1. An Assessment of the Carbon Balance of Arctic Tundra: Comparisons among Observations, Models, and Atmospheric inversions A. David McGuire and Co-authors U. Alaska Fairbanks and U.S. Geological Survey AGU Fall 2011 Meeting, GC41F-01 8 December 2011

2. Co-authors: Dan Hayes – Oak Ridge National Laboratory, USA Arnaud Heroult – Lund University, Sweden Eugenie Euskirchen – University of Alaska Fairbanks, USA John Kimball – University of Montana, USA Charles Koven – Lawrence Berkeley National Lab, USA Peter Lafleur – Trent University, Canada Paul Miller – Lund University, Sweden Walt Oechel – San Diego State University, USA Philippe Peylin – LSCE, France Mathew Williams – University of Edinburgh, UK Lead Authors: A.D. McGuire – University of Alaska Fairbanks, USA T. R. Christensen – Lund University, Sweden

3. From Hayes et al. (2011, Global Biogeochemical Cycles) Is the CO 2 sink of N. High Latitudes Changing? SOURCE SINK

4. Simulated Arctic Basin Terrestrial C Budget Mean annual C fluxes and change in stocks (Tg C yr -1 ) for the terrestrial component of the Arctic Basin, 1997 - 2006 From McGuire et al. (2010, Tellus)

5. Arctic Tundra Domain as defined by the Regional Carbon Cycle Assessment and Processes (RECCAP) Synthesis Activity

6. Results Organization Estimates based on flux observations Process model estimates Atmospheric inversion analyses Comparison of the estimates Conclusions

7. Analysis of Observations Includes both chamber-based and tower-based studies 250 estimates of CO 2 and CH 4 exchange

8. Synthesis of Tundra Observations Annual exchange of CO 2 cannot be distinguished from neutral balance across the range of studies that have been conducted

9. Process-based Modeling Regional Applications of Models: - TEM6 – Permafrost, Vertical SOM, CH 4, Fire - LPJ-Guess WHyMe – Permafrost, CH 4, Fire - Orchidee – with Cryoturbation - Terrestrial Carbon Flux (TCF) – Diagnostic Model Global Applications of Trendy DGVMs: CLM4C, CLM4CN, Hyland, LPJ, LPJ-Guess, Orchidee N, SDGVM, Triffid Compared two decades: 1990 – 1999 and 2000 - 2006 Spatial domain defined by RECCAP Arctic Tundra mask

10. Model1990-19992000-2006 g C m -2 yr -1 (negative = sink) Regional Apps. LPJ-G WHyMe-21-24 Orchidee-28-34 TEM6-6-3 - Global Apps. CLM4C0 CLM4CN Hyland00 LPJ-20-3 LPJ-Guess-21-24 Orchidee N-3 SDGVM-18-16 TRIFFID-8-17 Mean NEE of Arctic Tundra Simulated by Process Models NEE ranged between 0 and 34 g C m -2 yr -1 sink; sink increases between decades

11. Mean Seasonal Cycle of Carbon Fluxes All of the models indicate that July is the month of maximum NPP and NEP.

12. Interannual Anomalies of Carbon Fluxes Three of the four models have a similar range of interannual variability in GPP, NPP, RH, and NEP. Correlations among the models indicate that GPP, NPP, and RH are generally well correlated among the models, but the interannual variability in NEP is poorly correlated among the models.

13. Atmospheric Inversion Modeling 10 models 1985 – 2009 depending on model Spatial domain defined by RECCAP Arctic Tundra mask

14. Mean NEE of Arctic Tundra Estimated by Inversion Models NEE ranged between 26 g C m -2 yr -1 source and 48 g C m -2 yr -1 sink Sink increases between decades Model1990-19992000-2006 gC m -2 yr -1 (negative = sink) C13_CCAM_law- 26 C13_MATCH_rayner- -31 JENA_s96_v3.3--13 JMA_2010-35-37 LSCE_an_v2.1--14 LSCE_var_v1.0 15 22 NICAM_niwa_woaia-19 -9 rigc_Patra--48

15. Mean Seasonal Cycle of Carbon Fluxes of Inversions All of the models indicate that July is the month of maximum NEE.

16. Interannual Variability of Carbon Fluxes from Inversions Interannual anomalies vary from 2.1 to 13.1 g C m -2 yr -1 (standard deviation) Correlation of interannual anomalies is poor (mean r=0.03, range: -0.38 to 0.99)

17. Comparison of Regional NEE among Methods (Tg C yr -1 ) Observations and Inversions – Can’t be distinguished from neutral balance Process-Model Simulations – Arctic tundra has been a sink in the 1990s and 2000s Only one central estimate is a source (observations in 1990s – North America) All methods indicate that Arctic tundra has become a stronger sink in the 2000s Time PeriodObservations Regional Process- Based Models Global Process- Based Models Inversion Models 1990 - 1999 Central Estimate77-166 -78 -13 1990 -1999 Uncertainty-436 to 275-255 to -55 -188 to 0 -321 to 140 2000 - 2006 Central Estimate-217-187 -93 -117 2000 – 2006 Uncertainty-621 to -21-312 to -28 -222 to -1 -439 to 243

18. Changes in the Seasonal Cycle of NEP Estimated by the Regional Process Models LPJ-Guess WHyMe and Orchidee estimate greater uptake in early and mid-growing season, while TEM6 estimates greater uptake in the late growing season

19. Arctic Tundra C Assessment Conclusions Estimates of NEE based on observations and inversions have large uncertainties that cannot be distinguished from neutral balance. Process models indicate that Arctic tundra acted as a sink for CO 2 in recent decades. Central estimates based on observations, process- models, and inversions each suggest stronger sinks in the 2000s than in the 1990s. Analyses of regional models identified that two of the models had increased CO 2 uptake in early and mid- growing season between decades, while a third model had greater uptake in the late growing season. Simulation of the difference between production and decomposition is important to improve for assessing responses of Arctic tundra to projected climate change