PLANETARY STRUCTURE

In this section we examine the structure of the planet from the disciplinary viewpoint, beginning with the atmosphere and ending at the Earth's core. We then describe the frontiers of our knowledge of the components of the physical Earth system and outline the primary challenges for study in the next decade.

Atmospheric Structure

The structure and variability of the Earth's atmosphere are controlled by a combination of chemical, biological, and dynamical processes that operate over a wide variety of spatial and temporal scales. On average, this variability produces an atmosphere that approximates the classical layered profile found in textbooks. This traditional description of "spheres" and "pauses" is usually defined by reference to the vertical temperature profile. It begins at the Earth's surface with the troposphere. The stratosphere, mesosphere, thermosphere, ionosphere, and magnetosphere, in ascending order, overlie the troposphere's upper boundary, the tropopause.

The lowest layer of the atmosphere, the troposphere or "turning sphere," is the environment in direct contact with terrestrial and oceanic ecosystems. It is home to both benign and hazardous weather and is the component of the climate system that determines the very habitability of the planet. The troposphere is the major reservoir and transport medium for the pollutants that are discharged in the course of human activities.

The dynamical instability of air within the troposphere, and hence all the rich complexity of our weather, occurs because the atmosphere is heated from below. Incoming solar radiation warms the land/ocean surface, which in turn, heats the atmosphere and creates large-scale pressure gradients that drive the global air circulation system. Convective air currents also cycle heat, water vapor, and other gaseous constituents from the Earth's surface into the upper atmosphere and back again. These cycles affect the troposphere through complex dynamics that respond to the forces exerted at the surface through friction, Earth's rotation, and the thermal/pressure balance between the land and oceans.

In the stratosphere, absorption of solar ultraviolet radiation by ozone creates a thermal inversion, leading to a stable region with limited transport. The composition and structure of the stratospheric ozone layer are essential to life on Earth because the ozone layer serves as a protective shield against harmful ultraviolet radiation. In this region pollutants, such as chlorofluorocarbons (CFCs) and their reaction products, can build up to levels which, if unchecked, lead to the destruction of the ozone layer. In fact, the chemical composition of the stratosphere is very complex and involves hundreds of species and thousands of chemical reactions, including key catalytic cycles that both maintain and threaten the ozone layer.

Landmark achievements have occurred in tropospheric and stratospheric research over the last 50 years. Notably, we have become progressively more successful at forecasting weather and climate and we now understand the chemical processes that produce the ozone "hole" over Antarctica. Such research resulted in the Montreal Protocol which mandated international limits on the use of CFCs. The basic vertical structure of the lower atmosphere, determined by density, pressure, and temperature, is understood. The resulting thermo dynamical structure can be described in terms of large-scale circulation patterns such as the Asian monsoon. However, our understanding is limited by observations that generally span only several decades and knowledge determined from proxy paleoclimate records such as tree rings, ice cores, and corals. Similarly, the basic chemical composition of the atmosphere is known, and many important trace gas constituents have been detected. However, our understanding of the reactivity of the atmosphere is still evolving, and the factors controlling the emission and removal of many important gases (including carbon dioxide) are still undetermined in many critical areas.

The scientific challenges that confront the study of the troposphere and stratosphere over the next decade include understanding the role of clouds, aerosols, and biogeochemical feedbacks on climate; understanding the complex interactions between atmospheric dynamics and thermodynamics over the full range of spatial and temporal scales; and understanding the role of the ocean in the control of atmospheric circulation and climate. Meeting these challenges will lead to future improvements in our capability to predict weather, climate, and the changing chemical state of the atmosphere.

Above the stratosphere, the Earth's atmosphere is affected by interactions with the troposphere and stratosphere below, but also by processes originating in the Sun's corona and in the Earth's magnetosphere above. The composition of the atmosphere changes from a mixture of neutral and ionized species in the mesosphere and thermosphere to a fully ionized medium in the magnetosphere. Large changes in the structure of the upper atmosphere can be caused by relatively small changes in energetic inputs, due to the low densities. The magnetosphere is supplied by plasma from the ionosphere below and from the solar wind above, and is a region of strong nonlinear plasma interactions and variable, topologically complex current systems. Our arrays of telecommunication satellites and other technologies in space are vulnerable to energetic radiation and to compositional and structural changes in the upper atmosphere and magnetosphere. Knowledge of the structure of the neutral upper atmosphere, ionosphere, and magnetosphere has been greatly enhanced by research programs that have concentrated on the coupling of these regions from a thermal, compositional, and dynamical perspective. An era of exploration is now giving way to one of quantitative analysis and understanding. With enhanced new observations and modeling, a predictive capability for the entire near-Earth space environment can be developed and tested.

In the area of upper atmospheric structure, the challenges to achieve this predictive capability in the next decade include understanding the global electrodynamics of the system from the surface to the upper atmosphere, including the variability of electric fields and the effects of lightning; understanding the effects of magnetosphere-ionosphere-thermosphere coupling, including particle precipitation in the aurora and associated compositional changes; understanding the compositional response of the magnetosphere and ionosphere/thermosphere system to solar storms and other geomagnetic disturbances; and understanding the role of winds, atmospheric waves and tides, and wave-wave coupling effects on upper atmospheric structure at all important temporal and spatial scales.

