Introduction: Biogeochemical Cycles As Fundamental Constructs for Studying Earth System Science and Global Change

Introduction: Biogeochemical Cycles as Fundamental Constructs for Studying Earth System Science and Global Change

Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe

1.1 Introduction This includes all of the atoms in your own body and in all other living things, which have also The latter part of the 20th century has seen been permanent residents of the Earth through remarkable advances in science and technology. the eons. This means that the Earth is an essen- Accomplishments in biochemistry and medi- tially closed system with respect to atomic cine, computer technology, and telecommunica- matter, and is therefore governed by the law of tions have benefited nearly everyone on Earth to conservation of mass. This law dictates that all one degree or another. Along with these of the Earth's molecules must be made of the advances that have improved our quality of same aggregation of atoms even though molec- life, scientific research into the study of the ular forms may vary, evolve, and be transported Earth has revealed a planetary system that is within and around the planetary system. Pollu- more complex and dynamic than anyone would tion, therefore, is a human-induced change in have imagined even 50 years ago. The Earth and the distribution of atoms from one place on the environment have become one of society's Earth to another. greatest concerns, perhaps as the result of these In order to understand the impact of pollution discoveries combined with the quick dissemina- on Earth, we must realize that the planet itself is tion of information that is now possible with not stagnant, but continually moving material modern telecommunications. around the system naturally. Any human The basis of most environmental issues is (anthropogenic) redistribution in the elements is pollution. But what is pollution? Keep in mind superimposed on these continuous natural that with very minor exceptions, virtually all of events. Energy from the sun and radioactive the atoms in the solid, liquid, and gaseous parts decay from the Earth's interior drive these pro- of the Earth have been a part of the planet for all cesses, which are often cyclic in nature. As a of its approximately 4.5 billion years of exis- result, almost all of the rocks composing the tence. Very few of these atoms have changed continents have been processed at least once (i.e., by radioactive decay) or departed to space. through a chemical and physical cycle involving

Earth System Science Copyright © 2000 Academic Press Limited ISBN 0-12-379370-X All rights of reproduction in any form reserved 4 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe weathering, formation of sediments, and sub- of the biota (all living things) on the planet. The duction, being subjected to great heat and pres- interfaces between the geospheres are often sure to produce new igneous rocks. The water in fuzzy and difficult to define. For example, the oceans has been evaporated, rained out, and ocean sediments contain water as well as rock returned via rivers and groundwater flow many and organic material; it is difficult to say exactly tens of thousands of times. The main gases in the where the hydrosphere ends and the lithosphere atmosphere (nitrogen and oxygen) are cycled starts. Part of the hydrosphere exists in the frequently through living organisms. The com- atmosphere as rain and cloud droplets. bined effect of these dynamic transports and The constant transport of material within and transformations is a planet that is in a state of through the geospheres is powered by the sun continual physical, chemical and biological evo- and by the heat of the Earth's interior. A simple lution. A bird's eye, cartoon view of the diagram of these geospheric concepts and the dynamic Earth system is shown in Plate 1. This energy that moves material within them is pre- book is about putting together all of the different sented in Fig. 1-1. The result of the interactions dynamic parts of this figure into an understand- shown in Plate 1 and Fig. 1-1 is an Earth system able, coordinated picture. In the last chapter of that is complex, coupled, and evolving. the book, we will revisit the topic of human In addition to the natural evolution of the modification of the system in detail. interacting geospheres, human activities have brought about an entirely new set of perturba- tions to the system. Because many political and T. T. 1 Biogeochemical Cycles and Geospheres social issues surround the problem of human induced global change, there are both basic and Aside from the cyclic systems listed above, applied scientific motivations to study biogeo- there is a complementary set of chemical cycles chemical cycles and their roles in the Earth that we can describe for each of the most system. The need for development and applica- important biological elements (carbon, nitro- tion of basic science to the broad policy issues of gen, oxygen, sulfur, and the trace metals). dealing with global change have inspired the These biogeochemical cycles are descriptions of formation of a new integrative scientific disci- the transport and transformation of the ele- pline. Earth system science (NASA, 1986). ments through various segments of the Earth The subject offers a number of challenges that system, called geospheres. We use these con- are important for the scientific community to structs to compartmentalize the larger Earth address. Probably the largest challenge is inte- system into more manageable, chemically grating knowledge and material from many definable parts. disciplines. This is a major theme of this chapter What are the geospheres? One of them is and of this book, as will be seen in the sections easily definable and requires no special intro- that follow. If the scientific community is not duction: the atmosphere is the gas-phase enve- able to integrate the science necessary to lope surrounding the globe of the Earth. describe biogeochemical systems, it seems un- Another geosphere is the hydrosphere, which likely that it will be easy for society to derive includes all of the oceans, and freshwater solutions for the problems raised by global bodies of water on the planet. The lithosphere is change. the entirety of rocks on Earth, including rocks The principal obstacles facing us as scientists exposed to the atmosphere, under the waters of studying Earth system science are the finite the hydrosphere, and the entire interior parts of resources of most educational institutions. the planet. The pedosphere (literally that upon Development of this subject requires that we which we walk) comprises the soils of the Earth. think of novel ways to do interdisciplinary work The geospheres listed thus far are more or less in a setting dominated by traditional disciplines. geographically definable, but there is a geo- Although we can draw heavily on work being sphere that can exist within all of the other done in recently formed disciplines such as geospheres: the biosphere, which is the collection chemical oceanography, stable isotope geo- Introduction

