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2 A New Science of the Earth The photographic images relayed to a rapt earth-bound au- dience in 1969, when a human first set foot on the moon, were rapidly inscribed on the human psyche. Seen from space, our planet was breathtaking in its loveliness, startling in its solitude. The image brought home as never before that our home is, after all, a planet small, self-contained, and in some ways perhaps, fragile. In the ensuing 20 years, that image of the earth has become a cliche, but the ramifications of those hard-won insights persist. The earth's land masses, oceans and atmosphere, and biological communities are increasingly seen by scientists, as well as by the public, as part of a unified system. Consequently, scientists can no longer adhere to the academic definitions of the classical scientific disciplines. Scientists are turning for help to colleagues in diverse fields, and integrating their studies as they develop a science of the earth. As the pervasive effects of human activity on the earth sys- tem become clear, the worId's scientists face an urgent challenge: Can they apply the scientific understanding and technology that 15

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16 THE EARTH AS A SYSTEM have allowed us, for instance, to venture into space, to develop the scientific understanding necessary to address the challenges we face in protecting the global environment on our own planet? To understand how human-induced changes global warming, depletion of the protective ozone layer, acid rain, deforestation, and possibly other changes that have not yet been detected- affect and are affected by the earth system, scientists are study- ing the interactions among processes in the atmosphere, oceans, and land surfaces, and the plants and animals that inhabit them. In some ways, this new push to understand the earth is a natural outcome of those first glimpses from the moon, two decades back. The quest to understand how the earth works may not match the excitement of man's footprint on the age-old lunar dust or the thrill of a manned trip to Mars. But what this quest lacks in glamor it makes up in importance for the future of the earth's environment: In one of the broadest sci entific inquiries in human history, physical and social scientists are drawing on every resource of technology and intellect to advance understanding of both the natural variability of the earth's processes and the effects of human activities on them. This new approach to the study of our planet is referred to as earth system science. Its practitioners strive to understand how the world works on a global scale by describing how its parts and their interactions evolved, how they function today, and how they may be expected to function in both the near and distant future. In this light, the earth system is seen as a set of interacting subsystems characterized by processes that vary on spatial scales from milluneters to the circumference of the earth, and on time scales from seconds to billions of years. It has become ever more clear that despite wide separations in distance or time, many processes are connected, and that a change in one component can propagate through the entire system. A 1988 report of the Earth System Sciences Committee to the NASA Advisory Council noted, for example, that "volcanic activity occurs widely along intersections of the earth's crustal plates and is driven by mantle convection on long time scales; yet the effects of eruptions are felt locally within hours or days and then,

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A NEW SCIENCE OF THE EARTH 17 over larger areas, for months or years because of deposition of dust and gases in the atmosphere." The NASA report explains that a science of the earth sys- tem must aim toward understanding processes governing global change over five broad time scales. Processes operating over the longest time scale, millions or billions of years, encompass Me evolution of solid-earth structures and include the internal core and mantle processes that generate the earth's magnetic fielcl. As the ages pass, crustal movements rearrange the continents, oceans open and close, and mountains erode. The oceans and atmosphere formed, their chemical compositions were cleter- mined, and life evolved within this long time frame. Over the time scale of hundreds of thousands of years to millions of years, the earth system witnesses oscillations be- tween ice ages and interglacial periods, development of soils, and shifts in the distribution of biological species, largely in response to cyclical changes in the earth's orbit around the sun. Decades and centuries, the span of a few human genera- tions, are the time scale over which the oceans, atmosphere, and biota interact to form the physical climate system. These systems are linked by the flow of moisture over the globe in the form of vapor, liquid water, and ice, and they change in response to processes and interactions that occur over much shorter periods ranging from seasons to hours. In this time frame, the earth's biosphere responds to and influences cycles that move key substances such as carbon, nitrogen, phosphorus, and sulfur through the global environment. Within the fourth time frame, days to seasons, the earth responds to weather, changes in ocean currents, growth and melting of the polar ice caps and sea ice, surface runoff and erosion, and the annual cycles of plant growth and decay. Finally, each day sees a cycle of heating and cooling, growth and decay, that moves heat, water, and a host of substances among land, air, oceans, and biota. Earthquakes and vol- canic eruptions occur suddenly on this shortest time scale in response to adjustments occurring within the solid earth over much longer periods.

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18 THE EARTH AS A SYSTEM Human influence is superimposed on the natural earth sys- tem processes operating over these time scales. Human civi- lization is characterized by modification of the environment- beginning with fire and then agriculture but until fairly re- cently, we did not profoundly alter the planet as a whole. Over the past few centuries, however, the sheer expansion in the number of the earth's human inhabitants and the growth in our technological ability to modify the landscape and exploit the earth's bounty of minerals, water, and fossil fuel have pro- foundly changed the entire earth system. The extent and conse- quences of these changes are only beginning to be understood. With all that is known and yet to be learned, how do we synthesize the vast body of knowledge necessary to describe the interactive system that is our earth? It is not enough to simply enumerate processes that are important. Participants in the effort to develop an earth system science have devised a schematic mode! of the earth system a working hypothesis of how the parts of the system work together- atmospheric and ocean circulation and dynamics, atmospheric chemistry, terres- trial ecosystems, and the global hydrologic cycle. All of these parts of the system interface continually with human activities and with changes in natural inputs from the sun, from voIca- noes, and from other natural causes. Although processes operating on all time scales influence the earth system, for this conceptual mode} it is the middle time scale~ecades to centuries- that is most relevant to the urgent inquiry into global environmental change. Within individual scientific disciplines, the most advanced models developed for use on this time scale focus on the physics and dynamics of the atmosphere. Models of ocean dynamics and atmospheric chem- istry are fairly well developed. The least developed models are those describing terrestrial ecosystems and marine biogeochem- ical systems, which are difficult to predict and subtle in their nature. Francis Bretherton, director of the Space Science and Eng~- neering Center at the University of Wisconsin at Madison, led the committee that developed the NASA report on earth system

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A NEW SCIENCE OF THE EARTH 19 science. He explains that schematic models of this sort offer a vehicle by which scientists of different backgrounds can share, in a useful way, the knowledge that has been acquired so la- boriously by the work of the worId's scientists. Models also indicate which aspects of the earth system may be the most im- portant ones to measure and help scientists test whether their understanding of how the system works is correct. Although the global environmental changes ctiscussed in this book are partly due to the by-products of technologies cle- veloped cluring and since the industrial Revolution, it is our technological prowess that enables scientists to measure and observe the changes and processes under way and engineers to develop sophisticated technologies that reduce the burden on the environment. Scientists are confident that within the next two decades many answers will emerge as data are acquired, cross-referenced, and interpreted. As Bretherton cautions, how- ever, "Our vulnerability to error is greatest not from the things that we include in the model, but from prophesies we leave out entirely."