There are also significant observational requirements to extend our knowledge of both lower and upper atmospheric structure. Global observations of atmospheric parameters will require more powerful and more widely deployed facilities, such as large arrays of monitoring instruments (e.g., radar, optical instruments, and in situ chemical sensors) and space-borne instruments, as well as the development and dissemination of large integrated data sets and the development of new predictive models with sophisticated data-ingestion capabilities.

Ocean Structure

The oceans cover more than 70% of the Earth's surface. The oceans are clearly critical to society in many ways. They influence climate, provide food, support transportation (over 90% of all U.S. cargo crosses water), foster tourism and recreation, yield oil and gas, and are critical to our national security. The oceans also sequester about 30% of the CO2 emissions.

Major scientific advances have come about through technology that allows extensive, systematic measurements charting much of the basic structure of the Earth's oceans. Global data sets are now available describing ocean temperature, salinity, velocity, chemical tracers, and other water properties as a function of time, depth, and geographic location. A global climatology has been developed, based on all available data collected over several decades.

Long-term, open-ocean measurements have revealed large-scale fluctuations in ocean currents over weeks to months and systematic global variations in the intensity of these fluctuations. Oceanographers now have an estimate of the global distribution of small-scale vortices in the global ocean. These observational advances have been matched by progress in our theoretical understanding of ocean dynamics. On smaller scales, processes such as instability, turbulence and diffusion have been quantified and their role in controlling larger scale processes has been identified. Past achievements in quantifying small-scale turbulent mixing in the upper ocean, for example, have far-reaching implications for our understanding of global oceanic circulation patterns and all physical, biological, chemical, and geological marine processes that depend on the geographic distribution of mixing.

There has been a revolution in our understanding of equatorial oceanography and its linkage with meteorology. The most notable and practical success of the collaboration among physical oceanographers, meteorologists, and climate modelers is probably our understanding of how the ocean and atmosphere interact in producing El Niño and the consequent prediction of short- term climate. Current research seeks to identify longer-term processes in other ocean regions which may also extend our capability for ocean prediction.

The historical record of ocean circulation contained in marine sediments demonstrates the variability of the ocean system on time scales as short as decades and as long as many millennia. The data show that the present deep ocean circulation is fundamentally different from corresponding modes of the past. Importantly, ocean circulation patterns are now known to account for major changes in atmospheric and marine chemistry. We have learned that physical and chemical processes in the oceans operate at a variety of spatial scales ranging from global patterns of ocean circulation to highly localized processes occurring within a few kilometers or less. This spectrum of processes is fundamental to understanding ecosystem structure, carbon flux, and local and global changes in the climate.

Our knowledge of the structure of the coastal ocean has developed rapidly. Coastal margins are known to provide regions of very high biological productivity, carbon flux, and sedimentary carbon storage. There has been improved understanding of cross-shelf transport of nutrients, temperature, salinity, dissolved organic material, and particulate matter. Detailed knowledge of coastal ocean processes clearly offers the potential for better coastal management at a substantially reduced cost.

The scientific challenges for the next decade of study of the world's ocean fall under three general themes.

The first theme involves understanding and quantifying the ocean's role in transporting, storing, and exchanging heat, freshwater, mass and chemical constituents. The key questions here include: identifying the time and space scales of the variations in the transport, storage, and exchange processes (e.g., water-mass formation and modification); understanding the relation between small-scale turbulent mixing and the large-scale meridional overturning circulation; determining the role of the ocean in decadal to millennial climate variability and in the hydrological cycle; and understanding how the ocean influences climate change through its ability to absorb radiatively important gases.

The second theme concerns biocomplexity – the role of the physical ocean system in controlling the structure of ecosystems and the variability of living resources. Key questions here include: identifying the processes governing the cycling of nutrients and dissolved organic matter within the oceans; assessing the importance of the structure of the marine food web to its overall functioning; determining how the ecosystem structure and productivity will change with long- term variations in climate, fishing, or inputs of nutrients and anthropogenic chemicals; and learning what measures will prove effective for maintaining both biodiversity and the productivity of desirable species.

The third set of challenges relates to our understanding of the coastal oceans. Important questions here include: learning how materials are transported and exchanged among land, estuaries, the continental shelf, and the deep ocean, all the while undergoing physical, biological, and chemical transformations; unraveling the role of ocean margins as processors of material in the ocean and how these areas are affected by the massive human colonization of the coastline; identifying the chemical characteristics and fluxes of fluids emanating from rivers and arising from sediments on continental margins overlying aquifers on continental margins; understanding the relative roles of natural and anthropogenic variability in influencing these characteristics and fluxes; and understanding the structure and evolution of divergent and convergent margins, sedimentology, and stratigraphy of continental shelf and shore settings.

