an understanding of the records of past global change. Through the record in the rocks, the past has become the key to the future.

THE GLOBAL ENVIRONMENTS: A GEOLOGICAL PERSPECTIVE

The Changing Land Surface

The processes of weathering, erosion, and soil formation that together degrade upland areas have operated throughout earth history. Variations in the way the processes operate have generally been dominated by climate. Glaciers form in polar or subpolar regions or at high altitudes; deserts develop around 20° from the equator; and rain forests, with their great rivers, grow in equatorial and temperate latitudes.

Against this image it has been surprising to learn from new methods of dating and from quantitative studies of material fluxes that the times in which we live are unusual—not for one but for two reasons. The first is that since Northern Hemisphere glaciation developed about 2.5-million-years ago, fluctuations in ice volume have been large enough to repeatedly change world sea level by as much as 100 m over time scales of tens to hundreds of thousands of years. Despite recent fluctuations, Antarctic ice caps have remained stable for as long as 40-million-years. For this reason the dominant major cause of sea level change during the past 2.5-million-years has been the accumulation and ablation of Northern Hemisphere ice sheets. As an example of fast change, consider that 18,000 years ago, when sea level was about 100 m lower, rivers reached the sea at the edge of the continents. Since then they have retreated so much in response to rising sea level that some river mouths (e.g., the Susquehanna) now lie far back from the continental shelves in estuaries like Chesapeake Bay. Such short-term changes prevent the establishment of an equilibrium in weathering, erosion, and deposition, and during the past 2.5-million-years they have rendered the surface an unusually dynamic place.

We ourselves provide the other reason for unusual conditions. The condition of the soil and processes of erosion have been substantially changed since agriculture began. These changes have increased in recent decades because of such processes as dam building, forest destruction, widespread irrigation, and flood control. For instance, sediments that formerly were carried to the Mississippi delta are now being impounded behind dams; this is contributing to the encroachment of the Gulf of Mexico upon the delta. The problems of marine transgression along the Gulf Coast are discussed throughout this report; here, attention is drawn to progress in understanding the major processes that dominate the change in land surface above sea level, with emphasis on the peculiar problems and opportunities that result from the apparently exceptional time in which we live.

Landforms

Landforms are continually changing, but most changes are subtle and generally escape notice. Although great attention is rightly given to catastrophic events such as earthquakes, volcanic eruptions, and landslides, the time scales of importance in geomorphology—the study of landforms and the processes that shape them—range from seconds to millions of years, and the space scales range from single hillsides to global dimensions. The challenge is to characterize the ways in which landforms respond to both common and uncommon events. The geomorphic record contains information about the ways in which present and past environmental changes have modified the processes operating at the surface, both in intensity and duration. A long-term view is essential because the time spans of contemporary monitoring are too short to represent the range of possible conditions. Long-range perspectives from the geomorphic record permit testing of models of environmental change, whether they apply at global, regional, or local scales.

A landscape can provide information about the magnitudes and return frequencies of natural processes. This information can lead to identification of geomorphological thresholds that precede disastrous events. Some responses are immediate—floods, landslides, and debris flows—but others can be spread over years or decades—upland soil erosion, glacial to interglacial cycles, river-channel change, and sea level change. The rates of some of these processes have been greatly accelerated by human activities.

Geomorphic events of the past, which are recorded in landforms and stratigraphy, can provide usable analogs of anticipated environmental change.



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