assessing rates and patterns of evolution and extinction are enlivening interpretations of this record. A new context for this work has emerged with the expansion of research on large-scale changes in ancient oceans, atmospheres, and continental configurations. Models of the coupled ocean-atmosphere system are being tested against geological data and related to the history of life. Studies of ice cores, pollen, and marine plankton are yielding detailed pictures of environmental and biotic changes during the past several thousand years—changes that can serve as models for understanding events of the future.

Global extinctions are of great current interest. A variety of evidence has convinced many geologists that the impact of one or more meteorites or comets caused the extinction of the dinosaurs, but other great extinctions have been attributed to climatic changes driven by plate tectonics. The search for evidence of large impacts continues, and the potential role of massive volcanism is also under investigation.

Top-Priority, Theme I: The Past 2.5 Million Years

Environmental and biological changes since the onset of the Northern Hemisphere glaciation.

The most recent past has been chosen as a top-priority for half a dozen reasons. First, the onset of Northern Hemisphere glaciation during this period represents the most radical environmental change on Earth within the past several tens of millions of years. Second, its closeness to the present means that the record is generally more complete. Methods of study not applicable to older times can be used, such as dating techniques using 14C and other cosmogenic nuclides. Third, surface features such as soils and landscapes, including mountains and river systems, have largely developed or have been strongly modified within this geologically short interval. Fourth, the relatively complete understanding that is attainable makes this interval of peculiar importance as an analog of older, less readily or completely analyzable parts of the geological record. Fifth, as inhabitants of a rapidly changing environment, the human race will find it is useful to have a full appreciation of what has happened in the geologically recent past. Students of global change have emphasized the importance of the record of progressively more recent times. And, finally, the fossil record for this interval includes many living species—for example, humans—and many extinct species with close living relatives.

Other High Priorities

For Theme I Table 7.6 lists two additional high-priority programs, effectively recommending the pursuit of similar research back through time: first, during the relatively recent interval over the past 150-million-years, and then during the long interval before that back to 3.8-billion-years. The background for these investigations is outlined briefly in Table 7.4 and discussed at length in Chapter 3. Important programs already support these three priorities, which aim at integrating understanding but approach the problems in diverse ways. One simple way of dividing activities is their environment: the land surface, shallow subsurface, river system, frozen ground, glacial, lacustrine, and marine environments. These are all the focus of dedicated programs in a variety of agencies, federal and local.

Programs and Infrastructure

Preeminent among single programs relevant to understanding the environment and biological change on the 2.5-million-year time scale is the Ocean Drilling Program (ODP). Although the program operates only in the two-thirds of the Earth occupied by the oceans and their margins, no other single program can rival it in comprehensive scope. Its results embody an unrivaled record of the evolution of the atmosphere-ocean system and of ocean biology and biogeochemistry, and for this reason it is accorded the highest priority. In 1992 the NRC's Board on Earth Sciences and Resources and the Ocean Studies Board reviewed the ODP program and its forward plans.

The study of how the Earth has behaved during the past 2.5-million-years involves diverse agencies and numerous programs. For example, hydrology entails an understanding of rivers and landforms as well as soil development and groundwater movement on the 2.5-million-year time scale. A variety of organizations play a part, ranging from federal agencies (e.g., Department of Agriculture, USGS, Department of Energy (DOE), Environmental Protection Agency, Army Corps of Engineers, and NASA) through the state geological surveys and water authorities to individual counties and cities. Pools of relevant information have grown, such as

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