Okay, now let's tie this all together. Modern society is inextricably tied to the availability of energy, as we explored in Lesson 1. We just went through more than two full lessons outlining a lot of reasons to be concerned about the sustainability of modern society, in terms of the 3E's of sustainability and otherwise. Putting these two broad concepts together begs the question: What is sustainable energy?

At risk of sounding glib, the short answer is that there is no short answer. You will probably not be surprised to know that there is no single or even "correct" answer, that is to say, an answer that everyone can agree with. This has a lot to do with the fact that a singular definition of sustainability remains elusive, but in addition to that there is a lot of uncertainty with regards to both the long- and short-term impacts of energy use, and even how much energy (non-renewable in particular) is left to harvest. I want to be clear that the analysis that follows is not meant to answer the question once and for all, but to help frame some of the key considerations to make when answering the question. As you'll see, I've divided the analysis into sections for a number of energy sources, and subsections that provide information regarding supply, feasibility, and sustainability impacts.

Important Note to Keep in Mind

One last thing you should consider prior to reading through this lesson: No matter what mixture of energy sources/technologies that we decide to use, we cannot continue to emit CO₂ at the current rate for long. As detailed in the previous lesson, the reality of anthropogenic climate change and its negative impacts have near universal agreement among experts.  The Intergovernmental Panel on Climate Change (IPCC) has determined that we need to limit warming to 1.5 degrees C (about 3 degrees F) above pre-industrial levels to maximize our chances of avoiding climate catastrophe but no more than 2 C. (It is nearly 1.5 C, aka 2.6 F warmer already!) These are the goals of the Paris Climate Agreement, which you can read more about here. The following is a quote from the latest report from the IPCC, which was published in 2023. This is from the Summary for Policymakers. (A smorgasbord of climate change information for you energy policy nerds out there!) 

Pathways that limit warming to 1.5C (>50%) with no or limited overshoot reach net zero CO2 in the early 2050s, followed by net negative CO2 emissions. (source: Synthesis Report of IPCC Sixth Assessment Report, Summary for Policymakers, p. 21.)

In case you don't speak climate scientist, this means that we need to be 100% carbon neutral by around 2050 globally, followed by net negative emissions if we want the best chance of preventing the worst impacts of climate change. The UN states that, as of 2025, to reach net zero goals, we need to cut global emissions by 43% by 2030 (that's really not long from now!) See the image below for a visualization the global "carbon budget" that we have in order to keep emissions below the Paris Agreement targets.

Figure 4.1: Global Carbon Budgets to keep warming below 1.5 degree Celsius and 2.0 degrees Celsisus. 

Click for a text description of Figure 4.1.

This image contains two column charts that visually represent the amount of carbon dioxide that can be emitted globally and achieve different percent chances of keeping warming below 1.5 degrees Celsius (chart 1) and 2.0 degrees Celsius (chart 2). The purpose is to provide a visual representation of the global "carbon budget" in metric tons of carbon dioxide. The image shows the global emissions in 2022 of 41 tonnes, then columns to the right that show the tonnes of carbon dioxide that can be emitted and the percent chance that warming will be kept below the target temperatures. 

To keep warming below 1.5 C: 

• 83% chance: 100 tonnes
• 67% chance: 150 tonnes
• 50% chance: 250 tonnes
• 33% chance: 300 tonnes
• 17% chance: 500 tonnes

To keep warming below 2.0 C:

• 83% chance: 800 tonnes
• 67% chance: 950 tonnes
• 33% chance: 1450 tonnes
• 17% chance: 2000 tonnes

Credit: Our World in Data, CC-BY

In case you were wondering, global emissions have have only increased since the start of the Industrial Revolution (see below). In addition, a report authored by 13 federal agencies in the U.S. found that consequences for the U.S. will be dire if emissions are not significantly reduced. This report was particularly notable because it was released by the Trump Administration in 2018, which was no friend to climate regulation. (It was only released because it is mandated by Congress, and was immediately downplayed by the Administration, but still...)

