glen peters center for international climate and environmental research – oslo (cicero)

Glen PetersCenter for International Climate and

Environmental Research – Oslo (CICERO)

SBSTA Meeting (Bonn), 3-4 April, 2012

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

SF6N2O

CH4

Within about 5-10% for a range of TH (except for BC)

But why?

SBSTA Meeting (Bonn), 3-4 April, 2012

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 Energy-Balance 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 modelF T/λ

c1

c3

c2

k1

k2

Mixed-layer

Intermediate

Deep

SBSTA Meeting (Bonn), 3-4 April, 2012

1 layer box model

F T/λ

SBSTA Meeting (Bonn), 3-4 April, 2012

1 layer box model

F

T/λ

Energy going in

Energy going out

Change in energy

SBSTA Meeting (Bonn), 3-4 April, 2012

1 layer box model

F

T/λ

Instantaneous RF

Instantaneous T

Rate of change T

For a pulse emission…

SBSTA Meeting (Bonn), 3-4 April, 2012

1 layer box model

AGWP

iAGTP/λ

AGWP

~iAGTP

~AGTP

Integrate for a pulse emission…

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

BC

SF6

N2O

CH4

1. A pulse emission decays to zero, 2. thus the energy in the system decays to zero3. and for energy balance the accumulated

energy in equals the energy out

iAGTP AGWP

“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

SF6N2O

CH4

Within about 10% for a range of TH (except for BC)

But why?

SBSTA Meeting (Bonn), 3-4 April, 2012

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

SF6N2O

CH4

CO2 does a bad job for BCCO2 does a good job for N2O

Can estimate if CO2 over/under estimates

Why is GWP≠GTP? The path is different…

SBSTA Meeting (Bonn), 3-4 April, 2012

CO2 as a reference for CH4

RF

iAGTPAGTP

AGWP

SBSTA Meeting (Bonn), 3-4 April, 2012

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?