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Foundations of Environmental Science

WHAT IS ENVIRONMENTAL SCIENCE?

Environmental science is the study of the impacts of human activities on environmental systems. These human

activities include large-scale actions, such as clearing land for agriculture, fishing the oceans for food, mining

the land for minerals and fuels, and changing our planet’s climate through the emissions of decades worth

of greenhouse gases. These activities also include everyday individual actions, like driving a car to the store,

turning on your lights, and choosing whether to use plastic, paper, or reusable containers.

The environment is the sum total of all the conditions and living and nonliving factors that surround an

organism, including the others of its kind, its food sources (prey), any predators that may feed on it, the weather,

the landscape, and any other aspect of the world in which it lives. A local environment is the area immediately

surrounding an organism or person; an environment, however, can encompass an area of greater scale. An

environment can be as small as a pond or as large as a complete mountain range or an ocean. The immensely

complicated global environment is the sum of all the aspects of the Earth. Environmental science is interdisciplinary, covering

many aspects of biology, earth and atmospheric sciences,

fundamental principles of chemistry and physics,

human population dynamics, and biological and natural

resources. Environmental science is a science-based

discipline, meaning it is based on the scientific method

that includes observations, hypothesis testing, field and

laboratory research, and other practices, which we will

discuss later in this section.

One way of studying the environment is to study

its different systems and the ways they interact. A

system is a set of living and/or nonliving components

connected in such a way that changes in one part of the system affect other parts. A particular system can usually

be isolated and studied apart from other systems. The Earth is a system and so is an ant colony, a lake ecosystem,

and a farm. Because systems are so important to an understanding of the environment, we will devote much of

Section I to looking at environmental systems in detail. But first, we will explore how environmental scientists

monitor human impacts on environmental systems.

ENVIRONMENTAL INDICATORS

If we wanted to determine whether a person is healthy, we might measure body temperature, heart rate, blood

pressure, and respiration rate. If something is amiss with one or more of these indicators, it is usually a reliable

signal that something is wrong in the human body. What indicators can we use to determine the vitality of the

planet? Evaluating the health of the Earth, or even a specific environment, is much more complex, and we cannot

measure every single component. However, as with individuals, assessing certain key aspects of the environment

gives us an indication of its health.

An environmental indicator is a measure that reflects the environmental health of a system. For example, the

amount of new growth on trees might be used to indicate the state of a forest. Unfortunately, at present, there

is no single indicator that effectively assesses the whole planet. In addition, the same environmental indicator

can tell a very different story depending on when or where the measurement is taken. Measuring new growth

on trees over the summer will yield very different data than the same measurements taken over the winter.

Likewise, some parts of the world are experiencing declines in annual precipitation, while others are seeing

increases. Rates of change are also important when considering environmental indicators. This is analogous to

taking a person’s temperature multiple times during a day to see if it is stable and, if not, how fast it is changing.

The importance of a measurement may be best understood in the context of a pattern of measurements: Is

growth increasing? Decreasing? Are the changes global? Or regional?

The table below lists a number of commonly used environmental indicators; some are appropriate for studying

small-scale situations, while others are global. On a global scale, some of the most common indicators are the

size of the human population, food production, species diversity, global temperature, and the concentration of

atmospheric CO2

. Each has advantages and limitations. The differing opinions about the status of the planet that

you might observe among scientists, the media, and the general public depend in part on what indicators and

which time periods are used to make the assessment.

Some Common Environmental Indicators

Environmental Indicator Unit of Measure

Human population individuals

Ecological footprint hectares of land

The amount of new growth on trees can be used as an

indicator of the health of a forest.

Image Source: Smithsonian Environmental Research Center

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Per capita food production kg of grain/person

Total food production kg of grain/hectare of land

Carbon dioxide concentration in air (ppm)

Global temperature degrees Centigrade

Sea level change mm

Annual precipitation mm

Species diversity number of species per functional group

Fish consumption advisories present or absent; or number of fish allowed per week

Ambient water quality (toxics) concentration

Ambient water quality

(conventional)

concentration; presence or absence of bacteria

Atmospheric deposition rates quantity per unit area per time

Fish catch or harvest weight of fish per annum or weight of fish per effort expended

Extinction rate Number of mammal species per 10,000 species per 100 years

Habitat loss rate land cleared or “lost” per year

Infant mortality rate Number of deaths of infants under age 1 per 1,000 live births

Life expectancy Average number of years a newborn infant can be expected to

live under current conditions.

This long list of indicators can be grouped into the six indicators on which we will focus:

6 Biological diversity

6 Human population growth

6 Food Production

6 Resource consumption

6 Global temperature and atmospheric greenhouse gas levels

6 Pollution levels

Biological Diversity

Overall biological diversity describes the diversity of genes, species, habitats, and ecosystems on Earth. The

number of species on Earth, and whether that number is increasing or decreasing, can help us measure the

biological status of the planet. A species is defined as a group of organisms that is distinct from other groups

in morphology (body type), physiology, or biochemical properties. Individuals within a species can breed and

produce viable offspring. There are approximately 1.8 million “known”—that is, identified and catalogued—

species on Earth today. The actual number of species, while highly debated, is likely to be more than ten times

that number because most species, especially microbial species, have not yet been identified or catalogued.

