Science Resource Guide 2024-2025 PDF

OUR CHANGING CLIMATE Almeta Crawford High School - Rosharan, TX...

OUR CHANGING CLIMATE Almeta Crawford High School - Rosharan, TX SCIENCE An Introduction to Environmental Science Resource Guide 2 0 24 – 2 0 25 The vision of the United States Academic Decathlon® is to provide students the opportunity to excel academically through team competition. Toll Free: 866-511-USAD (8723) Direct: 712-326-9589 Fax: 712-366-3701 Email: [email protected] Website: www.usad.org This material may not be reproduced or transmitted, in whole or in part, by any means, including but not limited to photocopy, print, electronic, or internet display (public or private sites) or downloading, without prior written permission from USAD. Violators may be prosecuted. Copyright ® 2024 by United States Academic Decathlon®. All rights reserved.  Table of Contents INTRODUCTION.................. 6 The Human Component of Environmental Systems............................ 23 SECTION I: FOUNDATIONS OF System Analysis: Determining How ENVIRONMENTAL SCIENCE......... 7 Matter and Energy Flow in the What Is Environmental Science?.... 7 Environment...................... 23 Environmental Indicators............ 8 Inputs, Outputs, and Flux............. 23 Biological Diversity................... 9 Steady State........................ 24 World Human Population............. 11 Almeta Crawford High School - Rosharan, TX Food Production..................... 12 Environmental Science Case Study: Resource Consumption............... 12 Mono Lake—an I nput–Output System Global Temperatures and Greenhouse Analysis........................... 25 Gases.............................. 13 Mean Residence Time................ 28 Accumulation and Depletion.......... 29 Environmental Science Case Study: Feedbacks.......................... 29 M easuring Greenhouse Gases in Ice.....14 Overshoot.......................... 31 Air and Water Pollution............... 16 Regulating Population Systems....... 32 The Scientific Method.............. 16 Environmental Science Case Study: H umans An Illustration of the Scientific Method.. 18 and Elephants in Africa—Feedback and The Role of Repetition in Science...... 19 Understanding How to Interpret Scientific Regulation in I nteracting Population Studies............................. 19 Systems............................ 32 The Limitations of Environmental Environmental Science Case Study: Red Science........................... 20 Spruce in the Northeastern U nited The One Earth Problem............... 20 States—an Environmental System I mpacted Inconsistent Units of Measure for by the I nteraction of Natural and Energy............................. 20 H uman-Caused Factors............... 34 Subjectivity......................... 20 Unpredictable Consequences of Environmental Science Case Study: Preferences and Policies............. 20 Managing Environmental Systems in the Environmental Systems.............. Florida Everglades.................. 36 System Dynamics.................... 21 Section I Summary................ 38 Matter and Energy Exchange.......... 21 Open and Closed Systems............ 22 2024–2025 Science Resource Guide 2 SECTION II: BIODIVERSITY: FROM Ecological Communities........... 67 LOCAL TO GLOBAL.............. 40 Food Webs.......................... 67 Biodiversity....................... 40 Keystone Species.................... 68 What Is Biodiversity and Why Does It Succession......................... 70 Matter?.............................40 Environmental Science Case Study: A The Value of Biodiversity............. 41 Genetic Diversity..................... 41 Simple Ecosystem—Organ Cave....... 72 Expressions of Genetic Diversity....... 43 Species Diversity..................... 45 Productivity....................... 73 Primary Productivity................. 73 Evolution......................... 45 Energy Transfer Efficiency............ 74 Adaptation through Natural Selection... 47 Adaptation to a Changing Environment.. 48 Major Aspects of Ecosystems....... 76 Ecosystem Boundaries............... 77 Nonadaptive Evolutionary Processes... 48 The Biotic Components of Ecosystems.. 77 The Pace of Evolution................ 50 The Impact of Ecosystem Change on Its Changes in Environmental Conditions Biotic Components................... 77 and Extinctions....................51 Biomes........................... 78 Almeta Crawford High School - Rosharan, TX The Fossil Record................... 51 The Global Climate and Biomes........ 78 Mass Extinctions.................... 52 Biomes and Global Biodiversity....... 81 Estimating Extinction Rates from Habitat Loss............................... 53 The Cycle of Elements within the Human Activity and Biodiversity... 54 Biosphere........................ 85 The Elements on Earth............... 85 Habitat Fragmentation............... 54 Biogeochemical Cycles...............85 The Introduction of Exotic Species.....54 The Hydrologic Cycle................. 86 Linking Biodiversity and Evolution to The Carbon Cycle.................... 87 The Nitrogen Cycle................... 89 Ecology.......................... 55 The Ecological Perspective........... 55 Section II Summary............... 