1. Uncertainties in soil and terrestrial carbon response to 20th century human CO 2 emissions J.-F. Exbrayat 1, Q. Zhang 2, A. J. Pitman 3, G. Abramowitz 1 and Y.-P. Wang 4 1 Climate Change Research Centre, UNSW Sydney 2 College of Global Change and Earth System, Beijing Normal University, Beijing, China 3 ARC Centre of Excellence for Climate System Science, UNSW, Sydney 4 Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research CoE CSS Annual Workshop, Hobart 26/09/2012

2. Global carbon cycle Source: IPCC AR4 (image downloaded from http://www.gfdl.noaa.gov/anthropogenic-carbon-cycle)

3. Terrestrial Carbon Budget in a nutshell Homeostatic pre-industrial conditions: carbon uptake = release (GPP ~ R h ) GPP enhanced by anthropogenic increase in atmospheric CO 2 (fertilisation) Release (R h ) favoured by increasing temperatures Up to now, the gain in GPP is higher than the gain in R h = net uptake From Wania et al. [2012 GMD]

4. Limitations in nutrient availability slow down the carbon cycle R h response to soil temperature and soil moisture Sources of uncertainty From Zhang et al. [2011 GRL] From Exbrayat et al. [2012 under review]

5. Biogeochemical model within CABLE Coupled to CSIRO Mk3L GCM (3.2 lat x 5.6 lon) [Phipps et al., 2011 GMD] Several nutrient modes with corresponding limitations on C cycle: C-only, CN, CNP CASA-CNP model From Wang et al. [2010 BG]

6. R h parameterisation in soil biogeochemical models

7. Experiments 27 model versions: each combination of a moisture response function, a temperature response function and a nutrient mode Spin-up all model versions with pre-industrial atmospheric CO 2 (284.7 ppmv) Transient runs of increasing atmospheric CO 2 from 1850 to 2005 Prescribed SSTs from CSIRO Mk3.6 driven by CMIP5 historical emission data [Rotstayn et al., 2012 ACP]

8. Simulated 20 th century NEA Emission data from Carbon Dioxide Information Analysis Center [Boden et al., 2010]

9. Comparison with independent estimates

10. Regional impact Regions in CDIAC data [Boden et al., 2010]

11. Regional offset

12. Conclusion Introducing NP limitations reduces the NEA but also narrows the uncertainty introduced by different parameterisations of R h NP limited models are well in agreement with independent estimates when considering different time windows of the period 1959-2005 NP limitations reduce or even cancel the capability of regions to offset their emissions

13. Way forward What are the policy / trade scheme relevant implications of the regional results? Will the land surface remain a net sink? What is the effect of a more detailed N cycle (with separation of different inorganic N species) within CASA-CNP?

14. Thank you for your attention Questions? e: j.exbrayat@unsw.edu.au