Do Simplified Models Have any Useful Skill ?

1. Do Simplified Models Have any Useful Skill ? Thomas Stocker Climate and Environmental Physics University of Bern, Switzerland www.climate.unibe.ch Collaborators: R. Knutti, F. Joos, G.-K. Plattner, A. Schmittner

2. Content of Presentation 1. A model hierarchy 2. Simplified models: progress or confusion? 3. When are simplified models useful? • extensive parameter studies • developing novel model strategies • coupling biogeochemistry and other tracers 4. Prospects of simplified modeling

3. Hierarchy of Climate Models

4. approx. CPU time for 250-year simulation

5. Example 1: Multiple equilibrium states of the Atlantic overturning circulation

6. „One wonders whether other, quite different states of flow are permissible in the ocean or some estuaries and if such a system might jump into one of these with a sufficient perturbation. If so, the system is inherently fraught with possibilities for speculation about climatic change.“ Stommel (1961)

7. Bryan (1986)

8. next 1000 yrs ? last 8000 yrs last glacial Multiple Equilibria of the Atlantic MOC Stocker & Marchal (2000)

9. Simplified models propose a unified explanation of abrupt climate change

10. Example 2: Ocean circulation during the last glacial maximum (20,000 years ago)

11. modern LGM Atlantic MOC at the Last Glacial Maximum • zonally averaged ocean model • statistical dynamical atmosphere • modified Arctic freshwater fluxes Ganopolski et al. (1998)

12. modern LGM Atlantic MOC at the Last Glacial Maximum • 3d ocean model • energy-moisture balance atmosphere • modified continental run-off Weaver et al. (1998)

13. Atlantic MOC at the Last Glacial Maximum DWF shifted south MOC at similar strength deep ocean less ventilated DWF as today MOC reduced by > 60 % deep ocean less ventilated

14. Specific realisations of simplified models are too strongly influenced by model setup and parameter choices. • No information on the robustness of the results.

15. What constitutes a useful contribution of simplified models to science progress ? • extensive parameter studies • developing novel model strategies • coupling biogeochemistry

16. Simplified models are useful ... to perform extensive parameter studies and to explore uncertainties

17. Change of the Atlantic meridional overturning IPCC TAR (2001, Chapter 9)

18. Atlantic MOC Beyond 2100 Greenhouse Gas Forcing (Stocker & Schmittner, 1997)

19. Atlantic MOC Beyond 2100 Threshold of North Atlantic MOC (Stocker & Schmittner, 1997)

20. Parameter dependence of MOC threshold (Stocker & Schmittner, 1997)

21. Parameter dependence of MOC threshold (Stocker & Schmittner, 1997)

22. Predictability of Atlantic MOC Knutti & Stocker (2002)

23. Simplified models are useful ... to develop novel modeling strategies to outline possible application by more complex models

24. [IPCC WG1, SPM]

25. Estimating a PDF requires ~5,000 model simulations

26. Influence of mixing schemes on climate sensitivity

27. Climate sensitivity in different comprehensive models: IPCC TAR WG1, Table 9.A1 „It seems likely that models with higher sensitivity, those predicting the most drastic anthropogenic climate changes in the future, may have difficulty satisfying the ocean constraint.“ (Barnett et al., 2001)

28. global ocean heat uptake from the mid 50s to the mid 90s: ~ 21023 J Levitus et al. (2000)

29. Coupled model simulation including anthropogenic forcing: • observed GHG • direct aerosol effect • no indirect aerosol effect ocean heat uptake consistent for small climate sensitivity. Barnett et al. (2001)

30. Simulated ocean heat uptake 1955 - 1995 — complete forcing — no aerosol cooling Knutti et al. (2002)

31. How well do observations constrain climate sensitivity? complete forcing Knutti et al. (2002)

32. How well do observations constrain climate sensitivity? no aerosol cooling complete forcing Knutti et al. (2002)

33. Simplified models are useful ... to include biogeochemical cycles and various tracers to investigate processes in the coupled system

34. Climate model of reduced complexity • zonally averaged ocean, 3 basins • energy balance atmosphere • thermodynamic sea ice model • annual mean or seasonal cycle • prognostic carbon cycle

35. Anthropogenic CO2 in the Atlantic Plattner et al. (2002)

36. Outgassing of O2 in the Atlantic Plattner et al. (2002)

37. Revision of land uptake estimates Plattner et al. (2002)

38. Do simplified models have any useful skill?

39. Simplified models: pros and cons • positive: • great tools, efficient, lots of sensitivity studies • don‘t need big computers • parameterisations can be kept transparent • negative: • implication of reduced dynamics not testable • what was the model designed for? • time and spatial scales?

40. Simplified models: pros and cons • contribution to progress: • exploration of parameter space • estimation of uncertainty • development of novel methodologies • long-term simulation of coupled system • not useful: • „singular simulation syndrom“ • data fitting • „over-parameterisation“

41. Judicious use of simplified models: • simplified models cannot replace complex models. Reduced dynamics may, however, exhibit more clearly the dominating processes and feedback mechnisms at work. • simplified models are useful if applied judiciously. Beware of „singular simulations“! Robustness of results can be tested much better than in more complex models. • simplified models serve to test hypotheses, and explore their implications for other components of the climate system. They help target experiments with more expensive models.

42. Prospects of simplified models: • They shall not disappear! Simplified models remain valuable helpers in understanding the Earth system. • Simplified models will be used in feasibility studies before expensive simulations are started. • Simplified models will be used to interpret paleoclimatic data, and to test/determine transfer functions for proxy indicators. • Simplified models will be increasingly used for long-term (millennial) simulations for past and future climate changes.