Species extinction is a natural part of the process of life on Earth. Roughly 99.9 percent of the species that have

ever lived on Earth are now extinct. Though it is difficult to determine what the “background” rate of extinction

was before people played a role, estimates have been made using “quiet” periods in the geologic record (that is,

time periods with no massive environmental or biological upheaval). “Background” extinction rates are now

estimated to be two mammal extinctions per 10,000 species per one hundred years.1

From recent studies, it is clear that human beings have greatly accelerated species extinction rates to up to a

hundred times higher than background. The loss and degradation of habitat by human beings is considered the

major cause of species extinction today. Attempts to estimate species loss by relating it to the area of land that has

been altered by human activity suggest that as many as 40,000 species per year may be going extinct. Gains have

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been made in saving certain species, particularly those

that attract the attention of people, such as the American

bison, peregrine falcon, bald eagle, and California

condor, but overall, the number of species on Earth is

declining at a rate to rival past mass extinction events,

such as the extinction of the dinosaurs. (See Figure 2.)

Species such as the Bengal tiger, the snow leopard,

and the West Indian Manatee are endangered and

may go extinct if present trends are not reversed. And

the loss of species of particular importance within an

ecosystem—keystone species—can cause a cascade of

extinction of species dependent on them, resulting in

harm to or loss of entire ecosystems. The overall rate

at which species go extinct on Earth not only tells us

how biological diversity on Earth is decreasing but is an

important indicator of the state of land, water, and air

on the planet. If we use species diversity as an indicator

of environmental quality, we must conclude that the situation is getting worse and is not sustainable.

Snow leopards are endangered and may go extinct if present

trends are not reversed.

FIGURE 2

The five past mass extinctions events. Current human impacts may be causing another such extinction event.

Source: National Geographic

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World Human Population

According to the United Nations, the global human

population reached eight billion people in November

2022. Roughly 378,000 infants are born and 148,000

people die each day resulting in 230,000 new inhabitants

on Earth each day, or almost a million new people

on Earth every four days. Until the 1960s, the world

population was undergoing exponential growth, which

is growth that increases as a percentage of the numbers

already in the population. While human population

growth has slowed and is no longer exponential, world

population size will nonetheless continue to increase

for at least fifty to a hundred years. The United Nations

projects that world population will level off somewhere

between 8 and 12 billion people by the year 2150.

The growing human population on the Earth creates a greater

demand on Earth’s finite resources.

FIGURE 3

Human population size estimates from 1960 to today and a projection to 2100.

Credit: Katie Peek; Data Source: World Population Prospects 2022, United Nations Population Division. Image Source: Scientific American

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Can the Earth sustain so many people? If we use

the human population on Earth as an environmental

indicator, it is encouraging that the rate of population

growth has slowed, but we still should be concerned that

the total population will continue to increase for at least

the next fifty years and possibly longer. The additional

people on Earth will create a greater demand on Earth’s

finite resources, including energy, food, water, and land

and—unless we dramatically change our industrial

society—will produce more pollution and waste for the

foreseeable future.

Food Production

Food grains such as wheat, corn and rice provide more

than half the calories eaten by humans. Worldwide

grain production is a result of the quality of soils,

climatic conditions, land area under cultivation, human labor, energy, and water expended on growing food,

and other influences. Therefore, an increase or decrease in the amount of grain grown worldwide for human

consumption is an environmental indicator.

The term “intensity” in the context of agriculture refers to how much food is grown per hectare or acre of land.

The agricultural practices used to produce food vary widely from high-intensity monoculture (one crop) to low-

intensity polyculture (many crops). The yield (tons of grain per unit area of land) from a given area can indicate

both the intensity of agricultural methods and the quality of the land. High-intensity agricultural practices often

lead to soil erosion, runoff of fertilizers and animal wastes into waterways, and buildup of pesticides, all of

which reduce the quality of the land. As land becomes more degraded, its yield begins to decline.

Resource Consumption

Sustainable use occurs when present-day consumption of resources allows an adequate supply to remain for

future generations. Although there is no single way to determine the sustainability of a given society, the rapid

depletion of a resource is a clear indication that its use is not sustainable. The human consumption of resources,

energy, and land all contribute to a decrease in the sustainability of not only human activities, but of the natural

ecosystem on which all species, including humans, depend. However, many of the same human activities that

cause adverse impacts can improve the overall quality of life among human beings. Somehow, there must be a

balance between utilizing resources to improve life today, saving them for future generations, and protecting the

natural environment.

