Metrics for quantification of
influence on climate
Ayite-Lo Ajovan,
Paul Newman,
John Pyle,
A.R. Ravishankara
Co-Chairs, Science Assessment Panel
 Described in detail in the 2014 SAP report
 Will be available at the end of this year
 Simple parameters (a form of common currency) for
comparing atmospheric gases for their climate effects
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Atmospheric lifetime: Concept
Assume that L is
proportional to n.
100
Atmospheric concentration
Lifetime
t = 1/k
Infinitely long
lifetime
No loss,
No recovery
37
0
Time
Longer t
slower recovery
Shorter t
faster recovery
stop
emission
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Time
• Production determines how much
accumulates.
• The lifetime determines how rapidly the
atmosphere is “cleansed.”
Atmospheric lifetime is also a metric
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What is Greenhouse Effect?
Earth
Atmosphere
 Greenhouse Gas (GHG) absorbs Infra Red (IR) radiation; heats
surface (& lower atm)
 Changes in GHG abundance change the energy balance - hence
climate.
- How much? When?
 Arrhenius predicted (over 100 years ago) that temperature
would increase if CO2 increases.
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Ein
Eout
Energy going out
(proper units)
What is Greenhouse Effect?
Energy distribution governed by
fundamental laws of physics
CO2
O3
IR Wavelength
 Clearly see absorption by CO2 and other greenhouse gases
 Greenhouse effect is REAL! You can see it in the spectrum
(Otherwise, earth would be a frozen planet!).
 Adding gases traps more energy, but it takes time for the entire
Earth Land-Atmosphere-Ocean system to warm.
 Eventually the system comes to equilibrium
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Radiative Forcing
Earth
Atmosphere
Radiative Forcing is a metric
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Radiative Forcing
Earth
Atmosphere
• Instantaneous change in the outgoing flux
• A measure of what has happened… a look back
• Does not account for time dependence of
emission
Definition: Change in net irradiance at the tropopause (top of the radiative atm.)
subsequent to a perturbation after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with
surface and tropospheric temperatures and state held fixed at the unperturbed values
RF (wm-2) is a direct measure of the extent to which the Earth’s energy budget is
out of balance to stray away from its “normal” state. (Balance between incoming solar radiation and
outgoing infrared radiation)
RF is not measured for GHGs - it is calculated using input from basic lab
measurements of the gases.
Why use this?: DT = l x RF ;
Positive RF – heating; Negative RF– cooling; a good measure of
relative GHG contributions. Now extended to aerosols…
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Radiative Forcing
Global warming potential (GWP)
GWP =
Area A
Area CO2
Area A
Area CO2
20 Years since emission 100
Red: 15 yr.
Blue- CO2: 5-200 yr.
“lifetime” (from C- cycle
modeling)… CO2
“lifetime” is comlex
o 100 year GWP has
become the coin for
policy makers
o The time horizon is a
policy choice – not a
scientific choice
CO2-eq emission = emissions (e.g., in metric tonnes) x GWP (100)
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Global Warming Potentials (GWP)
 Estimate of “time-integrated” radiative forcing for a species
relative to that of the same mass of CO2
 Developed for the first IPCC assessment (Derwent, Rodhe, and
Wuebbles, 1990)--- very similar to ODP!
 Extensively used in national and international policy
 GWPs are a relative measure of the total energy added to the
climate system by a component in question relative to that added
by CO2.
 GWPs do not lead to equivalence with the temporal evolution of
the temperature response or that of other climate variables.
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Temperature Change
Global Temperature Potential (GTP)
DT(X)
GTP(X) =
DT(CO2 )
DT(X)
Rate of
decrease
determined
by property
of Earth
systmem
DT(CO2)
t1
Years after emission
t2
Blue = CO2
Red = gas w/ ~15 yr lifetime
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Global Temperature change Potentials
 A “temperature outcome” metric and does not give other climate
responses.
 An estimate of the relative temperature increase on a per unit
mass of emissions basis due to emissions of a greenhouse gas
relative to that due to CO2 emissions for the chosen time horizon
(Shine et al., 2005)
 First included in IPCC AR5.
 Accounts for climate sensitivity and the exchange of heat
between the atmosphere and the ocean, GTPs include physical
processes that GWPs do not.
 GTPs also incorporate extra uncertainties (compared to GWPs)
by including the climate response in the analysis.
 GTPs are somewhat sensitive to the specific climate model used
to calculate them and also to the background scenario.
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GTP vs. GWP
Molecule
CFC-11
CFC-12
Halon-1301
HCFC-22
HFC-23
HFC-32
HFC-134a
GWP (100yr)
4,660
10,200
6,290
1,760
12,400
677
1,300
GTP (100 yr)
2,920
8,590
4,700
265
12,800
98
214
o Similar trends
o GTP ≈ GWP when lifetime roughly equal to, or greater than, 100 yrs.
o GTP< GWP for shorter -lived substances
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Summary
1. There are a number of metrics for measuring
climate change due to emissions of a substance
2. Most common to date is Global Warming
Potential (GWP)
3. The GWP time horizon is NOT determined by
science; 100 year is the most commonly used
time horizon.
4. There is rough correspondence between GWP
and GTP. Higher GWP implies a higher GTP.
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THANK YOU
FOR
YOUR ATTENTION
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Extra slide
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Factors that influence energy & hence climate
Earth’s orbit around
the sun
Ocean (heat
storage/circulation)
Land processes and Ice-cover
solar intensity
Atmospheric Processes
Timescales of changes that are of current interest….
Decadal to century timescales…
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