90 Environmental Conditions............ 56 Resources.......................... 57 Population Ecology............... 58 SECTION III: THE HUMAN IMPACT ON Density-Dependent Growth........... 58 NATURAL RESOURCES........... 93 The Logistic Growth Model............ 59 The Human Population............ 93 Growth Rate........................ 93 Environmental Science Case Study: The Lower- and Higher-Income Countries...95 Challenge of Managing Population Population Size and Resource Use..... 96 Growth........................... 60 Factors Affecting Population Growth....96 Density-Independent Growth.......... 62 Fertility.............................. 97 Life Expectancy and Infant Mortality.. 97 Metapopulations.................... 62 Populations and Biodiversity.......... 63 Age Structure....................... 98 Community Ecology............... 63 The Elements on Earth............ 101 Interspecific Competition............. 63 The Cycles of Calcium, Magnesium, Predation........................... 65 Potassium, and Sulfur............... 103 Mutualism.......................... 67 Soil.............................. 103 2024–2025 Science Resource Guide 3  What Is Soil?....................... 103 The Scientific Management of Soil Horizons....................... 104 Fisheries.......................... 130 State Variables and Soil Formation... 105 Managing Fisheries for a Sustainable Soil Degradation....................108 Future.............................132 An Economic Approach to Fishery Water Resources................. 108 Management....................... 132 The Long-standing Challenge of Integrating an Ecological Perspective into Accessing Clean Water..............108 Fishery Management................133 Water’s Importance to Earth’s Environmental and Human Systems... 109 Environmental Science Case Study: Groundwater and Surface Water..... 109 Managing an Endangered Fishery...... 134 Transport of Water..................110 Desalination....................... 111 Forestry Resources................136 Water Use......................... 111 Principles of Forestry............... 136 Water Shortages................... 111 Harvesting Methods.................136 Floods............................. 112 Intensive Forestry.................. 137 Ecologically Sustainable Forestry.... 138 Water Pollution................... 112 Forestry in the Tropics.............. 139 Almeta Crawford High School - Rosharan, TX Types of Water Pollutants............113 Nonchemical Pollutants............. 116 Environmental Science Case Study: Ocean and Shoreline Pollution....... 117 Selective Logging and Butterfly Diversity Solid Waste Pollution............... 117 in Borneo......................... 139 Wastewater Treatment.............. 117 North American Forests.............141 Improvements in U.S. Water Quality... 118 Section III Summary.............. 141 Agricultural Resources............ 119 The Beginnings of Agriculture........ 119 Traditional Agricultural Methods......120 SECTION IV: SCIENCE FOR A The Green Revolution............... 121 SUSTAINABLE FUTURE.......... 146 The Status of World Food Production.. 121 Air Pollution and Atmospheric Food Insecurity—Hunger in the World. 122 Science......................... 146 Conventional Land-Use and Planting Major Air Pollutants................ 148 Techniques.........................124 Sulfur Dioxide....................... 148 Mechanization and Intensive Nitrogen Oxides..................... 148 Working of the Soil.................. 124 Carbon Monoxide.................... 148 Irrigation............................ 125 Lead................................ 148 Monoculture.........................125 Particulate Matter.................... 149 Chemical Fertilizers.................. 126 Ground-Level Ozone................ 149 Chemical Pesticides.................. 126 Secondary Pollutants............... 150 High-Density Farming of Animals.... 127 Natural Sources of Air Pollution......150 GMOs..............................128 Atmospheric (Thermal) Inversion..... 151 Genetic Engineering................. 128 The GMO Controversy................ 128 Environmental Case Study: Using Sustainable Agriculture............. 129 Models to Predict Pollution.......... 152 Fishery Resources.................130 Energy Use and Sources...........153 Overfishing and the Decline of Units of Energy.....................153 Fisheries.......................... 130 Worldwide Patterns of Energy Use... 153 2024–2025 Science Resource Guide 4  The Current Fuel Mix in the United Qualitative and Quantitative Risk States.............................153 Assessment....................... 175 Energy Efficiency................... 156 Environmental Risk Analysis......... 178 Energy Use for Transportation....... 157 Risk Assessment................... 178 Finding the Right Energy Source for the Job................................158 Environmental Science Case Study: Generating Electricity............... 158 Risk Assessment.................... 179 The Power Grid..................... 160 Risk Acceptance....................181 Risk Management................... 182 Nonrenewable Energy Sources....... 160 Coal................................ 160 Global Climate Change........... 