Obviously, the larger the population, the greater the consumption of resources. So, more people, regardless of their

lifestyle or where they live, means a greater environmental impact. But resource use per person, which varies from

region to region and by type of economy and country, is also critical. Patterns of resource consumption differ vastly

in different parts of the world. For example, a country where most people live in relatively small houses will have

less impact than a country where most people live in large houses, all other factors being equal. And the way people

heat and light their homes (with kerosene, candles, or electricity, for example), will produce different environmental

impacts.

For some resources, a very small portion of the world’s population may be responsible for most of the consumption.

The United Nations Development Program reports that the twenty percent of the people in the world who live in

developed countries consume forty-five percent of all meat and fish, fifty-eight percent of total energy, and eighty-

four percent of all paper, and own eighty-seven percent of the world’s automobiles and trucks. The poorest twenty

percent of the people in the world consume or use five percent or less of each of these items. Thus, while it is true

that a larger population translates to more consumption, more pollution, and more environmental impact, the way

Combustion of fossil fuel is the primary human activity that

produces carbon dioxide.

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people live is also an important predictor of environmental impact.

Global Temperatures and Greenhouse Gases

The temperature of the Earth is regulated by many factors, including incoming solar radiation, absorbed solar

heat emanating from the Earth, the surface area of ice caps and ocean, and the concentration of certain gases

that surround the Earth. These gases trap heat around the Earth, warming the atmosphere—much like the glass

around a greenhouse traps heat—so they are sometimes called greenhouse gases. Carbon dioxide and methane

are two greenhouse gases that are present in the atmosphere due to both natural processes and human activities.

Combustion of fossil fuel is the primary human activity that produces carbon dioxide.

For the past 130 years, global temperatures have fluctuated but show an overall increase. (See Figure 4.) During

the same period, atmospheric carbon dioxide and methane concentrations also increased steadily. (See Figure 5.)

Virtually all scientists agree that the increase in carbon dioxide during the last two centuries is anthropogenic (a

result of human activity), coming especially from the combustion of fossil fuels and destruction of forests. ENVIRONMENTAL SCIENCE CASE STUDY: Measuring Greenhouse

Gases in Ice

As we have seen, tracking changes in the concentration of gases over time helps us assess the state of

Earth’s atmosphere. However, one of the biggest challenges in environmental science is determining the

concentration of chemical elements that existed on Earth in ancient times. For example, scientists report

that over the past 160,000 years, global temperatures and atmospheric concentrations of carbon dioxide and

methane have fluctuated frequently. (See Figure 6.) But how do we know it? Ice gives us the answer.

Ice sheets and glaciers in Greenland and Antarctica contain layers of snow and ice. As new snow falls,

the old snow is buried and slowly turns to ice. Annual layers of snow/ice, which are sometimes visible

to the naked eye like the annual rings in trees, can accumulate to thousands of meters in thickness. Each

layer contains bubbles of trapped gases (including human-produced air pollutants in more recent layers)

in concentrations that reflect their atmospheric concentrations at the time the layer was sealed off from the

atmosphere.

Researchers interested in estimating atmospheric concentrations of elements and gases from thousands of

years ago must drill into the layers of buried ice and carefully remove an ice core. The ice core is kept frozen

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and brought to a laboratory, where a researcher assigns a

date to each annual layer corresponding to the year when

it was deposited on the surface as snowfall. The ice for a

given year is then removed in a slice, and air bubbles in

the ice are analyzed for their chemical content. Carbon

dioxide concentrations can be measured directly from the

air released as the ice melts. Relative temperature (e.g.,

warmer or cooler than today) can be inferred by the ratios

of different oxygen atoms of varying masses (oxygen

isotopes) that are released from the air bubbles.

FIGURE 6

800,000 years of ice core records for atmospheric carbon dioxide and temperature change in Antarctica. The last 160,000

years (right side of graph) show variation but an overall decline in both, until recently.

Source: British Antarctic Survey

Researchers interested in estimating atmospheric

concentrations of elements and gases from thousands

of years ago can drill into the layers of buried ice and

carefully remove an ice core.

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Air and Water Pollution

The metal lead (chemical symbol Pb) is very useful

because it is soft, malleable (can be shaped with just

a hammer), and resists corrosion, but it also impairs

human central nervous system function and is toxic

to most plants and animals. Developing brains (in

fetuses and children) are particularly sensitive to

lead. The amount of lead in the atmosphere, water,

soils, and plants and animals is an indicator of the

amount of pollution that has been introduced into the

natural environment and an indirect indicator of the

amount of harm that may have occurred from human

manipulation of the natural environment.

From five thousand years ago until fairly recently,

the global production, or mining, of lead has increased. In the early years of lead production, relatively small

amounts of the metal were liberated to the atmosphere during separation and refinement of the lead from other

metals. Changes in refining techniques that came with the Industrial Revolution led to greater releases into the

atmosphere. In addition, coal and oil contain small amounts of lead, and as more of these fuels were burned,

more lead was released to the atmosphere. Lead was also used as an additive to gasoline to improve engine

performance of the automobile engine. As the automobile became more widely used throughout the world, the

use of lead increased as well, and much of the lead production and emissions in the twentieth century were a

result of this use.