Surface and Deep Earth Structure

Planet Earth transports thermal energy from the deep interior to the oceans and atmosphere through a series of separate but linked convecting layers. The ultimate source of the heat energy is a combination of radioactive decay and cooling of the planet from its molten state following accretion 4.6 billion years ago. The deepest source is the metallic core, which liberates heat as it solidifies.

At the boundary between the solid inner core and the molten metallic outer core, growth of the inner core by freezing of the ever-shrinking outer core liberates sufficient latent heat to drive the turbulent, gyrating convective flow that results in the geodynamo. Convection in the core can transport heat no more rapidly than it can be removed via sluggish convection in the overlying mantle. The boundary between the core and mantle, the so-called "D-layer," is the site of intense chemical reactions. The boundary is warped by the stresses from convection in the overlying mantle. This region is thought to be the source of hot upwelling mantle plumes whose surface manifestations are hotspot volcanic edifices such as Hawaii, Yellowstone, and Iceland.

Whether convection in the 3,000-km thick mantle occurs in one or two separate layers is still under vigorous debate. The evidence from high-resolution seismic tomographic imaging is that some of the tectonic plates that founder into the mantle at subduction zones sink to the core- mantle boundary, while geochemical observations require several reservoirs that remain unmixed over geologic time.

The crust and upper mantle of the planet, beneath both land and ocean, contain the building block of life and are the source of our mineral resources. Geophysical and geological techniques have provided increasingly detailed pictures of the structure of the crust including mountain ranges, mid-ocean ridges, continental margins, and sedimentary basins. These studies have provided valuable details of plate tectonics — the surface manifestations of mantle convection. Plate tectonic creation of new oceanic crust at ocean ridges and its ultimate destruction deep beneath ocean trenches, represents a major cycling of mass, heat, water and chemical constituents between the Earth's surface and interior. This fundamental process plays a major role in controlling the biogeochemistry of the ocean and atmosphere. Plate boundaries are the loci of most earthquakes, volcanic activity, and exchange of heat and volatiles between the interior and the oceans and atmosphere.

There are two primary challenges to expanding knowledge of the surface and deep Earth structure over the next decade. The first is the use of combined approaches to infer the physical structure of the Earth's interior. Promising techniques include the mapping, at increasingly higher spatial resolution, of the vertical and lateral variations in crust, mantle, and core structure using seismic imaging and the use of such images with geodetic, geochemical, magnetic field, and magnetotelluric information to infer the composition and physical properties of Earth's interior. The second challenge is to understand the interactions of the mantle plate boundaries, as described above. This enterprise will involve observations, theory, and computation.

---

The Primary Challenges

Eight primary challenges to expanding knowledge of planetary structure in the next decade are:

1. understanding the details of the complex interactions between atmospheric and ocean dynamics and thermodynamics over the full range of spatial and temporal scales;



2. determining the role of clouds, aerosols, and biogeochemical feedbacks in the radiative balance of the atmosphere and climate;



3. understanding and predicting the response of the near-Earth space environment to solar storms and geomagnetic disturbances;



4. understanding and quantifying the ocean's role in transporting, storing, and exchanging heat, freshwater, mass, and chemical constituents;



5. understanding the processes that control the state and variability of the coastal oceans;



6. determining the nature and variability of the global hydrological cycle in order to quantify, monitor, and understand the transportation of water by streams, the recharge of aquifers, and water quality and storage;



7. enhancing the resolution of lateral and vertical variations of fine structure throughout the solid Earth; and



8. understanding the structural relationships between the mantle, the overlying crust and lithosphere, and the underlying core.

PLANETARY ENERGETICS AND DYNAMICS

Much of the basic research to describe the structure and composition of the solid, liquid, and gaseous components of Earth, particularly in the geologic past remains to be accomplished. Nevertheless, rapid progress is being made to understand the dynamics of the mass and energy fluxes within and among components of the planetary system. This system is driven by energy from two huge reservoirs: the Sun and the heat produced and stored in the deep interior of the planet. Radiation from the Sun drives the motions of the atmosphere and hydrosphere, and heat from within drives the dynamics of the solid Earth from core to crust.

Solar energy at visible wavelengths is absorbed at the surface of the Earth and in the lower part of the atmosphere with an excess in tropical latitudes. The global imbalance resulting from this nonuniform heating drives the Earth's two great heat engines: the ocean and the atmosphere. Global three-dimensional oceanic currents and embedded eddy structures serve to convey heat from the equatorial regions to the poles, fundamentally maintaining the oceanic energy balance. Winds, storm systems, and large circulating cells perform a similar function in the atmosphere, while exhibiting tremendous variability across time and space. At shorter, more energetic wavelengths, solar radiation and energetic particles in the solar wind are absorbed at much higher altitudes, generating electrically conducting layers, driving a complex space environment and creating the protective ozone shield.

The solid Earth is a third great heat engine. Separate but linked convection cells within the core and mantle transport heat derived from radioactive decay, accretion, and the latent heat of crystallization of the core from the Earth's interior to the hydrosphere/atmosphere system. Thermally driven convection also transports mass within and on the Earth, leading to plate motions, the Earth's magnetic field, and over geologic time, the large-scale chemical differentiation of the planet.