Solar Radiation ^oooooooo

Saline Hydrosphere

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Sedimentary Rock

Magma

< ^ O O O ^ Radioactive Decay Fig. 1-1 Diagram of cyclic processes and fluxes between the major reservoirs on Earth. chemistry, and atmospheric chemistry, we may functioning of the planet was by James Hutton be able to glean some clues in how to accom- (1788), who viewed the Earth as a "superorgan- plish our goal of integrating the disciplines by ism, and that its proper study should be by examining the history of Earth system study. physiology." More than 100 years later, a classic Indeed, the recognition that biogeochemical paper by Svante Arrhenius (1896) appeared, issues may be significant for mankind goes called "On the influence of carbonic acid in the back several hundred years. air on the temperature of the ground." This work provided a paradigm for quantitatively connecting the greenhouse effect of carbon diox- 1.2 History ide to climate, as well as to the global biogeochemical cycle of carbon. A truly meaningful Som^e of the earliest work in the study of biogeo- study of these issues clearly requires input from chemical cycles and their role in the physical nearly all of the natural sciences (chemistry. 6 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe physics, biology, geology, meteorology, etc.). back-based integrative science geophysiology These scientific disciplines, which were still in and the system itself Gaia, which is a Greek their early formative stages when Arrhenius' word meaning Mother Earth. pioneering work emerged, have since evolved This view of the coupled nature of the Earth over the past century into highly refined and system has not dominated the historical devel- useful, but largely separated entities. Accord- opment of the key disciplines of the natural ingly, most scientists have adopted a reductionist sciences. Many evolutionary biologists have approach to biogeochemistry (i.e., simplifying viewed the changing physical climate of the large scientific problems into smaller parts to be Earth as an externally imposed factor to which examined by an individual discipline). In con- the biosphere must adapt. Likewise, many trast, as we stated above, one of the goals of geologists and geophysicists have viewed the Earth system science (referred to early on as evolution of the planet as being governed natural philosophy) is to integrate the natural primarily or exclusively by chemical and phys- sciences to strive towards understanding the ical processes. Under this perspective, free entire system. oxygen in the atmosphere is viewed as a con- In addition to the work of Arrhenius, several stant factor. The disparate views of the biologi- other scientists have made notable contribu- cal and geologic communities have coexisted tions that have helped to mold the approach for nearly a century, and (with few exceptions) and content of Earth system science (and of this their merging has been controversial (see, e.g., book). The term biosphere was originally coined Dawkins, 1976). by the Austrian geologist Eduard Suess as a The Gaia hypothesis was put forward by Love- way of defining the parts of the Earth's surface lock together with Lynn Margulis (Lovelock and that support life (Suess, 1875). The global view Margulis, 1974) to provide a single scientific portrayed by Arrhenius' lengthy quote of the basis for integrating all components of the paper on the carbon cycle by Arvid Hogbom Earth system. This theory suggests that Earth's set the tone for coupling the biosphere to biota as well as the planet itself are parts of a geochemistry (Arrhenius, 1896). This coupling quasi-living entity that "has a capacity for was strongly emphasized and promoted by the homeostasis" (or self-regulation) (Lovelock, Russian mineralogist Vladimir Ivanovich Ver- 1986). The earliest version of Lovelock's Gaia nadsky, who published a series of lectures on hypothesis contained phrases like "by and for the subject, first in Russian (1926), then in the biosphere," which implied a sense of pur- French (1929), and recently translated into Eng- posefulness on the part of the biota to evolve in lish (1997). In these writings, Vernadsky devel- ways that would suit its own continued exis- oped an integrative definition of the biosphere as tence. As the Gaia hypothesis has evolved, the all living things and everything connected to interdependence of the evolution of biota and them. He strongly believed that the Earth is a geophysical/geochemical systems is described set of connected parts that can only be studied in non-teleological terms. Lovelock (1991) him- in an indivisible, holistic way. Vernadsky went self recently stated what seems obvious: "In no so far as to suggest that geologic phenomena at way do organisms simply 'adapt' to a dead the Earth's surface are inherently caused by life world determined by physics and chemistry itself, so his definition of the biosphere includes alone. They live in a world that is the breath all of the Earth's geologic features. Vernadsky's and bones of their ancestors and that they are teachings remained relatively obscure until now sustaining." Hutchinson (1970) popularized them in a famous article in Scientific American. Vernadsky came to be called the father of modem biogeo- 1.3 Evidence for the Coupled Nature of the chemistry by James Lovelock (1972), who went Earth System on to suggest that feedbacks in the biosphere- climatic system lead to homeostasis of basic The biosphere is ultimately what ties the major Earth processes. Lovelock called this new feed- systems of the Earth together. Studying the Introduction