Beginning in 1975, clean air legislation required that new cars sold in the United States use gasoline without

lead, and gradually the same requirements were imposed in many other parts of the world. This switch from

leaded to unleaded gasoline is primarily responsible for the decreases in lead emissions. While there is still a

great deal of toxic lead produced and emitted throughout the world, the substantial decline in lead emissions is

certainly a positive step. If we use global lead emissions as an environmental indicator, we should conclude that

the situation is improving. However, this “easy fix” simply stopped adding a harmful element to gasoline. There

are still significant quantities of lead emitted in coal, oil and even gasoline that we call “unleaded.”

Lead was also a major ingredient in paint. Although houses built after 1960 tend to have much lower

concentrations of lead in paint, there are many houses built before 1960 that are covered with peeling paint that

can be composed of 50 percent lead. This paint can add to the indoor air concentration of lead, and when it

peels, it is sometimes ingested by young children. While not part of the atmospheric measurement of lead, this is

another important pathway of lead pollution to human beings.

However, the major source of lead contamination in the U.S. today is our drinking water—particularly from

lead pipes and other plumbing material that will corrode over time, especially if the water is highly acidic. While

many of these lead pipes have been replaced with safer materials, lead plumbing fixtures are still prevalent,

especially in lower income communities. Lead is but one example of how human activities contaminate our air,

water, and land.

THE SCIENTIFIC METHOD

The scientific information that we will cover—including the information just presented on environmental

indicators—has been collected, analyzed, and synthesized through a process called the scientific method.

The scientific method is an objective way to explore the natural world, draw inferences from it, and predict

the outcome of certain events, processes, or alterations. This method is used by scientists in many parts of the

world and is the generally accepted way to conduct science. A simple experiment conducted by a first-year

college student follows the same principles as a large, multi-million-dollar experiment conducted by a group of

The major source of lead contamination in the U.S.

is drinking water.

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investigators at a research institution.

FIGURE 7

The process of scientific inquiry.

Source: University of California Berkeley, Understanding Science 101

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Let’s look at each of the major steps in the scientific method.

6 Observe the natural world, with or without human interference, and ask questions about those

observations.

6 Generate a hypothesis. Make a general statement about the organisms or processes under observation

that could answer the questions posed. The hypothesis must be testable and falsifiable—that is, the

researcher must be able to determine whether it is incorrect.

6 Based on existing information, make a preliminary determination of whether the hypothesis is true

or false. Based on the hypothesis, the observations, and questions, it is possible to make an informed

projection about the hypothesis.

6 Test the hypothesis with an experiment. Hypotheses should make predictions about the world. Determine

whether the hypothesis is false using an observational experiment or a manipulation experiment, testing

these predictions.

An observational experiment is conducted by observing phenomena in the natural world without any

interference by the researcher. When a wildlife biologist observes hundreds of interactions between moose

and wolves, they are conducting an observational experiment. A manipulation experiment is conducted by

changing some aspect—the experimental variable—of a natural or controlled environment. The elements

being studied are divided into two groups: the experimental and the control. The experimental group is the one

that is manipulated; the control group is left undisturbed for comparison. These two groups should be treated

identically in every way, with the exception of the one variable that is being tested in the experimental group.

It is important to have a large enough sample size—the number of individuals tested or samples collected—so

that the data gathered are representative of the entire population. For example, if you are testing the effect of a

pollutant on the growth rate of a plant species, you would want to test the effect on ten or a hundred plants, not

just one or two.

6 Accept, revise, or reject the hypothesis. Reconcile any differences between the predictions and the

results. If findings differ from the hypothesis, the hypothesis is modified and retested. This may continue

until there is general agreement between the hypothesis and the experiment.

6 Report findings to others. An essential part of the scientific method is to inform others of what has been

done. Reporting to others can take place through peer-reviewed written communications in publications

or formal presentations of the results at conferences and scientific meetings.

6 Replicate the experiment. For any given hypothesis, the process described above is generally repeated

over and over by different scientists. When a given hypothesis is tested and accepted by many

investigators, it may become a scientific finding. If a hypothesis is widely accepted, it becomes a theory.

If a theory is widely accepted and appears to apply universally without any exceptions, it is called a

universal law. An example of a universal law is the First Law of Thermodynamics, which says that

energy cannot be created or destroyed, it simply changes form. Even though we use the term “law,”

no scientific finding is considered definitively proven, because there is always the possibility of new

information that would change the conclusions. Therefore, scientific laws are considered not disproven.

An Illustration of the Scientific Method

Let’s consider a hypothetical example to see how the scientific method is applied. Scientists have observed that

species diversity, one of our environmental indicators, is affected by the alteration of habitat.

