1. Climate models in (palaeo-) climatic research How can we use climate models as tools for hypothesis testing in (palaeo-) climatic research and how can we apply this to understand climate change from the Cretaceous to the near future? Nanne Weber

2. Temperature 1900-2100 Different scenario’s for GreenHouse (GH) Gases +0.6C

3. History of climate last 300 Myr last 3 Myr last 1000yrlast 50 kyr cold  warm

4. Outline of this talk 1.What is a climate model? 2.Middle Holocene (6000 years Before Present =6 kyr BP): stable climate with warm NH summers 3.The Last Glacial Maximum (LGM, 21 kyr BP): cold, low GH Gas levels and large continental caps 4.The Paleocene-Eocene Thermal Maximum (PETM, 55 Myr BP): warm and high GH Gas levels

5. A climate model describes the Earth’s climate system

6. Computations are done on a grid with finite size You have to choose a grid Grid size is always limited Small systems are not simulated, so have to be parameterised Small ~ grid distance Model can only be validated at large spatial scale

7. Important processes that have to be parameterised: Turbulent transport of heat, impuls, moisture Clouds Precipitation Boundary layers Radiation Climate models tend to be very sensitive to these processes!!

8. Completeness of model: this is determined by availability, computer resources and research question

9. Model= a set of mathematical equations which are solved on a grid by a computer *Equations describe many different processes (e.g. incoming radiation, cloud formation, heat transport, snow melt) in one or more components (e.g. atmosphere, ocean, vegetation) of the climate system *With a given spatial resolution *With given boundary conditions (e.g. glacial ice sheets, Greenhouse gases for 2100, Cretaceous land-sea mask) Climate models

10. The middle Holocene (6 kyr BP) Northern Africa wet southern Europe and India also wet, northern Europe dry

11. Het midden Holoceen Middle Holocene

12. The middle Holocene (6 kyr BP) Hypothesis: changes in precipitation (and vegetation) at 6 ka due to orbital forcing This holds also for cyclic patterns in sedimentary records

13. Orbital parameters: main cycles

14. Orbital parameters at 6kyr BP: higher NH summer insolation Changes in monthly-mean insolation as a function of latitude

15. The middle Holocene changes in summer temperature and precipitation (6k minus 0k)

16. Monsoon precipitation over Africa at 6 kyr BP: 1) models underestimate the signal as indicated by biomes 2) vegetation and ocean feedbacks help! Figure from IPCC Third Assessment Report (2001)

17. The middle Holocene (6 ka BP) Hypothesis: changes in precipitation (and vegetation) at 6 ka due to orbital forcing OK???

18. The Last Glacial Maximum LGMPI CO 2 ppm 185280 CH 4 ppb 350760 N 2 O ppb 200270 LGM versus Pre-Industrial (PI) climate: lower GH-gas concentrations LGM

19. The Last Glacial Maximum (LGM) period of maximum extent of NH continental ice sheets

20. Hypothesis: changes in annual temperature and precipitation (21k minus 0k) are due to low GH-gas and ice sheets

21. Regional cooling

22. Cooling over the north Atlantic: data (diamonds) and 16 different models Figure from Kageyama et al. (2006)

23. Hypothesis OK? Yes for regional- mean cooling, but we do not (yet) understand spatial details

24. If we do a transient run, with prescribed insolation (upper), CO2 (middle) and ice (lower), do we find a realistic Antarctic temperature?

25. If we do a number of transient runs, each with separate forcings (insolation, CO2 and ice), where do we find a response to what?

26. Different aspects of climate forced by different factors!! SAT – CO2, ice Monsoon – orbital

27. What forces variations methane? One hypothesis is: variations in the wetland source Wetland area for today and LGM (Kaplan, 2002)

28. Extended hypothesis: in the cold and dry LGM climate wetland area is reduced and emissions are much lower than today

29. Methane model: compute wetland area and emissions from climatic fields (temperature, bottom moisture, etc) black: NH high-latitudes green: tropics yellow: SH high-latitudes

30. PETM: extremely warm, because of high GH-gas concentrations? Zachos et al. (2001) Large ‘mismatch’ in temperature between model (red line) and data (dots: red=PETM, yellow=just before or after PETM) Fig. from Sluijs (2006)

31. PETM simulation with EC-Earth, with CO2=1400 ppm and prescribed sea surface temperatures. Results for DJF surface temps

32. EC-Earth runs: polar warmth seems to be due to combination of feedbacks (sea-ice and snow, clouds,..) Comparison of simulated temps with data (green dots)

33. Ten Can one step twice into the same river?