Introduction
Definition of a System
Introduction to the Global Climate System
Summary of Major Influences on Global Climate
Interconnectedness of the Earth System
Earth System Science is a relatively new
field of study that focuses on the operation of the whole Earth,
including the atmosphere, hydrosphere, biosphere, and geosphere.
These four spheres can be thought of as four machines or systems that
are connected together to make one larger machine -- the whole Earth
system. Earth System Science is especially concerned with the
interactions between these different spheres and how these
interactions control the global climate. This field of study
incorporates and integrates material from traditional geology,
meteorology, oceanography, ecology, atmospheric chemistry, and other
fields.
Why has this new field of study has emerged
recently? There are perhaps two related causes: 1) a growing sense of
urgency to understand the dynamics of the global climate system; and
2) the rapid advances in computing and satellite technology that
enabled us to observe and model the planet on a global scale. Why the
sense of urgency to understand the workings of the climate system?
Observations of the change in atmospheric composition (ozone
depletion and CO2
increase) and rising global temperatures indicate that we are
altering the global climate system, possibly with serious
consequences. If we can understand the climate system, we can make
good predictions about how it will change in the future, which will
help us to decide how we need to change our behavior and what we need
to prepare for in the future.
Because of the integrative, cross-disciplinary nature, Earth System Science demands a broad, holistic view of our planet and requires collaboration and communication between disciplines that were traditionally somewhat isolated. Isolated, reductionist thinking, in which you focus in on some small detail, is clearly important to the development of our knowledge, but by its very nature, this mode of thinking cannot illuminate the interconnections that are so important to the operation of the whole Earth system. The holistic view in no way dismisses the very careful, detailed studies that have taken place within all of the sub-disciplines encompassed in Earth System Science; instead, it builds on this accumulated knowledge and attempts to understand how it all fits together.
Earth System Science also involves some
exciting challenges -- we need to cultivate a broad and deep
understanding of all of the sciences. We also need to develop a grasp
of how to analyze the behavior of systems that involve important
interconnections -- this leads us to computer modeling which can
range from the fairly simple versions explored in this book, to
models that push the computing capabilities of the most powerful
supercomputers.
Definition of a
System
Before going any further, we need to discuss
what is meant by a system; it is a somewhat vague term that means
different things to different people. In this book, we will be
talking about dynamic systems -- systems that involve change --
because change at all timescales is a major theme in the study of the
Earth. Some aspects of Earth, like its weather, change dramatically
on short timescales, while others, like the positions and sizes of
the continents, show dramatic change only when viewed over hundreds
of millions of years. But, getting back to the definition, dynamic
systems are coordinated or related sets of processes and reservoirs
(places where things can reside or forms in which matter or energy
exists) through which material or energy flows, characterized by
continual change. This is a complex definition, but I think it is
fairly precise, so it is worth thinking about carefully. A few
examples will help.
A bathtub is a simple example of a dynamic
system. Water flows into the tub through a faucet and leaves the tub
through a drain. The faucet and the drain represent processes that
are related because they both involve water moving into and out of
the same reservoir, which is the tub itself. The balance between the
inflow through the faucet and the outflow through the drain
determines how much water accumulates in the tub. If the inflow and
outflow rates are the same, then there will be no change in the
amount of water in the tub, so the system would appear to be
unchanging and not a true dynamic system. But of course, individual
water molecules are moving through the system; material is flowing
through this system.
Another example of a dynamic system is a pot
of water set on a burner. In this case, energy, rather than matter,
flows through the system. Energy is added to the pot via the burner
and it is absorbed by the water in the pot, raising the temperature
of the water. Energy escapes the system via infrared radiation (heat
waves) and through vapor loss. When water changes from a liquid to a
vapor, a process called evaporation, it requires a good deal of
energy and this energy comes from the body of liquid water. This is
why evaporation from a film of water covering your skin cools you. So
in this system, there is one process adding energy to the system and
two processes removing energy to the system and a reservoir of water
through which the energy passes. We'll look into these systems in a
more detailed manner in the next chapter.
While we're on the topic of basic
definitions, we should also consider the global climate system and
its important components -- the atmosphere, hydrosphere, biosphere,
and geosphere -- so that you have a clear idea of what it meant by
these terms.
The Global Climate
System
Climate is simply the average weather of
some area, so you might think of the global climate in terms of
average global temperature (about 15°C these days). The global
climate system then is the set of related, coordinated processes and
reservoirs that determine the energy balance of the surface
environment, which determines the temperature. A simplified drawing
of what this system involves is shown in FIGURE 1.1, which is worth
taking a close look at. The global climate system involves the flow
of energy as well as matter through the geosphere, the atmosphere,
the hydrosphere, and the biosphere. All of these parts of the larger
system play important parts in determining how the solar energy, in
the form of visible and ultraviolet light, is absorbed, reflected,
transformed to infrared (heat) energy, how that heat is emitted and
absorbed, ultimately determining how much heat is stored in the
system and how that heat sets various parts of the system in
motion.
The incoming solar energy, called insolation
(not to be confused with insulation) shines down on the Earth -- most
intensely at the equator -- and is either absorbed or reflected.
