C O R E C A S E S T U D Y

leaving each forested valley had to flow across a dam where sci-
entists could measure its volume and dissolved nutrient content.

In the first experiment, the investigators measured the
amounts of water and dissolved plant nutrients that entered and
left an undisturbed forested area (the control site) (Figure 2-1,
left). These measurements showed that an undisturbed mature
forest is very efficient at storing water and retaining chemical
nutrients in its soils.

The next experiment involved setting up an experimental
forested area. One winter, the investigators cut down all trees
and shrubs in one valley (the experimental site), left them where
they fell, and sprayed the area with herbicides to prevent the
regrowth of vegetation. Then they compared the inflow and
outflow of water and nutrients in this experimental site (Fig-
ure 2-1, right) with those in the control site (Figure 2-1, left) for
3 years.

With no plants to help absorb and retain water, the amount
of water flowing out of the deforested valley increased by
30–40%. As this excess water ran rapidly over the ground, it
eroded soil and carried dissolved nutrients out of the deforested
site. Overall, the loss of key nutrients from the experimental for-
est was six to eight times that in the nearby control forest.

Carrying Out a Controlled
Scientific Experiment

One way in which scientists learn about how nature works is
to conduct a controlled experiment. To begin, scientists isolate
variables, or factors that can change within a system or situation
being studied. An experiment involving single-variable analysis
is designed to isolate and study the effects of one variable at
a time.

To do such an experiment, scientists set up two groups. One
is the experimental group in which a chosen variable is changed
in a known way, and the other is the control group in which the
chosen variable is not changed. If the experiment is designed and
run properly, differences between the two groups should result
from the variable that was changed in the experimental group.

In 1963, botanist F. Herbert Bormann, forest ecologist
Gene Likens, and their colleagues began carrying out a clas-
sic controlled experiment. The goal was to compare the loss of
water and nutrients from an uncut forest ecosystem (the control
site) with one that was stripped of its trees (the experimental
site).

They built V-shaped concrete dams across the creeks at the
bottoms of several forested valleys in the Hubbard Brook Experi-
mental Forest in New Hampshire (Figure 2-1). The dams were
anchored on impenetrable bedrock, so that all surface water

Science, Matter, Energy,
and Systems2

Figure 2-1 Controlled field experiment to measure the effects of deforestation on the loss of water and soil nu-
trients from a forest. V–notched dams were built into the impenetrable bedrock at the bottoms of several forested
valleys (left) so that all water and nutrients flowing from each valley could be collected and measured for volume
and mineral content. These measurements were recorded for the forested valley (left), which acted as the control
site. Then all the trees in another valley (the experimental site) were cut (right) and the flows of water and soil nutri-
ents from this experimental valley were measured for 3 years.

Links: refers to the Core Case Study. refers to the book’s sustainability theme. indicates links to key concepts in earlier chapters. 29

Key Questions and Concepts

2-1 What is science?
C O N C E P T 2 – 1 Scientists collect data and develop theories,
models, and laws about how nature works.

2-2 What is matter?
C O N C E P T 2 – 2 Matter consists of elements and compounds,
which are in turn made up of atoms, ions, or molecules.

2-3 How can matter change?
C O N C E P T 2 – 3 When matter undergoes a physical or chemical
change, no atoms are created or destroyed (the law of conservation
of matter).

2-4 What is energy and how can it be changed?
C O N C E P T 2 – 4 A When energy is converted from one form to
another in a physical or chemical change, no energy is created or
destroyed (first law of thermodynamics).

C O N C E P T 2 – 4 B Whenever energy is changed from one form to
another, we end up with lower-quality or less usable energy than
we started with (second law of thermodynamics).

2-5 What are systems and how do they respond to
change?
C O N C E P T 2 – 5 A Systems have inputs, flows, and outputs of
matter and energy, and their behavior can be affected by
feedback.

C O N C E P T 2 – 5 B Life, human systems, and the earth’s life-
support systems must conform to the law of conservation of matter
and the two laws of thermodynamics.

Note: Supplements 1 (p. S2), 2 (p. S4), 5 (p. S31), and 6 (p. S39) can be used with this
chapter.