182 Petroleum............................ 161 The Sun–Earth Heating System...... 184 Oil................................... 161 Greenhouse Gases..................185 Natural Gas......................... 162 Hydraulic Fracturing................. 162 Evidence of Temperature Change Nuclear Power.......................163 over Time..........................186 Nuclear Accidents................ 164 Indicators of Climate Change.........187 Radioactive Waste................ 165 Models............................ 188 Feedback in the Global Greenhouse Renewable Energy.................. 166 Almeta Crawford High School - Rosharan, TX System............................ 188 Direct and Indirect Solar Energy...... 166 The Temperature–CO2 Feedback Passive Solar Energy................. 167 Loop................................ 189 Active Solar Energy..................167 The Temperature–Permafrost Solar Water Heating.............. 167 Feedback Cycle...................... 189 Solar Generation of Electricity.....167 The Ice–Albedo Feedback Loop....... 190 Wind Energy........................ 168 Effects of Global Warming........... 190 Advantages and Disadvantages of Solar Predicted Future Effects of Global and Wind Energy.................... 169 Warming........................... 191 Hydroelectric Power................. 170 Run-of-the-River Hydro........... 170 Water Impoundment.............. 171 Section IV Summary............. 193 Biomass Around the World...........172 CONCLUSION.................... 196 Modern Carbon vs. Fossil Carbon......173 Ethanol..............................173 Geothermal and Tidal Energy.........173 GLOSSARY...................... 197 Conservation and Efficiency......... 174 Reducing Peak Demand.............. 175 NOTES.......................... 206 Human Environmental Impacts and BIBLIOGRAPHY.................. 207 Human Health Risks.............. 175 2024–2025 Science Resource Guide 5  Introduction Environmental science is the study of the impacts that In Section III we will turn to the natural resources on human activities have on the environment, including which human society depends—how we impact them the pollution impact of turning on your lights, the loss and how we can use science to sustainably manage of biodiversity from deforestation and the overfishing them. Natural resources include minerals that provide of the oceans, and the many global impacts of adding the raw materials for many of our modern products, billions of tons of greenhouse gases to the atmosphere. as well as the soils in which we grow our food. They However, environmental science is also a tool for also include water resources—critical for drinking and developing ways to manage those impacts so that for agriculture—and the natural resources we harvest humans, and the other species with whom we share the from ecosystems, such as timber from forests and Almeta Crawford High School - Rosharan, TX Earth, can have a sustainable future. fish from the ocean. Unfortunately, we have depleted many of these resources through poor management. To study these impacts requires an interdisciplinary Environmental scientists are working to develop new, approach, relying on many aspects of biology, earth science-based management strategies that will allow and atmospheric sciences, fundamental principles for the sustainable use of natural resources. of chemistry and physics, and human population dynamics. Section I starts with an overview of how In Section IV, we deal with four related topics: 1) environmental science is conducted—what types of atmospheric science and air pollution; 2) our various scientific approaches are used and what unique issues energy sources and how their use causes pollution environmental science has to deal with. We then and other environmental impacts; 3) human health explore how the Earth is made up of interconnected impacts from pollution and other human activities; and systems—living (humans and other species) and 4) how economic development and increased energy nonliving components (e.g., air, water, minerals) use has impacted our atmosphere and the global joined together by the flow of energy and matter. environment—global change in general and global Environmental science deals with natural systems like climate change in particular. The human act of adding lakes and forests and also domesticated systems like greenhouse gases to the atmosphere is impacting us cities and farms. today and will likely impact people in the future even more. We will explore what environmental science can Section II deals with biodiversity, from genes to add to the work of maintaining a sustainable Earth. ecosystems. You will learn the basics of ecosystem ecology—the study of the different ways that living and nonliving components are organized together in NOTE TO STUDENTS: Throughout this resource guide, nature. We will then focus on nonhuman species— you will notice that some terms have been bold-faced. Bold- how they have evolved, what controls their distribution face indicates a key term, and these terms are defined in the and abundance, how they interact with each other, and glossary of terms at the end of the resource guide. how human activities impact them. You will also learn some of the ways that environmental scientists develop strategies to protect species and their ecosystems. 