An environmental scientist in an area of Southern California that is being developed for housing poses the

question, “What will happen to the diversity of species of small mammals and shrubs if the size of a natural

area i

Energy (food)

Energy

Water Water

Energy lost as heat

Isopods/Amphipods

compete for energy

Crayfish prey upon

Isopods/Amphipods (use

energy for growth of

individuals and

population)

Energy used to grow

larger individuals

& populations

(Energy from

Isopods/Amphipods

that is not used by Crayysh

(Energy not captured

by Isopods/Amphipods)

FIGURE 8

A diagram of a simple cave system showing the flow of water and energy through a system. The study of all systems starts with a

similar modeling of the inputs and outputs.

Energy Matter

Input:

Solar

radiation

Outputs:

Heat

energy,

reeected

light

No (major)

inputs

No (major)

outputs

(a) Open system (b) Closed system

FIGURE 9

Open and closed systems. (a) Earth is an open system with respect to energy. Solar radiation enters the Earth system, and

energy leaves it in the form of heat and reflected light. (b) However, Earth is essentially a closed system with respect to matter

because very little matter enters or leaves the Earth system. The white arrows indicate the cycling of energy and matter.

Source: Friedland, Andrew and Rick Relyea, Essentials of Environmental Science 2nd ed. W.H. Freeman, New York (2016).

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The Human Component of Environmental Systems

Because environmental systems almost invariably include people or run up against human influence in one form

or another, many areas of human endeavor, some of which are not scientific at all, are important to a systems-

based understanding of the environment. Some of the most important areas that we will touch on are:

6 Economics

6 Social structures and institutions, including various levels of government

6 Law

6 Policy

6 Environmental advocacy and action

For example, new scientific data on global warming will affect new policies or laws related to greenhouse gas

production, as well as ways to adapt to a changing climate.

SYSTEM ANALYSIS: DETERMINING HOW MATTER AND ENERGY FLOW IN

THE ENVIRONMENT

Inputs, Outputs, and Flux

People who examine systems often conduct a system analysis to determine what goes in, what comes out, and

what has changed within a given system. This type of analysis is very similar to the kind of analysis you might

perform on your personal checking account to learn your financial status. In your checking account, you start

with a sum of money called your balance. Systems analysts call that balance a pool. If you deposit money into

your checking account, you are adding an input. You also have expenditures—you write checks against your

checking account balance or withdraw money from your account. Systems analysts call this an output.

In order to determine your financial status, you start with your balance at the beginning of a month, add inputs

(deposits), and subtract outputs (checks and withdrawals). This gives you your checkbook balance at the end of

the month, or the change in the pool of money. Systems analysts call that change a flux. If you quantify your

income in terms of so many dollars per month, you are describing a flux rate, a flow per unit of time.

FIGURE 10

If inputs are greater than outputs, then flux is positive.

INPUTS – OUTPUTS = TOTAL FLUX

The same kind of analysis can be done for water in a bucket, pollutants in the atmosphere, or nutrients in the

ocean. It tells an environmental scientist if the size of the pool is increasing, decreasing, or staying the same.

Because it was designed to be done for materials that have mass, it is often called a mass balance analysis—an

accounting of the inputs and outputs to determine the fluxes in a given system. All types of balance analyses,

whether they be mass, energy, or monetary, can be represented as:

Net Flux = Inputs − Outputs

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Steady State

The most important aspect of conducting a mass, energy, or monetary balance analysis is learning if your system

is in steady state—that is, if input equals output and the size of the pool does not change over time. The first step

is to determine the siz

the Mono Lake story.

The Natural Water System

The first system we’ll consider is the natural water system

that, analogous to the simplified bucket examples just

discussed, involves the inflow (input) and outflow (output)

of water. Mono Lake is called a terminal lake because

under “normal conditions” water flows ifor the birds but an

ultimate decline in the prey population from over-predation, which was followed in turn by a decline in

the bird population. Secondly, as the lake level went down, alkaline dust was exposed, leading to vast dust

storms affecting bird and other nearby wildlife populations.

Lowered water levels had another, less obvious, but even more critical effect on Mono Lake’s environmental

system. The salts that were once diluted by the lake’s original large volume were now concentrated in a

smaller volume of water, leading to a dramatic increase in salinity. The algae, shrimp, and other Mono Lake

residents could survive with the natural salt concentrations, but this drastic and rapid increase in salts proved

difficult for them. The most significant effect was on algae, which are the base of the food chain. Higher

salinity slows the uptake of nitrogen from the decayed animals and their excretions. Since nitrogen is a

critical element for growth, slower nitrogen uptake led to slower growth of the algae population and less food

for the flies and shrimp and thus eventually for the birds. By the early 1980s, Mono Lake and the populations

that depended upon it were dying.

The Mono Lake story up to this point is a real-world example of input, output, and steady state in a mass

balance system. Before 1941, the water system of Mono Lake was in an approximate steady state with the

outflow of water from evaporation more or less matching the inflow from streams. The salt-balance system

FIGURE 13

Mono Lake’s input/output water system.

Source: Modified image from Mono Lake Committee

Evaporation

is the only

outflow of

water from

the lake

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was not in a steady state and was slowly moving toward increased salt concentrations. The food chain had

been able to compensate (at least over recent ecological history) for the increasing salinity but was not able to

adapt to the rapid changes resulting from the addition of the new, human water use system.

