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Findings |
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2 Transient Dynamics |
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VEMAP 2
Model Results - Conterminous U.S.
(Note: Model experiments were limited in scope to the conterminous
U.S.; we do not present model results from the Alaska domain.)
Background and Experimental Design:
The Vegetation-Ecosystem Modeling and Analysis Project (VEMAP) was a large,
collaborative, multi-agency program to simulate and understand ecosystem
dynamics for the continental U.S. Two principal model experiments
were run. First, a series of ecosystem models were run from 1895 to 1993
to simulate current ecosystem biogeochemistry. Second, these same models
were integrated forward using the output from two climate system models
(CCC (Canadian Climate Centre) and Hadley Centre models) using climate
results translated into the VEMAP grid and re-adjusted for high-resolution
topography for the simulated period 1994-2100.
Findings:
The models agree fairly well for the mean fluxes in the late 20th century,
suggesting a sink due to CO2 fertilization, climate, and agriculture of
less than 0.1 gigatons of carbon (Gt C) per year. The models agree in the
mean within about 25%, although they differ somewhat in their simulation
of interannual variability. The models agree in simulating a high sensitivity
to drought, which they predict should release carbon to the atmosphere
(Schimel et al., 2000 [Abstract]).
The estimated value for the CO2 fertilization sink, 0.08 Gt C per year,
is a small fraction of the sink actually estimated for the U.S. using direct
observations (0.3-0.7) and suggests that changes in land use practices,
including agricultural abandonment and fire suppression, dominate the sink
in the U.S. The models provide an interesting view of the future. Three
of the models used do not include any disturbance (e.g., fire or harvest)
practices. They agree reasonably well in suggesting a steady sink from
CO2 fertilization interacting with climate change (This figure
shows results from the VEMAP models, showing the historical (1895-1993)
and future climate scenario results.).Three of the models include disturbance.
In two models, LPJ (Lund-Potsdam-Jena) and MC1 (MAPSS-Century 1), disturbance
is included via a prognostic fire model. In the third, Century, the model
is forced by assumed fire and harvest return frequencies. All three models
suggest a much lower accumulation of carbon because of chronic losses due
to disturbance. One model (MC1) suggests that climate changes in the mid-century
could trigger large scale fires with substantial losses of carbon. The
other is less sensitive to the mid-century climate but shows losses accelerating
in the late 21st century. Clearly, the role of disturbance and land management
must be the priority for the next round of model development, testing,
and applications.
Key
Publications
Model Variables:
Available model
variables differ for each ecosystem model group. We provide a
description
of the variable, including the units and file naming convention.
Contacts