2024–2025 Science Resource Guide 6  Section I 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, Almeta Crawford High School - Rosharan, TX 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. FIGURE 1 Environmental science is the study of how human activities impact environmental systems. Source: Odum, E.P., Ecology: A Bridge Between Science and Society, 3rd Ed. Sinauer, 1997. 2024–2025 Science Resource Guide 7 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. The amount of new growth on trees can be used as an indicator of the health of a forest. One way of studying the environment is to study Image Source: Smithsonian Environmental Research Center 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 Almeta Crawford High School - Rosharan, TX 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 2024–2025 Science Resource Guide 8  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 concentration; presence or absence of bacteria (conventional) 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 Almeta Crawford High School - Rosharan, TX 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 2024–2025 Science Resource Guide 9 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 Snow leopards are endangered and may go extinct if present how biological diversity on Earth is decreasing but is an trends are not reversed. 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. Almeta Crawford High School - Rosharan, TX FIGURE 2 The five past mass extinctions events. Current human impacts may be causing another such extinction event. Source: National Geographic 2024–2025 Science Resource Guide 10 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 The growing human population on the Earth creates a greater projects that world population will level off somewhere demand on Earth’s finite resources. between 8 and 12 billion people by the year 2150. FIGURE 3 Almeta Crawford High School - Rosharan, TX 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 2024–2025 Science Resource Guide 11 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 Combustion of fossil fuel is the primary human activity that Food grains such as wheat, corn and rice provide more produces carbon dioxide. 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. Almeta Crawford High School - Rosharan, TX 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 2024–2025 Science Resource Guide 12 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. FIGURE 4 Almeta Crawford High School - Rosharan, TX History of global temperature change and causes of recent warming. Source: IPCC 2024–2025 Science Resource Guide 13 FIGURE 5 Almeta Crawford High School - Rosharan, TX Atmospheric carbon dioxide. Source: NOAA 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 2024–2025 Science Resource Guide 14  FIGURE 6 Almeta Crawford High School - Rosharan, TX 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 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. 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. 2024–2025 Science Resource Guide 15 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 The major source of lead contamination in the U.S. manipulation of the natural environment. is drinking water. 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, Almeta Crawford High School - Rosharan, TX 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 2024–2025 Science Resource Guide 16 investigators at a research institution. FIGURE 7 Almeta Crawford High School - Rosharan, TX The process of scientific inquiry. Source: University of California Berkeley, Understanding Science 101 2024–2025 Science Resource Guide 17 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 Almeta Crawford High School - Rosharan, TX 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 is reduced from ten hectares to one hectare?” (1 hectare = 2.47 acres). The scientist phrases this question as a hypothesis: “Reducing the size of the natural area will result in a significant loss in small mammal and shrub species” and predicts that the hypothesis is correct. (Alternatively, they could predict that it is incorrect.) 2024–2025 Science Resource Guide 18 The researcher then uses the following situation as an experiment to test their hypothesis: Five housing developments are to be constructed in the suburbs of a major city in Southern California, each on a ten-hectare plot of land with one hectare in the middle left as a “natural area.” Before the developments begin, the investigator conducts inventories of all the species on all five ten-hectare areas destined to become housing developments, and five similar ten-hectare areas that will remain undisturbed and act as controls. The survey determines an average number of species per hectare. The scientific method is an ongoing discussion among In this example, the experimental variable is the researchers. reduction of habitat size from ten hectares to one hectare. Ten years after the housing developments have been completed, the investigator returns to the survey