Welcome to Lesson 5

As we explored in Lesson 1, energy can be generated from many sources—fossil fuels, nuclear power, hydropower, wind, solar, and more. But here's a critical truth: no energy source is completely impact-free. Every method of producing energy affects the environment in some way, whether through emissions into the atmosphere, water use, land disruption, or waste generation. Understanding these trade-offs is essential for making informed decisions about our energy future.

In this lesson, we will examine the environmental consequences of our energy choices. We'll start by analyzing the major pollutants released when burning fossil fuels—including carbon dioxide, nitrogen oxides, sulfur dioxide, and particulate matter—and how these emissions affect air quality, human health, and climate. Next, we'll highlight historical environmental successes, such as global cooperation to reduce sulfur emissions and heal the ozone layer, demonstrating that science, policy, and innovation can drive meaningful progress. We'll also investigate the challenges of nuclear waste management and the long-term stewardship required for radioactive materials. Finally, we'll explore the water-energy nexus, comparing how much water different technologies consume and why that matters in water-stressed regions.

Learning Objectives

By the end of this lesson, you will be able to:

1. Identify the primary environmental impacts associated with major energy sources (fossil fuels, nuclear, hydropower, wind, solar, and biomass).
2. Compare the trade-offs between energy technologies across multiple dimensions: greenhouse gas emissions, air pollution, water consumption, land use and change, and waste generation.
3. Explain how combustion of fossil fuels releases pollutants that affect human health, ecosystems, and climate—and describe the chemical processes behind key emissions (CO₂, NOₓ, SO₂, PM).
4. Analyze the water-energy nexus by evaluating withdrawal vs. consumption across different electricity generation methods and assessing regional implications.
5. Evaluate the challenges and strategies for managing high-level nuclear waste, including storage technologies and policy considerations.
6. Recognize examples of successful environmental policy—such as the Montreal Protocol and the U.S. Acid Rain Program—and explain how science, technology, and international cooperation enabled positive change.

In Lesson 3 (Energy Supply and Demand), we explored how the United States and the world source their energy. One key takeaway: fossil fuels—petroleum, natural gas, and coal—still dominate the global energy mix.

Want the latest data? Check out the U.S. Energy Information Administration's U.S. Energy Facts Explained for up-to-date statistics. As of 2024, fossil fuels supply more than 80% of total U.S. energy consumption.

 How Fossil Fuels Release Energy

When we use fossil fuels for energy, we typically burn (combust) them. This chemical reaction releases stored energy—but also emits substances into the atmosphere.

A simplified representation of hydrocarbon combustion looks like this:

$${\text{C}}_{\text{x}}{\text{H}}_{\text{y}}{\text{ + O}}_{\text{2}}{\text{ (from air) → CO}}_{\text{2}}{\text{ + (y/2) H}}_{\text{2}}\text{O + other products}$$

Important note: The "air" in this reaction isn't just oxygen. Earth's atmosphere is ~78% nitrogen (N₂), ~21% oxygen (O₂), and ~1% argon and other gases. At high combustion temperatures, nitrogen can react to form nitrogen oxides (NO_x)—a key pollutant we'll discuss shortly.

What's Actually in Fossil Fuels?

Fossil fuels are primarily made of carbon (C) and hydrogen (H)—the same building blocks found in all living things. That's no coincidence: fossil fuels formed over millions of years from ancient plants and microorganisms that were buried, compressed, and shielded from decay.

In addition to carbon and hydrogen, fossil fuels contain small amounts of:

• Sulfur (S) → can form SO₂ (a contributor to acid rain)
• Nitrogen (N) → contributes to NO_x formation
• Oxygen (O), trace metals, and mineral matter

Clarification: Not everything in the fuel is emitted unchanged. During combustion, chemical bonds break and reform, producing new compounds—some useful (like energy and water vapor), others harmful (like CO₂, NO_x, SO₂, and particulates).

In this unit, we'll examine the major pollutants released from combustion, how they affect human health and the environment, and what strategies exist to reduce their impact.

Text description of the Fossil Fuel Composition image.

Natural gas is composed of carbon, hydrogen, nitrogen, sulfur, and oxygen.

Petroleum is composed of carbon, hydrogen, nitrogen, sulfur, oxygen, and minerals.

Coal is composed of carbon, hydrogen, nitrogen, sulfur, oxygen, and minerals.

Credit: © Penn State is licensed under CC BY-NC-SA 4.0

Instructions: Click on the purple hot spot shown above the piece of coal below to determine what products are formed from each during combustion.

The most common products of combustion we will discuss in this module are as follows.

The greenhouse effect is the natural process in which gases in the atmosphere trap heat from the sun. This process was first identified by scientists in the 1800s. This process is what makes earth habitable for life.

