standing historical geology, and, during the past 20 years, flooding and emergence have been found to be approximately simultaneous for at least three continents. This coincidence has led to the suggestion that the main driver of sea level change is within the world ocean. The amount of seafloor spreading going on at a particular time seems a strong candidate for the dominant control. Young seafloor is hot and shallow, and aged seafloor is cold and lies deep. The average rate of spreading and the total length of spreading center at any one time can be quantitatively associated with the extent of flooding of the continents.

Conceptual models that are progressively refined may be considered successful for parts of the earth system but are not really comprehensive. The question of continental flooding helps to show why this is so. The amount of water that is locked up in ice has decreased since about 20,000 years ago with the melting of North American and Eurasian ice sheets. This decrease has accounted for about 100 m of sea level rise over the same interval. This is a very large change in sea level compared with the range of continental elevation. About 80 percent of the continental area lies within 1 km of the present sea level. Although we can estimate the change in the average age of the ocean floor for the past 100-million-years we cannot do so well for the amount of water locked up in ice sheets. Evidence that there were no major ice sheets 100-million-years ago is strong, but it is not clear how long ago the great Antarctic ice sheet formed, nor can we tell whether it has fluctuated much or even disappeared and been renewed. This example illustrates a general situation. Comprehensive modeling of earth systems is very difficult, because although some parts of the system are understood, fluctuations in critical variables for other parts of the system are too poorly known to justify the construction of elaborate models. The challenge represented by the fact that some parts of the earth system can be readily modeled whereas others cannot has been met by concentrating on modeling those subsystems that can be handled well and by seeking innovative ways of quantifying information about subsystems that cannot. A further problem is linking the models of system parts that operate at different rates, such as the atmosphere, which changes on a very short time scale, and the ocean, which changes more slowly. The most successful and productive models reconstruct cyclic changes involving limited components of the earth system. Only a few components, or variables, can be tested at one time, but every successful simulation provides further understanding.

Some aspects of paleoceanography and paleoclimatology lend themselves to quantitative modeling in isolation from the whole. However, most are not, and, because of the interdependencies, the models should be global in scale, link adjacent water masses, and couple the atmosphere with the oceans, so great is the interdependence of these two great fluid envelopes. Atmosphere and ocean are linked especially by exchanges of heat, momentum, carbon dioxide, and, of course, water. The surface temperatures of the ocean influence these fluxes. The salinity of waters that reach the ocean bottom, a variable related to climate, also affects oceanic circulation. And changes in global sea level affect the levels of atmospheric carbon dioxide, which in turn influence global climate through the greenhouse effect. Outstanding successes include modeling of the glacial cycles of the past 0.8-million-years, as controlled by variations in orbital parameters, and modeling of sedimentary basin fill, as controlled by loads applied to the lithosphere.

Modeling of atmospheric systems, which mainly use energy balance models and general circulation models (GCMs), has grown to be extremely sophisticated in recent years, and solid-earth scientists have found some community of interest with atmospheric modelers. For example, at the time of the most recent glacial maximum, the solar energy input at the surface was less than it is today. A GCM of that time also indicates wind and temperature distributions that are very different from those of today. These indications have been compared with the geological record of the most recent glacial maximum with some success. Simplified energy balance models for the atmosphere for 100 million and 375-million-years ago, when continental distributions were very different and much of the surficial climate was warmer than it is now, have also met with some qualified success. There are some problems, too—for example, existing GCMs for the glacial maximum will not predict conditions that would grow the large ice sheets that existed then.

Intellectual Frontiers

Understanding the processes that are active today in establishing the surface environment and understanding how those processes have operated throughout geological time are the basic challenges addressed in this chapter. Intellectual frontiers relate to questions about the surface environment that have not been asked in the past, either because it was not possible to ask them or because it was not

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