Interactions Among Air Quality and Climate Policies: Lectures 7 and 8 (abridged versions)

Radiative forcing of climate (1750 to present): Important contributions from air pollutants IPCC, 2007

Methane: Connecting global climate and ozone pollution Arlene M. Fiore (GFDLNOAA) Jason West (University of North Carolina) Larry Horowitz (GFDL/NOAA) Vaishali Naik (GFDL/NOAA)

Year Variations of CH 4 Concentration (ppb) Over the Past 1000 years [Etheridge et al., 1998] Historical increase in atmospheric methane

Global Methane Emissions <25% uncertainty in total emissions ANIMALS 90 LANDFILLS + WASTEWATER 50 GAS + OIL 60 COAL 30 RICE 40 TERMITES 20 WETLANDS 180 BIOMASS BURNING + BIOFUEL 30 GLOBAL METHANE SOURCES Natural ~200 (Tg CH 4 yr -1 ) Anthro ~300 (Tg CH 4 yr -1 ) PLANTS? Keppler et al., Sanderson et al., Kirschbaum et al., Ferretti et al., 2006 Clathrates? Melting permafrost? [EDGAR 3.2 Fast-Track 2000; Olivier et al., Wang et al., 2004]

Modern Methane cycle The cycle is relatively simple since the dominent sink is well known (over 90% due to oxidation by OH radicals). The sources are another story. The total atmospheric burden is ~5Pg (~1,780ppbv) with an atmospheric lifetime of ~9 years, which is modestly dependent on [CH 4 ] itself. Interestingly for a greenhouse gas with over 50% anthropogenic sources, its level in the atmosphere has stopped increasing over the last decade.

Observed trend in surface CH 4 (ppb) 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 NOAA GMD Network Global Mean CH 4 (ppbv) Can we explain this? Many hypotheses : 1. Approach to steady-state 2. Source Changes Anthropogenic Wetlands/plants (Biomass burning) 3. Transport changes 4. Sink Changes (CH 4 +OH) Humidity Temperature OH precursor emissions overhead O 3 columns

100 Year IPCC scenarios for methane emissions 2100 SRES A Longterm Projections Are Very Uncertain (Tg CH 4 ) to 2100

West et al., 2006 Double dividend of methane controls: Improved air quality and reduced greenhouse warming 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) 20% anth. NO x 20% anth. CH 4 20% anth. NMVOC 20% anth. CO CLIMATE: Radiative Forcing (W m -2 ) 20% anth. NMVOC 20% anth. CH 4 20% anth. NO x 20% anth. CO Steady-state results from MOZART-2 global chemical transport model NO x  OH  CH 4

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 global surface O 3 A.M. Fiore

How much can methane be reduced? 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, % of anth. emissions 20% of anth. emissions Methane potential reduction (Mton CH 4 yr -1 ) (industrialized nations)

July surface O 3 reduction from 30% decrease in anthropogenic CH 4 emissions Globally uniform emission reductionEmission reduction only in Asia Fiore et al., JGR, 2008 Take Home 1. Ozone reduction is independent of location of methane reduction [pick the cheapest option] 2. Ozone reduction is generally largest in polluted regions [high nitrogen oxides] 3. Methane reduction is a win-win for climate and air quality

CONCLUSIONS Methane reduction is a win-win for climate and air quality. This is a robust result across global chemical transport models. A 10% reduction should pay for itself and another 10% can be paid for with modest carbon credits. The maximum impact on air quality is in high NOx regions. The location of the methane reduction is not important for either climate or air quality, so pick the least expensive options.

On To Lecture 8

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

Multi-model study shows similar surface ozone decreases over NH continents when global methane is reduced Full range of 12 individual models  >1 ppbv O 3 decrease over all NH receptor regions  Consistent with prior studies TF HTAP 2007 report draft available at EUROPEN. AMER.S. ASIAE. ASIA

Will methane emissions increase in the near future? Anthropogenic CH 4 emissions (Tg yr -1 ) Current Legislation (CLE) Scenario Dentener et al., ACP, 2005 A2 B2 MFR

Possible Emission Trajectories in the Near Future (2005 to 2030) Anthropogenic CH 4 Emissions (Tg yr -1 ) Control scenarios reduce 2030 CH 4 emissions relative to CLE by: A) -75 Tg (18%) – cost-effective now B) -125 Tg (29%) – possible with current technologies C) -180 Tg (42%) – requires new technologies A B C CLE Baseline Surface NO x Emissions 2030:2005 ratio A.M. Fiore

Summary: Climate and Air Quality Benefits From CH 4 Control Significant CH 4 reductions can pay for themselves Benefits are independent of reduction location  Target cheapest controls worldwide Complementary to NOx, NMVOC controls and maximum benefit in high NOx regions Robust response over NH continents across models  ~1 ppbv surface O 3 for a 20% decrease in anthrop. CH 4 Decreases hemispheric background O 3  Opportunity for joint international air quality-climate management