Science is an adventure of the human spirit.
It is essentially an artistic enterprise, stimulated largely by curiosity,

served largely by disciplined imagination,
and based largely on faith in the reasonableness, order,

and beauty of the universe.

WARREN WEAVER

Science Is a Search for Order
in Nature
Have you ever seen an area in a forest where all the
trees were cut down? If so, you might wonder about
the effects of cutting down all those trees. You might
wonder how it affected the animals and people living
in that area and how it affected the land itself. That is
what scientists Bormann and Likens (Core Case
Study) thought about when they designed their
experiment.

Such curiosity is what motivates scientists. Sci-
ence is an endeavor to discover how nature works
and to use that knowledge to make predictions about
what is likely to happen in nature. It is based on the
assumption that events in the natural world follow or-

derly cause-and-effect patterns that can be understood
through careful observation, measurements, experi-
mentation, and modeling. Figure 2-2 (p. 30) summa-
rizes the scientific process.

There is nothing mysterious about this process. You
use it all the time in making decisions. Here is an ex-
ample of applying the scientific process to an everyday
situation:

Observation: You try to switch on your flashlight and
nothing happens.

Question: Why didn’t the light come on?

Hypothesis: Maybe the batteries are dead.

Test the hypothesis: Put in new batteries and try to
switch on the flashlight.

2-1 What Is Science?
CONCEPT 2-1 Scientists collect data and develop theories, models, and laws about
how nature works.

▲

30 CHAPTER 2 Science, Matter, Energy, and Systems

Result: Flashlight still does not work.

New hypothesis: Maybe the bulb is burned out.

Experiment: Replace bulb with a new bulb.

Result: Flashlight works when switched on.

Conclusion: Second hypothesis is verified.

Here is a more formal outline of steps scientists of-
ten take in trying to understand nature, although not
always in the order listed:

• Identify a problem. Bormann and Likens
(Core Case Study) identified the loss of water
and soil nutrients from cutover forests as a
problem worth studying.

• Find out what is known about the problem. Bormann
and Likens searched the scientific literature to find
out what was known about retention and loss of
water and soil nutrients in forests.

• Ask a question to be investigated. The scientists asked:
“How does clearing forested land affect its ability to
store water and retain soil nutrients?

• Collect data to answer the question. To collect data—
information needed to answer their questions—
scientists make observations of the subject area
they are studying. Scientific observations involve
gathering information by using human senses of
sight, smell, hearing, and touch and extending
those senses by using tools such as rulers, micro-
scopes, and satellites. Often scientists conduct
experiments, or procedures carried out under
controlled conditions to gather information and test
ideas. Bormann and Likens collected and analyzed
data on the water and soil nutrients flowing from
a patch of an undisturbed forest (Figure 2-1, left)
and from a nearby patch of forest where they had
cleared the trees for their experiment (Figure 2-1,
right).

• Propose a hypothesis to explain the data. Scientists sug-
gest a scientific hypothesis, a possible and test-
able explanation of what they observe in nature
or in the results of their experiments. The data
collected by Bormann and Likens show a decrease
in the ability of a cleared forest to store water and
retain soil nutrients such as nitrogen. They came
up with the following hypothesis to explain their
data: When a forest is cleared, it retains less water
and loses large quantities of its soil nutrients when
water from rain and melting snow flows across its
exposed soil.

• Make testable predictions. Scientists use a hypothesis
to make testable or logical predictions about what
should happen if the hypothesis is valid. They of-
ten do this by making “If . . . then” predictions.
Bormann and Likens predicted that if their original
hypothesis was valid for nitrogen, then a cleared
forest should also lose other soil nutrients such as
phosphorus.