sites and inventories the species again. If the one hectare “natural areas” in the housing developments have fewer species Almeta Crawford High School - Rosharan, TX than the undisturbed areas, the investigator could conclude that their hypothesis was not falsified—reducing the size of the natural area did decrease species diversity—and report their findings to the scientific community. Not all scientific studies have a clear experimental variable or manipulation. Sometimes, an observation is made after an event has occurred, and an environmental scientist must determine what has happened without having data from before the event. This kind of analysis is a little bit like detective work, but certain aspects of the scientific method still apply. The Role of Repetition in Science The scientific method is an ongoing discussion among researchers. Scientists frequently disagree about hypotheses, experimental conditions, results, and the interpretation of results. Two investigators may even obtain different results from similar experiments, or two interpretations may explain the same set of observations. Any single finding has limited significance. It is when the same finding is repeated over and over by different investigators that we can begin to trust that the observed phenomenon is real and significant. In the meantime, the disagreements and the discussion about contradictory findings are not only normal, but are a valuable part of the scientific process. While reporting on two studies that reached opposite conclusions, the popular press often assumes the discrepancy is the result of bad science or confusion on the part of scientists. Particularly when a scientific issue is of great popular interest or concerns policy—questions such as global warming, toxicity of pollutants, or species extinction, for example—individual preliminary results may be reported to the general public before scientists have had a chance to reconcile apparent or actual differences. Understanding How to Interpret Scientific Studies If it is important to view scientific findings critically, how can we judge whether a report is based on good science? Sometimes scientific investigators do not differentiate between the control group of subjects and the experimental group. Other times, there is not a large enough sample size to draw general conclusions. Alternatively, the conclusions may be made about one group (for instance, mature trees) when the experiment was done on a different group (seedlings). In order to conduct a scientifically sound study, the investigators must use a large enough sample size and have a distinct difference between the experimental group and the control group. Further, they must demonstrate a cause-and-effect relationship between a manipulation and a result and be able to identify a mechanism that would give rise to the observed result. A simple correlation between one event and another—that is, the two occurring together—does not constitute scientific evidence that one caused the other. 2024–2025 Science Resource Guide 19 THE LIMITATIONS OF ENVIRONMENTAL SCIENCE Because of the nature of what is studied and the way the research is conducted, applying the traditional scientific method to environmental science presents a number of challenges and limitations that are not usually found in other scientific fields. The One Earth Problem The greatest challenge is the fact that there is no undisturbed baseline with which to compare the contemporary Earth. Virtually every part of the planet has been altered by human beings in some way. When it is difficult or impossible to decide which of two Though some remote parts of the Earth appear to be alternative actions, such as using a paper bag or a plastic bag, is better or worse for the environment overall, our assessments undisturbed, we can find quantities of lead—produced and our choices ultimately involve value judgments and by smelters during the time of the Roman Empire— personal decisions. in the Greenland ice sheet, traces of the organic compound PCB in the fatty tissue of penguins, and Almeta Crawford High School - Rosharan, TX species carried by ship to remote tropical islands from other parts of the world. This situation makes it difficult to know the “original” levels of lead or species diversity before humans began to alter the Earth and, consequently, how the current situation deviates from those unknown levels. Inconsistent Units of Measure for Energy Although the sources may differ, all energy is essentially the same. However, we use it in many different forms, and our society describes virtually every form in a different way. For example, we purchase gasoline in gallons and electricity in kilowatt hours. When we buy an air conditioner, its energy use is reported in watts or amps, but the amount of work it does in the form of extracting heat from the air is measured in British thermal units per hour, and we measure its effect on our home environment in degrees Fahrenheit or Celsius, depending on where we live. Scientists, on the other hand, measure energy in joules or calories. The lack of consistent values makes it very hard to see how much energy we are using overall. Subjectivity