1. Why care about methane Daniel J. Jacob

2. Global present-day budget of atmospheric methane Wetlands: 160 Fires: 20 Livestock: 110 Rice: 40 Oil/Gas: 70 Coal: 50 Waste: 60 Other: 40 EDGAR anthropogenic emissions + LPJ wetlands (Tg a -1 ) CH 4 Atmospheric oxidation by OH radical Lifetime 9 years Global distribution of emissions Emission 550  60 Tg/year Atmospheric concentration: 1800 ± 50 ppb - well-mixed in troposphere - declines in stratosphere above 10-18 km

3. Rising atmospheric methane The last 30 years (remote sites) Methane The last 1000 years (ice cores) IPCC [2014]

4. Radiative forcing of climate change Solar flux F in Terrestrial flux F out = σ T 4 Global radiative equilibrium: F in = F out Perturb greenhouse gases or aerosols radiative forcing  F = F in - F out Surface equilibrium temperature responds as  T SURF ~  F T ATM T SURF

5. Radiative forcing referenced to emissions, 1750-present Radiative forcing from methane emissions is 0.97 W m -2, compared to 1.68 W m -2 for CO 2 Together methane and black carbon (BC) have radiative forcing comparable to CO 2  they have made comparable contribution to 1750-present climate change But atmospheric lifetimes of methane (10 years) and BC (~1 week) are shorter than CO 2 (> 100 years) What does that mean for priorities in controlling future emissions? [IPCC, 2014]

6. Climate policy metrics consider the integrated future impact of a pulse unit emission of a radiative forcing agent Inject 1 kg of agent X at time t = 0 time Concentration C(t) from pulse time Impact from pulse = f(C(t)) time Discount rate Climate metric = (impact)  (discount rate)  dt …usually normalized to CO 2

7. Standard IPCC metric: Global Warming Potential (GWP) Integrated radiative forcing over time horizon [0, H] CO 2 methane BC Radiative forcing  F vs. time for pulse unit emission of X at t = 0 GWP for methane vs. chosen time horizon: 28 for H = 100 years  1 Tg CH 4 = 28 Tg CO 2 (eq) IPCC [2014] GWP is easy to compute but does not correspond to any physical impact Methane GWP is 28 for 100 years but 84 for 20 years; which to use? H Discount rate: step function time 100-y GWP 20-y GWP

8. Paris Climate Conference (December 2015) Countries pledge to keep global warming to less than 2 o C (“two degrees of danger”). What does such a goal mean in terms of climate policy?

9. Global temperature potential (GTP) metric introduced by IPCC AR5 Global mean surface temperature change at t = H CO 2 methane BC Temperature change vs. time for pulse unit emission at t = 0 Temperature response to actual 2008 emissions taken as a 1-year pulse IPCC [2014] Methane as important as CO 2 for 10-year horizon, unimportant for 100-year horizon Discount rate: Dirac function H time Methane GTP 20 = 67 GTP 100 = 4

10. Why does methane cause only a short-term temperature response? ToTo ToTo T o +  T o ToTo F in t < 0 t = 0 t = 20 years t = 100 years climate equilibrium emission pulse climate response back to original equilibrium F out  F = 0  F < 0  F > 0

11. Implication of GTP-based policy for near-term climate forcers GTP potential Right now we’ll just worry about CO 2. But in 70 years please start acting on methane, and in 95 years go all after black carbon, baby! Aiming to optimize for a maximum temperature change on a 100-year horizon: IPCC [2014]

12. Sole focus on temperature change over long-term horizon fails to address immediate climate problems No summer Arctic sea ice in 20 years? Sea level rise increasing hurricane damage?

13. Methane should be part of climate policy for reasons totally different than CO 2 It addresses climate change on time scales of decades – which we care about It offers decadal-scale results for accountability of climate policy It is an alternative to geoengineering by aerosols It has air quality co-benefits Reducing methane emissions makes money Solution is to have two climate metrics, for 20-year and 100-year horizons

14. 4 th -highest annual maximum of daily 8-h average ozone, 2010-2012 EPA [2014] Ozone production mechanism: Production RATE can be VOC- or NO x -limited: O3O3 VOC O3O3 NO x New standard: 70 ppb Methane as a precursor of ozone air pollution over US it is mainly NO x -limited

15. VOCs increase ozone production efficiency (OPE) per unit NO x emitted HO 2 OH NO 2 NO HNO 3 VOC hv O3O3 Emission Deposition Methane (9-year lifetime) increases global background tropospheric ozone in two ways: It is the principal sink of OH and so increases OPE; Methane oxidation produces formaldehyde (HCHO), which photolyzes to produce HO 2

16. Background ozone is increasingly relevant for meeting NAAQS Mean ozonesonde data in summer 2013 NAAQS Ozone in middle troposphere is routinely in excess of NAAQS; Downwelling to surface can cause NAAQS exceedances Travis et al. [2016] Observations GEOS-Chem model

17. North American ozone background over the US 4 th highest annual North American background ozone (GEOS-Chem model) Zhang et al. [2011] defined as the surface ozone concentrations that would be present in the absence of North American anthropogenic emissions Background makes large increment towards NAAQS

18. Source attribution of ozone in Intermountain West NA background ≡ simulation with no anthropogenic sources in N America 2006 o Most ozone is from non US sources o Non US anthropogenic sources contribute ~15 ppb; half is from methane Zhang et al. [2014] Stratospheric intrusion MDA8

19. ~ 1 ppb decrease in surface ozone across the northern hemisphere co-benefit of climate policy; impractical as air quality policy driver range over 18 models Effect of ~25% decrease in global anthropogenic methane emissions North America Europe East Asia South Asia Reducing methane anywhere would benefit surface ozone globally Fiore et al. [2009]