 4?? Methane (and CO) emission control is an effective way of simultaneously meeting air quality standards and abating global warming --- EMEP/CCC-Report 1/2005

7 Impacts of O 3 precursor reductions on U.S. summer afternoon surface O 3 frequency distributions GEOS-Chem Model Simulations (4 x5 ) NO x controls strongly decrease the highest O 3 (regional pollution episodes) CH 4 controls affect the entire O 3 distribution similarly (background) Results add linearly when both methane and NO x are reduced Fiore et al., 2002; West & Fiore, ES&T, 2005

8 Rising background O 3 has implications for attaining air quality standards Surface O 3 background appears to be rising [e.g. Lin et al., 2000; Jaffe et al., 2003, 2005; Vingarzan, 2004; EMEP/CCC-Report 1/2005 ] Pre-industrial background Europe seasonal WHO/Europe 8-hr average U.S. 8-hr average Current background Future background? O 3 (ppbv)

9 Methane trends and linkages with chemistry, climate, and ozone pollution 1) Methane Trends from 1990 to 2004 Are emission inventories consistent with observed CH 4 trends? Role of changing sources? Role of changing sinks? 2) Climate and air quality benefits from CH 4 controls Characterize the ozone response to CH 4 control Compare with traditional controls on NO x, NMVOC Incorporate methane controls into a future emission scenario Research Tool: MOZART-2 Global Chemical Transport Model [Horowitz et al., 2003] NCEP, 1.9 x1.9, 28 vertical levels Fully represent methane-oh relationship Test directly with observations 3D model structure

10 More than half of global methane emissions are influenced by human activities ~300 Tg CH 4 yr -1 Anthropogenic [EDGAR 3.2 Fast-Track 2000; Olivier et al., 2005] ~200 Tg CH 4 yr -1 Biogenic sources [Wang et al., 2004] >25% uncertainty in total emissions Clathrates? Melting permafrost? PLANTS? Keppler et al., Sanderson et al., Kirschbaum et al., Ferretti et al., 2006 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

11 Observed trend in surface CH 4 (ppb) Global Mean CH 4 (ppb) NOAA GMD Network Hypotheses for leveling off discussed in the literature: 1. Approach to steady-state 2. Source Changes Anthropogenic Wetlands/plants (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 (CH 4 +OH) Humidity Temperature OH precursor emissions overhead O 3 columns Can the model capture the observed trend (and be used for attribution)?

12 Bias and correlation vs. observed surface CH 4 : BASE simulation EDGAR 2.0 emissions held constant Global Mean Surface Methane (ppb) OBSERVED MOZART-2 Mean Bias (ppb) Overestimates interhemispheric gradient Overestimates but matches trend Captures flattening post but underestimates abundance r 2 Correlates poorly at high N latitudes S Latitude N

13 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 ); different distribution ANTH + BIO Apply climatological mean (224 Tg yr -1 ) post-1998

14 Bias & Correlation vs. GMD CH 4 observations: Global Mean Surface Methane (ppb) 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 S Latitude N

15 Bias & Correlation vs. GMD CH4 observations: Global Mean Surface Methane (ppb) 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 Fiore et al., GRL, 2006 S Latitude N

16 How does meteorology influence methane abundances? Why does BASE run with constant emissions level off post-1998? Examine sink τ = [CH 4] k[oh][ch 4 ] CH 4 Lifetime (τ) against Tropospheric OH τ Temperature (88% of CH 4 loss is below 500 hpa ) Humidity Photolysis Lightning NO x τ = 0.17 yr = 1.6%) What drives the change in methane lifetime in the model?

17 Small increases in temperature and OH shorten the methane lifetime against tropospheric OH Deconstruct τ (-0.17 years) from to into individual contributions by varying OH and temperature separately τ OH + = Global Lightning NO x (TgN yr -1 ) 0 T(+0.3K) OH(+1.2%) BASE climt climoh BASE 3 LNOx (TgN/y) Year An increase in lightning NO x drives the OH increase in the model But lightning NO x is highly parameterized how robust is this result?

18 Work in progress: Additional evidence for a global lightning NO x increase? Estimate lightning NO x changes using options available in the GFDL Atmospheric General Circulation Model: Convection schemes (RAS vs. Donner-deep) Meteorology (free-running vs. nudged to NCEP reanalysis) Lightning NO x % change (91-95 to 00-04) 20% MOZART RAS Donner 18% 16% 14% More physically-based lightning NO x scheme [Petersen et al., 2005] Evidence from observations? % change 12% 10% 8% 6% 4% LIS/OTD Flash counts 2% 0% NCEP(nudged) Lightning NO x increase robust; magnitude depends on meteorology c/o L.W. Horowitz free-running GCM Magnetic field variations in the lower ELF range [e.g. Williams, 1992; Füllekrug and Fraser- Smith, 1997; Price, 2000] Negev Desert Station, Israel

19 Recap: Methane trends Global Mean Surface Methane (ppb) OBS ANTH+BIO BASE ANTH Available emissions estimates for the past decade are fairly consistent with CH 4 observations τ decreases by 1.6% in BASE from to due to higher temp. (+0.3K) and OH (+1.2%); OH rise from enhanced lightning NO x NEXT: Climate and air quality benefits from CH 4 controls 1) Magnitude, spatial variability, linearity 2) Comparison with traditional O 3 control strategies 3) Future scenarios

20 Characterizing the methane-ozone relationship with idealized model simulations Reduce global anthropogenic CH 4 emissions by 30% 0 0 Surface Methane Abundance (ppb) Tropospheric O 3 Burden (Tg) Simulation Year -12 Model approaches a new steady-state after 30 years of simulation Is the O 3 response sensitive to the location of CH 4 emission controls?