fundamental differences between a planet full of intimate contact with the solid phases of the life (the Earth) versus one that lacks life (e.g.. planet and exchange substances with them. Mars) evidences this. Dead planets are in or near Since the atmosphere and hydrosphere are dis- a state of perpetual thermodynamic equili- tributed globally and because they each are brium, but as pointed out by Lovelock (1972), mixed on large, often global spatial scales, the the spheres of the Earth and the chemical con- chemical influences of the biosphere are evident stituents within them are far out of equilibrium everywhere on Earth. in a thermodynamic sense, as evidenced by the A rich base of empirical evidence convinc- following points: ingly illustrates the complexity of the couplings within the Earth system. One of the most impor- • O2, N2, and H2O are the main molecular forms tant pieces of evidence of this interconnected- coexisting in the atmosphere, but the condi- ness is the chronology provided by chemical tion of thermodynamic equilibrium would and isotopic analysis of ice cores from the require that HNO3 be formed from these deepest, oldest ice on the planet. The Vostok ice gases and subsequently dissolve in the core (Lorius et ah, 1985; Saltzman, pers. comm., oceans. 1998) currently yields information covering • O2 coexists with combustible biomass in more than 400 000 years, from the current inter- plants. glacial time, through the most recent ice age, • Acidic materials in the atmosphere (e.g. CO2 through the previous interglacial time and back and H2CO3, SO2 and H2SO3, HNO3 and so to an earlier ice age. Figure 1-2 shows plots of on) coexist with basic materials in both several chemical variables against time. The igneous and sedimentary rocks (e.g. FeS2 data are derived from detailed analyses of the and CaC03). chemical composition of the ice. These data will We cover each of these types of examples in be discussed in more detail in Chapter 17, but separate chapters of this book, but there is a even a cursory examination reveals features that clear connection as well. In all of these examples, indicate regulation of the system by the bio- the main factor that maintains thermodynamic sphere: disequilibrium is the living biosphere. Without • The isotopically inferred temperature shows the biosphere, some abiotic photochemical reac- two relatively stable climatic states: ice ages tions would proceed, as would reactions and interglacial periods. The temperature associated with volcanism. But without the con- during the current interglacial is about the tinuous production of oxygen in photosynthesis, same as during the past one. Likewise, the various oxidation processes (e.g., with reduced temperature during the coldest part of the last organic matter at the Earth's surface, reduced ice age was about the same as the minimum sulfur or iron compounds in rocks and sedi- temperature during the previous glacial ments) would consume free O2 and move the period. atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of • CO2 is higher in interglacial times and lower the planet is thus intimately tied to the bio- in ice ages, indicating significant differences sphere. in photosynthesis between these two climatic states. All living organisms require at least one • Sulfate and methane sulfonic acid (MSA) mobile phase (gas or liquid) in order to exist. (both of which are produced primarily by Life on Earth as we know it would be impos- photosynthetic algae in the ocean) are low sible without the involvement of the liquid during interglacials and high in ice ages, phase of water. The gas phase is necessary for opposite to the CO2 trend in Antarctica life forms that consume gaseous substances or (although not in Greenland). that produce gaseous waste products. Hence, the very functioning of the biosphere implicitly These chemical constituents all vary in syn- depends on the existence of the mobile atmos- chrony and two climatic states as defined by phere and hydrosphere, both of which are in temperature coincide with the "climatic states'' 8 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe

Age (kyr) 50 100 150 200 250 -420

-440 5 D (%o) -460 h

-480 280 260 CO2 (ppmv) 240 220 200

600

CH4 (ppbv) 500

400 60

Methane sulfonic ^^ \~ acid (ng/g) 20

Non seasalt 500 sulfate SO4 400 (ng/g) 300 200 100 150 200 Age (kyr) Fig. 1-2 Chemical data from the Vostok ice core. The graph of dD can be taken as a proxy for temperature changes, as described in Chapter 18. CO2 and CH4 are greenhouse gases and vary in the same direction as temperature. Non-seasalt sulfate and methane sulfonic acid are both sulfur species existing in the particle phase, and are positively correlated with each other, but negatively with T. Major variations for all of these variables seem to correlate either positively or negatively with each other, indicating a coupled system. SD, non-seasalt sulfate, and methane sulfonic acid data kindly provided by Dr Eric Saltzman. CO2 data are from Bamola et al (1987) and Jouzel et al (1993). CH4 data are from Chappellaz et al. (1990) and Jouzel et at. (1993). (ppmv = parts per million by volume; ppbv = parts per billion by volime) as defined by variation in the chemical vari- the Earth system into one or another preferred ables. Although these correlations and anti- climatic and biogeochemical state. Currently, correlations do not prove that the Earth no viable alternative hypothesis has been functions as an integrated biogeochemical advanced to explain these correlations. To system, it does strongly suggest that the many unravel the detailed functioning of the entire subsystems involved are coupled. Furthermore, Earth system, w^e must first establish a basis for the presence of the quasi "set points" of the understanding the individual "spheres," and temperature and chemical variables suggests the individual biogeochemical cycles that are the possible existence of feedbacks that steer involved. Introduction

1.4 Philosophy of Using the Cycle Approach to There are, however, some disadvantages: Describe Natural Systems on Earth • The analysis is by necessity superficial. It Viewing the Earth system as a set of coupled provides little or no insight into what goes on biogeochemical cycles that both depend on and inside the reservoirs or into the nature of the influence the climate allows us to conceptually fluxes between them. simplify the movement of material on Earth and • It gives a false impression of certainty. Typi- its couplings to climate. Much of this book is cally at least one of the fluxes in a cycle is presented using this approach, giving budgets calculated by the imposed mathematical for the flux of material into and out of various necessity of balancing a steady-state budget. reservoirs. Take for instance the circulation of Such estimates may erroneously be taken to water between the oceans, atmosphere, and represent solid knowledge. continents. In this example, the reservoirs • The analysis is based on averaged quantities would be "the oceans," "the water in the atmos- that cannot always be easily measured phere," "the ground water," etc. Using the most because of spatial variation and other compli- basic description of the cyclic processes that cating factors. take place, we can mathematically and quantitatively model these cycles to describe and Also, many important geophysical problems predict the distribution of the important chemi- cannot be studied using a simplified cycle cal constituents of the planet. The fundamental approach. Weather forecasting, for example, concepts that govern modeling of biogeochem- requires a detailed knowledge about the distri- ical cycles are covered in Chapter 4, but we bution of winds, temperature, etc. within the introduce some of the main ideas here, since atmosphere. Weather cannot be forecast using a these concepts and definitions are useful for the reservoir model with the atmosphere as one of entire text. the reservoirs. It would not be much better even A basic goal of the cycle approach is to if the atmosphere were divided into several determine how the fluxes between the reservoirs reservoirs. A forecast model requires a resolu- depend on the content of the reservoirs and on tion fine enough to resolve explicitly the struc- other external factors. In many cases the details ture of the most important weather phenomena of the distribution of an element within each such as cyclones, anticyclones, and wave pat- reservoir are disregarded, and for the most terns on spatial scales as small as a few hundred simplified calculations, the amounts of material kilometers. Spatially explicit models are based in each reservoir are assumed to remain con- either on a division of the physical space into a stant (i.e., there is a condition of steady state). large but finite number of grid cubes (grid point This allows a chemical budget to be defined for models) or on a separation of the variables into the entire cycle. different wave numbers (spectral models). A steady-state, flux-based approach to describing the physical-chemical environment on Earth has advantages as well as disadvantages. Some advantages are that: 1.5 Reservoir Models and Cycles - Some Definitions • It provides an overview of fluxes and reservoir contents. The models used to study biogeochemical cycles • It gives a basis for quantitative modeling. are described by a set of terms whose definitions • It helps to estimate the relative magnitudes of must be clearly understood at the outset. We anthropogenic and natural fluxes. define them here as they are used throughout • It stimulates questions such as, "Where is the the book. material coming from? Where is it going next?" Reservoir (box, compartment). An amount of • It helps to identify gaps in knowledge. material defined by certain physical, chemical 10 Michael C Jacobson, Robert J. Charlson, and Henning Rodhe