How it works (simplified):

• Sunlight passes through the atmosphere and warms Earth's surface
• Earth radiates that heat back toward space as infrared radiation (invisible heat energy)
• Greenhouse gases (GHGs) absorb and re-emit some of that infrared radiation
• This trapped heat warms the lower atmosphere—like a blanket around the planet

What is the Greenhouse Effect? (2:29)

Climate change refers to long-term shifts in global temperatures and weather patterns, typically measured over decades or longer. It's important to distinguish between weather the short-term conditions outside today, like rain, sunshine, or a cold snap and climate, which is the average of weather patterns over 30 years or more. Just because a particular day is cold or a region experiences a severe snowstorm does not mean climate change isn't happening or that the Earth isn't warming overall; climate is defined by long-term trends, not single events

Natural vs. Human-Caused Change

• Climate can change naturally (volcanic eruptions, solar activity)
• Since the 1800s, human activities have been the MAIN driver of recent climate change
• The primary cause? Burning fossil fuels: coal, oil, and natural gas

Humans are responsible for global warming

Climate scientists have showed that humans are responsible for virtually all global warming over the last 200 years. Human activities like the ones mentioned above are causing greenhouse gases that are warming the world faster than at any time in at least the last two thousand years.

The average temperature of the Earth’s surface is now about 1.42°C warmer than it was in the late 1800s-prior to the industrial revolution-and warmer than at any time in the last 100,000 years. Scientists can gather information on the environment going back hundreds of thousands of years through ice cores. Ice core samples trap small amounts of air bubbles which allow scientists to gather information on the environment at that time, including the amount of carbon dioxide in the atmosphere.

The last decade (2015-2024) was the warmest on record, and each of the last four decades has been warmer than any previous decade since 1850.

How Ancient Ice Proves Climate Change Is Real (12:25)

Many people think that climate change means warmer temperatures. But increasing temperatures are only the beginning of the story. Because the Earth is a system where everything is connected, changes in one area can influence changes in all others.

The consequences of climate change include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.

People are experiencing climate change in diverse ways

Climate change can affect our health, ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where entire communities have had to relocate, while protracted droughts are putting people at risk of famine. In the future, the number of people displaced by weather-related events is expected to rise.

Read more about climate change on the United Nations What is Climate Change page.

Crash Course What is Climate Change

What is Climate Change? (13:57)

Climate Change Solutions

Since climate change is a worldwide problem that requires international cooperation. It will affect everyone, although the impacts of climate change are more readily seen in island populations as sea levels rise. 

Paris Agreement is an agreement signed in 2015 at the UN Climate Change Conference (COP 21) in Paris. This agreement set long term goals to guide all nations to 

• Reduce Global Greenhouse Emissions to hold global temperatures to well below 2°C above pre-industrial levels.
• Periodically assess the collective progress towards meeting long term goals 
• Provide financing to developing countries to mitigate climate change, strengthen resiliency and adapt to climate impacts

What is the Paris Agreement (1:39)

It’s easy to feel overwhelmed by the environmental impacts of energy production, but history shows that meaningful progress is possible. Throughout the 20th century, scientists and policymakers collaborated to solve several major environmental crises; issues that once dominated the headlines but are rarely discussed today. These success stories demonstrate how science and policy, when working together, can drive real, lasting change.

Nuclear power plays a significant role in the U.S. energy mix, providing about 18–19% of the US’s electricity in 2024, and roughly 9–10% of global electricity generation 

Like coal or natural gas plants, nuclear facilities generate electricity by heating water to create steam that spins a turbine. The key difference is the heat source: nuclear plants use fission—splitting atoms of uranium or plutonium—rather than burning fossil fuels. This process produces no direct carbon emissions during operation, but it does generate radioactive waste, primarily in the form of spent nuclear fuel rods that remain hazardous for thousands of years.

In the United States and most countries, spent nuclear fuel is not recycled. While reprocessing—chemically separating usable materials from waste—is technically feasible and practiced in a few nations like France, the U.S. has maintained a policy against commercial reprocessing since 1977 due to cost, proliferation concerns, and technical challenges 

 As a result, spent fuel is treated as high-level waste and must be safely stored until its radioactivity decays to safe levels. Radioactive materials are classified by hazard: low-level waste (contaminated tools, clothing) requires minimal shielding, while high-level waste (spent fuel) demands robust, long-term isolation. Because some isotopes have half-lives spanning millennia—meaning it takes that long for half their radioactivity to decay—storage solutions must be secure for geological timescales.

Currently, all commercial nuclear waste in the U.S. is stored on-site at power plants. After removal from reactors, spent fuel rods are first cooled in water-filled pools for several years. Once sufficiently cooled, they are transferred to dry cask storage: sealed steel cylinders encased in concrete or steel structures, designed to withstand earthquakes, floods, and other hazards 

Credit: U.S. Nuclear Regulatory Commission (Public Domain)

These casks are monitored and maintained above ground, with the expectation that a permanent deep geological repository—such as the long-proposed Yucca Mountain site—will eventually provide a final solution. However, political and technical debates have delayed such a facility for decades, leaving interim storage as the de facto national strategy.