Clouds are highly reflective and so is ice; land is generally more
reflective than water, and deserts are more reflective than forests.
So, the locations of clouds and continents and the nature of land
surface are important to the climate system. Why is location
important? A land mass at the equator reflects more energy than an
identical land mass near the pole because the intensity of the
insolation is much greater at the equator. As we will see, land
masses do move quite a bit over periods of many millions of years, so
this is something to keep in mind.
The insolation that is not reflected is
absorbed at the surface, and warms the surface, which then radiates a
different kind of energy, infrared radiation -- heat -- back up from
the surface. Much of the heat is actually transported by water; as it
evaporates, it takes heat energy from the surface, and when it
condenses up in the atmosphere to form a cloud or a rain drop, it
releases that heat. Plants are important in this process since they
absorb water from the soil and release it to the atmosphere.
Fortunately for us, not all of that heat escapes to outer space. Some
of that heat is absorbed by gases like carbon dioxide, water, and
chlorofluorocarbons (CFCs) in the atmosphere and those gases then
return some of that heat back to the surface -- this is the famous
greenhouse effect, and without it, the Earth would be intolerably
cold, about 33°C colder than the present. To put this in
perspective, the global temperature during an ice age is about
8°C colder than the present. These greenhouse gases can be
thought of as a global blanket, keeping out planet warm.
Because the Earth's surface receives varying
intensities of solar energy, temperature differences occur and sets
winds and ocean currents in motion. The basic job of these fluids is
to transfer heat from the equatorial region to the poles. While winds
are the primary driving agents of surface ocean currents, variations
in temperature and salinity of the ocean water produce currents that
stir up the deeper parts of the oceans. Polar ice plays an important
role in generating these cold, deep currents that sweep along the
ocean floor for great distances; regions of the oceans with little
rainfall lead to water that is higher in salinity and therefore
denser than normal ocean water that can also lead to deep currents.
These deep ocean currents appear to be very important in switching
our global climate from a glacial age to an interglacial age (the
present).
The right-hand side of Fig. 1.1 shows a
mountain range where weathering and erosion are occurring, two
processes that may not seem obviously connected to the global climate
system. But in fact, these processes are quietly involved in
moderating the balance of CO2
in the atmosphere. Weathering of rocks includes several processes
that break the rocks down into smaller pieces; some of these
processes involve physically breaking the rock, while other involve
chemical alteration and decomposition. These chemical reactions
proceed very slowly and often use weak acids to do the job. One of
the most important of these acids is called carbonic acid and is
formed from CO2
and water, which come from the atmosphere. Thus, chemical weathering
uses up atmospheric CO2
and erosion sweeps the by-products of weathering aside, exposing
fresh rocks to the surface so that they can be weathered. The
by-products of weathering are transported by rivers to the oceans,
where some of them are used by plankton to make skeletons of
carbonate (CaCO3)
that end up being deposited on the ocean floor, forming sedimentary
rocks. The rates of chemical weathering are sensitive to the
temperature, which is related to the amount of
CO2
in the atmosphere. This means that on a global scale, when the Earth
is hotter, weathering is faster and has a cooling effect since it
removes CO2
from the atmosphere -- it stabilizes our planet's temperature, but it
acts slowly, allowing for dramatic short-term changes.
Summary of Major Influences on Global Climate
This first glance at the whole Earth System and how it determines the state of the global climate should give you a good sense its complexity and some of the important interconnections. You might also begin to see that the operation of the system involves several cycles (carbon cycle, water cycle, etc.), which are systems that are more or less closed. In these cycles, material passes from place to place in a cyclical fashion. If you track a particular water molecule over millions of years, you may see it move from the atmosphere to the land to the oceans to the deep interior, then up through a volcano to the atmosphere. Many of these cycles represent important subsystems of the global climate system, and we will explore these cycles throughout this book.
Interconnectedness of the Earth System
In addition to understanding how different parts of the Earth System affect the global climate, it is important to understand how these different parts are linked together &emdash; how they are interconnected. The graph below represents these interconnections in the form of connecting arrows; each arrow represents some set of processes that operate within one of the Earth's four "spheres" that influences the "sphere" that the arrow points to.
A New Way of Thinking
One of the benefits of studying the dynamics of Earth systems is that it leads to a better understanding of your environment -- how that environment came to be, how it is maintained, how it changes naturally, and how human activities can be affected by the environment. Ideally, this kind of appreciation and understanding of the Earth can be obtained in most introductory geology classes; what this new approach -- the Earth System Science approach -- adds is a new way of thinking that entails looking at phenomena as parts of a system and then trying to understand the connections within that system and its dynamics -- how it changes over time. When you start thinking about the different parts of your surroundings, or the larger world as parts of a system, you are forced to consider things like the connections between the subject at hand and its surroundings. This way of thinking also encourages you to consider how the system will change in the future, how it has changed in the past, and how those changes affect the behavior of the system. One of the key features about this new way of thinking, a feature that I think is highly important, is that is motivates you to ask fundamental questions, questions that are directed towards understanding how systems work.
