global changes and those changes resulting from modification of the environment by human beings.

To predict continued environmental changes and their effects on the biosphere, we need as a historical perspective the results of new and continuing studies on the reconstruction of global warming trends and other atmospheric/hydroshereric changes, as determined from geochemical cycles, paleoclimatology, evolution of life, and extinctions.

The scientific background for this objective is covered mainly in Chapter 3, with some applications to global change appearing in Chapter 5. Global change is popularly associated with change in climate, but scientists recognize broader implications, extending to such phenomena as sea level, groundwater quality, and biodiversity. The record of the past is vital to an understanding of global change. We need to be able to characterize variability within the past record and to identify the causes of that variability as well as to understand system states very different from those of today. System transitions, especially abrupt ones, have much to teach us.

The past is also particularly useful for testing the credibility of new climatic models. If they cannot reproduce what has already occurred, their ability to predict conditions in the future is likely to be limited.

The influence of volcanism on climate has been documented, although its contribution to global extinctions is still debated. The implications of a massive ash eruption for climate change and agriculture may be a serious threat to global habitability, but the frequency of such eruptions is low.

The role of orbital variation in modifying temperature at the surface over the past few hundred thousand years has been demonstrated by geologists studying a deep-sea core from the Indian Ocean. Awareness is growing that extraterrestrial influence might be discerned operating in much of the earth system.

The intensity of the magnetic field has varied by more than 10 percent in the past few hundred years, and there is some evidence that much larger changes in intensity accompany episodes of magnetic reversal. Very little is known, or even guessed, about whether and how magnetic-field variation might affect the environment, especially living plants and animals, but research is warranted.

Pollution of groundwater in a variety of ways is a major and costly problem in many areas, and it is evident that humans are polluting these water resources at an ever-increasing rate. Research is growing, too, but it is not clear that the full expertise of the solid-earth science community is being involved on a large enough scale.

Top Priority, Theme D: Develop the Ability to Remediate Polluted Groundwater

The need, the opportunity, the potential for success, and the scientific challenge represented by thorough understanding of the environment and how polluted groundwater might be remediated, particularly with microbial methods.

Large and increasing quantities of organic waste material occupy unconfined areas in the United States, both at the surface and underground, including groundwater. Removal of these materials from their present sites is becoming increasingly difficult, and the need for in situ modification is clear. Research into such activities as breeding and using specialized microorganisms to modify and render harmless organic waste materials in a broad range of environments is needed and offers a unique challenge. The subsurface science programs of DOE and EPA represent prominent initiatives in this field.

Other High Priorities

Two other high-priorities for Theme D are listed in Table 7.13. The first is related to the top-priority selection in the sense that secure isolation of toxic and radioactive wastes will in the future reduce the pollution of water supplies. Microbiological techniques may be used here as well, but the immediate need is to characterize geologically secure repository sites and to determine how most efficiently to convert nuclear waste into solid glass and ceramic materials. The second topic represents an opportunity for multidisciplinary research into the health risks of some minerals and elements; with proper characterization of the geological materials



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