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Ideas: The Research Agenda — Goals
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PLANETARY STRUCTURE
![](https://webarchive.library.unt.edu/eot2008/20081106185814im_/http://www.nsf.gov/images/bluefade.jpg) 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:
- understanding
the details of the complex interactions between atmospheric and ocean
dynamics and thermodynamics over the full range of spatial and temporal
scales;
- determining the
role of clouds, aerosols, and biogeochemical feedbacks in the radiative
balance of the atmosphere and climate;
- understanding
and predicting the response of the near-Earth space environment to solar
storms and geomagnetic disturbances;
- understanding
and quantifying the ocean's role in transporting, storing, and exchanging
heat, freshwater, mass, and chemical constituents;
- understanding
the processes that control the state and variability of the coastal
oceans;
- 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;
- enhancing the
resolution of lateral and vertical variations of fine structure throughout
the solid Earth; and
- understanding
the structural relationships between the mantle, the overlying crust
and lithosphere, and the underlying core.
PLANETARY ENERGETICS AND DYNAMICS
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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:
- 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;
- 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;
- understanding
how hydrologic processes interact with weather and climate at multiple
scales to alter landscapes and shape aquifers;
- 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;
- 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;
- obtaining extensive
observations of the composition, dynamics, and energetics at the interfaces
of the various Earth systems; and
- developing comprehensive
models that can ingest coupled data sets and provide quantitative understanding
and prediction of Earth system processes.
PLANETARY ECOLOGY
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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:
- understanding
how land surface biophysical processes interact with regional climate
and modify patterns of climate and associated hydrologic variability;
- incorporating
the land surface state into predictions of weather, seasonal to interannual
climate, and hydrologic processes;
- analyzing how
the large-scale atmosphere-ecosystem exchange of water and energy might
change in a world high in carbon dioxide;
- understanding
how the role of marine ecosystems will change with future changes to
ocean circulation, temperature, and nutrient/toxic inputs;
- 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;
- understanding
what effect the functional diversity of species has on ecosystem function
within biomes and at the global level;
- 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
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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:
- determining how
the biogeochemical cycles of carbon, nitrogen, oxygen, phosphorus, and
sulfur are coupled;
- enumerating what
energy transformations control the biosphere and climate systems;
- understanding
how biological and social processes and the evolution of life regulate
the Earth system and its climate states;
- determining what
the short-term and long-term history of planetary metabolic changes
has been; and
- developing sufficiently
sophisticated models to explain historic events and predict future changes
in planetary metabolism.
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