• Test the predictions with further experiments, models,
or observations. To test their prediction, Bormann
and Likens repeated their controlled experiment
and measured the phosphorus content of the soil.
Another way to test predictions is to develop a
model, an approximate representation or simula-
tion of a system being studied. Since Bormann and
Likens performed their experiments, scientists have
developed increasingly sophisticated mathematical
and computer models of how forest systems work.
Data from Bormann and Likens’s research and that
of other scientists can be fed into such models and

Scientific law
Well-accepted
pattern in data

Scientific theory
Well-tested and
widely accepted

hypothesis

Accept
hypothesis

Revise
hypothesis

Perform an experiment
to test predictions

Use hypothesis to make
testable predictions

Propose an hypothesis
to explain data

Analyze data
(check for patterns)

Perform an experiment
to answer the question

and collect data

Ask a question to be
investigated

Find out what is known
about the problem
(literature search)

Identify a problem

Test
predictions

Make testable
predictions

Figure 2-2 What
scientists do. The es-
sence of science is this
process for testing
ideas about how na-
ture works. Scientists
do not necessarily fol-
low the exact order of
steps shown here. For
example, sometimes a
scientist might start by
formulating a hypoth-
esis to answer the ini-
tial question and then
run experiments to
test the hypothesis.

CONCEPT 2-1 31

SCIENCE FOCUS

Easter Island: Some Revisions to a Popular Environmental Story

as a source of protein for the long voyage)
played a key role in the island’s permanent
deforestation. Over the years, the rats multi-
plied rapidly into the millions and devoured
the seeds that would have regenerated the
forests.

Another of Hunt’s conclusions was that
after 1722, the population of Polynesians on
the island dropped to about 100, mostly from
contact with European visitors and invaders.
Hunt hypothesized that these newcomers in-
troduced fatal diseases, killed off some of the
islanders, and took large numbers of them
away to be sold as slaves.

This story is an excellent example of how
science works. The gathering of new scientific
data and reevaluation of older data led to a
revised hypothesis that challenges our think-
ing about the decline of civilization on Easter
Island. As a result, the tragedy may not be as
clear an example of human-caused ecologi-
cal collapse as was once thought. However,
there is evidence that other earlier civilizations
did suffer ecological collapse largely from
unsustainable use of soil, water, and other
resources, as described in Supplement 5 on
p. S31.

Critical Thinking
Does the new doubt about the original Easter
Island hypothesis mean that we should not
be concerned about using resources unsus-
tainably on the island in space we call Earth?
Explain.

eroded, crop yields plummeted, and famine
struck. There was no firewood for cooking
or keeping warm. According to the original
hypothesis, the population and the civiliza-
tion collapsed as rival clans fought one an-
other for dwindling food supplies, and the
island’s population dropped sharply. By the
late 1870s, only about 100 native islanders
were left.

In 2006, anthropologist Terry L. Hunt,
Director of the University of Hawaii Rapa Nui
Archeological Field School, evaluated the
accuracy of past measurements and other
evidence and carried out new measurements
to estimate the ages of various artifacts. He
used these data to formulate an alternative
hypothesis describing the human tragedy on
Easter Island.

Hunt came to several new conclusions.
First, the Polynesians arrived on the island
about 800 years ago, not 2,900 years ago.
Second, their population size probably never
exceeded 3,000, contrary to the earlier esti-
mate of up to 15,000. Third, the Polynesians
did use the island’s trees and other vegetation
in an unsustainable manner, and by 1722,
visitors reported that most of the island’s
trees were gone.

But one question not answered by the
earlier hypothesis was, why did the trees
never grow back? Recent evidence and
Hunt’s new hypothesis suggest that rats
(which either came along with the original
settlers as stowaways or were brought along

or years, the story of Easter Island
has been used in textbooks as

an example of how humans can seriously
degrade their own life-support system. It
concerns a civilization that once thrived and
then largely disappeared from a small,
isolated island in the great expanse of the
South Pacific, located about 3,600 kilome-
ters (2,200 miles) off the coast of Chile.

Scientists used anthropological evidence
and scientific measurements to estimate the
ages of certain artifacts found on Easter
Island (also called Rapa Nui). They hypothe-
sized that about 2,900 years ago, Polynesians
used double-hulled, seagoing canoes to colo-
nize the island. The settlers probably found a
paradise with fertile soil that supported dense
and diverse forests and lush grasses. Accord-
ing to this hypothesis, the islanders thrived,
and their population increased to as many as
15,000 people.

