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Tropical vs. extratropical terrestrial CO2 uptake and implications for carbon-climate feedbacks Outline: • How we track the fate of anthropogenic CO2 • Historic estimates of latitudinal distribution of forest sinks • Implication of sink estimates for future climate change • A new synthesis of global carbon cycle budgeting techniques Britton Stephens, NCAR Earth Observing Laboratory Fossil-fuel CO2 emissions and atmospheric growth rate are well known Scripps Institution of Oceanography CO2 Program The Global Carbon Budget Historically: “missing CO2 sink” = global land 2000-2009: Global Carbon Project, 2014 Net land sink is calculated as a residual from other annual mean terms IPCC AR5, 2013 Data from Le Quéré et al., ESSDD, 2014 IPCC AR4 (2007) numbers come from 3 methods: atmospheric O2, ocean CFC, ocean inversion IPCC AR5 (2013) used ocean inversion and pCO2 methods only GCP 2014 uses same three methods and time period as AR4 Three ways to estimate global spatial distribution of CO2 fluxes Atmospheric CO2 observations with inverse atmospheric transport models Bottom-up forest inventory data plus statistical models MLO Bradford et al., Ecol. Arch., 2014. Dynamic global vegetation models [CO2] – Transport = Flux Carlye Calvin Since 1990s: “missing CO2 sink” = northern land Global pCO2 data set implies a northern land sink of 2.0-3.4 PgCyr-1 for 1981-1987 Tans, Fung, Takahashi, Science, 1990 TransCom3 Atmospheric Inverse Model Intercomparison Study Northern Land = -2.4 ± 1.1 PgCyr-1 Combined global atmosphere, fossil-fuel, and ocean constraint Model results are systematically dependent on atmospheric transport Northern Land Tropical Land Observed value Three inverse models selected by annual mean vertical CO2 gradients Northern Land = -1.5 ± 0.6 PgCyr-1 • A northern sink would most likely be land-use change driven, and diminishing • A tropical sink would most likely be driven by CO2 fertilization, and growing • Climate response expected to have unique latitudinal signature CO2 response () Climate response (g) 24 % IPCC AR5 RECCAP Atmospheric Inverse Model Intercomparison Study Fluxes estimated for 2001-2004 Peylin et al., Biogeosciences, 2013 Models have converged and Trop. vs. North relationship has tightened Northern Land = -2.2 ± 0.6 PgCyr-1 A complete global forest inventory estimate Northern Land = -1.2 ± 0.1 PgCyr-1 Pan et al., Science, 2011 Inventories only agree with global constraints with intact forest sink TRENDY comparison of dynamic global vegetation models S1 = CO2 forcing only S3 = Climate, CO2, and land-use forcing Northern Land = -1.0 ± 0.3 PgCyr-1 Sitch et al., Biogeosciences, 2015 Models only agree with global constraints with CO2 fertilization sink Long-term growth in land CO2 sink inferred from global constraints IPCC AR5, 2013 Growth in observed land sink and modeled CO2 effect both parallel accelerating growth in atmospheric CO2 Estimated global CO2 effect = - 2.5 ± 0.3 PgCyr-1 Up to 25% of present-day anthropogenic CO2 and 60% of total terrestrial CO2 sink Conclusions 1) Convergence of 4 independent constraints: a) Global atmospheric, fossil-fuel, and ocean budgets b) Vertical gradient selection of atmospheric inversions c) Bottom-up forest inventories d) Dynamic global vegetation modes 2) Available estimates suggest a strong CO2 effect and negative feedback to climate change, but with significant caveats 3) There is a strong need to resolve discrepancies between atmospheric inverse model estimates 4) Ongoing work to apply HIPPO Global Campaign CO2 measurements to validate state-of-the-art atmospheric inverse models