Forests and climate change: Observations of the role of terrestrial ecosystems in the earths carbon

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Forests and climate changeObservations of the
role of terrestrial ecosystems in the earths
carbon cycle

• Ken Davis
• Department of Meteorology
• The Pennsylvania State University

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Outline

• Is atmospheric CO2 changing?
• Are humans responsible?
• Why should atmospheric CO2 alter climate?
• Is the climate changing?
• Are humans responsible?
• Why study the terrestrial carbon cycle?
• Atmospheric methods
• Eddy covariance
• Atmospheric budgets
• Some results
• Some conclusions

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• Is atmospheric CO2 changing?
• Yes, without question.

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IPCC, 2001
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• Are humans responsible?
• Yes, without question.

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The Global Carbon Cycle

• Carbon is the stuff of life

Sarmiento and Gruber, 2004
Sarmiento and Gruber, 2002, Physics Today
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Global fluxes of CO2 in PgC yr-1 based on CO2 and
O2 observations. Error bars are one standard
deviation uncertainty, and not interannual
variability, which is much greater (IPCC, 2001)
Interdecadal global CO2 budget
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• Why should atmospheric CO2 alter climate?

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• Do you have a board handy?

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Brook, 2005, Science
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• Is the climate changing?
• Yes (always).

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• Are human responsible (for recent changes)?
• The balance of evidence suggests a discernible
  human influence on global climate (IPCC, 2001).

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• So why study the terrestrial carbon cycle?

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Global fluxes of CO2 in PgC yr-1 based on CO2 and
O2 observations. Error bars are one standard
deviation uncertainty, and not interannual
variability, which is much greater (IPCC, 2001)
Interdecadal global CO2 budget
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Sink mechanisms

• Ocean Increase pCO2 in the atmosphere, and
  ocean water pCO2 will increase to reach
  equilibrium
• Terrestrial ecosystems No dominant mechanism is
  obvious.

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1 ppm yr-1 2 PgC yr-1. Fossil fuel emissions
are 6 PgC yr-1. Sink is implied! Interannual v
ariability!
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Scientific questions

• Why does this terrestrial carbon sink exist?
• How large is it, really?
• Where is it located?
• What is the cause of the large degree of
  interannual variability in the global system?
• How is this terrestrial sink likely to change
  with time? (Does the interannual variability in
  the system tell us anything about this?)
• How can/will terrestrial ecosystems alter the
  global carbon cycle in future decades?

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Geopolitical interests

• Can this sink be manipulated to further slow the
  changes in climate forcing?
• Should nations receive credit for these
  terrestrial sinks in international agreements?

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Possible terrestrial carbon sink mechanisms

• Regrowth of logged forests or woody enroachment
  in grasslands
• Nitrogen or CO2 fertilization
• Longer growing seasons/better growing conditions

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Hypotheses

• Northern hemisphere ecosystems are responsible
  for a majority of the terrestrial sink of CO2.
• The current terrestrial sink of CO2 is caused by
  regrowth of forests.
• Current temporal variability in the global carbon
  budget is largely due to terrestrial ecosystems.
• Terrestrial ecosystems will become a more
  significant sink of atmospheric CO2 in the coming
  century.
• This sink will be too small to prevent serious
  climatic change, but will be an important factor
  in slowing the process.

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Predicted future carbon cycle

• But models diverge in coupled simulations of the
  future

IPCC, 2001
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• Atmospheric methods
• Eddy covariance
• Atmospheric budgets

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Methods
Change in atmospheric concentration of CO2
over time inversion or ABL budget approach.