Measurements made by scientists
seemed to indicate that over time, the
Polynesians began living unsustainably by
using the island’s forest and soil resources
faster than they could be renewed. When
they used up the large trees, the islanders
could no longer build their traditional sea-
going canoes for fishing in deeper offshore
waters, and no one could escape the island
by boat.

Without the once-great forests to ab-
sorb and slowly release water, springs
and streams dried up, exposed soils were

F

used to predict the loss of phosphorus and other
types of soil nutrients. These predictions can be
compared with the actual measured losses to test
the validity of the models.

• Accept or reject the hypothesis. If their new data do not
support their hypotheses, scientists come up with
other testable explanations. This process continues
until there is general agreement among scientists in
the field being studied that a particular hypothesis
is the best explanation of the data. After Bormann
and Likens confirmed that the soil in a cleared for-
est also loses phosphorus, they measured losses
of other soil nutrients, which also supported their
hypothesis. A well-tested and widely accepted sci-
entific hypothesis or a group of related hypotheses
is called a scientific theory. Thus, Bormann and
Likens and their colleagues developed a theory that
trees and other plants hold soil in place and help it

to retain water and nutrients needed by the plants
for their growth.

Important features of the scientific process are curi-
osity, skepticism, peer review, reproducibility, and openness to
new ideas. Good scientists are extremely curious about
how nature works. But they tend to be highly skepti-
cal of new data, hypotheses, and models until they can
be tested and verified. Peer review happens when sci-
entists report details of the methods and models they
used, the results of their experiments, and the reason-
ing behind their hypotheses for other scientists working
in the same field (their peers) to examine and criticize.
Ideally, other scientists repeat and analyze the work
to see if the data can be reproduced and whether the
proposed hypothesis is reasonable and useful (Science
Focus, below).

For example, Bormann and Likens (Core
Case Study) submitted the results of their for-

32 CHAPTER 2 Science, Matter, Energy, and Systems

to explain some of their observations in nature. Often
such ideas defy conventional logic and current scien-
tific knowledge. According to physicist Albert Einstein,
“There is no completely logical way to a new scientific
idea.” Intuition, imagination, and creativity are as im-
portant in science as they are in poetry, art, music, and
other great adventures of the human spirit, as reflected
by scientist Warren Weaver’s quotation found at the
opening of this chapter.

Scientific Theories and Laws
Are the Most Important Results
of Science
If an overwhelming body of observations and measure-
ments supports a scientific hypothesis, it becomes a sci-
entific theory. Scientific theories are not to be taken lightly.
They have been tested widely, are supported by exten-
sive evidence, and are accepted by most scientists in a
particular field or related fields of study.

Nonscientists often use the word theory incorrectly
when they actually mean scientific hypothesis, a tentative
explanation that needs further evaluation. The state-
ment, “Oh, that’s just a theory,” made in everyday con-
versation, implies that the theory was stated without
proper investigation and careful testing—the opposite
of the scientific meaning of the word.

Another important and reliable outcome of science
is a scientific law, or law of nature: a well-tested
and widely accepted description of what we find hap-
pening over and over again in the same way in nature.
An example is the law of gravity, based on countless ob-
servations and measurements of objects falling from
different heights. According to this law, all objects fall
to the earth’s surface at predictable speeds.

A scientific law is no better than the accuracy of the
observations or measurements upon which it is based
(see Figure 1 in Supplement 1 on p. S3). But if the data
are accurate, a scientific law cannot be broken, unless
and until we get contradictory new data.

Scientific theories and laws have a high probabil-
ity of being valid, but they are not infallible. Occasion-
ally, new discoveries and new ideas can overthrow a
well-accepted scientific theory or law in what is called a
paradigm shift. It occurs when the majority of scien-
tists in a field or related fields accept a new paradigm, or
framework for theories and laws in a particular field.

A good way to summarize the most important out-
comes of science is to say that scientists collect data and
develop theories, models, and laws that describe and
explain how nature works (Concept 2-1). Scientists use
reasoning and critical thinking skills. But the best sci-
entists also use intuition, imagination, and creativity
in asking important questions, developing hypotheses,
and designing ways to test them.