Since the early 1980s, the history of Mono Lake is an example of the interaction of environmental

science with other human components mentioned earlier—environmental policy, environmental law, and

environmental advocacy. The effects of Los Angeles’s water use on the Mono Lake environmental systems

was first noticed by ecologists and environmental scientists, who provided information to environmental

advocates and lawyers to bring a series of lawsuits and legislative proposals seeking to stop water

withdrawals.

At the same time, environmental advocates attempted to change the water-use policy through a public

campaign advertising both the beauty and the fragility of Mono Lake. These early attempts to use

environmental science and advocacy to inform environmental law and policy failed. However, in 1983, the

California Supreme Court ruled that it was the duty of the California government to protect the environment

of Mono Lake. This court decision led to new laws requiring federal and state agencies to better manage

Mono Lake. The result is the current reduction in water withdrawals and increase in the lake’s water level.

In 2023, the water level further increased from snowmelt in the Sierra Nevada mountains and the resulting

increase in in-flows from tributaries. The final answer to preventing the death of Mono Lake proved simple:

increase inflow and decrease the diversion of water to Los Angeles until the bucket filled back up.

Mean Residence Time

The Mono Lake example demonstrates that even a basic understanding of input–output system dynamics can

be useful in solving some environmental problems. However, in many situations, for example if we want to

determine how long it will take for a pollutant to be flushed from a lake, it is valuable to know the rate at which a

pool turns over—that is, how long it takes for the contents of the pool to change.

If a pool is in steady state, we can calculate a mean residence time (MRT), which is the average time that a

portion of the pool remains in the system. The mean residence time is the pool divided by the input or the output:

MRT = (pool)/(flux in or out)

Note that we can calculate the mean residence time using either the flux in or the flux out. Because the system is

in steady state, the flux in and out are equal, and so either flux will give the same answer. For example, consider

the bucket discussed earlier, with a pool of ten liters, a flux in of one liter per minute, and a flux out of one liter

per minute:

MRT = (10 liters ) / (1 liter / minute)

MRT = 10 minutes

The MRT value tells you that an average quantity of water—say, a milliliter—will remain in the bucket for ten

minutes before being flushed out. In fact, some water may remain for a longer time, and some may remain for a

shorter time, but the mean residence time is an average.

Though we determined the mean residence time for water, if we have information on the pool and flux of

something dissolved in water, such as a particular pollutant, we can determine the mean residence time for

that substance as well. We can also calculate residence times for air pollutants. In that case, MRT is usually

defined as the period that an average molecule will remain chemically active in the atmosphere. Residence times

(also referred to as atmospheric lifetime) have been estimated for several gases known to be involved in the

greenhouse effect and in the depletion of the ozone layer:

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Gas Residence Time (years)4

Carbon dioxide 100*

Methane 11.8

Nitrous oxides 109

Chlorofluorocarbons 100

Hydrofluorocarbons 222

It is important to note that the estimated residence time

for carbon dioxide is a particularly rough estimate since

this gas is not destroyed in the atmosphere but is cycled

through different parts of the global carbon cycle at

different rates—ranging from a few years to thousands

of years. (We will learn more about the global carbon

cycle in Sections II and IV of this resource guide.) As

we discuss the impacts of human activities on global

climate change in later sections, these values will take

on important significance.

Accumulation and Depletion

It is important to remember that mean residence time

is valid only if the system is in steady state. If a system

is not in steady state, we may want to determine the rate at which it is accumulating or losing material. For

example, if a pollutant is accumulating in a drinking water reservoir, it may be valuable to know the time when

pollutant concentrations will become toxic to organisms in the reservoir or to humans drinking the water in the

reservoir. We can calculate accumulation or depletion rates by using the formula for net flux:

Net Flux = Inputs – Outputs

For example, assume that a pollutant is slowly decreasing in concentration in the water because it is interacting

with the sediment that lines the bottom of the reservoir. A calculation of the change in the system can indicate

when that water will be safe to drink. Suppose the reservoir holds 1,000,000 liters of water, and the pollutant is

at a concentration of 10 mg/L. Assume no additional pollutant is entering the reservoir, and that 1,000 mg/day

interacts with the sediment. We can calculate:

Flux = 0 – 1,000 mg/day

Flux = – 1,000 mg/day

At the start, the reservoir holds 10 mg/L X 1,000,000 liters = 10,000,000 mg of the pollutant.

Losing 1,000 mg/day, the reservoir will contain no pollutant in 10,000 days. In other words, it will take 10,000

/365 days/yr = 27.4 years before the pollutant is totally gone from the reservoir.

Feedbacks

So far, we have presented fairly simple systems with easily defined inputs and outputs. Any change in the system

involves simply increasing or decreasing the inputs or outputs. Even Mono Lake, a major environmental system,

could be described as a simple input/output system. For other environmental systems, the important factors are

not the input and output themselves, but the mechanisms that control, or regulate, these flows. In these regulatory

mechanisms, a change in the system either leads to further change or returns the system to its original state.

