Earth system science consists of both reductionist and integrated components. Parts of this transdisciplinary field are clearly disciplinary, exemplified by the chapters in Part Two. Other parts are more integrative in the sense that the disciplines of the Earth Sciences plus chemistry, physics, and biology are all needed in the study of the elemental cycles as found in Part Three. Yet other levels of integration emerge when specific aspects of the Earth system are examined. We include in Part Four examples ranging from an integrated view of the acid-base and redox balances of Earth, to the coupling of biogeochemical cycles and climate, to a closely related view of the record of biogeochemistry as found in ice sheets and glaciers, to the many ways that humans modify the biogeochemistry of Earth. Undoubtedly, there are many other examples of integration that could be developed; however, incomplete though these may be, they do convey a variety of ways in which Earth systems can be viewed. As we cautioned in Chapter 1, the whole of Earth system science as an integrative discipline is a new and emerging branch of science. It is far from complete; yet, it is clear from the examples in Part Four that many aspects of science that are important to the life and even the existence of humans require explicit integration in order to be applied. We repeat that this integration does not happen on its own and that it requires a global systems view that is often or usually missing in reductionist science (see Section 1.7 of Chapter 1).
Chapter 16 considers a broad but closely related set of chemical processes that control the acid-base and oxidation-reduction (or redox) balances of the planet. It will be seen that both sets of processes depend greatly on the chemical properties and the biogeochemical cycling of the same key elements (C, N, O, S) and of course, water. Further, it will become apparent that all of the reservoirs of the planet from Part Two are involved. While some of the acid-base and redox processes can be viewed usefully from a single sphere or single discipline perspective, they all eventually have connections of one or another sort to the Earth system and the broader integrative picture.
Climate and its connections to biogeochemistry are the focus of Chapter 17. The fundamental nature of the inherently variable climate of Earth presents numerous puzzles, especially the currently inexplicable stability of climate for the last ca. 104 years. It seems possible that this stability is somehow connected to biogeochemistry, again through the key elements C, N, O, and S. In order to explore these connections, it is necessary to define the key processes that contribute to climate. It also is necessary to clearly define and delineate the concepts of forcings, feedbacks, and responses. This delineation allows examination of the roles of biogeochemistry, and most especially of feedbacks. The latter can be viewed as a hierarchy that range from the simplest physical ones to the most complex biogeochemical ones.
Gaining a historical perspective on how the Earth functions in a climatic and biogeochemical sense is made possible largely through studies of the chemistry and isotopic character of ice in the large ice sheets (mainly antarctic and Greenland) and in glaciers. Chapter 18 presents an integrated review of the record of the Earth’s biogeochemistry that is obtained by chemical and physical analysis of old ice, some of which has been sequestered for hundreds of thousands of years. Importantly, all of this old ice originated in the atmosphere; materials trapped in it were subject to atmospheric transport and processing, requiring an understanding of atmospheric chemistry for interpretation of the record. While some evidence, especially for times greater than are represented in old ice (e.g., more than a few hundred thousand years) is provided by sedimentary and even metamorphic rocks, the old ice constitutes the most detailed and complete record – by far. Perhaps the most important record is that of gases (especially CO2 and CH4) trapped in the ice, literally providing paleosamples of the atmosphere. Isotopes, especially 2H (deuterium, D) and 18O in the frozen water itself yield proxy information of temperature.
Finally, Chapter 19 addresses the increasingly apparent modification of the biogeochemical processes of Earth by humans. Major changes such as the ca. 30% increase in atmospheric CO2, increases in methane and N2O concentrations, and rainwater acidification are among the more obvious examples, while changes in the biosphere such as de- and re-forestation and changes in species diversity are evident. Again, it will be seen that the key elements of C, N, O, and S come into play. Isotopes of H, C, and O play important roles, especially for inference of climatic changes.