Even with secure containment, nuclear waste management raises important questions about intergenerational responsibility, environmental justice, and energy policy trade-offs. While nuclear power offers reliable, low-carbon electricity, its waste legacy requires careful planning, transparent decision-making, and sustained investment. As we evaluate energy options for a sustainable future, understanding both the benefits and challenges of nuclear power—including how we steward its waste—is essential for informed citizenship and engineering innovation.

Water and energy are deeply interconnected in what scientists call the water-energy nexus. In the twentieth century alone, global energy use grew ten-fold while water use grew six-fold, and both continue to rise with population growth and increasing affluence. Understanding water use in electricity generation requires distinguishing between water withdrawal (water removed from a source) and water consumption (water that is evaporated, transpired, or otherwise not available for immediate reuse). Electricity is the fastest-growing form of energy, making water consumption across different generation methods—typically measured in liters per megawatt-hour (L/MWh)—a critical metric for sustainable energy planning.

The water intensity of electricity generation varies dramatically across different technologies. Wind and solar photovoltaic (PV) systems have the lowest water consumption because they require no cooling; their minimal water use occurs upstream during mining and manufacturing of components, plus occasional panel cleaning. In contrast, thermal power plants (coal, natural gas, nuclear, and biomass) require substantial water for cooling, depending on the cooling technology. Hydropower and biomass show the widest ranges: hydropower averages can appear high due to reservoir evaporation, while biomass water use depends heavily on whether feedstock crops are rain-fed or irrigated.

For thermal power plants, which account for over 70% of utility-scale electricity generation, the cooling system type is the primary determinant of water use. Once-through cooling draws water from rivers or lakes, passes it through the plant once, and returns it—resulting in high withdrawal but relatively low consumption. However, this method creates thermal pollution, returning water at higher temperatures that reduces oxygen levels and severely disrupts aquatic ecosystems. Wet cooling (recirculating) systems use cooling towers where heat dissipates through evaporation, reducing withdrawal but increasing consumption as water is lost to the atmosphere. Dry cooling systems use no water for cooling, relying instead on air conduction and convection, but they are more expensive and less efficient than wet systems.

Hydropower and biomass present unique water challenges. While water flowing through hydroelectric turbines isn't considered consumptive (it remains available downstream), large reservoirs significantly increase surface area and evaporation rates, creating consumptive losses that vary widely by location and climate. Similarly, biomass electricity consumes substantial water through crop irrigation, plant transpiration, and processing—with irrigated crops using dramatically more water than rain-fed alternatives. These site-specific factors make average water consumption estimates for both hydropower and biomass highly variable and context-dependent.

Even when water isn't consumed, energy production still impacts the environment. Thermal power plants using once-through cooling return warmed water to rivers and lakes, creating thermal pollution that degrades aquatic habitats. Nuclear plants are often sited near large water bodies specifically to access cooling water, concentrating environmental impacts in those ecosystems. As climate change intensifies droughts and water scarcity, the water-energy nexus becomes increasingly critical: choosing low-water technologies like wind and solar PV isn't just about reducing emissions—it's about building resilient energy systems that can operate sustainably in a water-constrained future.

As we’ve explored in this lesson, every energy source carries environmental trade-offs. Fossil fuel combustion releases pollutants that impact air quality, human health, and global climate. Thermal power plants draw and consume vast quantities of water, with cooling system choices dramatically shaping local ecosystems. Nuclear power offers reliable, low-carbon electricity but requires secure, long-term management of radioactive waste. And while renewables like wind and solar PV minimize operational water use and emissions, their manufacturing, land use, and material supply chains still carry environmental footprints. The key insight isn’t that any single technology is perfect, but rather that informed energy decisions require weighing impacts across multiple dimensions.

Yet this lesson also demonstrates that environmental challenges are not insurmountable. The successful global phaseout of ozone-depleting chemicals and the dramatic reduction of acid rain-causing sulfur emissions prove that science, technology, and coordinated policy can drive real progress. These achievements weren’t built on waiting for flawless solutions—they emerged from iterative innovation, regulatory frameworks, and public engagement. Today, that same collaborative approach is guiding efforts to decarbonize grids, deploy water-smart cooling systems, advance nuclear waste stewardship, and scale renewable integration.

As you move forward in this course, keep the water-energy-emissions-waste nexus in mind. Energy choices ripple through ecosystems, economies, and communities, and understanding those connections is essential for designing resilient, equitable systems. In our next unit, we’ll shift from assessing impacts to exploring energy efficiency and demand-side solutions—because the cleanest, most affordable, and lowest-impact energy is often the energy we never have to generate. Reflect on how the trade-offs we’ve discussed might shape infrastructure planning, policy debates, or your own career path in the evolving energy sector.