Glen Peters Center 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

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 SF 6 N2ON2O CH 4 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 model FT/λT/λ c1c1 c3c3 c2c2 k1k1 k2k2 Mixed-layer Intermediate Deep

SBSTA Meeting (Bonn), 3-4 April, layer box model FT/λT/λ

SBSTA Meeting (Bonn), 3-4 April, layer box model F T/λT/λ Energy going in Energy going out Change in energy

SBSTA Meeting (Bonn), 3-4 April, layer box model F T/λT/λ Instantaneous RF Instantaneous T Rate of change T For a pulse emission…

SBSTA Meeting (Bonn), 3-4 April, 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 SF 6 N2ON2O CH 4 1. A 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 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 SF 6 N2ON2O CH 4 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 CH 4 and twice RF What is GWP, GTP, iGTP? GWP=GTP=iGTP=0.5 N 2 O is the “universal” gas GWP~GTP~iGTP

SBSTA Meeting (Bonn), 3-4 April, 2012 Does GWP=iGTP? Depends on the reference gas… Does CO 2 represent the other species well?

SBSTA Meeting (Bonn), 3-4 April, 2012 CO 2 response can capture time scales of CH 4, N 2 O, and SF 6 CO 2 response can’t capture time scale of BC

SBSTA Meeting (Bonn), 3-4 April, 2012 CO 2 as a reference BC SF 6 N2ON2O CH 4 CO 2 does a bad job for BC CO 2 does a good job for N 2 O Can estimate if CO 2 over/under estimates Why is GWP≠GTP? The path is different…

SBSTA Meeting (Bonn), 3-4 April, 2012 CO 2 as a reference for CH 4 RF iAGTPAGTP AGWP

SBSTA Meeting (Bonn), 3-4 April, 2012 Does GWP=iGTP? Depends on the reference gas AND metric… CO 2 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 (CO 2 ) is good enough for integrated metrics GWP≠GTP since the reference gas (CO 2 ) 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 CO 2 as a reference gas is underappreciated Reference gas may be more important for instantaneous metrics (e.g., GTP) How does CO 2 affect metric values?