Comparing Global Carbon Cycle Models to Observations is Hard but Better Than the Alternative

1. Comparing Global Carbon Cycle Models to Observations is Hard but Better Than the Alternative Britton Stephens, National Center for Atmospheric Research [illustration by Mercer Mayer]

2. Outline: Why model-data comparisons are important Why model-data comparisons are hard Atmosphere example: vertical profiles of CO2 and latitudinal flux partitioning Ocean example: seasonal O2 and CO2 cycles and Southern Ocean ventilation Land example: Ecosystem respiration response to widespread beetle outbreak

3. It’s all about trajectories. . . .

4. Climate projections are sensitive to human decisions and carbon cycle feedbacks. . . [IPCC, 2007] How hot is it going to get?

5. 6 or 12? 3 or 7? . . . and human decisions are sensitive to scientific knowledge feedbacks [IPCC, 2007] How much can we burn?

6. How well can we predict human factors? [Raupach et al. 2007, PNAS; www.globalcarbonproject.org] Fossil-fuel emissions have already exceeded highest scenario used for IPCC projections

7. How well can we predict climate feedbacks? [Stroeve, et al., GRL, 2007] Arctic summer sea ice levels have already exceeded the lowest model estimates

8. How well can we predict carbon-cycle feedbacks? C4MIP Projections [Friedlingstein, et al., J. Climate, 2006] In 2050, combined anthropogenic offsets have a range of 3 to 15 PgC/yr At $32/ton CO2, ± 6 PgC/yr = ± $700 Billion/yr

9. Annual fluxes are small relative to balanced seasonal exchanges and to standing pools Annual residuals Pools and flows Net Oceanic Sink Land-Based Sink Uncertainties on natural annual-mean ocean and land fluxes are +/- 25 to 75 % The global carbon cycle for the 1990s, showing the main annual fluxes in GtC yr –1. [IPCC, 2007]

10. Global atmospheric inverse models and surface data can be used to make regional flux estimates Forward: Flux + Transport = [CO2] Inverse: [CO2] – Transport = Flux

11. 800 m 360 m 120 m Model-data fusion is hard because: • Models often don’t predict something that can be measured • Observations don’t measure something that can be predicted • A cultural divide

12. Annual mean TransCom3 Level 1 Results Measurement uncertainty ≈ 0.2 ppm Continental site “Data error” ≈ 2.2 ppm “For most regions, the between-model uncertainties are of similar or smaller magnitude than the within-model uncertainties. This suggests that the choice of transport model is not the critical determinant of the inferred fluxes.” [Gurney et al, Nature, 2002]

13. 12 Model Results from the TransCom 3 Level 2 Study Systematic trade off between northern and tropical land fluxes

14. Regional land flux uncertainties are very large • All model average and standard deviations: Northern Land = -2.4 ± 1.1 PgCyr-1 Tropical Land = +1.8 ± 1.7 PgCyr-1

15. Bottom-up estimates have generally failed to find large uptake in northern ecosystems and large net sources in the tropics

16. A helpful discovery about the nature of the model disagreements Tropical Land and Northern Land fluxes plotted versus vertical CO2 gradient Systematic trade off is related to vertical mixing biases in the models

17. 12 Airborne Sampling Programs from 6 International Laboratories Northern Hemisphere sites include Briggsdale, Colorado, USA (CAR); Estevan Point, British Columbia, Canada (ESP); Molokai Island, Hawaii, USA (HAA); Harvard Forest, Massachusetts, USA (HFM); Park Falls, Wisconsin, USA (LEF); Poker Flat, Alaska, USA (PFA); Orleans, France (ORL); Sendai/Fukuoka, Japan (SEN); Surgut, Russia (SUR); and Zotino, Russia (ZOT). Southern Hemisphere sites include Rarotonga, Cook Islands (RTA) and Bass Strait/Cape Grim, Australia (AIA).

18. 12 Airborne Sampling Programs from 6 International Laboratories

19. Vertical CO2 profiles for different seasonal intervals

20. Comparing the Observed and Modeled Gradients • 3 models that most closely reproduce the observed annual-mean vertical CO2 gradients (4, 5, and C): • Northern Land = • -1.5 ± 0.6 PgCyr-1 • Tropical Land = • +0.1 ± 0.8 PgCyr-1 • All model average: • Northern Land = • -2.4 ± 1.1 PgCyr-1 • Tropical Land = • +1.8 ± 1.7 PgCyr-1 Most of the models overestimate the annual-mean vertical CO2 gradient Northern Land Tropical Land Observed value

21. Observational and modeling biases evaluated: • Interlaboratory calibration offsets and measurement errors • Diurnal biases • Interannual variations and long-term trends • Flight-day weather bias • Spatial and Temporal Representativeness WLEF Diurnal Cycle Observations All were found to be small or in the wrong direction to explain the observed annual-mean discrepancies [Schulz et al., Environ. Sci. Technol. 2004, 38, 3683-3688]

22. Seasonal vertical mixing [figure courtesy of Scott Denning]

23. 1 1Faraday, 1855 • Airborne measurements suggest: • Northern forests, including U.S. and Europe, are taking up much less CO2 than previously thought • Intact tropical forests are strong carbon sinks and are playing a major role in offsetting carbon emissions [Stephens et al., Science, 2007] However, large (O ~ 2 PgCyr-1) flux uncertainties associated with modeling atmospheric CO2 transport remain

24. Transcom3 Fossil Fuel Response pressure N S N S N S N S ppm ppm latitude

25. TransCom3 Seasonal Ocean O2 Amplitude [T. Blaine, SIO Dissertation, 2005]

26. HIAPER Pole-to-Pole Observations of Atmospheric Tracers HIPPO (PIs: Harvard, NCAR, Scripps, and NOAA): A global and seasonal survey of CO2, O2, CH4, CO, N2O, H2, SF6, COS, CFCs, HCFCs, O3, H2O, and hydrocarbons NCAR Airborne O2 Instrument 5 loops over next 3 years, starting in January 2009

27. Southern Ocean Air-Sea CO2 Fluxes solubility anthropogenic biology solubility

28. Solubility Pump Biological Pump The Southern Ocean will play a key role in future anthropogenic CO2 uptake, mediated by strong opposing solubility and biological influences 2056-65 Global Warming Simulation [Sarmiento et al., Nature 1998 ]

29. Air-Sea Flux ComparisonContemporary Fluxes 1992-6 [courtesy A. Jacobsen]

30. Direction and magnitude of response to increased circumpolar winds is uncertain ORCA-PISCES-T sea to air = positive UVic-ESCM [Zickfeld et al., Science 2008] [Le Quéré et al., Science 2007]

31. Southern Ocean Air-Sea CO2 and O2 Fluxes Solubility (thermal) and biological processes have discernable effects on atmospheric O2 and CO2

32. SIO Gradient = PSA – (CGO+SPO)/2 SIO Stations PSA SPO CGO SeaWiFS Summer Chlorophyll a [PCTM runs courtesy David Baker]

34. Fuel-cell technique can be used on ships to greatly increase data coverage in Southern Ocean R/V Lawrence M. Gould

35. Effects of large-scale Mountain Pine Beetle outbreaks on ecosystem carbon fluxes Model for British Columbia Fraser Experimental Forest, Colorado [Kurz et al., Nature, 2008]

36. Fraser Experimental Forest Noctural Respiration Signals Ecosystem respiration decreases, because reduction in autotrophic respiration is greater than increase in heterotrophic respiration

37. So what is the ideal form of modeler - observationalist interaction?