Why care about methane Daniel J Jacob Global
Why care about methane Daniel J. Jacob
Global present-day budget of atmospheric methane CH 4 Emission 550 60 Tg/year Waste: 60 Other: 40 Atmospheric oxidation by OH radical Lifetime 9 years Atmospheric concentration: 1800 ± 50 ppb - well-mixed in troposphere - declines in stratosphere above 10 -18 km Wetlands: 160 Global distribution of emissions Coal: 50 Fires: 20 Oil/Gas: 70 Rice: 40 Livestock: 110 EDGAR anthropogenic emissions + LPJ wetlands (Tg a-1)
Rising atmospheric methane The last 1000 years (ice cores) Methane The last 30 years (remote sites) Methane IPCC [2014]
Radiative forcing of climate change Terrestrial flux Fout = σ T 4 Solar flux Fin TATM TSURF • Global radiative equilibrium: Fin = Fout • Perturb greenhouse gases or aerosols radiative forcing F = Fin - Fout • Surface equilibrium temperature responds as TSURF ~ F
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]
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 Concentration C(t) from pulse time Impact from pulse = f(C(t)) time Discount rate time Climate metric = (impact) (discount rate) dt …usually normalized to CO 2
Standard IPCC metric: Global Warming Potential (GWP) Integrated radiative forcing over time horizon [0, H] Radiative forcing F vs. time for pulse unit emission of X at t = 0 CO 2 methane BC Discount rate: step function time H IPCC [2014] GWP for methane vs. chosen time horizon: 28 for H = 100 years 1 Tg CH 4 = 28 Tg CO 2 (eq) • 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? 20 -y GWP 100 -y GWP
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?
Global temperature potential (GTP) metric introduced by IPCC AR 5 Global mean surface temperature change at t = H CO 2 methane BC Temperature change vs. time for pulse unit emission at t = 0 Discount rate: Dirac function Methane GTP 20 = 67 GTP 100 = 4 H time IPCC [2014] Temperature response to actual 2008 emissions taken as a 1 -year pulse Methane as important as CO 2 for 10 -year horizon, unimportant for 100 -year horizon
Why does methane cause only a short-term temperature response? Fin Fout To To t<0 t=0 climate equilibrium emission pulse F = 0 F > 0 To + To To t = 20 years t = 100 years climate response back to original equilibrium F < 0 F = 0
Implication of GTP-based policy for near-term climate forcers Aiming to optimize for a maximum temperature change on a 100 -year horizon: 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! IPCC [2014]
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?
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 has air quality co-benefits • It is an alternative to geoengineering by aerosols • Reducing methane emissions makes money Solution is to have two climate metrics, for 20 -year and 100 -year horizons
Methane as a precursor of ozone air pollution 4 th-highest annual maximum of daily 8 -h average ozone, 2010 -2012 EPA [2014] New standard: 70 ppb Ozone production mechanism: Production RATE can be VOC- or NOx-limited: O 3 VOC NOx over US it is mainly NOx-limited
VOCs increase ozone production efficiency (OPE) per unit NO x emitted VOC HO 2 NO O 3 Emission OH HNO 3 hv NO 2 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
Background ozone is increasingly relevant for meeting NAAQS Mean ozonesonde data in summer 2013 • Ozone in middle troposphere is routinely in excess of NAAQS; • Downwelling to surface can cause NAAQS exceedances Observations GEOS-Chem model NAAQS Travis et al. [2016]
North American ozone background over the US defined as the surface ozone concentrations that would be present in the absence of North American anthropogenic emissions 4 th highest annual North American background ozone (GEOS-Chem model) Background makes large increment towards NAAQS Zhang et al. [2011]
Source attribution of ozone in Intermountain West NA background ≡ simulation with no anthropogenic sources in N America MDA 8 Stratospheric intrusion 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]
Reducing methane anywhere would benefit surface ozone globally Effect of ~25% decrease in global anthropogenic methane emissions range over 18 models Fiore et al. [2009] North America Europe East Asia South Asia • ~ 1 ppb decrease in surface ozone across the northern hemisphere • co-benefit of climate policy; impractical as air quality policy driver
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