Clearly, the climate system is quite complex. The parameters involved in climatic change interact in an intricate system of positive and negative feedbacks that make it difficult to predict the specific results of an individual perturbation. Adding to the difficulty is the fact that individual perturbations are rare. Parameters usually change simultaneously, and the climate system is continuously responding to each push and pull. Because of this complexity there may be significant lag times between a parameter's change and the climate system's response. So climatic changes and their forcing factors are not easily linked. In spite of this uncertainty, being able to understand and predict climate change is critical in a world where the human population already approaches five and a half billion people.

To deal with the complexities of the climate system, scientists rely heavily on the preserved climate record and computer models of climate systems. We only have a relatively short and imperfect historical record of climate change, but scientists can also use the geological record to observe how the climate system has changed through time (Table 2). The examples provided earlier, of Cretaceous warmth, Little Ice Age cooling, and "the year without a summer" all provide evidence for long- and short-term, naturally-induced climate variation. This proxy record allows us to place limits on the historical range of natural variability. With these limits, we can evaluate which of the climatic changes observed in the post-industrial world occurred as a result of human activity. Paleoclimate records are also used in computer simulations of ancient climate. By adjusting different variables, scientists are able to reproduce changes that occurred decades to millions of years ago. In this way, we can improve our understanding of the magnitude of climate change with respect to a variety of forcing functions.

Computer simulations of present and past climates help researchers place limits upon the influence of various perturbations, both natural and anthropogenic, and also help to untangle the system's complex feedbacks. General circulation models (GCMs) are three-dimensional mathematical models programmed to solve mathematical expressions at many points on and above the globe. Scientists use them to predict and understand cause and effect relationships within the climate system. Although these models have become increasingly more sophisticated over the years, they still suffer from limitations. The models can only handle a limited number of variables at any one time and they are unable to adequately represent the more complex (and less understood) systems like deep ocean water circulation or cloud formation. A complex GCM also requires a tremendous amount of computer time and expense. One way to simplify GCMs is by increasing the spacing between grid points at which the solutions will be calculated. This decreases the amount of computer time that is needed, but it also decreases the resolution of the models. Hence, GCMs can often produce fairly reasonable models of global climate conditions, but relatively poor models of climate change on a more local scale. Despite these limitations, computer models remain a useful tool. By varying individual parameters, scientists can determine the relative importance of different factors on climate change.

Global climate models suggest that in the future, temperatures will rise and precipitation patterns change due to increases in the atmospheric concentrations of greenhouse gases. However, the rate and magnitude of climate change is far from certain, as the system self-adjusts through a multitude of feedbacks. Because the biosphere and climate system are so intimately intertwined, changes in climate will cause significant responses in the biosphere. Specifically, changes in temperature and hydrology will shift vegetation zones and affect the related fauna. Rates of extinction would be enhanced as plants and animals struggle to locate and acquire limited resources. The impact on human life would also include altered agricultural belts, shifting fisheries, and changes in habitable land.

It is extremely difficult to predict global climate change, or even to assess the relative importance of natural and anthropogenic factors in climatic systems. The problem is exacerbated as "all of the changes will be occurring simultaneously. Thus, we must consider that CO 2 increases, temperature changes, rainfall changes, UV-B increases, and increases in oxidants will be superimposed all at once on the biota. What will happen to individuals or single species faced with multiple changes in the physical and chemical climate? And what implications will the multitude of changes have for the intricate intertwined relationships of organisms at the ecosystem level? The possible number of interactions is staggering" (Ennis and Marcus, 1993, p. 38).

We still do not understand many parts of the climate system, yet changes (both natural and anthropogenic) are occurring. The fear remains that these changes may adversely affect the Earth's ability to support an rapidly expanding human population.

Barron, E.J., 1994, Climatic Variation in Earth History, Understanding Global Climate Change: Earth Science and Human Impacts, Global Change Instruction Program #108, UCAR, 23 pp.

Bradley, R.S., 1985, Quaternary Paleoclimatology, Unwin Hyman, Cambridge MA, 472 pp.

