System Dynamics: A butterfly stirring the air in Beijing can influence weather patterns in New York a month later. This poetic statement focuses on highlighting the interconnectedness of Earth's systems. Environmental studies focus on systems made up of living and nonliving components that affect one another. Different scientists define and study these systems in diverse ways: a physiological scientist may examine humpback whale survival, a population biologist analyzes changes in whale numbers, a community ecologist explores interactions between whales and their prey, and a conservation biologist assesses the impacts of human fisheries on whale populations. While these systems are interconnected, the primary concern for environmental science is the effects of human activity, especially regarding fishery policies and economics. Ultimately, the largest system studied by environmental scientists is the Earth itself, with system dynamics describing the interactions within these systems.
Matter and Energy Exchange: Environmental systems, whether small or large, involve the exchange of matter (materials) or energy. The most important one in this instance is water. In different environmental systems, the exchange of energy plays a critical role. This involves the intake of energy (food) by individual animals, the flow of energy through ecosystems, the fossil fuel energy that powers contemporary human society, and the essential energy sourced from the Sun, which all environmental systems ultimately rely upon.
Open and Closed Systems: Systems can be categorized as open or closed. An open system allows for the exchange of matter and energy with other systems, while a closed system does not allow such exchanges. The Earth system is considered open in terms of energy, as solar energy comes in and heat energy escapes. However, it is closed regarding matter since, aside from occasional meteorites or space shuttles, no material enters or leaves the Earth. In contrast, the ocean is an open system for both energy and matter, receiving energy from the Sun and matter like sediment and nutrients through rivers and streams.
The Human Component of Environmental Systems: Because of environmental systems involving human interaction or activity, many areas of human endeavor are important to an understanding of an environment. Some important ones are: economics, social structures and institutions, law, policy, and environmental advocacy.
Inputs, Outputs, and Flux: System analysis involves examining what inputs, outputs, and changes occur within a system. This process is analogous to analyzing a personal checking account to assess financial status. The initial balance in a checking account is referred to as a 'pool.' Deposits represent inputs, while checks and withdrawals are considered outputs. To evaluate financial status, one starts with the initial balance, adds inputs, and subtracts outputs to determine the month-end balance or 'change in the pool,' known as 'flux.' The rate of income, expressed in dollars per month, is described as 'flux rate,' indicating flow per unit of time. This can be represented as (INPUTS - OUTPUTS = TOTAL FLUX). The same kind of analysis can be done for anything, all which tells environmental scientists if the size of the sample space is increasing, decreasing, or staying the same. This was designed for materials that have mass, therefore called a mass balance analysis. All types of balance analyses can be represented as (NET FLUX = Inputs - Outputs).
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 measure the size of the pool. Next, we want to measure, estimate, or calculate the net flux into and out of the system(input and output). To measure net flux into and out of a system, we can use an analogy of a bucket with holes and a faucet. If the bucket has a capacity of 10 liters, an input flux of 1 liter per minute from the faucet, and an output flux of 1 liter per minute from the holes, the net flux is 0, meaning there is no change in the system over time. The water level in the bucket remains constant at 10 liters until one of the fluxes changes, indicating that the system is in a steady state. Net flux into and out of a system can be illustrated using a bucket analogy. A bucket with a capacity of 10 liters experiences an input flux of 1 liter per minute from a faucet and an output flux of 1 liter per minute from holes at the bottom. With equal input and output, the net flux is 0, resulting in a constant water level of 10 liters, indicating a steady state until one of the fluxes changes.
Environmental Science Case Study: Mono Lake—An Input–Output System Analysis: Mono Lake is one of the oldest lakes in North America, located northeast of Los Angeles at the border of the Sierra Nevada and Great Basin. Its story involves four interconnected environmental systems: the Natural Water System, where the inflow from tributaries equals the outflow through evaporation; the Salt-Balance System, which is characterized by increasing salt concentrations due to evaporation; the Ecological System, which includes a food chain starting with algae and ending with gulls; and the Water-Use System, where water withdrawals for Los Angeles have led to a significant decline in lake levels. Since 1941, Los Angeles has been diverting water at a rate of approximately 80.4 million gallons/day, causing the lake level to drop 40 feet over 40 years, affecting all other environmental systems.
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) 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.
Accumulation and Depletion: Mean residence time is valid only in steady-state systems. If not in steady state, we assess the accumulation or depletion of material, such as pollutants in a reservoir. For instance, to determine when pollutant concentrations will become toxic, we calculate accumulation or depletion rates using net flux formulas: Net Flux = Inputs – Outputs Example: If a reservoir holds 1,000,000 liters of water with a pollutant concentration of 10 mg/L, and no new pollutant is added while 1,000 mg/day interacts with sediment:
[ Flux Calculation: 0 – 1,000 mg/day = -1,000 mg/day] (Initial pollutant amount: 10 mg/L x 1,000,000 L = 10,000,000 mg) Time to full depletion: 10,000,000 mg / 1,000 mg/day = 10,000 days = 27.4 years. Thus, it will take approximately 27.4 years for the pollutant to be entirely removed from the reservoir.
Feedbacks: Mean residence time is valid only in steady-state systems. If a system is not in steady state, it's important to assess the accumulation or depletion of material, such as pollutants in a reservoir. For example, determining when pollutant concentrations will become toxic involves calculating accumulation or depletion rates using the formula: Net Flux = Inputs – Outputs. In a scenario with a reservoir holding 1,000,000 liters of water with a pollutant concentration of 10 mg/L, and no additional pollutant entering while 1,000 mg/day interacts with sediment, the net flux is -1,000 mg/day. Initially, the reservoir contains 10,000,000 mg of the pollutant. At a loss rate of 1,000 mg/day, it will take approximately 10,000 days, or 27.4 years, for the pollutant to be fully eliminated from the reservoir. 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. Feedback systems are crucial in environmental science, particularly the Earth's heating system feedback loop. Warmer surface temperatures increase evaporation, adding moisture to the atmosphere, which enhances heat-trapping gases. This creates a positive feedback loop, leading to further warming and increased evaporation. However, more evaporation can also result in greater cloud cover, which reflects sunlight and may lower temperatures, creating a possible negative feedback loop. The outcome of these interactions on overall temperatures remains uncertain. The balance of various environmental systems relies on effective feedback loops, but conflicting factors can disrupt these loops, particularly in relation to resource exploitation and human use
Overshoot: A key concept in feedback systems is the time delay between when a signal is generated and when it is acted upon. For instance, if you notice a decline in your bank account balance, you may change your spending habits. However, without timely updates, this delay could lead to continued overspending, resulting in overshoot, which is when a system exceeds its stable set point. In nature, many systems also face delays in information transmission leading to overshoot. This is particularly relevant in population systems; when birth rates are high, controlling factors like disease and reduced fertility may not respond quickly enough, causing the population to exceed its carrying capacity. This often leads to a significant population crash due to disease or starvation.
Net Population Change = Input (Births + Immigration) – Output (Deaths + Emigration). Although births and deaths typically overshadow immigration and emigration, the regulation of these flows is complex. Population sizes can be regulated by both abiotic (nonliving) environmental components and biotic components (other populations). For example, an increasing deer population may experience a decrease in available food, leading to fewer births—this is negative feedback. In more intricate systems, one population may regulate another; for instance, an increase in deer may attract more wolves, leading to increased predation and ultimately stabilizing the deer population through negative feedback. This document also sets up a discussion of real-world environmental systems impacted by human activity, which will be examined in more detail later. |