21 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) DU mw m -2 Tropospheric O 3 column response is independent of CH 4 emission location except for small (~10%) local changes Target cheapest controls worldwide

22 Decrease in summertime U.S. surface ozone from 30% reductions in anthrop. CH 4 emissions MAXIMUM DIFFERENCE (Composite max daily afternoon mean ozone JJA) NO ASIAN ANTH. CH 4 Largest decreases in NO x -saturated regions

23 Tropospheric O 3 responds approximately linearly to anthropogenic CH 4 emission changes across models X MOZART-2 [West et al., PNAS 2006; this work] TM3 [Dentener et al., ACP, 2005] GISS [Shindell et al., GRL, 2005] GEOS-CHEM [Fiore et al., GRL, 2002] IPCC TAR [Prather et al., 2001] Anthropogenic CH 4 contributes ~50 Tg (~15%) to tropospheric O 3 burden ~5 ppbv to surface O 3

24 How much methane can be reduced? Ozone reduction (ppb) Cost-saving reductions 0.7 North America Rest of Annex I Rest of World (industrialized nations) <$10 / ton CO 2 eq. 1.4 All identified reductions 10% of anth. emissions 20% of anth. 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 West & Fiore, ES&T, 2005

25 Impacts of O 3 precursor reductions on global surface O 3 Steady-state change in 8-hr daily maximum surface O 3 averaged over 3-month O 3 season from 20% reductions in global anthropogenic emissions NO x NMVOC ppbv CO CH 4 MOZART-2 model (2.8 x 2.8 ) West et al., submitted

26 Double dividend of methane controls: Improved air quality and reduced greenhouse warming AIR QUALITY: Change in population-weighted mean 8-hr daily max surface O 3 in 3-month O 3 season (ppbv) CLIMATE: Radiative Forcing (W m -2 ) NO x OH CH 4 20% anth. NMVOC 20% anth. CO 20% anth. CH 4 20% anth. NO x 20% anth. NMVOC 20% anth. CO Steady-state results from MOZART-2 20% anth. CH 4 20% anth. NO x West et al.,submitted

27 Will methane emissions increase in the future? Anthropogenic CH 4 emissions (Tg yr -1 ) Dentener et al., ACP, 2005 A2 B2 MFR Current Legislation (CLE) Scenario PHOTOCOMP for IPCC AR-4 used CLE, MFR, A2 scenarios for all O 3 precursors [Dentener et al., 2006ab; Stevenson et al., 2006; van Noije et al., 2006; Shindell et al., 2006] Our approach: use CLE as a baseline scenario & apply methane controls

28 Policy-relevant methane control scenarios to 2030 Anthropogenic CH 4 emissions (Tg yr -1 ) CLE 200 Scen A 150 Scen B 100 Scen C 50 0 c/o J.J. West % from 2005 to 2030 under CLE 2030 decrease relative to CLE: A: -75 Tg (18% of 2030 anthrop. CH 4 emis.) B: -125 Tg (29%) C: -180 Tg (42%) Transient, full-chemistry simulations in MOZART-2; NCEP meteorology, recycled for Other O 3 precursor emissions follow the CLE baseline: Anthrop. NO x emissions increase by 5.3 Tg N (+19%) Anthrop. CO emissions decrease by 44 Tg CO (-10%)

29 Preliminary results: Impacts on climate and global surface ozone Radiative Forcing (W m -2 ) 2005 to Net Forcing Global Annual Mean Mean Surface Surface Ozone O 3 CLE A B CLE C CLE A B C Scenario C stabilizes despite 5 Tg N increase in anth. NO x Cost-benefit analysis in progress...

30 Regional control efforts (even under optimistic scenarios) may be offset by increases in hemispheric ozone pollution By 2030 under the CLE scenario, the benefit of European emission control measures is significantly counterbalanced by increasing global O 3 levels [Szopa et al., GRL, 2006] U.S. air quality degrades despite domestic emissions controls (A1 2030) 1995 Base case 2030 A1 IPCC 2030 Scenario Anthrop. NO x emis. Global U.S. Methane emis. A1 +80% -20% +30% GEOS-Chem Model (4 x5 ) [Fiore et al., GRL, 2002] longer O 3 season International approach to ozone abatement?

31 TF HTAP multi-model assessment of intercontinental source-receptor relationships Co-Chairs: Terry Keating (U.S. EPA), Andre Zuber (EC) Intercontinental Source-Receptor Regions Experiment Set 1 (~20 models): Decrease precursor emissions by 20% in source regions Estimate pollutant response over receptor regions Inform 2007 review of CLRTAP Gothenburg Protocol

32 Methane trends: Connecting climate and O 3 chemistry Cost-saving reductions <$10 / ton CO 2 eq. All identified reductions Methane (ppb) + = OBS BASE ANTH ANTH+BIO τ OH to climt T climoh OH BASE BASE North America Rest of Annex I Rest of World Methane reduction potential (Mton CH 4 yr -1 ) Global mean surface CH METHANE TRENDS FROM 1990 TO 2004 Simulation with time-varying emissions and meteorology best captures observed CH 4 distribution Model trend driven by increasing T, OH Trends in global lightning activity? Potential for climate feedbacks (on sources and sinks) CLIMATE AND AIR QUALITY BENEFITS FROM CH 4 CONTROL Independent of reduction location Target cheapest controls worldwide Reduces climate forcing and surface O 3 Complementary to NO x, NMVOC controls Rising hemispheric background O 3 may offset domestic efforts to reduce pollution Opportunity for international air quality management