Paper or plastic? When choosing between two alternative actions, such as using a paper bag or a plastic bag, we often try to compare their environmental impacts. How can we know for sure which is best? There are techniques for determining what harm may come from using benzene to make a plastic bag and techniques for determining what environmental or human damage may come from using chlorine to make a paper bag. However, different substances tend to affect the environment differently: benzene may pose more of a risk to people while chlorine might pose a greater risk to organisms in a stream. It is difficult, if not impossible, to decide which is better or worse for the environment overall. Ultimately, our assessments and our choices involve value judgments and personal decisions. Unpredictable Consequences of Preferences and Policies Understanding natural science is the major focus of this resource guide, but it is not the sole answer to our environmental problems. An advancement in scientific understanding or a development in technology may appear capable of achieving fabulous environmental gains. But human action and cooperation are necessary, and they are not always forthcoming. A recent example is the changes that have occurred in passenger vehicle fuel efficiency in the United States. Since 1975, technological improvements have increased the fuel efficiency of most cars in the United States from an average of thirteen miles per gallon to more than thirty miles per gallon by 2021.2 We might assume that the overall average miles per gallon of all vehicles in the U.S. should have steadily 2024–2025 Science Resource Guide 20 increased during this time period. Unfortunately, that’s not what happened. Due to consumer preferences and personal choice, more and more people have purchased sport utility vehicles, light trucks, and minivans, which often get less than twenty miles to the gallon, which brought the overall average vehicle fuel efficiency down in the 1990s. However, new polices promoting the use of electric and hybrid vehicles along with the popularity of smaller SUVs, has changed this trajectory for the better. It is vital to remember that no matter how great the scientific or technological gains may be, human choices, opinions, action, or lack of action are equally important in the ultimate effect on the environment. Although we have identified a number of limitations in environmental science, the conclusions reached by environmental scientists are based on data from many areas of the physical and natural sciences, gathered according to the scientific method. Environmental science can help us understand the world we live in. ENVIRONMENTAL SYSTEMS System Dynamics A butterfly stirring the air in Beijing can affect weather patterns in New York a month later. This often- paraphrased statement is a poetic way of describing the interconnectedness of systems on Earth. The study of the environment is the study of systems. Recall that 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. In practice, systems are defined by the person looking at them. For example, one scientist may spend an entire career studying the physiological Almeta Crawford High School - Rosharan, TX and anatomical systems of individual humpback whales to learn how they can dive so deep, swim so far, and survive in such a range of marine environments. In contrast, the population biologist will focus on gathering data on population changes over time, while the community ecologist will be interested in the whale’s interaction with other species, such as their prey. And the conservation biologist will likely be most concerned with the adverse impacts of human fisheries—and the gear used in those fisheries—on the whale population. While all these systems are connected, it is this final one—where human activity has it most serious impacts—where environmental science will interact with fishery policies, law, and economics. Throughout this resource guide, we will define systems in terms of the particular environmental issue we are studying. The largest system studied by environmental scientists, and the one of which all others are part, is our global system—the Earth. The interactions of systems and components within systems are known as system dynamics. Matter and Energy Exchange Environmental systems, whether small or large, involve the exchange of matter (materials) or energy. One of the most important materials involved in environmental systems is water; some others are fuels (oil, coal, etc.), chemicals, and gases (e.g., oxygen). For other environmental systems, the exchange of energy is the important process. This includes the energy (food) intake of a single animal, the energy flows through an ecosystem, the fossil fuel energy used to drive modern human society, and the energy on which all environmental systems ultimately depend: the energy from the Sun. 2024–2025 Science Resource Guide 21  FIGURE 8 Inflow Outflow Isopods/Amphipods Crayfish prey upon compete for energy Isopods/Amphipods (use energy for growth of individuals and population) Energy Energy (food) (Energy from Isopods/Amphipods that is not used by Crayysh (Energy not captured by