Closing in on the Missing Carbon Sink: Implications for Climate Research and Mitigation

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Closing in on the Missing Carbon Sink
Implications for Climate Research and Mitigation

• Dr. Britton Stephens
• National Center for Atmospheric Research

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Outline

• Current state of scientific knowledge concerning
  global carbon cycling
• Climate change mitigation challenges associated
  with knowledge gaps
• New northern and tropical land uptake estimates
  from airborne CO2 measurements
• Implications of new results for
• strategies to address climate change

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Careful atmospheric CO2 measurements since the
1950s show that about half of fossil fuel
emissions remain in the atmosphere
FF
Atm
IPCC, 2007
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How can we separate the natural uptake between
land and ocean?
Sabine et al, Science, 2004
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Annual fluxes are small relative to balanced
seasonal exchanges and to standing pools
Annual residuals
Pools and flows
Land-Based Sink
Net Oceanic Sink
The global carbon cycle for the 1990s, showing
the main annual fluxes in GtC yr 1. IPCC, 2007
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Carbon Units

• 1 Petagram C (PgC) 1 Gigaton C (GtC) 1
  Billon Tons C 3.7 Gigaton CO2 (GtCO2)

C
CO2
H2CO3
C2H6O
CaCO3
CH4
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Uncertainties on natural ocean and land fluxes
are /- 25 to 75
1990s 2000-2005
Atmospheric Increase Atmospheric Increase 3.2 0.1 4.1 0.1
Fossil Fuel Emissions Fossil Fuel Emissions 6.4 0.4 7.2 0.3
Ocean-to-Atmosphere Ocean-to-Atmosphere 2.2 0.4 2.2 0.5
Land-to-Atmosphere Land-to-Atmosphere 1.0 0.6 0.9 0.6
Land-use Change 1.6 (0.5 to 2.7) 1.5
Residual Land Sink -2.6 (4.3 to 0.9) -2.4
IPCC, 2007 and Canadell et al., PNAS 2007
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Global atmospheric inverse models and surface
data have been used to make regional flux
estimates
Forward Flux Transport CO2
Inverse CO2 Transport Flux
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12 Model Results from the TransCom 3 Study
Systematic trade off between northern and
tropical land fluxes
Model Model Name
1 CSU
2 GCTM
3 UCB
4 UCI
5 JMA
6 MATCH.CCM3
7 MATCH.NCEP
8 MATCH.MACCM2
9 NIES
A NIRE
B TM2
C TM3
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Regional land flux uncertainties are very large

• All model average and standard deviations
• Northern Land -2.4 1.1 PgCyr-1
• Tropical Land 1.8 1.7 PgCyr-1
• At 30/ton of CO2
• 1.5 PgCyr-1 165 Billion

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Challenges associated with flux uncertainty

• 1) Verification

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Challenges associated with flux uncertainty

• 2) Prediction

How much can we burn?
IPCC, 2007
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Challenges associated with flux uncertainty

• 3) Detection

Thermohaline circulation
Melting permafrost
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• All model average and standard deviations
• Northern Land -2.4 1.1 PgCyr-1
• Tropical Land 1.8 1.7 PgCyr-1

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Bottom-up estimates have generally failed to find
large uptake in northern ecosystems and large net
sources in the tropics
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An helpful discovery about the nature of the
model disagreements
Tropical Land and Northern Land fluxes plotted
versus vertical CO2 gradient
Systematic trade off is related to vertical
mixing biases in the models
Model Model Name
1 CSU
2 GCTM
3 UCB
4 UCI
5 JMA
6 MATCH.CCM3
7 MATCH.NCEP
8 MATCH.MACCM2
9 NIES
A NIRE
B TM2
C TM3
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12 Airborne Sampling Programs from 6
International Laboratories
Northern Hemisphere sites include Briggsdale,
Colorado, USA (CAR) Estevan Point, British
Columbia, Canada (ESP) Molokai Island, Hawaii,
USA (HAA) Harvard Forest, Massachusetts, USA
(HFM) Park Falls, Wisconsin, USA (LEF) Poker
Flat, Alaska, USA (PFA) Orleans, France (ORL)
Sendai/Fukuoka, Japan (SEN) Surgut, Russia
(SUR) and Zotino, Russia (ZOT). Southern
Hemisphere sites include Rarotonga, Cook Islands
(RTA) and Bass Strait/Cape Grim, Australia (AIA).
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12 Airborne Sampling Programs from 6
International Laboratories
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Vertical CO2 profiles for different seasonal
intervals
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Comparing the Observed and Modeled Gradients