For a superb look at how science works and what sci-
entists do, see the Annenberg video series, The Habitable
Planet: A Systems Approach to Environmental Science (see

est experiments to a respected scientific journal. Before
publishing this report, the journal editors had it re-
viewed by other soil and forest experts. Other scientists
have repeated the measurements of soil content in un-
disturbed and cleared forests of the same type and also
in different types of forests. Their results have also been
subjected to peer review. In addition, computer models
of forest systems have been used to evaluate this prob-
lem, with the results subjected to peer review.

Scientific knowledge advances in this way, with sci-
entists continually questioning measurements, making
new measurements, and sometimes coming up with
new and better hypotheses (Science Focus, p. 31). As
a result, good scientists are open to new ideas that have
survived the rigors of the scientific process.

Scientists Use Reasoning,
Imagination, and Creativity
to Learn How Nature Works
Scientists arrive at conclusions, with varying degrees of
certainty, by using two major types of reasoning. In-
ductive reasoning involves using specific observations
and measurements to arrive at a general conclusion or
hypothesis. It is a form of “bottom-up” reasoning that
goes from the specific to the general. For example, sup-
pose we observe that a variety of different objects fall to
the ground when we drop them from various heights.
We can then use inductive reasoning to propose that
all objects fall to the earth’s surface when dropped.

Depending on the number of observations made,
there may be a high degree of certainty in this conclu-
sion. However, what we are really saying is “All objects
that we or other observers have dropped from various
heights have fallen to the earth’s surface.” Although it
is extremely unlikely, we cannot be absolutely sure that
no one will ever drop an object that does not fall to the
earth’s surface.

Deductive reasoning involves using logic to ar-
rive at a specific conclusion based on a generalization
or premise. It is a form of “top-down” reasoning that
goes from the general to the specific. For example,

Generalization or premise: All birds have feathers.

Example: Eagles are birds.

Deductive conclusion: Eagles have feathers.

THINKING ABOUT
The Hubbard Brook Experiment and Scientific
Reasoning

In carrying out and interpreting their experiment,
did Bormann and Likens rely primarily on inductive or
deductive reasoning?

Deductive and inductive reasoning and critical think-
ing skills (pp. 2–3) are important scientific tools. But
scientists also use intuition, imagination, and creativity

CONCEPT 2-1 33

the website at www.learner.org/resources/series209
.html). Each of the 13 videos describes how scientists
working on two different problems related to a certain
subject are learning about how nature works. Also see
Video 2, Thinking Like Scientists, in another Annenberg
series, Teaching High School Science (see the website at
www.learner.org/resources/series126.html).

The Results of Science Can Be
Tentative, Reliable, or Unreliable
A fundamental part of science is testing. Scientists insist
on testing their hypotheses, models, methods, and re-
sults over and over again to establish the reliability of
these scientific tools and the resulting conclusions.

Media news reports often focus on disputes among
scientists over the validity of data, hypotheses, models,
methods, or results (see Science Focus, below). This
helps to reveal differences in the reliability of various

scientific tools and results. Simply put, some science is
more reliable than other science, depending on how
carefully it has been done and on how thoroughly the
hypotheses, models, methods, and results have been
tested.

Sometimes, preliminary results that capture news
headlines are controversial because they have not been
widely tested and accepted by peer review. They are
not yet considered reliable, and can be thought of as
tentative science or frontier science. Some of these
results will be validated and classified as reliable and
some will be discredited and classified as unreliable. At
the frontier stage, it is normal for scientists to disagree
about the meaning and accuracy of data and the va-
lidity of hypotheses and results. This is how scientific
knowledge advances.

By contrast, reliable science consists of data, hy-
potheses, theories, and laws that are widely accepted
by scientists who are considered experts in the field
under study. The results of reliable science are based on

SCIENCE FOCUS

The Scientific Consensus over Global Warming

view. Typically, they question the reliability
of certain data, say we don’t have enough
data to come to reliable conclusions, or
question some of the hypotheses or mod-
els involved. However, in the case of global
warming, they are in a distinct and declining
minority.