Isopods/Amphipods) Energy used to grow larger individuals & populations Water Water Energy lost as heat 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 Almeta Crawford High School - Rosharan, TX similar modeling of the inputs and outputs. Open and Closed Systems Systems can be either open or closed. An open system is one where the exchange of matter or energy between it and other systems occurs. In a closed system, exchange does not occur. The Earth system is open with respect to energy. Solar energy enters the Earth’s atmosphere, and heat energy escapes from the Earth’s atmosphere. However, the Earth system is closed with respect to matter, such as chemical elements. Except for the occasional meteorite or space shuttle, no material enters or leaves the Earth system. The ocean is a system that is open to both energy and matter. Energy from the Sun enters the ocean, and energy from the ocean is easily transferred to other systems such as the atmosphere. And matter, such as sediment and nutrients, enters the ocean via rivers and streams. FIGURE 9 Energy Matter Outputs: Input: Heat No (major) No (major) Solar energy, inputs outputs radiation reeected light (a) Open system (b) Closed system 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). 2024–2025 Science Resource Guide 22 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 Almeta Crawford High School - Rosharan, TX 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 INPUTS – OUTPUTS = TOTAL FLUX If inputs are greater than outputs, then flux is positive. 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 2024–2025 Science Resource Guide 23 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 size of the pool. Sometimes we can measure the pool directly. With a bucket of water, for example, we could empty the bucket into a measured container to determine the size of our pool. If we are trying to determine the size of a large or immobile pool, such as a flock of birds or an ocean, we have to calculate or estimate the pool size. Next, we want to measure, estimate, or calculate the net flux into and out of the system (input and output). For example, imagine that someone has punched holes in the bottom of our bucket, so the water is leaking out, and at the same time we are running water into it from a faucet. We can measure input from the faucet and output from the holes. As Figure 11 shows, the bucket has a pool of 10 liters, a flux in of 1 liter per minute, and a flux out of 1 liter per minute. Since the input equals the output, net flux = 0, and over time there will be no change in this system. The pool will remain at 10 liters until someone changes one of the fluxes, perhaps by turning off the faucet or plugging up the holes in the bottom of the bucket. This system is in steady state. FIGURE 11 Almeta Crawford High School - Rosharan, TX A steady state system does not change over time. 2024–2025 Science Resource Guide 24 Many pools in the natural world are at steady state; the water in the atmosphere is an example. The amount of water that enters the atmosphere from evaporation in any given time period is roughly equal to the amount that leaves the atmosphere as precipitation over the same time. The oceans are also at steady state; the water that enters from rivers and streams is roughly equal to the water that evaporates. When a community bans watering lawns and washing cars and declares a drought emergency during a dry summer, it is because their water supply is no longer at steady state; more water is being lost from the reservoir than is replenished by precipitation and streams. If a resource, such as a water supply, is decreasing in size, it means that the system is not being utilized in a sustainable way. FIGURE 12 Almeta Crawford High School - Rosharan, TX A system that is not in a steady state changes over time. It is important to realize that one part of a system can be in steady state while another part is not. For example, though water in the atmosphere is in a steady state, carbon dioxide in the atmosphere is not; it is slowly increasing, as we will discuss later. ENVIRONMENTAL SCIENCE CASE STUDY: Mono Lake—An Input– Output System Analysis Mono Lake is a large, deep, and old lake—one of the oldest lakes in North America, estimated to be between 1 and 3 million years old.3 It is located about three hundred miles northeast of Los Angeles on the border between the Sierra Nevada Mountain range and the Great Basin Desert. Four interconnected environmental systems are critical to 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) Mono Lake, one of the oldest lakes in North America, is estimated to be between 1 and 3 million years old. of water. Mono Lake is called a terminal lake because Source: Cal Matters under “normal conditions” water flows into the lake (from tributaries bringing water from snowmelt in the Sierra Nevada Mountains), but since it is the lowest point in the landscape, no water flows out into streams or rivers. Although the water level of the lake fluctuates over the years, for any given time period the water flux in from precipitation and river input is roughly equal to 2024–2025 Science Resource Guide 25 the water flux out through