Will computer models enable scientists to accurately predict the future?
Running says it is too early to answer that question, but some
tantalizing results are emerging from current Earth-system modeling. For
example, if the current climate models are correct in predicting higher
temperatures coupled with greater rainfall in the next century, then the
biospheric models predict a response of generally higher plant productivity and
longer growing seasons. However, this seemingly positive prediction is tempered
by the predictions that agricultural areas will gradually migrate to higher
latitudes, leaving some current croplands too hot for production and severely
disrupting local economies.
Running notes that his biospheric model results are directly coupled with
climate model results. If higher temperatures occur without a corresponding
increase in rainfall, then plant productivity is predicted to drop due to more
frequent and widespread droughts. |
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Can we trust computer models?
Of course, models have limitations too. For instance, they can
over-simplify the systems they are designed to simulate; thereby leading to
erroneous results (Waring and Running 1998).
"If you were looking at a complex system like the biosphere and you
tried to completely represent all the complexity of that system, you would drown
in details," Running states. "Models are elegant simplifications of
reality. There are many details that are left out because the models are
generally focused on certain central tendencies of the ecosystem."
Therefore a model must be thoroughly tested in a wide range of conditions so
that its strengths and weaknesses are well understood. Running also cautions
that while they are important research tools, models cannot ever verify what the
truth is (or will be). Only direct measurements can establish "scientific
truth." But models can tell scientists where conventional understanding is wrong
and encourage them to make the critical measurements that might not
otherwise be made.
The terrestrial biosphere in the 21st century... |
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Using the same models that generated the trio of
net primary productivity images on page 1, this set
compares the model outputs based on predicted climate if atmospheric carbon dioxide doubled
(710 parts per million). Scientists evaluate the accuracy of computer models both by
comparing them to observations and comparing several models run with the same initial
data. (Image courtesy of the University of Montana
Numerical Terradynamic Simulation Group) |
Running sees larger things in store for Earth
scientists after the fall
1999 launch of NASAs Earth Observing System flagship satelliteTerra.
This new satellite carries a payload of sensors that will provide a new suite
of data products, which Running feels will ultimately yield a new generation of
models. With daily global measurements of land surface vegetative cover, forest
fires and the amount of biomass burned, total leaf area and production rates of
foliage, and incoming photosynthetically active (solar) radiation, Terra will
provide direct measurements of many variables that today we can only model.
Moreover, Terra will make measurements that apply to a wide range of atmospheric
and oceanic disciplines, as well as land-based, so it addresses the Earth system
as a whole.
"Finally, we will be able to answer questions like Where is the
missing terrestrial carbon sink? " Running surmises.
"Terras measurements of the net primary production of green
vegetation will help us quantify the terrestrial biospheres role in the
global carbon cycle."
References
- Arrhenius, S., 1896: "On the influence of carbonic acid in the air
upon the temperature of the ground." Philosophical Magazine and Journal
of Science, 41, pp. 237-276.
- Ford, Ray, Steven Running, and Ramakrishna Nemani, 1994: "A Modular
System for Scalable Ecological Modeling." IEEE Computational Science
& Engineering, Fall 1994, pp. 32-44.
- Knox, Robert G., Virginia L. Kalb, and Elissa R. Levine, 1997: "A
Problem-Solving Workbench for Interactive Simulation of Ecosystems."
IEEE Computational Science & Engineering, 4, pp. 52-60.
- Levine, Elissa R. and Daniel S. Kimes, 1998: "Predicting Soil Carbon
in Mollisols Using Neural Networks." Soil Processes and the Carbon Cycle.
Edited by Rattan Lal, et al. CRC Press, pp. 473-484.
- Levine, Elissa R. Personal interview, 1999.
- Running, Steven W., Richard H. Waring, and R.A. Rydell, 1975:
"Physiological Control of Water Flux in Conifers." Oecologia,
18, pp. 1-16.
- Running, Steven W. Personal interview, 1999.
- Waring, Richard H. and Steven W. Running. Forest Ecosystems: Analysis at
Multiple Scales, 2nd Edition. Academic Press, 1998.
-Weishampel, John F., Robert G. Knox, and Elissa R. Levine, 1999: "Soil
saturation effects on forest dynamics: scaling across a southern/northern
hardwood landscape." Landscape Ecology, 14, pp. 121-135.
How to Build Better Climate Models
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The Terra satellite, as it will appear after the
solar panel and antenna deploy. (Image by Reto Stöckli,
NASA GSFC Visualization Analysis Lab) |