Friis-Christensen, E.F., and Lassen, K., 1991, Length of the Solar cycle: An Indicator of Solar Activity closely Associated with Climate, Science, 254, 698-700

Duxbury, A.C. and Duxbury, A.B., 1994, An Introduction to the World's Oceans, fourth edition Wm. C. Brown, Dubuque, Iowa, 472 pp.

Emiliani, C., 1978, The cause of the ice ages, Earth and Planetary Science Letters, 37, 349-352.

Ennis, C.A., and Marcus, N.H., 1993, Biological Consequences of Global Climate Change, Understanding Global Climate Change: Earth Science and Human Impacts, Global Change Instruction Program #107, 51 pp.

Farman, J.C., Gardiner, B.G., and Shanklin, J.D., 1985, Nature, 315, 207-210.

Firor, J., 1990, The Changing Atmsophere: A Global Challenge, Yale University Press.

Houghton, J.T., Meira Filho, L.G., Bruce, J., Hoesung Lee, Callander, B.A., Haites, E., Harris, N., and Maskell, K.(editors), 1994, Climate Change 1994, Cambridge University Press, 339 pp.

Jones, A.E. and Shanklin, J.D., 1995, Continued decline of total ozone over Halley, Antarctica, since 1985, Nature, 376, 409-411.

Martin, J.H., Gordon, R.M., and Fitzwater, S.E., 1990, Iron in Antarctic waters, Nature, 345, 156-158.

Martin, J.H., Coale, K.H., Johnson, K.S., Fitzwater, S.E., Gordon, R.M., Tanner, S.J., Hunter, C.N., Elrod, V.A., Nowicki, J.L., Coley, T.L., Barber, R.T., Lindley, S., Watson, A.J., Van Scoy, K., Law, C.S., Liddicoat, M.I., Ling, R., Stanton, T, Stockel, J., Collns, C., Anderson, A., Bidigare, R., Ondrusek, M., Latasa, M., Millero, F.J., Lee, K., Yao, W., Zhang, J.Z., Friederich, G., Sakamoto, C., Chavez, F., Buck, K., Kolber, Z., Greene, R., Falkowski, P., Chisholm, S.W., Hoge, F., Swift, R., Yungel, J., Turner, S., Nightingale, P., Hatton, A., Liss, P., and Tindale, N.W., 1994, Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean, Nature, 371, 123-129.

Mouginis-Mark, P.J., Pieri, D.C., and Francis, P.W., 1993, Volcanoes, in: Atlas of Satellite Observations related to Global Change, ed. Gurney, R.J., Foster, J.L., and Parkinson, C.L., Cambridge University Press, pp. 341-357.

Rampino, M.R., Self, S., and Stothers, R.B., 1988, Volcanic Winters, Annual Review of Earth Planetary Sciences , 16, 73-99.

Revkin, A., 1992, Global Warming, Understanding the Forecast, American Museum of Natural History and Environmental Defense Fund.

Rind, D., and Overpeck, J., 1993, Hypothesized Causes of Decade-To-Century-Scale Climate Variability: Climate Model Results, Quaternary Science Reviews, 12, 357-374.

Siegenthaler, U., and Sarmiento, J.L., 1993, Atmospheric Carbon Dioxide and the Ocean, Nature, 365 , 119-125.

Sigurdsson, H., 1990, Assessment of the Atmospheric Impact of Volcanic Eruptions, Geological Society of America, special paper 247, 99-110.

Shaw, G., 1992, Clouds and Climate Change, Understanding Global Change: Earth Science and Human Impacts, Global Change Instruction Program, UCAR, 20 pp.

Streete, J., 1991, The Sun-Earth System, Understanding Global Change: Earth Science and Human Impacts, Global Change Instruction Program #102, UCAR, 33 pp.

WAIS, The West Antarctic Ice Sheet Initiative, A Multidisciplinary Study of Rapid Climate Change and Future Sea Level, National Science Foundation.

World Meteorological Organisation, 1975, The Physical Basis of Climate and Climate Modelling, GARP Publications, Series No. 16, WMO Geneva.