Tropical vs. extratropical terrestrial CO 2 uptake and implications for carbon-climate feedbacks Outline: How we track the fate of anthropogenic CO 2 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 CO 2 emissions and atmospheric growth rate are well known Scripps Institution of Oceanography CO 2 Program

Global Carbon Project, 2014 The Global Carbon Budget IPCC AR5, : Historically: “missing CO 2 sink” = global land Net land sink is calculated as a residual from other annual mean terms

Data from Le Quéré et al., ESSDD, 2014 IPCC AR4 (2007) numbers come from 3 methods: atmospheric O 2, ocean CFC, ocean inversion IPCC AR5 (2013) used ocean inversion and pCO 2 methods only GCP 2014 uses same three methods and time period as AR4

[CO 2 ] – Transport = Flux Three ways to estimate global spatial distribution of CO 2 fluxes Atmospheric CO 2 observations with inverse atmospheric transport models Bradford et al., Ecol. Arch., MLO Carlye Calvin Bottom-up forest inventory data plus statistical models Dynamic global vegetation models

Tans, Fung, Takahashi, Science, 1990 Global pCO 2 data set implies a northern land sink of PgCyr -1 for Since 1990s: “missing CO 2 sink” = northern land

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 Observed value Northern Land Tropical Land

Northern Land = -1.5 ± 0.6 PgCyr -1 Three inverse models selected by annual mean vertical CO 2 gradients

24 % IPCC AR5 A northern sink would most likely be land-use change driven, and diminishing A tropical sink would most likely be driven by CO 2 fertilization, and growing Climate response expected to have unique latitudinal signature CO 2 response (  ) Climate response (  )

Peylin et al., Biogeosciences, 2013 RECCAP Atmospheric Inverse Model Intercomparison Study Fluxes estimated for

Northern Land = -2.2 ± 0.6 PgCyr -1 Models have converged and Trop. vs. North relationship has tightened

Pan et al., Science, 2011 A complete global forest inventory estimate Northern Land = -1.2 ± 0.1 PgCyr -1

Inventories only agree with global constraints with intact forest sink

TRENDY comparison of dynamic global vegetation models Northern Land = -1.0 ± 0.3 PgCyr -1 Sitch et al., Biogeosciences, 2015 S1 = CO 2 forcing only S3 = Climate, CO 2, and land-use forcing

Models only agree with global constraints with CO 2 fertilization sink

IPCC AR5, 2013 Long-term growth in land CO 2 sink inferred from global constraints

Growth in observed land sink and modeled CO 2 effect both parallel accelerating growth in atmospheric CO 2

Estimated global CO 2 effect = ± 0.3 PgCyr -1 Up to 25% of present-day anthropogenic CO 2 and 60% of total terrestrial CO 2 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 CO 2 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 CO 2 measurements to validate state-of-the-art atmospheric inverse models