or biological characteristics that, for the pur- steady state, i.e., M does not change with time. poses of analysis we consider to be reasonably Usually some fluxes are better known than homogeneous. Examples: others. If steady state prevails, a flux that is unknown a priori can be estimated by difference • Oxygen in the atmosphere. from the other fluxes. If this is done, it should be • Carbon monoxide in the southern hemi- made very clear in the presentation of the sphere. budget which of the fluxes is estimated as a • Carbon in living organic matter in the ocean difference. surface layer. • Amount of ocean water having a density Turnover time. The turnover time of a reservoir between pi and p2- is the ratio of the content M of the reservoir to • Sulfur in sedimentary rocks. the sum of its sinks S or the ratio of M to the If the reservoir is defined by its physical sources Q. The turnover time is the time it will boundaries, the content of the specific element take to empty the reservoir in the absence of is called its burden. We will denote the content of sources if the sinks ren\ain constant. It is also a a reservoir by M. The dimension of M would measure of the average time spent by individual normally be mass, although it could also be, e.g., molecules or atoms in the reservoir (more about moles. this will be presented in Chapter 4). Flux. The amount of material transferred from Cycle. A system consisting of two or more con- one reservoir to another per unit time, in general nected reservoirs, where a large part of the denoted by f (mass per time). Examples: material is transferred through the system in a cyclic fashion. If all material cycles within the • The rate of evaporation of water from the system, the system is closed. Many systems of ocean surface to the atmosphere. connected reservoirs are not cyclic, but instead • The rate of oxidation of N2O in the strato- material flows unidirectionally. In such systems sphere (i.e., flux from the atmospheric N2O- some reservoirs (at the end of the chain) may be nitrogen reservoir to the stratospheric NO;^- accumulative, whereas others remain balanced nitrogen reservoir). (nonaccumulative); cf. Holland (1978). • The rate of deposition of phosphorus on marine sediments. Biogeochemical cycle. As discussed early in the chapter, this term describes the global or In more specific studies of transport pro- regional cycles of the 'Tife elements" C, N, S, cesses, flux is normally defined as the amount and P with reservoirs including the whole or of material transferred per unit area per unit part of the atmosphere, the ocean, the sedi- time. To distinguish between these two conflict- ments, and the living organisms. The term can ing definitions, we refer to the latter as "flux be applied to the corresponding cycles of other density.'' elements or compounds. Source. A flux (Q) of material into a reservoir. Budgets and cycles can be considered on very different spatial scales. In this book we concen- Sink. A flux (S) of material out of a reservoir - trate on global, hemispheric and regional scales. very often this flux is assumed to be propor- The choice of a suitable scale (i.e. the size of the tional to the content of the reservoir (S = kM). In reservoirs), is determined by the goals of the such cases the sink flux is referred to as a first- analysis as well as by the homogeneity of the order process. If the sink flux is constant, inde- spatial distribution. For example, in carbon cycle pendent on the reservoir content, the process is models it is reasonable to consider the atmos- of zero order. Higher-order fluxes, i.e. S = kM^ phere as one reservoir (the concentration of with a > 1, also occur. CO2 in the atmosphere is fairly uniform). On Budget A balance sheet of all sources and sinks the other hand, oceanic carbon content and of a reservoir. If sources and sinks balance and carbon exchange processes exhibit large spatial do not change with time, the reservoir is in variations and it is reasonable to separate the Introduction 11 surface layer from the deeper layers, the Atlantic tion of the entire system; however, we can from the Pacific, etc. Many sulfur and nitrogen provide a set of examples of integrated subsys- compounds in the atmosphere occur in very tems that will illustrate the nature of the interac- different concentrations in different regions of tions that constitute major pieces of the whole. the world. For these compounds regional bud- Key features of the process of integrating nat- gets tell us more about local dynamics than do ural-science fundamentals are (1) some degree global budgets. of globality or global applicability, (2) interaction of two or more biogeochemical cycles, and (3) coupling of the interacting geospheres. While 1.6 The Philosophy of Integration as a Basis for there are many such subsystems, we will men- Understanding the Earth System tion a few of them to which we will return in the closing chapters. These examples of integrated 7.6.1 Recognizing the Interconnected Nature subsystems also introduce and maintain a three- of the Earth System way focus on one or more geosphere(s), the biogeochemical cycles, and on systems integra- Although it is clearly established that it is neces- tion. While these topics might appear to be non- sary to study all of the geospheres and all of the parallel, they share the key feature of an inte- biogeochemical cycles in order to understand grative global view. and predict the workings of the entire Earth system, it is not sufficient to stop there. The 7.6.2.7 The hydrologic cycle functioning of the biosphere and each of the individual physical spheres of the planet Far from being just the processing of water on involves continuous and strong interactions, Earth, this cycle is the basis for a wide range of making all parts of the Earth system dependent meteorologic, geochemical, and biological sys- to some degree on all the other parts. Earth tems. Water is the transport medium for all system science attempts to understand current nutrients in the biosphere. Water vapor con- conditions by extending analyses backwards densed into clouds is the chief control on plane- and forward in time - to include the earliest tary albedo. The cycling of water is also one of stages of the evolution of life on Earth, as well as the major mechanisms for the transportation of projections of human-induced global change in sensible heat (e.g. in oceanic circulation) and the future. latent heat that is released when water falls The task of integrating the spheres and the from the air. biogeochemical cycles emerges as a necessary if daimting challenge. Disciplinary science has 7.6.2.2 Acid-base and oxidation-reduction provided little in the way of precedent for us, systems although substantial guidance is provided by the pioneers like Arrhenius, Vemadsky, and It is often taken for granted that the oxygen Lovelock who presaged these global develop- content of the air is nearly constant at ca. 20% ments. Another major contributor to this kind of of the atmospheric volume, that most of the thinking is the field of systems analysis or systems liquid water on the planet is aerobic (i.e. con- engineering, and the field of cybernetics, though tains O2), and that most water has pH values these lack any degree of focus on the Earth relatively close to "neutral" (close to 7). How- system. ever, these circumstances are not mere coinci- dences but are in fact consequences of the interaction of key global biogeochemical cycles. 1.6.2 Examples of Integration of Global For instance, the pH of rainwater is often deter- Systems mined by the relative amounts of ammonia and sulfuric acid cycled through the atmosphere, a In the chapters that follow, we cannot provide a clear example of interaction between the nitro- complete and unified description for the integra- gen and sulfur cycles. 72 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe

1.6.23 Ice-age/interglacial climatic flip-flops provide the basic disciplinary components, starting with fundamental concepts of model- Although these are often viewed as being ing. Earth science, biology, and chemistry. merely changes in physical climate as indexed Having reviewed these basic scientific building by temperature, they really represent changes in blocks we move on to a survey the biogeochem- a large range of biogeochemical phenomena. ical cycles of key elements. Following this, we The rich and rapidly growing body of data present a set of integrative topics. from ice cores and sediments strongly supports The three user groups of this book (students, the notion that the Earth functions as a coupled teachers, and researchers) will all discover that system. the challenge of understanding Earth system science is at least as much of a problem of 7.6.2.4 The climate system with biogeochemical integration of well developed fundamental feedbacks fields into a global context as it is to refine the disciplinary pieces themselves. Users who are Climate is often viewed as the aggregate of all of practiced in a traditional field utilizing reduc- the elements of weather, with quantitative defi- tionism will need to expand their thinking into nitions being purely physical. However, because other disciplines as well as into the problem of of couplings of carbon dioxide and many other how to combine their own field into the larger atmospheric species to both physical climate picture. In doing so, they will find an opportu- and to the biosphere, the stability of the climate nity to extend the scope of their own discipline system depends in principle on the nature of towards and into the global context. feedbacks involving the biosphere. For example, the notion that sulfate particles originating from the oxidation of dimethylsulfide emitted by References marine phytoplankton can affect the albedo (reflectivity) of clouds (Charlson et al, 1987). At Arrhenius, S. (1896). On the influence of carbonic acid this point these feedbacks are mostly unidenti- in the air upon the temperature of the ground. fied, and poorly quantified. Philosophical Magazine and Journal of Science S.5, Vol 4, No 251, p. 237 ff. 1.6.2.5 Anthropogenic modification of the Earth Bamola, J. M., Raynaud, D., Korotkevich, Y. S., and system Lorius, C. (1987). Vostok ice core provides 160000 year record of atmospheric CO2. Nature 329, 408- 413. Again, the myriad influences of human activity Chappellaz, J., Barnola, J. M., Raynaud, D., Korotke- are usually viewed as separate effects (global vich, Y. S., and Lorius, C. (1990). Atmospheric warming, acid rain, ozone loss, urban pollution, methane record over the last climatic cycle revealed etc.) However, these individual symptoms by the Vostok ice core. Nature 345,127-131. clearly have major interdependences that must Charlson, R. ]., Lovelock, J. E., Andreae, M. O., and be understood if humans are to learn how to Warren, S. G. (1987). Oceanic phytoplankton, at- coexist with a stable Earth system. mospheric sulphur, cloud albedo and climate. Nature 316, 655-661. Dawkins, R. (1976). "The Selfish Gene." Oxford Uni- 1.7 The Limitations and Challenges of versity Press, New York. Understanding Earth Systems Holland, H. D. (1978). "The Chemistry of the Atmos- phere and Oceans." p. 351. Wiley-Interscience, New York. Earth system science is a young science with Hutchinson, G. E. (1970). The Biosphere. Sclent. Am. great potential, but we must exercise caution in September, 45-53. not overlooking important details of traditional, Hutton, J. (1788). "Theory of the Earth; or an investi- disciplinary science in our attempt to develop gation of the laws observable in the composition, this new and integrative science. The foundation dissolution and restoration of land upon the upon which we will proceed in this book is to globe." R. Soc. Edin. Trans. 1,209-304. Introduction 13