During the past 30 years, our understanding of planetary energetics and dynamics has been transformed through observational and theoretical studies that have tracked the flows of mass, energy, and momentum through the Earth system. For example, we can now describe mantle convection and plate tectonics in some detail. Our ability to forecast weather with a reasonably high level of accuracy from day to day is taken for granted, yet it rests on a huge body of basic and applied research carried out during the latter part of the 20th Century. Our steadily increased understanding of a full range of oceanic motion on many spatial scales has led to the development of numerical models that can resolve small-scale eddies in the oceans. Our understanding of the roles that biogeochemical cycles play in defining the climate system has been greatly enhanced and we are now poised to make meaningful predictions about the implications of climate change on national and regional scales. Our understanding of the space environment and how it responds to variations in solar forcing is new and may pave the way to a predictive capability.

Much of this progress has been primarily discipline-based, with somewhat less attention paid to processes that simultaneously involve several Earth system components. Our challenge and our imperative is to extend and build upon these past efforts to reach a more profound and holistic understanding of the complete Earth system. Such an understanding will require investigations into the myriad processes interacting at a variety of spatial and temporal scales and domains from the innermost core of the Earth through the atmosphere to the outer fringes of the Sun.

The following sections describe some of the challenges in greater detail.

Life inside a Stellar Atmosphere

The Earth's location in the inner part of the solar system causes it to be continually bathed in short-wave energetic radiation and exposed to the solar wind, the outbound flow of energetic particles from the heated solar chromosphere and corona. The outgassing of the planetary interior and the long-term impact of biological processes have provided Earth with an envelope of air that is a safe haven for life to flourish in an otherwise hostile environment. It is this special set of circumstances that makes Earth a unique planet within the solar system.

The stratosphere is largely controlled by radiative processes that result from the balance between the incoming short-wave radiation from the Sun and the long-wave emissions from the Earth's surface and atmosphere. In the lower stratosphere, photochemical processes generate the ozone layer that protects life from damage from solar ultraviolet radiation. The dynamics and chemistry that control the quantity of ozone in the stratosphere have largely been explained in the last 15 years, but many questions remain concerning the nonlinear exchange of energy and chemical constituents between the troposphere and stratosphere.

Above the stratosphere, solar radiation heats and stirs the outer edges of the Earth's atmosphere and strips neutral atoms of electrons, creating the electrified ionosphere. Solar energy also gives rise to atmospheric gravity waves and tides, perturbations which, in turn, control atmospheric structure and composition. Winds, waves, and tides in the upper regions of the atmosphere propagate globally and vertically, affecting the environment where we conduct our space activities.

In addition, the Sun continuously emits a stream of charged particles with a velocity of nearly a million miles per hour — the solar wind. This plasma carries with it a magnetic field that interacts with the Earth's own magnetic field creating a dynamic cavity known as the magnetosphere. The solar wind plasma moving past the Earth generates electric currents and fields within the magnetosphere that are transmitted down to the Earth, often resulting in 100,000 volts applied across the ionospheric polar cap. These strong electric fields move the ionospheric plasma at speeds greater than anywhere else on the globe and, ultimately, play a key role in setting the entire upper atmosphere in motion. Exploring the physics of solar disturbances, as well as studying the interactions between the solar wind and the magnetosphere, are high-priority research goals.

Interactions between the Planetary Interior and Exterior

The convecting systems in Earth's interior are closely linked to and interact with each other. For example, it is now accepted that a solid inner core along with the magnetic field that shields us from the solar wind are required for the geodynamo to exist. Our sister planet, Venus, lacks a magnetic field, probably because its center is too hot for iron to exist in a solid phase. Flow in the outer core changes over many decades. As a result, torques on the core mantle boundary change the length of day. These are larger, albeit more gradual, than those caused by changes in the atmospheric circulation. The outer core also exerts a torque on the inner core, causing it to spin at a different rate than the rest of the planet. Changes in the mantle convection pattern over geologic time affect the temperature at the core-mantle boundary, resulting in variations both in the frequencies of geomagnetic reversals and in the amount of heat transported via hot plumes in the mantle. Variations in plume flux result in variations in magmatic activity. The largest known outpouring of flood basalts, the Siberian traps, is now known to coincide with the most massive extinction of life in the history of the planet, the great Permian extinction. This extinction coincides with a dramatic lowering of carbon isotopic ratios of the oceans, although the links between these phenomena are at present unclear. The interaction among the mantle heat engine, oceans, and the biosphere is spectacularly demonstrated by the high temperature vents on the sea floor, with their exotic biological communities and their real-time ore deposition. This interaction could have been an important energy source for primordial life.

From space-based and surface-based techniques, we are now assembling synoptic views of the shape and structure of the Earth at many scales. These help us describe the surface environment and understand its evolution. At various resolutions and degrees of completeness, synoptic views are available of the whole Earth for topography, the shape of the geoid, magnetic field, gravity field, various climate parameters, temperature, vegetation, etc. The geoid is now known with sufficient resolution to reveal deep Earth structure and the shape of the ocean floor. Global topography is known with sufficient resolution to be used as a quantitative tool in tectonic and climate studies. Images of the magnetic field can be used in regional and continental-scale mapping.

