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

Global Carbon Project, 2014

The Global Carbon Budget

IPCC AR5, 2013

2000-2009:

Historically:“missing CO2 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 O2, ocean CFC, ocean inversionIPCC AR5 (2013) used ocean inversion and pCO2 methods onlyGCP 2014 uses same three methods and time period as AR4

[CO2] – Transport = Flux

Three ways to estimate global spatial distribution of CO2 fluxes

Atmospheric CO2 observations with inverse atmospheric transport models

Bradford et al., Ecol. Arch., 2014.

MLO

Carlye Calvin

Bottom-up forest inventory data plus statistical models

Dynamic global vegetation models

Tans, Fung, Takahashi, Science, 1990

Global pCO2 data set implies a northern land sink of 2.0-3.4 PgCyr-1 for 1981-1987

Since 1990s:“missing CO2 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 CO2 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 CO2 fertilization, and growing

• Climate response expected to have unique latitudinal signature

CO2 response () Climate response (g)

Peylin et al., Biogeosciences, 2013

RECCAP Atmospheric Inverse Model Intercomparison Study

Fluxes estimated for 2001-2004

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 = CO2 forcing only

S3 = Climate, CO2, and land-use forcing

Models only agree with global constraints with CO2 fertilization sink

IPCC AR5, 2013

Long-term growth in land CO2 sink inferred from global constraints

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 budgetsb) Vertical gradient selection of atmospheric inversionsc) Bottom-up forest inventoriesd) 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