Transcript Combination of mechanisms responsible for the missing carbon sink

658A Carbon Cycle and Climate Past, Present and Future
Combination of mechanisms responsible for the
missing carbon sink using bottom-up approach
Haifeng Qian
March 29, 2006
Outline
Brief background of carbon cycle and
“missing carbon”
 Concerns to be addressed
 Literary reviews
 Progress and challenge on bottom-up
approach
 Conclusion and discussion

“Missing carbon” in the carbon cycle balances (1980’s)
Emission + Land-use = Terrestrial + Ocean + Atmosphere
5.4
*: more uncertain
1.7 (*)
1.9 (*)
2.0
3.2
“missing carbon”
 History of “missing sink”: coined by ocean modelers (1970’s) of the carbon
cycle who were unable to account for all the carbon released to the atmosphere
from fossil fuel burning and land-use changes in oceanic sinks, but were
uncomfortable attributing the unaccountable carbon to a terrestrial sink, without
having a detailed model to explain terrestrial biosphere carbon storage
mechanisms.
 During the past two decades, a substantial fraction of the carbon sink has
been on land, in the temperate and boreal latitudes of the Northern Hemisphere
the mechanisms and the detailed spatial pattern of this Northern Hemisphere
terrestrial sink remain elusive.
 Quantifying these sinks and understanding the underlying mechanisms are
top priorities for understanding Earth's major biogeochemical cycles and for
establishing how changes in their magnitude could affect the future trajectory of
atmospheric CO2 concentration.
Schindler, David S. “The Mysterious Missing
Sink.” Nature. March 1999
Questions:






What is debate of missing carbon sinks
Why is missing carbon sinks important
Modeling approaches for this “ missing” carbon
Mechanisms responsible for “missing” carbon
Progress and challenge of bottom-up approach
Perspectives of future carbon
Bottom-up
Top-down
Most important advantage of bottom-up approach is that can give the estimates
based on the mechanistic hypotheses about the processes that control the fluxes
Possible mechanisms for missing carbon





Rising temperatures, atmospheric CO2
concentrations, and nitrogen deposition,
Climate change
Land use
Forest regrowth
Fire suppression
The Not-So-Big US Carbon Sink ...
 There is a sink for carbon of about 2 Pg/year north of approximately
30°N, although these analyses were unable to constrain the longitudinal
distribution of the sink.
 Fan et al.(science 1998) suggest on the basis of atmospheric and
oceanic data and modeling that the Northern Hemisphere carbon sink
is predominantly North American, south of 51ºN, with a magnitude
about that of U.S. fossil fuel emissions.
 Fung et al(science 1999):U.S. ecosystems accumulated
carbon at a rate of 0.15 to 0.35 PgC/yr, equivalent to about 10 to 30%
of U.S. fossil fuel emissions.
 The apparent contrast between the conclusions of these two studies
highlights the differences between and uncertainties associated with
atmospheric "top-down" and terrestrial ecosystem "bottom-up"
approaches.
Mechanism 1: Rising temperatures, elevated atmospheric CO2
concentrations, and nitrogen deposition , and climate change
Greeness?
NDVI increase (1982-1990)
Myneni et al. 97 nature
Fertilization mechanism is not all:

Elevated CO2 concentrations and nitrogen deposition
stimulate plant growth in many ecosystems
However, Long-term observation suggests such CO2 fertilization
effects are too small to explain.

Experiments (DUKE)with elevated co2
concentration show a rapid decrease of the
fertilization effect after an initial
enhancement.
 Soil fertility limits
carbon sequestration by
forest ecosystems in
CO2-enrich atmosphere
Ram Oren: 2001, Nature 411
Caspersen et al. science 290
Contributions of Land-Use History to Carbon Accumulation
in U.S. Forests
B(1) = B(0) + G(0) –μB(0)
Forest enhance << forest regrowth (land use)
 Land use is the dominant factor governing the rate of carbon
accumulation in these states, with growth enhancement contributing
far less than previously reported.
 The estimated fraction of aboveground net ecosystem production
due to growth enhancement is 2.064.4%, with the remainder due
to land use.
Mcguire 2001: Analysis of co2,
climate and land use effect with
four process-based ecosystem
model
Cumulative net carbon storage
Consider effect of increasing
co2, climate change and
cropland establishment and
abandonment
Net lease associated with
cropland establishment and
abandonment
Predicting the future
source
Sink
? Narrow or better
understanding
IPSL
UMD
Hadley
UMD work: These results suggest that different CO2
fertilization strengths explain part of the UMD-IPSL
differences and soil decomposition and turnover time
explain partly the UMD-Hadley differences in the coupled
runs.
Zeng: GRL 2004
Mechanism 2: re-growth
The ecosystem recovery
processes that are
primarily responsible for
the contemporary U.S.
carbon sink will slow over
the next century, resulting
in a significant reduction of
the sink.
Hurtt: PNAS,2002
Mechanism 3:fire suppression US)
Fig. 2. Estimated average annual air-to-ground net flux in Pg C/y from
1700 to 1990. Positive values indicate a land sink and negative values
indicate a source to the atmosphere. Light line, Houghton estimate(1999
science) without fire suppression. Dark line, Houghton estimate with fire
suppression. Dark line with ■ ED (Ecosystem Demography
model).
Hurtt: PNAS,2002
Conclusion and discussion
“ Missing” Carbon still not well known
 Progress made but challenge big
 Mechanisms in bottom-up approach
 No magic bullets to avoid higher co2
concentration in the next century
 Better understanding of carbon cycle is
indeed needed

Reference








Sarmiento, J.L. and N. Gruber. Sinks for
anthropogenic carbon, Physics Today, 55(8), 30-36,
2002.
J. P. Caspersen et al., Science 290, 1148 (2000).
W. H. Schlesinger, J. Lichter, Nature 411, 466 (2001).
S. W. Pacala et al., Science 292, 2316 (2001).
G. C. Hurtt et al., Proc. Natl. Acad. Sci. USA 99, 1389
(2002).
Zeng et al . GRL, 2004, 31 L20203
Christopher B. F et al .Science 1999. 544 – 545
Mcguire et al. GBC, 2001 ,15,183
Thanks