Consider your own or your parents’ behavior with respect to a bank account. If you notice that your pool of

money (your checkbook balance) is decreasing, you may spend less money to reduce the flux of dollars out of

your checkbook, or you may work more hours to increase the flux of dollars into your checkbook. Essentially,

Warmer temperatures at the Earth’s surface lead to greater

evaporation from oceans and lakes. The additional moisture

in the atmosphere from evaporation enhances the layer of

heat-trapping gases, including water vapor, that cover the

Earth, which makes the Earth warmer, which leads to greater

evaporation, and more warming, creating a positive

feedback loop.

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2024–2025 Science Resource Guide

you alter your behavior in one or more ways in order to change your cash flow situation. These changes in

behaviors, called feedbacks, are adjustments made by a system in response to behavior or events.

Balancing your checkbook is an example of a negative feedback loop, in which the behavior always brings the

system variable—in this case, your money—back to a starting point. By contrast, a gambler, who bets more and

more money as they begin losing, will not return to the starting point—the loss of money will cause increased

betting and more losses, until all the money is gone. This is an example of a positive feedback loop, in which the

system variable is continuously moved away from the stable point—what we often call a vicious cycle.

FIGURE 14

Negative and positive feedback loops.

Source: Friedland, Andrew and Rick Relyea, Essentials of Environmental Science, 2nd ed. W.H. Freeman, New York (2016).

Reduced

surface

area

Less

evaporation

Level

rises

Level

drops

Lake level

Population

increase

(a) Negative feedback loop (b) Positive feedback loop

Births

births

Population

increase

Feedback systems are found throughout the environment. One major feedback system that is of great importance

to environmental scientists, policy makers, and citizens is the Earth’s heating system feedback loop. In general,

warmer temperatures at the Earth’s surface lead to greater evaporation from oceans and lakes. The additional

moisture in the atmosphere from evaporation enhances the layer of heat-trapping gases, including water vapor,

that cover the Earth. This helps to make the Earth warmer, which leads to greater evaporation, and more

warming, and the cycle continues.

In the absence of other factors that compensate for or balance the warming, this positive feedback loop could

continue making temperatures warmer and warmer, driving the system away from the starting point. However,

more evaporation also leads to more cloud cover, which would reflect more incident sunlight and possibly lower

temperatures, resulting in a negative feedback loop. It is unknown whether or not the sum of these loops would

lead to an increase or decrease in temperature.

The balance in many environmental systems is dependent on the smooth operation of feedback loops. Sometimes

conflicting factors lead to a breakdown in the negative feedback loop and send the environmental system away

from its set point, the stable value for the parameter under examination. This is particularly true when the

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2024–2025 Science Resource Guide

system involves a conflict between ecological control factors regulating a natural resource, such as a commercial

fish or an energy source, and economic and social factors driving human use of such resources. As you study the

exploitation of natural resources, both living and nonliving, try to determine what factors may be disrupting the

negative feedback loop of those systems.

Overshoot

One last system dynamics concept to consider in both positive and negative feedback systems is the time between

when a signal is generated and when it is received and responded to. Consider the bank account example. As soon

as you notice that your balance is steadily decreasing, you might alter your spending habits or try to make some

more money. But what if you don’t have an app that provides you with a continuous update of your account? This

delay in receiving the signal might mean that you would keep overspending and exceed your intended balance.

Exceeding the stable set point of a system is known as overshoot. In the natural world, many systems experience

delays in the transmittance of information that lead to overshoot. Overshoot is an important part of human and

nonhuman population systems. When a population’s birth rate is high, the factors controlling population growth

(disease, reduced fertility, etc.) cannot compensate fast enough, and the population will grow past the maximum

number of individuals that can be supported by its environment, known as the carrying capacity. The result of

such an overshoot will usually be a dramatic population crash from disease or starvation.

FIGURE 15

Because of a slow response to a signal, an action continues long after it should. This is known as overshoot.

Source: Paul Chefurka, “Population: The Elephant in the Room.”

Carrying Capacity Overshoot

Degraded

Carrying

Capacity

Time

tio

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2024–2025 Science Resource Guide

Regulating Population Systems

Environmental scientists define a population as a group

of individuals of a single species. We will discuss

populations in detail in Section II, but for the purpose

of illustrating feedback systems, we need to introduce

some basic population concepts here.

The size of any population is controlled by two inputs—

the number of births and the amount of immigration—

and two outputs—the number of deaths and the amount

of emigration (individuals leaving the population).

Net Population Change = Input (Births + Immigration)

– Output (Deaths + Emigration)

For most—though not all—populations, births and

deaths greatly outnumber immigration and emigration,

so it is the former two “flows” that we will concentrate

on. Environmental scientists usually find it easy to

estimate birth rates and death rates. What is more difficult, and more interesting, is determining how these two

flows are regulated.