• 3 models that most closely reproduce the observed
  annual-mean vertical CO2 gradients (4, 5, and C)
  
• Northern Land
• -1.5 0.6 PgCyr-1
• Tropical Land
• 0.1 0.8 PgCyr-1
• All model average
• Northern Land
• -2.4 1.1 PgCyr-1
• Tropical Land
• 1.8 1.7 PgCyr-1

Northern Land Tropical Land
Most of the models overestimate the annual-mean
vertical CO2 gradient
Model Model Name
1 CSU
2 GCTM
3 UCB
4 UCI
5 JMA
6 MATCH.CCM3
7 MATCH.NCEP
8 MATCH.MACCM2
9 NIES
A NIRE
B TM2
C TM3
Observed value
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• Airborne CO2 measurements indicate
• Northern forests, including U.S. and Europe, are
  taking up much less CO2 than previously thought
• Intact tropical forests are strong carbon sinks
  and are playing a major role in offsetting carbon
  emissions

• Outcomes of this work
• Helps to resolve a major environmental mystery of
  the past two decades
• ? Northern missing carbon sink may not have
  been found because it is not there
• Improved understanding of processes responsible
  for carbon uptake will improve predictions of
  climate
• change and assessment
• of mitigation strategies

Stephens et al., Science, 2007
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Implications for Climate Mitigation Strategies

• 1) National emission estimates

Temperate North America Tropical America
Fan et al, 1998 -1.4 0.5 NA
Transcom 3, 2004 -0.9 0.4 0.7 1.1
U.S. SOCCR -0.5 0.25 NA
NOAA CarbonTracker -0.5 0.6 0.0 0.7
PgCyr-1
Models 4, 5, and C still estimate a strong sink
in Temperate North America (-0.9) and a strong
source in Tropical America (0.8) but with large
compensating flux revisions in other regions.
Zonal partitioning by these models is highly
uncertain.
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Implications for Climate Mitigation Strategies

• 2) Processes and prediction
• Northern uptake will slow as forests mature
• A tropical fertilization sink will continue as
  long as CO2 continues to increase
• A tropical climate change sink may decrease or
  even reverse depending on feedbacks

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Implications for Climate Mitigation Strategies

• 3) Deforestation releases C pool and removes C
  sink

Nominal C Pool (tC/hectare) Sink spread evenly over all forest (tC/hectare/year)
Boreal Forest 35 1
Temperate Forest 60 1
Tropical Forest 120 ( 200 w/peat) 2
A palm oil plantation can only offset fossil CO2
emissions at 3 tC/hectare/year
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Implications for Climate Mitigation Strategies

• 4) Caveat concerning short-term nature of
  afforestation and reforestation offsets

IPCC, 2007
On century time-scale oceanic and geologic
sequestration will be required
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Conclusions

• Large uncertainties exist in estimates of
  regional carbon fluxes and limit our ability to
  make optimal mitigation decisions
• There is a strong need for expanded atmospheric
  and oceanic observational networks related to
  carbon cycling
• Airborne CO2 data suggest that tropical forests
  are taking up a lot more and northern much less
  carbon than previously believed
• Preventing tropical deforestation is important
  for preserving existing carbon pools and the
• capacity to absorb future CO2 emissions