Interactions between the Atmosphere and the Earth's Surface

The role of the coupled ocean-atmosphere in climate is one of the critical challenges for study, and the economic benefits of improving our understanding and ability to predict the variability of the system are enormous. The oceans are a giant energy, water and chemical reservoir for the planet. The great ocean currents and atmospheric wind systems transport heat from the equator to the poles, thereby maintaining the thermal balance of the planet. Recent advances in knowledge of physical and chemical atmosphere-ocean interactions and the resulting development of more accurate seasonal forecasts have demonstrated the value and utility of fundamental research in this critical area. Among the interactions in question are the exchanges of momentum and energy at the air-sea boundary that, in part, drive the ocean currents and feed atmospheric convection. Continued research will focus on interactions on a diverse set of geographic and temporal scales ranging from regional variability over periods of months to global changes over decades to centuries.

Interactions in the boundary between land and atmosphere hold particular research interest, given the critical nature of this boundary to human life. In the coming years, research can be expected to focus on the impacts of volcanic activity on weather and climate, the influence of different land-surface features on local atmospheric patterns, and interactions among hydrologic and precipitation regimes. Associated with the last topic is a major activity to study the hydrological cycle in the fullest sense: weather and climate, runoff, water quality, etc.

The nature and timing of tectonic forces are responsible for the evolution of the continents and the ocean basins. These factors, in combination with wind and water erosion, are responsible for shaping the landscape. Landscape is the major controlling factor in the development of agriculture and population centers. For example, coastal regions are particularly fertile areas for study by geoscientists because atmospheric, oceanic, and terrestrial components all interact there. Biologically, coastal realms are critically important because they account for a considerable share of the world's biological production, as well as the world's major fisheries. They are also the primary locale for accumulation of sediments, and are critical zones of meteorological transition. But in addition, an increasingly large share of the world's people lives along or near coastlines despite the presence of tectonic or climatic hazards such as earthquakes, tornadoes, hurricanes, or water shortages.

The vitality of all life on Earth and our economic prosperity depend on water. A reliable water supply will be one of the greatest challenges to future expansion of population centers. Water continuously circulates over and through the Earth, and as it moves, the hydrologic cycle sustains life. Moving water carries nutrients and dissipates wastes through dilution, deposition, or chemical transformation. Water couples Earth physical and chemical systems with biota through multiple nonlinear feedbacks and thresholds that operate differently at different spatial and temporal scales. At the land and sea surface, the evaporation of water provides a constant source of latent heat for the atmosphere. In the form of water vapor, it is the Earth's most powerful greenhouse gas and the primary carrier of atmospheric energy. Within land reservoirs (e.g., wetlands, lakes, aquifers, river basins, soils, and forests) water forms the basis for all biological habitats. Nevertheless, the characterization of the water cycle from evaporation to transportation, precipitation, and storage is poorly understood. Consequently, determining how natural fluctuations and anthropogenic factors may impact the water cycle is a major scientific and societal challenge for the coming years.

Humans have not only achieved the ability to alter the planet's surface; they now stand more squarely than ever in the path of nature's processes. The growth of human population has forced humanity onto less productive lands and into parts of the landscape that have never before been occupied in such large numbers. This leaves much of the population increasingly in harm's way — on steep slopes, on flood plains, and on very arid lands. It is the alteration of the planet's surface that most greatly impacts society; the frequencies and intensities of the surface processes driven by climate affect the human condition. In most places on the Earth, the soil on which our agriculture depends is seldom more than a few meters thick, and in some places it is much thinner. This thin layer of soil can be altered chemically (e.g., be depleted of nutrients) and be very mobile. In order to understand the interactions of humankind with the Earth, one must understand the biochemical evolution and movements of that fragile layer of soil which nurtures plants, harbors animals, buttresses our architectural foundations. Often mudslides and landslides inundate our lowlands, and sometimes bury communities or even civilizations in their wake.

We are altering the planet by changing the trace gas content of the atmosphere. Yet it may not be changes in the gas that we breathe that threaten the human condition. It may be other effects such as the alteration of the hydrologic cycle; the frequency or intensity of storms; the warming of the Earth's surface and consequent melting of sea ice and glaciers; the change in the seasonality of precipitation and hence the timing and duration of runoff; or the change in intensity and location of hurricane landfall. Often it is not the rain or its absence itself, but rather the geomorphic processes that are triggered — landslides, floods, debris flows, deflation, dune migration, soil destruction — that most impact humans. The prediction and mitigation of these geomorphic responses to global change will challenge the geosciences community in the coming decade.