Jouzel, J., Lorius, J. R., Petit, C. et al (1993). Vostok ice- Lorius, C., Jouzel, J., Ritz, C., Merlivat, L., Barkov, core - a continuous isotope temperature record N. I., Korotkevich, Y. S., and Kotlyakov, V. M. over the last climatic cycle (160000 years). Nature (1985). A 150000-year climatic record from Antarc- 329, 403-408. tic ice. Nature 316, 591-596. Lovelock, J. E. (1972). Gaia as seen through the atmos- NASA (1986). "Earth System Science - Overview - A phere. Atmos. Environ. 6,452-453. Program for Global Change." Earth System Lovelock, J. E. and Margulis, M. (1974). Atmospheric Sciences Committee, NASA Advisory Council, homeostasis by and for the biosphere. Tellus 2 6 ,1 - Washington, DC. 10. Suess, E. (1875). "Die Enstehung der Alpen" (The Lovelock, J. E. (1986). ''The Biosphere.'' New Scient. Origin of the Alps). W. Braunmiiller, Vienna. July 17, 51. Vemadsky, V. I. (1997). "The Biosphere." (D. B. Lovelock, J. E. (1991). Geophysiology - the science of Langmuir, transL; revised and annotated by Gaia. In "Scientists on Gaia" (S. A. Schneider and M. A. S. McMenamin), Copernicus Books, New P. J. Boston, eds), pp. 3-10. MIT Press, Cambridge, York. MA.