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What can we learn about sink efficiencies from trends in CO 2 airborne fraction ?

What can we learn about sink efficiencies from trends in CO 2 airborne fraction ?. M. Gloor, J. Sarmiento, N. Gruber University of Leeds, Princeton University, ETH Switzerland Article: ACP Discussions, 2010. Outline Introduction Time course of anthropogenic C flux

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What can we learn about sink efficiencies from trends in CO 2 airborne fraction ?

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  1. What can we learn about sink efficiencies from trends in CO2 airborne fraction ? M. Gloor, J. Sarmiento, N. Gruber University of Leeds, Princeton University, ETH Switzerland Article: ACP Discussions, 2010

  2. Outline • Introduction • Time course of anthropogenic C flux • Controls on Airborne fraction according • to linear models • Conclusions

  3. 1850 2000 2100 Cox et al. 2000 Rainforest Savannah

  4. How much anthropogenic carbon stays in the atmosphere ? => Airborne fraction (C.D. Keeling, 1973) ( ) Canadell et al. 2007, PNAS

  5. Recent statements from high-profile publications • Positive trend in AFFF+LU <=> decreasing trend of carbon ‘sink efficiency’ <=> positive carbon cycle - climate feedback • Canadell et al. PNAS, 2007, Raupach et al. 2008, LeQuere et al. • Nature Biogeoscience, 2009 • B. Constancy of AF fundamental property of system • Knorr Geophys Res. Lett. 2009 • Globally Carbon emissions accelerating Raupach et al. 2007

  6. Anthropogenic forcing

  7. Fossil Fuel Emissions (PgC yr-1) (Marland et al. 2006, updated) Fossil fuel e-folding time-scale (yr) Land use change Emissions (PgC yr-1) (Houghton et al. 2007) Year (AD)

  8. Airborne fraction predictions

  9. ‘Sink efficiency’ and linear models of the carbon cycle Here: cause = anthropogenic C flux to atmosphere effect = flux out of atmosphere to ocean and land carbon pools ‘Constant sink efficiency’ <=> with ; toc-1,tld-1‘sink efficiencies’ , atmosphere-ocean /-land C fluxes anthropogenic perturbation of atmospheric carbon

  10. Most simple linear model with ‘s’ stands for system Solution with G(t,t’) ‘Greens function’ model easily generalizable to multi-pool linear carbon cycle models by using multi-pool Greens function results very similar to the ones presented here

  11. Airborne fraction for simple cases (i) Flux to atmosphere , subscript stands for forcing, then and thus (ii) (iii)

  12. tf/ts=20/45=0.44 tf/ts=40/45=0.88 Airborne fraction (-) tf/ts=20/4.5=4.44

  13. =>Near constancy of AF reflects approximate exponential fossil fuel emissions’ rise, i.e. not fundamental system property (Bacastow and Keeling 1979) • Association between positive trend in AF and decrease in carbon sink efficiency does not hold • Need to remove variations in AF due to spin-up time effect and deviations from exponential function of anthropogenic emissions from AF if one wants to de- tect potential trends in sink efficiency from observed AF • Need an evolution equation for AF given anthropo- genic fluxes

  14. AF time evolution for actual anthropogenic fluxes to the atmosphere From the general solution for our simple model we can derive a differential equation for AF • Relative growth rate of anthropogenic emissions and relative changes in system response time-scale ts control variations in AF • Permits to predict AF variations by time-stepping equation using estimates of fossil fuel emissions and land use change

  15. Fossil fuel Emission’s Relative Growth Rate from least squares minimization

  16. Focus 1959 to present • Strong observed inter-annual variation missing in predicted AF • (as these due to non-anthropogenic forcings - e.g. volcanoes) • Residuals exhibit a trend - evidence for sink efficiency decrease ?

  17. Least squares estimation of flux corrections such that predicted and observed AF match and analyse them • One event (2002/2003) we cannot currently attribute to either external forcing or omissions in land use change fluxes • Trend in residuals disappears

  18. Signal to noise Airborne Fraction (-)

  19. Conclusions • Approximate constancy of AF due to approximate exponential increase of fossil fuel emissions • No association between positive trend in AF and negative trend in sink efficiency • May account for spin-up time and exponential growth rate deviations using predictive equation for AF • => no evidence for long-term decreasing trend in sink efficiency - potential exception: 2002/3 event • trends in AF not very good diagnostic because complicated signal and S/R quite small

  20. Canadell et al. PNAS, 2007, Raupach et al. 2008, LeQuere et al. Nature Biogeoscience, 2009 Knorr, Geophys. Res. Lett. , 2009

  21. Raupach et al. 2007 PNAS what about ‘peak oil’ ?

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