Environmental scientists study both single population systems and systems of interacting populations. In

both cases, the size of any one population can be regulated, through various feedbacks, by abiotic (nonliving)

components of the environment and by populations of other organisms (the biotic components of the environment).

For example, as a deer population increases in size, the amount of food available for each individual will probably

decrease. Less food means less energy for females to put into reproduction, resulting in fewer births, or less food for

newly born fawns—a negative feedback.

In more complex systems containing many interacting populations, one population may be regulated by the size

of another. A deer population may have enough food to fuel an increase in population size, but it may live in an

area with many wolves, which prey upon the deer. In most predator-prey systems, such as wolf-deer systems, the

amount of predation will increase as the number of prey increases (they become easier to find and to hunt). This

negative feedback cycle will drive the deer population back to its starting point.

Let’s now look in more detail at three different real-world examples of environmental systems that are impacted

by human activity and studied by environmental scientists.

In most predator-prey systems, such as wolf-deer systems,

the amount of predation will increase as the number of prey

increases. This negative feedback cycle will drive the prey

population back to its starting point.

ENVIRONMENTAL SCIENCE CASE STUDY: Humans and Elephants in

Africa—Feedback and Regulation in Interacting Population Systems

Elephants are found throughout most of central and southern Africa. Overall, the African elephant

population is declining, most notably in East Africa where few elephants are found outside of protected

nature reserves. There are two main reasons for this decline: the loss of habitat resulting from the conversion

of land to agricultural use and the poaching of elephants for their ivory. Both factors are related to the rapid

growth of the human population in Africa—a growing, mostly poor, population that needs food and money

(from the sale of ivory) to survive. The decline of the elephant population was the main motivation for the

1989 CITES (Convention on International Trade in Endangered Species) ban on the ivory trade. However, the

illegal trade in ivory continues, as does the use of former elephant habitat for farming.

Foundations of Environmental Science

What is Environmental Science?

Study of impacts of human activities on environmental systems.
Includes large-scale actions (e.g., clearing land, fishing, mining, climate change) and everyday actions (e.g., driving, electricity use, waste disposal).

Environment Definition

The sum of all living and nonliving conditions surrounding an organism (e.g., food sources, predators, weather, landscape).
Can be local (pond) or large scale (mountain range, global).

Interdisciplinary Nature

Covers biology, earth sciences, chemistry, physics, human dynamics, and resource management.
Science-based discipline utilizing scientific methods (observations, hypothesis testing, research).

Environmental Systems

A system consists of connected living and nonliving components.
Changes in one part affect others. Study systems like Earth, ecosystems, and farms.

Environmental Indicators

Measures reflecting environmental health; similar to health indicators for humans.
Cannot measure all components; specific indicators indicate health status.

Examples of Environmental Indicators

Size of human population, food production, species diversity, global temperature, atmospheric CO2 concentration.
Must assess measurements within context, including time and location.

Common Indicators Table

	Indicator Unit of Measure
Human population	individuals
Ecological footprint	hectares of land
Food production	kg of grain/person
Carbon dioxide concentration	ppm
Global temperature	degrees Centigrade
Sea level change	mm
Species diversity	number of species per functional group

Key Environmental Indicators to Focus On

Biological diversity
Human population growth
Food production
Resource consumption
Global temperature and greenhouse gas levels
Pollution levels

Biological Diversity

Measures the diversity of genes, species, habitats, ecosystems.
Extinction rates accelerated by humans; natural extinction is normal but current rates are vastly heightened.
Loss of keystone species harms ecosystems and reflects the state of land, water, and air quality.

World Human Population

Reached eight billion in Nov 2022; 230,000 new inhabitants daily.
Exponential growth until the 1960s; projected to level off between 8-12 billion by 2150.
Growth demands finite resources, increasing pollution and waste.

Food Production

Grains provide over half human calories.
Production influenced by soil quality, climate, and agricultural practices.
Intensity of agriculture determines food yield; practices can impact land quality.

Resource Consumption

Sustainable use of resources is vital for future generations.
Larger populations lead to greater consumption; however, lifestyle differences impact consumption rates.

Global Temperatures and Greenhouse Gases

Earth’s temperature regulated by solar radiation and greenhouse gas concentrations.
Most increase in CO2 is anthropogenic; temperatures show an overall increase over 130 years.

Ice Core Studies

Ice layers in Greenland/Antarctica reveal ancient gas concentrations and climate data.

Air and Water Pollution

Indicators of pollution (like lead levels) signal potential environmental harm.
Historical use of lead in various products and recent legislation aiding reduction of lead emissions.

The Scientific Method

Framework for scientific inquiry and testing hypotheses. Steps include observation, hypothesis generation, testing, and reporting.

Steps

Observe and question.
Hypothesize.
Test & experiment.
Analyze and report.
Repeat & replicate to confirm findings.

Limitations of Environmental Science

No undisturbed baseline makes assessment difficult.
Subjectivity in assessing environmental impacts.
Conflicting human preferences vs scientific understanding.

Environmental Systems

Studying systems involves recognizing input/output dynamics.
Systems can be open (