---

The Primary Challenges

The primary challenges for expanding knowledge of planetary energetics and dynamics include:

1. understanding the dynamical evolution of the deep Earth and the interactions between the planetary interior and exterior, using high-resolution seismic observations to determine the present state and theoretical models, constrained by paleomagnetic and geochemical data, to describe the evolution over geologic time;



2. understanding the dynamics of climate and paleoclimate, combining knowledge of radiatively active atmospheric gases with an understanding of the climatic impact of ocean processes, the role of clouds and aerosols, and the importance of natural and human-influenced biogeochemical cycles;



3. understanding how hydrologic processes interact with weather and climate at multiple scales to alter landscapes and shape aquifers;



4. understanding and charting the flows of mass, energy and momentum from the Sun into the magnetosphere and upper atmosphere and determining and predicting the response of the near- Earth space environment as a system to such flows;



5. determining the energetic and dynamic consequences of the interplay among the various scales of motion in the Earth system, from turbulence through mesoscale systems to global circulation of, for example, air, water, magma, and trace constituents;



6. obtaining extensive observations of the composition, dynamics, and energetics at the interfaces of the various Earth systems; and



7. developing comprehensive models that can ingest coupled data sets and provide quantitative understanding and prediction of Earth system processes.

PLANETARY ECOLOGY

Advances in understanding the planet, its energy and chemical flows, and its structures and dynamics position the scientific community well to address the living system of the planet at the large scale: planetary ecology. In facing this challenge, it is recognized that the terrestrial and marine biosphere consists of diverse ecosystems that vary widely in complexity and productivity, in the extent to which they are managed, and in their value to society. Ecosystems directly provide food, forage, timber, and fiber, as well as water cycling, climate regulation, recreational opportunities, and wildlife habitat. The proper functioning of ecosystems and sustained use of natural resources may be threatened by a number of global environmental changes. Some of the stresses or disturbances that have the greatest immediate potential to affect ecosystems adversely and alter their capability to support humanity include changes in land use and land cover, changes in climate, alterations in global nitrogen cycles, biodiversity loss, and species invasions. Subjecting ecosystems to one or more of these stresses simultaneously may have a significant negative impact on ecosystem function.

The linked climate-terrestrial biome system is a critical case in point. Climate affects the terrestrial biome over time scales from seconds or minutes to decades or centuries. At each step, the climate system integrates the shorter-term processes and applies feedbacks to the terrestrial biome. At the shortest time scales, the influence of temperature, radiation, humidity, and winds has a dramatic effect on the ability of plants to transpire. On longer time scales, integrated weather patterns regulate biological processes such as the timing of leaf emergence or excision, uptake of nitrogen by microbes and hence rates of organic soil decay, and turnover of inorganic nitrogen. The effect of climate at the annual or interannual scale defines the net gain or loss of carbon by the biota, its water status for the subsequent growing season, and even its ability to survive. Over longer periods ecosystem dynamics (both successional and ecosystem redistribution), which respond to climate and human land use, become central.

In another example, human-induced variations in climate will certainly affect our ability to sustain ocean ecosystems. Humans have already fundamentally influenced worldwide ocean ecosystems via fisheries, aquaculture, the introduction of nonnative species, modification or destruction of critical habitats, and the addition of nutrients and chemical pollutants to the oceans. Our future ability to track the various stresses on ocean ecosystems will depend on producing trustworthy measurements of the concentrations and cycling of key nutrients; uncovering the relationships between biological processing and chemical diagenesis in sediments; documenting pervasive influences of fluid motions on marine individuals, populations, communities, and ecosystems; improving understanding of the physical and biological controls on carbon cycling and CO2 uptake; and improving our understanding of the persistence of transients in structured ecological models.

It is now recognized that, at planetary scales, ecosystems are important regulators of the climate and chemistry of the atmosphere and that the interaction between the climate system and the biosphere is a two-way process. For instance, at large scales vegetation and soils influence climate, in part by controlling the amount of radiation reflected or absorbed and by regulating the rate of evapotranspiration. Advances in understanding this interplay between the planetary ecosystem and climate are evident in efforts to connect paleobiology and paleoclimate. Recent work on the sensitivity of climate models to changes in the distribution of vegetation has established, for example, that biogeophysical feedbacks can be of major significance to climate.

The climate of 6000 BP(Before Present) presents a particularly important illustration. Key features of the climate and ecosystem distribution of 6000 BP include the more poleward extent of northern forests and the greatly expanded African and Asian monsoon regions, with vegetation and lakes in areas that are now arid. The extensive coverage of paleodata describing the state of the terrestrial biosphere, in particular the distribution of ecotones at 6000 BP, provides a decisive standard against which to evaluate models of the climate system including the terrestrial biosphere. Models suggest that under present orbital conditions the coupled African vegetation-monsoon system possesses two stable states: the relatively arid present state and an alternative state in which part of the western Sahara is vegetated and the subtropical anticyclone is located far to the west. The alternative state can only be reached from a differently vegetated initial land surface. On the other hand, under 6000 BP orbital conditions, the coupled climate-dynamic vegetation system converges on a green Sahara consistent with the pattern of ecotones shown by the paleodata.

Terrestrial and marine ecosystems at the large scale also affect climate through their influence on atmospheric CO2 and as potent sources and sinks of other trace gases such as methane and nitrous oxide. The global carbon cycle provides another example of the direct link between the studies of planetary ecology and the Earth system.

