Download Tropical vs. extratropical terrestrial CO2 uptake and implications for

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