Uncertainties in soil and terrestrial carbon response to 20th century human CO2 emissions

J.-F. Exbrayat1, Q. Zhang2, A. J. Pitman3, G. Abramowitz1 and Y.-P. Wang4

1 Climate Change Research Centre, UNSW Sydney2 College of Global Change and Earth System, Beijing Normal University, Beijing, China3 ARC Centre of Excellence for Climate System Science, UNSW, Sydney4 Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research

CoE CSS Annual Workshop, Hobart 26/09/2012

Global carbon cycle

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

Terrestrial Carbon Budget in a nutshell

Homeostatic pre-industrial conditions: carbon uptake = release (GPP ~ Rh)

GPP enhanced by anthropogenic increase in atmospheric CO2 (fertilisation)

Release (Rh) favoured by increasing temperatures

Up to now, the gain in GPP is higher than the gain in Rh= net uptake

From Wania et al. [2012 GMD]

• Limitations in nutrient availability slow down the carbon cycle

• Rh response to soil temperature and soil moisture

Sources of uncertainty

From Zhang et al. [2011 GRL]

From Exbrayat et al. [2012 under review]

• 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]

Rh parameterisation in soil biogeochemical models

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 CO2 (284.7 ppmv)

Transient runs of increasing atmospheric CO2 from 1850 to 2005

Prescribed SSTs from CSIRO Mk3.6 driven by CMIP5 historical emission data [Rotstayn et al., 2012 ACP]

Simulated 20th century NEA

Emission data from Carbon Dioxide Information Analysis Center [Boden et al., 2010]

Comparison with independent estimates

Regional impact

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

Regional offset

Conclusion

Introducing NP limitations reduces the NEA but also narrows the uncertainty introduced by different parameterisations of Rh

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

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?

Thank you for your attentionQuestions?

e: j.exbrayat@unsw.edu.au