Media reports are sometimes confusing
or misleading because they present reliable
science along with a quote from a scientist
in the field who disagrees with the con-
sensus view, or from someone who is not
an expert in the field. This can cause public
distrust of well-established reliable science,
such as that reported by the IPCC, and may
sometimes lead to a belief in ideas that are
not widely accepted by the scientific com-
munity. (See the Guest Essay on environ-
mental reporting by Andrew C. Revkin at
CengageNOW.)

Critical Thinking
Find a newspaper article or other media
report that presents the scientific consensus
view on global warming and then attempts
to balance it with a quote from a scientist
who disagrees with the consensus view. Try
to determine: (a) whether the dissenting
scientist is considered an expert in climate sci-
ence, (b) whether the scientist has published
any peer reviewed papers on the subject, and
(c) what organizations or industries are sup-
porting the dissenting scientist.

climate changes, and project future climate
changes. The IPCC network includes more
than 2,500 climate experts from 70 nations.

Since 1990, the IPCC has published four
major reports summarizing the scientific con-
sensus among these climate experts. In its
2007 report, the IPCC came to three major
conclusions:

• It is very likely (a 90–99% probability) that
the lower atmosphere is getting warmer
and has warmed by about 0.74 C° (1.3 F° )
between 1906 and 2005.

• Based on analysis of past climate data and
use of 19 climate models, it is very likely (a
90–99% probability) that human activities,
led by emissions of carbon dioxide from
burning fossil fuels, have been the main
cause of the observed atmospheric warm-
ing during the past 50 years.

• It is very likely that the earth’s mean
surface temperature will increase by
about 3 C° (5.4 F° ) between 2005 and
2100, unless we make drastic cuts in
greenhouse gas emissions from power
plants, factories, and cars that burn fossil
fuels.

This scientific consensus among most of
the world’s climate experts is currently con-
sidered the most reliable science we have on
this subject.

As always, there are individual scientists
who disagree with the scientific consensus

ased on measurements and mod-
els, it is clear that carbon dioxide

and other gases in the atmosphere play a
major role in determining the temperature of
the atmosphere through a natural warming
process called the natural greenhouse effect.
Without the presence of these greenhouse
gases in the atmosphere, the earth would be
too cold for most life as we know it to exist,
and you would not be reading these words.
The earth’s natural greenhouse effect is one
of the most widely accepted theories in the
atmospheric sciences and is an example of
reliable science.

Since 1980, many climate scientists have
been focusing their studies on three major
questions:

• How much has the earth’s atmosphere
warmed during the past 50 years?

• How much of the warming is the result
of human activities such as burning oil,
gas, and coal and clearing forests, which
add carbon dioxide and other greenhouse
gases to the atmosphere?

• How much is the atmosphere likely to
warm in the future and how might this
affect the climate of different parts of the
world?

To help clarify these issues, in 1988, the
United Nations and the World Meteorological
Organization established the Intergovernmen-
tal Panel on Climate Change (IPCC) to study
how the climate system works, document past

B

34 CHAPTER 2 Science, Matter, Energy, and Systems

the self-correcting process of testing, open peer review,
reproducibility, and debate. New evidence and better
hypotheses (Science Focus, p. 31) may discredit or alter
tried and accepted views and even result in paradigm
shifts. But unless that happens, those views are consid-
ered to be the results of reliable science.

Scientific hypotheses and results that are presented
as reliable without having undergone the rigors of peer
review, or that have been discarded as a result of peer
review, are considered to be unreliable science. Here
are some critical thinking questions you can use to un-
cover unreliable science:

• Was the experiment well designed? Did it involve
enough testing? Did it involve a control group?
(Core Case Study)

• Have the data supporting the proposed
hypotheses been verified? Have the results
been reproduced by other scientists?

• Do the conclusions and hypotheses follow logically
from the data?

• Are the investigators unbiased in their inter-
pretations of the results? Are they free of a hid-

den agenda? Were they funded by an unbiased
source?

• Have the conclusions been verified by impartial
peer review?

• Are the conclusions of …