Techniques and data sets for inferring the behavior of ecosystems from large-scale observations evolved rapidly during the 1990s. The use of information about the spatial and temporal distribution of atmospheric CO2 concentrations, or inverse modeling, to deduce spatial and later temporal patterns of terrestrial and oceanic CO2 exchange improved our understanding of the likely nature of terrestrial sinks. Inverse modeling began in the geophysical community, where global CO2 observations were also developed, but use of the technique rapidly grew through collaborations with ecologists. Inverse modeling showed a sink of CO2 in northern latitudes, through discrepancies between the observed interhemispheric gradient of CO2 and the values predicted based on fossil emissions and characteristics of interhemispheric transport. Whereas initial analyses had reached different conclusions about the distribution of this sink between marine and terrestrial systems, later analyses using 13C in CO2 and measured O2 indicated a substantial terrestrial sink. This sink has been and remains difficult to quantify or even detect in classical forest and soil inventory measurements; extensive atmospheric measurements remain the most conclusive evidence for the location of the so-called "missing sink."

Applications of the inverse methodology over time have also suggested correlations between climate and terrestrial CO2 exchange at hemispheric to global scales. These observations remain preliminary but provide a foundation for future monitoring of global source and sink patterns. The relationship of terrestrial carbon storage to climate is fundamental to understanding the interactions between climate and ecosystems that may occur during future climate changes. More generally, climate cannot be viewed as a force external to ecosystems: ecosystems participate in the shaping of weather, climate, and atmospheric composition.

The coupling of the biosphere with the climate system is highlighted further in considering the evolution of climate under increased greenhouse forcing. As the climate system evolves in response to increasing greenhouse gases, the terrestrial and, perhaps, marine biotic systems will also evolve. This evolution must be considered concurrently since, as discussed above, the couplings are two-way. This important scientific challenge will require insight from a variety of disciplines and advanced observational and computational resources. More importantly, if the challenge is to be met, it must be addressed in a more coherent and aggressive manner in the next decade than it has been in the past decade.

Ecology at the planetary scale thus contributes a unique perspective to Earth science. While the geophysical sciences begin conceptually with a unified physical-chemical view of systems (based in fluid dynamics, thermodynamics, and photochemistry), the underlying paradigm of ecology emphasizes the diversity of ecosystems, resulting from evolutionary history of organisms and the soils and landforms or water bodies they inhabit. This perspective, of seeking understanding from the similarities and differences between processes across a range of environments, has become an important paradigm in interdisciplinary Earth system research. It is also central to the study of the planet, since humans are now altering the ecology of the planet, its chemistry, and potentially its climate. Moreover, these changes are coupled in an exceedingly complex manner, and interactions among biota and between biota and their environment may produce unpredictable results with cascading feedbacks. Consequently, recognizing the complex ways in which changes in the planet's environment can affect and be affected by biological systems at the large scale is a fundamental and important scientific challenge.

---

The Primary Challenges

The scientific issues facing planetary ecology are captured in seven primary challenges:

1. understanding how land surface biophysical processes interact with regional climate and modify patterns of climate and associated hydrologic variability;



2. incorporating the land surface state into predictions of weather, seasonal to interannual climate, and hydrologic processes;



3. analyzing how the large-scale atmosphere-ecosystem exchange of water and energy might change in a world high in carbon dioxide;



4. understanding how the role of marine ecosystems will change with future changes to ocean circulation, temperature, and nutrient/toxic inputs;



5. determining the interactions of changing land use, climate, nutrient and toxic inputs, and hydrology on ecosystems and their ability to support human activities and sustain biodiversity;



6. understanding what effect the functional diversity of species has on ecosystem function within biomes and at the global level;



7. establishing whether potential changes to global biodiversity and climate could affect global net primary production, trace gas exchange, and other critical aspects of ecosystem function.

PLANETARY METABOLISM

The preceding three research fronts dealing with Earth's structure, mass and energy cycling, and biogeochemical processes lead naturally to an integrated understanding of planetary metabolism and biocomplexity in the environment. For several decades geoscientists have found it necessary to incorporate basic laws from physics and chemistry into their geological framework. A new thrust in the 21st Century will be to recognize that Earth's past history and future course cannot be understood without an explicit integration of the effects of its biological activity, including that of humans.

Our understanding of the evolution of life on the planet is still developing, but it is increasingly clear that life arose relatively early in Earth history and that its effects on atmospheric composition and on geological formations have been enormous. Our oxygenated atmosphere is intrinsically unstable over millions of years and can only be maintained through biological processes. On the early Earth, primitive photosynthetic life set the stage for higher cellular life forms and ever-more-efficient metabolic pathways, allowing conversion of glucose to energy, and ultimately leading to the krebs cycle and human evolution.

Geochemical Cycles

Whether or not they contain life, planets are subject to solar energy pumping, which provides for a flux of matter from lower chemical potential forms to those at higher potential. The existence of planetary geochemical cycles is the inevitable result of the energy flow through such a system. Even on a planet devoid of an ocean and life, high and low chemical potential compounds occur, such as those observed on Mars.

