Can models show us the future?

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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.

   

 
Change in NPP
 

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)

TerraRunning sees larger things in store for Earth scientists after the fall 1999 launch of NASA’s Earth Observing System flagship satellite–Terra. 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. "Terra’s measurements of the net primary production of green vegetation will help us quantify the terrestrial biosphere’s 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.

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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)

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