evaporation. This is similar to a bucket of water in which evaporation is balanced by the slow drip of a faucet. The Salt-Balance System The second Mono Lake system involves the changing concentrations of salts in the lake. This system is dependent upon the water system since small concentrations of salts such as sodium and magnesium enter the lake via streams (even fresh water contains small concentrations of salts). The salt concentration increases over time because the evaporation process leaves salts behind while new salts continue to enter the lake. Therefore, Mono Lake, like other saline lakes, is slowly becoming saltier. We can calculate a mass balance to see quantitatively how this happens. We would have to use some very large numbers to do a mass balance for salt in Mono Lake, but we can use a hypothetical model lake, and one element, sodium (Na), to illustrate how salt water accumulates in a terminal lake. Since small concentrations of sodium enter the lake but none leaves, the mass balance for sodium in our lake involves a small, steady input and virtually no output. Remember the mass balance equation we introduced earlier? We can use that to estimate the change in salinity (concentration of salt in a liquid) over time (mg = milligrams): Net Flux = Inputs – Outputs Almeta Crawford High School - Rosharan, TX Input (from tributaries) = (1,000 liters water/day) x (5mg Na/liter) = 5,000 mg Na/day Output (from evaporation) = (1,000 liters water/day) x 0 mg Na/liter = 0mg Na/day Net Flux = + 5,000 mg Na/day This mass balance indicates that although the amount of water in our hypothetical lake remains roughly constant, the salt content steadily increases by 5,000 mg per day. Over time, our lake becomes saltier and saltier. This calculation explains why lakes like Mono Lake, Great Salt Lake, and the Dead Sea have such high salt contents. The Ecological System The third critical environmental system is the ecological system (ecosystem) of populations interacting with each other and with the physical environment of Mono Lake. We will discuss just one part of this ecosystem, the relatively simple food chain going from photosynthetic algae at the bottom of the food chain to gulls at the top. The algae, being green photosynthesizers, receive most of their energy from the Sun’s light. Brine shrimp and flies eat the algae and are eaten by gulls. The algae obtain most of their important nutrients (such as nitrogen) that they need to carry out photosynthesis from the excretions and decay of flies, shrimp, and birds. The Water-Use System The use of Mono Lake’s waters by the population of Los Angeles is the final key environmental system affecting the overall lake system. Los Angeles began withdrawing water from the non-salty Mono Lake tributaries in 1941 at a rate of approximately 80.4 million gallons/day. The effect of this water withdrawal on the total pool was exactly what we would expect from a bucket in which the in-flow is decreased (with water being to diverted to Los Angeles) without any compensatory decrease in evaporation (the outflow); the lake level dropped—forty feet in forty years. Given this significant change, what were some impacts of this change in the human water use system on the other three environmental systems of Mono Lake? 2024–2025 Science Resource Guide 26  FIGURE 13 Evaporation is the only outflow of water from the lake Almeta Crawford High School - Rosharan, TX Mono Lake’s input/output water system. Source: Modified image from Mono Lake Committee The lowering of Mono Lake’s water level had two very noticeable effects. One was the exposure of the previously water-covered tufa towers, which are the main habitat for the flies and shrimp on which the gulls depend for food. Exposure of these habitats did not directly lead to the death of the flies and shrimp, but it did make it easier for birds to prey upon them. This resulted in an initial glut of food for 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 2024–2025 Science Resource Guide 27  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: Almeta Crawford High School - Rosharan, TX 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: 2024–2025 Science Resource Guide 28 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 Warmer temperatures at the Earth’s surface lead to greater we discuss the impacts of human activities on global evaporation from oceans and lakes. The additional moisture climate change in later sections, these values will take in the atmosphere from evaporation enhances the layer of on important significance. heat-trapping gases, including water vapor, that cover the Earth, which makes the Earth warmer, which leads to greater Accumulation and Depletion evaporation, and more warming, creating a positive Almeta Crawford High School - Rosharan, TX It is important to remember that mean residence time feedback loop. 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, 2024–2025 Science Resource Guide 29 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 Population increase Reduced surface Less area evaporation More Births Almeta Crawford High School - Rosharan, TX births Level Level drops rises Population Lake level increase (a