Variations in the Earth's Orbit

It was recognized in the nineteenth century that variations in the Earth's orbit would cause changes in incoming solar radiation that could be important in controlling ice ages. Theoreticians first calculated how these variations would interact, and the study of deep-sea sediments has yielded persuasive evidence that the recurring ice ages are indeed closely associated with orbital cycles. The importance of orbital variations in controlling the climatic changes of the past 2-million-years has proved revolutionary. The critical studies of sediments from deep-sea cores were published less than 20 years ago, and since then application of the idea of orbital control has been applied to studies of ice cores and cave deposits. Researchers now have a yardstick to apply to the complicated record of the most recent past, which is helping to make their interpretations more quantitative. Problems being addressed currently include the extent to which evidence of orbital control can be recognized in older parts of the geological record and how orbital variations (which act directly on insolation) have affected not only such variables as sea-surface temperature but also atmospheric trace gas concentrations.

Humankind as a Geological Agent

The geological sciences have traditionally been responsible for finding and maintaining adequate supplies of fuels, minerals, and nonmineral resources at reasonable prices. The continuing ability to meet this demand depends on basic and applied research in the geosciences. But as human endeavors put pressure on the carrying capacity of settled regions, different components of the Earth's systems are affected. This report therefore does not limit its consideration of resources to the traditional meaning of the word.

Groundwater is an essential resource. In parts of the world the supply of groundwater is dwindling because it is being withdrawn from underground aquifers faster than it is being replenished. Accessible and stable places to live are also a resource that human populations are rapidly consuming. Soil erosion is accelerating in many areas around the world; for every pound of food consumed in the world, an average of 7 pounds of soil is lost to erosion. A habitable environment is a resource that pollutants released to the air, water, or solid-earth can compromise. Biological species of the world are also a resource, but human activities have caused extinctions at an accelerating rate. If current trends persist, a large proportion of the species existing today may disappear during the next few decades.

The consequences of human activity are a recent factor needed to understand earth systems. Before the control of fire, the environmental effects of human beings were comparable to those of other species. Then crop cultivation, animal domestication, and the subsequent appearance of urban civilization introduced a new set of forces onto the Earth. Today, human beings are changing basic earth processes in ways they have never been changed before because of the acceleration and concentration of the effects.

Each person in the United States uses an average of 16 metric tons—about 35,000 pounds—of minerals and fossil fuels each year. This amount includes only the use of materials; it does not include the material moved during the construction of homes, parking lots, factories, dams, and so on. On a worldwide basis, the human population uses nearly 50 billion metric tons of material each year. That is more than three times the amount of sediment transported to the sea by all the rivers of the world.

Clearly, humankind has become a geological agent that must be taken into account in considering the workings of the earth system. If future generations are to have resource supplies in the full sense of the word, decisions must be made within a context that considers the Earth as a total entity. The geological sciences offer information that will be invaluable in evaluating trade-offs and in balancing competing demands.


There are two fundamental reasons for pursuing the earth sciences. One is to learn more about the world where we live, to satisfy a basic human curiosity about our surroundings and our relationship to them. The other is that research in the geosciences can be used to improve the human condition. Geological knowledge is essential for making decisions regarding the use and preservation of resources or the protection of human life and habitats from the effects of natural disasters.

Stated this way, the distinctions between intellectual curiosity and utilitarian concern may appear fairly sharp, but in fact the two categories overlap extensively. Not all scientific studies have immediate applications, but experience shows that few findings in the geosciences remain unapplied for long. Today's theoretical science may very well be tomor-

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement