An Assessment of the Carbon Balance of Arctic Tundra:

Comparisons among Observations, Models, and

Atmospheric inversions

A. David McGuire and Co-authorsU. Alaska Fairbanks and U.S. Geological Survey

AGU Fall 2011 Meeting, GC41F-018 December 2011

Co-authors:Dan Hayes – Oak Ridge National Laboratory, USAArnaud Heroult – Lund University, SwedenEugenie Euskirchen – University of Alaska Fairbanks, USAJohn Kimball – University of Montana, USACharles Koven – Lawrence Berkeley National Lab, USAPeter Lafleur – Trent University, CanadaPaul Miller – Lund University, SwedenWalt Oechel – San Diego State University, USAPhilippe Peylin – LSCE, FranceMathew Williams – University of Edinburgh, UK

Lead Authors:A.D. McGuire – University of Alaska Fairbanks, USAT. R. Christensen – Lund University, Sweden

From Hayes et al. (2011, Global Biogeochemical Cycles)

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

Cum

ulat

ive

NEE

sin

ce 1

960

(Pg

C) BONA

BOAS

BOEU

(1)

(2)

Is the CO2 sink of N. High Latitudes Changing?

SOURCE

SINK

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)

Arctic Tundra Domainas defined by the Regional Carbon Cycle Assessment

and Processes (RECCAP) Synthesis Activity

Results Organization

• Estimates based on flux observations

• Process model estimates

• Atmospheric inversion analyses

• Comparison of the estimates

• Conclusions

Analysis of Observations

• Includes both chamber-based and tower-based studies

• 250 estimates of CO2 and CH4 exchange

Synthesis of Tundra Observations

Annual exchange of CO2 cannot be distinguished from neutral balance across the range of studies that have been conducted

Process-based Modeling• Regional Applications of Models:

- TEM6 – Permafrost, Vertical SOM, CH4, Fire - LPJ-Guess WHyMe – Permafrost, CH4, 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

Model 1990-1999 2000-2006g C m-2 yr-1

(negative = sink)Regional Apps.

LPJ-G WHyMe -21 -24

Orchidee -28 -34TEM6 -6 -3

-Global Apps.

CLM4C 0 -1CLM4CN -1 -1Hyland 0 0

LPJ -20 -3

LPJ-Guess -21 -24Orchidee N -1 -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

Mean Seasonal Cycle of Carbon Fluxes

• All of the models indicate that July is the month of maximum NPP and NEP.

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.

Atmospheric Inversion Modeling• 10 models

• 1985 – 2009 depending on model

• Spatial domain defined by RECCAP Arctic Tundra mask

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

Model 1990-1999 2000-2006gC 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

Mean Seasonal Cycle of Carbon Fluxes of Inversions

All of the models indicate that July is the month of maximum NEE.

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)

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 Period Observations

Regional Process-

Based Models

Global Process-

Based Models Inversion

Models

1990 - 1999Central Estimate 77 -166 -78 -13

1990 -1999Uncertainty -436 to 275 -255 to -55 -188 to 0 -321 to 140

2000 - 2006Central Estimate -217 -187 -93 -117

2000 – 2006Uncertainty -621 to -21 -312 to -28 -222 to -1 -439 to 243

Changes in the Seasonal Cycle of NEPEstimated 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

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 CO2 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 CO2 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