When life is present, this geochemical cycle can take another pathway. The basic chemical constituents of organic matter - carbon, nitrogen, oxygen, phosphorus, and sulfur - follow a closed loop or cycle through increasing molecular energy states, as the elements are incorporated into living tissue, and then through decreasing energy levels, as the tissues decompose, giving rise to biogeochemical cycles. The chemical disequilibrium observed on Earth is thus a signature of a living planet.

The significance of the role of living systems in all of the Earth's biogeochemical cycles is a relatively recent discovery. The recognition of biotic factors as controls of biogeochemical cycles has allowed for significant advances in our understanding of the natural metabolism responsible for the compositions of the atmosphere, oceans, soils, and sediments. Such a planetary metabolism is interactive — physical, chemical, and biological processes are inextricably linked. Therefore, quantification of the characteristic dynamics of these interlocked cycles and the controlling factors is essential for a better understanding of global processes generally.

The biogeochemical system is not only internally coupled, it is also fully intertwined with the planet's physical climate system. The climate obviously is a strong determinant of vegetation distribution and growth, litter decomposition and trace gas production, ocean circulation and marine production, and atmospheric transport and chemical change. However, the influence is not one-way. Vegetation strongly controls the exchange of water and energy between the Earth's terrestrial surface and the atmosphere. There is evidence that marine primary production partially controls the depth of the mixed layer in the ocean, and as previously mentioned, many biologically produced gases influence the planet's heat balance through the greenhouse effect. It is known from ice-core records that these gases have varied substantially in concentration on glacial and interglacial time scales; the paleoenvironmental records provide good reason to believe that some of them are directly involved in the regulation of natural climate changes.

Atmospheric Chemistry

One example of the emerging importance of the role of planetary metabolism in influencing the environment of Earth is the evolution of the field of atmospheric chemistry over the past twenty years. In the 1970s it was recognized that a combination of biological and industrial processes had the potential to change atmospheric composition on both local and global scales, with important societal implications. Issues such as deteriorating air quality and stratospheric ozone depletion highlighted the fact that identifying the processes that control atmospheric composition was important, yet the fundamental understanding of these processes was lacking. Over the past two decades, atmospheric chemistry has developed as a new and important sub-discipline of the geosciences. There is now a cadre of scientists from the geosciences, other physical sciences, biology, and engineering pursuing research on chemical, physical and biological processes affecting atmospheric composition. They are addressing the transport and deposition of biologically important chemicals, either nutrients or toxics, with significant impact on the health of natural and managed ecosystems. In a sense, atmospheric chemistry represents a model for the future of the geosciences. The atmosphere responds very rapidly to changing inputs and observational programs allow researchers to quickly detect and study this response, and to suggest and identify cause and effect relationships. For other components of the Earth system, response times to external forcing are much slower, and studying the interdependencies between components is more difficult. Current and future observational and computational techniques will make it possible to carry out detailed multidisciplinary studies of the interactions and feedbacks between the environment and life on the Earth.

Biodiversity

Biodiversity of every marine community is vastly greater than previously recognized, and both sampling statistics and molecular tools indicate that the large majority of marine species remains undescribed. Blooms of larger diatoms, capable of exporting carbon and nutrients to deeper communities, can be limited by trace elements such as iron. Spectacular evidence of this limitation came from a large-scale experiment that added dissolved iron to the open ocean. Biotic responses to this addition suggest that production in large regions of the open ocean can be affected by global-scale weather patterns, for example, delivering desert dust to oceanic systems. A challenge for ocean ecologists is to facilitate better stewardship of marine resources and ecosystems by understanding and predicting which perturbations and food-web alterations will cause collapses of marine communities and the ecosystem services that they provide; predicting and mitigating the effects of harmful algal blooms; understanding and predicting interactions among global climate, marine geochemical processes, and marine biota; and understanding and mitigating outbreaks of microbial pathogens that can decimate important marine populations. Meeting these challenges will require better understanding of the causes and consequences of change on time scales from hours to millennia.

Understanding how the fluxes of mass and energy among various components of the solid and fluid Earth link to biological activity on and beneath its surface represents a fundamental goal of current research. This understanding of planetary metabolism bears directly on key scientific questions concerning the co-evolution of different components of the Earth system including life, as well as on the most pressing environmental questions of our time. Present understanding of these issues is very incomplete; the attack on the problem will require extensive interdisciplinary collaboration and will rely upon the achievements of all. This attack will employ a hierarchy of models; it will include interdisciplinary problem analysis and the synthesis, interpretation, and application of global-scale data sets, including those obtained by continuous monitoring from space and from new land and ocean-based observing systems.

---

The Primary Challenges

There are five primary challenges facing researchers in the study of planetary metabolism:

1. determining how the biogeochemical cycles of carbon, nitrogen, oxygen, phosphorus, and sulfur are coupled;



2. enumerating what energy transformations control the biosphere and climate systems;



3. understanding how biological and social processes and the evolution of life regulate the Earth system and its climate states;



4. determining what the short-term and long-term history of planetary metabolic changes has been; and



5. developing sufficiently sophisticated models to explain historic events and predict future changes in planetary metabolism.