Glen Peters
Center for International Climate and
Environmental Research – Oslo
(CICERO)
Why iGTP (integrated GTP)?
Another metric just complicates matters?
SBSTA Meeting (Bonn), 3-4 April, 2012
A primer…
Integrated response to a pulse emission
equals
instantaneous response to a sustained emission
Mathematical property of a
convolution for a linear R
SBSTA Meeting (Bonn), 3-4 April, 2012
SBSTA Meeting (Bonn), 3-4 April, 2012
SBSTA Meeting (Bonn), 3-4 April, 2012
SBSTA Meeting (Bonn), 3-4 April, 2012
Why iGTP (integrated GTP)?
• In the early days of the GWP, research
focussed on iGTP!
• At some stage, the link from radiative forcing
to temperature was lost.
• Is the iGTP similar to the GWP?
SBSTA Meeting (Bonn), 3-4 April, 2012
iGTP=GWP?
BC
CH4
Within about 5-10% for a
range of TH (except for BC)
N 2O
But why?
SBSTA Meeting (Bonn), 3-4 April, 2012
SF6
Why? Two cases
• Absolute metrics
• AGWP, AGTP, iAGTP
• Relative metrics
• GWP, GTP, iGTP
• Reference gas important!
• Different reasons for similarities
SBSTA Meeting (Bonn), 3-4 April, 2012
Use Box-Diffusion EnergyBalance Model (EBM)
• Metrics use Impulse Response Functions
• IRFs 1-1 mapping with box-diffusion EBM
• Analytical solutions
• Easy problem (?)
• Don’t need a GCM to understand the
physics…
SBSTA Meeting (Bonn), 3-4 April, 2012
Example, 3 layer box model
F
Mixed-layer
Intermediate
T/λ
c1
c2
c3
Deep
SBSTA Meeting (Bonn), 3-4 April, 2012
k1
k2
1 layer box model
F
SBSTA Meeting (Bonn), 3-4 April, 2012
T/λ
1 layer box model
F
Energy going in
Change in energy
T/λ
SBSTA Meeting (Bonn), 3-4 April, 2012
Energy going out
1 layer box model
For a pulse emission…
F
Instantaneous RF
Rate of change T
T/λ
SBSTA Meeting (Bonn), 3-4 April, 2012
Instantaneous T
1 layer box model
Integrate for a pulse emission…
AGWP
AGWP
~AGTP
iAGTP/λ
~iAGTP
One equation linking the AGWP,
AGTP, iAGTP
SBSTA Meeting (Bonn), 3-4 April, 2012
EBM Interpretation
• AGWP: Cumulative energy added to the
system (integrated forcing)
• iAGTP/λ: Cumulative energy lost from the
system (feedbacks, back to space)
• OHC/AGTP: Energy currently in the system
• OHC: in the ocean
• AGTP: in the surface-mixed layer
SBSTA Meeting (Bonn), 3-4 April, 2012
Absolute metrics
CH4
1. ABC
pulse emission decays to zero,
2. thus the energy in the system decays to zero
3. and for energy balance the accumulated
energy in equals the energy out
SF6
iAGTP  AGWP
N 2O
“Inertia” dictates the difference
SBSTA Meeting (Bonn), 3-4 April, 2012
Normalized metrics
• If iAGTP  AGWP, then it does not imply
iGTP  GWP
• Why? The reference gas…
• The idea of a reference gas is to
represent the other gases
SBSTA Meeting (Bonn), 3-4 April, 2012
iGTP=GWP?
BC
CH4
Within about 10% for a range
of TH (except for BC)
N 2O
But why?
SBSTA Meeting (Bonn), 3-4 April, 2012
SF6
A thought experiment…
• Suppose X is the reference gas
• X has a lifetime as for CH4 and twice RF
• What is GWP, GTP, iGTP?
• GWP=GTP=iGTP=0.5
• N2O is the “universal” gas
• GWP~GTP~iGTP
SBSTA Meeting (Bonn), 3-4 April, 2012
Does GWP=iGTP?
• Depends on the reference gas…
• Does CO2 represent the other species
well?
SBSTA Meeting (Bonn), 3-4 April, 2012
CO2 response can capture time scales of
CH4, N2O, and SF6
CO2 response can’t capture time scale of
BC
SBSTA Meeting (Bonn), 3-4 April, 2012
CO2 as a reference
BC
CH4
CO2 does a bad job for BC
CO2 does a good job for N2O
Can estimate if CO2 over/under estimates
Why is GWP≠GTP? The path is different…
N 2O
SBSTA Meeting (Bonn), 3-4 April, 2012
SF6
CO2 as a reference for CH4
RF
AGTP
SBSTA Meeting (Bonn), 3-4 April, 2012
AGWP
iAGTP
Does GWP=iGTP?
• Depends on the reference gas AND metric…
• CO2 as a reference
• GWP~iGTP: GWP and iGTP are integrations
•
Except for very short lived species
• GWP≠GTP: GTP is a pathway (instantaneous)
SBSTA Meeting (Bonn), 3-4 April, 2012
Does GWP=iGTP=GTP?
• It is often argued
• “GWP and GTP are different since they are a
different responses”
• I would argue
• “GWP and GTP are different since the
reference gas is bad”
SBSTA Meeting (Bonn), 3-4 April, 2012
Details
• Possible to look into more technical details
• Climate model parameterisation
• Fluxes in and out of different ocean layers
• …
• Read the paper
SBSTA Meeting (Bonn), 3-4 April, 2012
Conclusion
• iAGTP  AGWP for a pulse emission since
RF,T  0 and energy balance requires the
energy in to equal the energy out
• iGTP  GWP since the reference gas (CO2)
is good enough for integrated metrics
• GWP≠GTP since the reference gas (CO2) is
not good enough for the pathway of RF
SBSTA Meeting (Bonn), 3-4 April, 2012
Policy Implications
• If integrated temperature is the goal of
climate policy, then GWP is a simple metric
with a similar response
• Common metrics are connected
• The importance of CO2 as a reference gas is
underappreciated
• Reference gas may be more important for
instantaneous metrics (e.g., GTP)
• How does CO2 affect metric values?
SBSTA Meeting (Bonn), 3-4 April, 2012