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Why care about methane
Daniel J. Jacob
Global present-day budget of atmospheric methane
Atmospheric oxidation by OH radical
CH4
Emission
550  60 Tg/year
Waste: 60 Other: 40
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 CO2
• Together methane and black carbon (BC)
have radiative forcing comparable to CO2 
they have made comparable contribution to
1750-present climate change
• But atmospheric lifetimes of methane (10
years) and BC (~1 week) are shorter than
CO2 (> 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

Climate metric =

0
time
(impact)(discount rate)dt
…usually normalized to CO2
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
CO2 methane BC
H
AGWP(X)   ΔFX (t )dt
0
GWP(X) 
AGWP(X)
AGWP(CO2 )
Discount rate: step function
time
H
IPCC [2014]
GWP for methane
vs. chosen time horizon:
28 for H = 100 years
 1 Tg CH4 = 28 Tg CO2 (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 2oC (“two degrees of danger”).
What does such a goal mean in terms of climate policy?
Global temperature potential (GTP) metric introduced by IPCC AR5
Global mean surface temperature change at t = H
CO2 methane BC
GTP ( X ) 
Temperature change vs. time
for pulse unit emission at t = 0
ΔTo , X (H )
ΔTo ,CO 2 (H )
Discount rate:
Dirac function
Methane GTP20 = 67
GTP100 = 4
H
time
IPCC [2014]
Temperature response
to actual 2008 emissions
taken as a 1-year pulse
Methane as important as CO2
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
t = 20 years
climate
response
F < 0
To
t = 100 years
back to
original
equilibrium
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 CO2.
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 CO2
• 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
4th-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:
O2
1. VOC  OH 
 HO2  products
O3
O3
2. HO2  NO 
 OH  NO2
O2
3. NO2  h 
 NO  O3
VOC
NOx
over US it is mainly NOx-limited
VOCs increase ozone production efficiency (OPE) per unit NOx emitted
ozone produced
OPE =
 as VOCs 
NO x emitted
VOC
HO2
OH
HNO3
NO
hv
NO2
O3
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 HO2
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
4th 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
MDA8
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