Glen Peters Center for International Climate and Environmental Research – Oslo (CICERO)

1. Glen Peters Center for International Climate and Environmental Research – Oslo (CICERO)

2. Why iGTP (integrated GTP)? • Another metric just complicates matters?

3. A primer… Integrated response to a pulse emission equals instantaneous response to a sustained emission Mathematical property of a convolution for a linear R

7. 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?

8. iGTP=GWP? BC CH4 Within about 5-10% for a range of TH (except for BC) But why? N2O SF6

9. Why? Two cases • Absolute metrics • AGWP, AGTP, iAGTP • Relative metrics • GWP, GTP, iGTP • Reference gas important! • Different reasons for similarities

10. 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…

11. Example, 3 layer box model F T/λ c1 Mixed-layer k1 c2 Intermediate k2 c3 Deep

12. 1 layer box model F T/λ

13. 1 layer box model F Energy going in Change in energy Energy going out T/λ

14. 1 layer box model For a pulse emission… F Instantaneous RF Rate of change T Instantaneous T T/λ

15. 1 layer box model Integrate for a pulse emission… AGWP AGWP ~AGTP ~iAGTP iAGTP/λ One equation linking the AGWP, AGTP, iAGTP

16. 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

17. Absolute metrics CH4 BC 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 SF6 N2O

18. 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

19. iGTP=GWP? BC CH4 Within about 10% for a range of TH (except for BC) But why? N2O SF6

20. 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

21. Does GWP=iGTP? • Depends on the reference gas… • Does CO2 represent the other species well?

22. CO2 response can capture time scales of CH4, N2O, and SF6 CO2 response can’tcapture time scale of BC

23. 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… N2O SF6

24. CO2 as a reference for CH4 RF AGWP AGTP iAGTP

25. 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)

26. 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”

27. Details • Possible to look into more technical details • Climate model parameterisation • Fluxes in and out of different ocean layers • … • Read the paper

28. 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

29. 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?