Flux of carbon across this plane tower or
aircraft flux approach



Change in forest biomass over time forest
inventory approach




Change in CO2 concentration in a small box over
time chamber flux approach
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Atmospheric approaches to observing the
terrestrial carbon cycle
Time rate of change (e.g. CO2)
Mean transport
Turbulent transport (flux)
Source in the atmosphere
Average over the depth of the atmosphere (or the
ABL)
F0C encompasses all surface exchange Oceans,
deforestation, terrestrial uptake, fossil fuel
emissions.
Inversion study Observe C, model U, derive
F Flux study Observe F directly
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Inverse Modeling of CO2
Changes in CO2 in the air tell us about sources
and sinks
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Method eddy covariance
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Theory
Yi et al, 2000
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Sonic anemometer
Infrared gas analyzer
Campbell Scientific, Inc. LI-COR, Inc.
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Hourly fluxes at WLEF for 1997, observed and
filled. Davis et al, 2003.
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Net ecosystem-atmosphere exchange of CO2 in
northern Wisconsin
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Methods for quantifying the terrestrial carbon
cycle
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• Some results concerning the terrestrial carbon
  cycle

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Pieter Tans et al, 1990, Science Observed
interhemispheric gradient in CO2 mixing ratio
is much less than the modeled gradient. Reasonabl
e spatial distributions of fossil fuel and ocean
fluxes included in model. Conclusion
Significant northern hemisphere terrestrial
CO2 sink must exist. (Southern ocean
source cant work dCO2/dt would be wrong.)
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Peylin et al, 2005, GBC
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Joint constraints! Complementary methods
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• A place
• Say, cheese

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• A cluster of stand-level flux towers

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Chequamegon Ecosystem-Atmosphere Study (ChEAS)
region
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• A very tall flux tower
• (the only one in the world to date)

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WLEF CO2 flux and mixing ratio observatory
Photo credit UND Citation crew, COBRA
WLEF tall tower (447m) CO2 flux measurements at
30, 122 and 396 m CO2 mixing ratio
measurements at 11, 30, 76, 122, 244 and 396
m
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NEE of CO2 at WLEF(forest scale)

• The region is a net source of CO2 to the
  atmosphere.
• Interannual variability is significant resolved
  by the measurements.
• Interannual variability is caused by changes in
  the timing of leaf-out, and correlated with
  changes in soil moisture.

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NEE and gross fluxes at ChEAS sites 1997-2002
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ChEAS upscaling test results

• Climate alone does not explain ChEAS CO2 fluxes.
  See differences among neighboring towers.
• WLEF fluxes cannot be explained as a linear
  combination of Lost Creek and Willow Creek
  fluxes.
• aspen? conifers? WLEF footprint dissimilar?
  Systematic error in flux tower measurements?
• Some ecosystem characteristic is missing. What
  is it?

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ChEAS upscaling test results

• Climate alone does not explain ChEAS CO2 fluxes.
• The WLEF footprint is a source of CO2 to the
  atmosphere.
• drying wetlands?
• disturbance/management?
• WLEF fluxes cannot be explained as a linear
  combination of Lost Creek and Willow Creek
  fluxes.
• aspen? conifers? WLEF footprint dissimilar?
  systematic errors that differ among flux towers?

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Sylvania Willow Creek flux tower comparison
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ChEAS upscaling test results

• Climate alone does not explain ChEAS CO2 fluxes.
• The WLEF footprint is a source of CO2 to the
  atmosphere.
• drying wetlands?
• disturbance/management?
• WLEF fluxes cannot be explained as a linear
  combination of Lost Creek and Willow Creek
  fluxes.
• aspen? conifers? WLEF footprint dissimilar?
  systematic errors that differ among flux towers?
• Sylvania (old growth) fluxes differ from Willow
  Creek (mature) fluxes as expected due to stand
  age (similar GEP, old R gt mature R).
• But soil respiration from chambers contradicts
  this result.

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North American Carbon Plan(NACP)http//www.carbo
ncyclescience.gov
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Complementary nature of inversion downscaling and
flux tower upscaling

• Inversion downscaling Flux tower upscaling
• Excellent spatial Intrinsically local
• integration measurements.
• Strong constraint on Difficult to upscale flux
• flux magnitude magnitudes. Variability
  easier.
• Poor temporal Excellent temporal resolution
• resolution
• Limited mechanistic Strong mechanistic
• understanding. understanding

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Can the terrestrial biosphere offset fossil fuel
emissions and prevent climate change?

• Forest type Biomass Soil carbon
• Tropical wet 156 255 PgC
• Temperate 73 142
• Boreal 143 179
• Fossil fuel emissions 7 PgC yr-1 x 50 years
  350 PgC.
• Schlesinger, Biogeochemistry.