Effect of parameter uncertainties on simulated land-atmosphere C flux

1. Effect of parameter uncertainties on simulated land-atmosphere C flux Zaehle, S.1,2, Sitch, S.1, Smith, B.3, Hattermann, F.1 1 Potsdam Institute for Climate Impact Research 2 Now: LSCE 3 Lund University Published as: Zaehle, S., Sitch, S., Smith, B., Hattermann, F., The effect of parameter uncertainties on the modeling of terrestrial biosphere dynamics, Global Biogeochemical Cycles, doi:10.1029/2004GB002395, 2005

2. Experiment Setup Plant physiology: 12 SOM dynamics: 3 Vegetation dynamics: 3 Plant structure: 9 Hydrology: 6 Disturbance: 3 Literature ranges for 36 parameters 1000 LatinHypercube Sample sets; Representative set of sites across all major global biomes Parameter importance for A, NPP, Rh, aAET, ecosystem C pools, vegetation composition etc. Identify main parameters contributing to model uncertainty Plant physiology: 6 SOM dynamics: 3 Vegetation dynamics: 2 Plant structure: 2 Hydrology: 1 Reduced set of 14 parameters 400 LatinHypercube Sample sets; Global uncertainty analysis (CRU2000 @ 3°x3°) Estimate of uncertainty in present day NEE Global Estimates of A, NPP, VegC compared against (top-down) benchmarks

3. Comparison with eddy-covariance data

4. Seasonal cycle of CO2 • Using setup of Heimann et al. (1998), i.e. TM2 (Kaminski 1999 )

5. Continental scale forest C budget (1990s) • potential natural vegetation, considering actual land-use patterns • 400 model realisations of 14 parameters sampled with latin-hypercube sampling

6. Uncertainty in future land-atmosphere flux

7. Global key model result ranges

8. Conclusions • Assessment of the (un)certainty of model results • Identification of key sensitive parameters for different ecosystem processes • Identification of systematic model-data biases not related to model parameterisation • Estimate of the range of land-atmosphere carbon flux consisent with the model structure and forcing considered • Parameter-based model uncertainty is of the same order as inter-model differences!

9. Uncertainty in global carbon cycle modeling Land-atmosphere C flux 1980s 1990s Residual terrestrial C sink (1980s) by IPCC TAR Model uncertainty conforming to top-down estimates of GPP and NPP, and vegetation C storage Full model uncertainty • Main Results: • Parameter based uncertainty in NPP greater than difference in NPP from six DGVMs (Cramer et al., 2001) • Seasonal cycle of NEE comparatively robust (compared to eddy-covariance and atmospheric CO2 measurements) • Interannual variability of NEE well constrained with robust decadal and longer time scale trends Zaehle et al., 2005, Global Biogeochem. Cycles

10. Spatial Pattern in Parameter Importance • RPCC for average NPP Maximum canopy conductance analogue (GM) Intrinsic quantum efficiency aC3