Please keep this in mind as you read through these summaries. There is near consensus that humans must significantly reduce net emissions to near zero by mid-century, or we face a very dire future. No energy solution should be considered sustainable in the long term if it emits any carbon dioxide, unless carbon reduction technologies are sufficient to offset these emissions. Right now, it is much cheaper to not emit in the first place than to capture and store them.

We'll start with the most straightforward aspect: how much do we have, and how long will it last? That is an obvious question to ask, because if we are going to run out anytime soon, it is clearly not sustainable.

Supply

First and foremost, note all of the various caveats issued in these reports. As the EIA notes: "The amount of much coal that exists in the United States is difficult to estimate because it is buried underground." (Probably not what you want to hear from the authority on the topic, but bear with me.) The same goes for the rest of the world, and it is more difficult in a lot of other countries because even less is known about the underground resources. Also, there is a difference in the basic assumptions regarding the statistics.

Terminology is important to keep in mind.

• Demonstrated reserve base is the amount of coal that is estimated to be in place and could conceivably be mined commercially. Some of this is difficult to get to because of access issues (technical, political, etc.).
• The Estimated recoverable reserves are the portion of the demonstrated reserve base that can be realistically recovered, taking into consideration restrictions (e.g., property rights, land use conflicts, and physical and environmental restrictions). Estimated recoverable reserves are a subset of the demonstrated reserve base, and thus are always smaller. They do not take economic feasibility into account.
• Recoverable reserves at active mines are "the amount of recoverable reserves that coal mining companies report to EIA for their U.S. coal mines that produced more than 25,000 short tons of coal in a year. Basically, they epresent the quantity of coal that can be mined from existing reserves at active mine. The EIA noted in an older page that they "essentially reflect the working inventory at producing mines."
• You will also sometimes see total resources identified, which are a scientific estimate of all coal in the U.S., including coal that has yet to be discovered. It is not likely that all of this can be accessed, and further, we would not want to because it would require widespread destruction of the landscape.

All of these quantities use some estimation, with the bigger reserves requiring more estimation, so take all of them with at least a grain of salt. They should be considered a good estimate, with the estimated recoverable reserves probably being a reasonably good (but possibly conservative) estimate of what's left and can be realistically mined. You don't need to memorize all of these terms, by the way! It wouldn't hurt, mind you, but the goal here is for you to understand that coal resources are known to varying degrees, and for you to be conscious of which estimates companies/organizations/people cite. (Note that the coal data below are from 2023, which is the last time the EIA page was updated.)

Figure 4.2: Levels of estimated coal reserves in the U.S. as of 2023. (Click on image to view more information and download Excel file of Figure data.)

Click for a text description of Figure 4.2.

This image is a bar chart titled "U.S. reserves of coal by type and mining method as of January 1, 2023." It visually represents the volume of coal reserves in the United States, categorized by the type of reserve and the method of mining—underground (blue) and surface (brown). The y-axis measures coal reserves in billion short tons, ranging from 0 to 500, while the x-axis is divided into three categories: Demonstrated Reserve Base, Estimated Recoverable Reserves, and Recoverable Reserves at Producing Mines.

The Demonstrated Reserve Base is the largest category, totaling approximately 450 billion short tons, with the majority attributed to underground mining. The Estimated Recoverable Reserves are significantly smaller, around 250 billion short tons, and are more evenly split between underground and surface mining. The third category, Recoverable Reserves at Producing Mines, is minimal—barely visible on the chart—indicating a very small volume of coal reserves currently accessible at active mining sites.

At the bottom left of the image, there is a logo for the U.S. Energy Information Administration (EIA), along with a citation: "Data source: U.S. Energy Information Administration, U.S. Coal Reserves, Table 15, October 2023." This chart provides a clear snapshot of the scale and accessibility of U.S. coal reserves, highlighting the vast difference between total reserves and those currently recoverable at active mines.

Credit: U.S. EIA

"Very impressive" you might be thinking, but what does it all mean for sustainability of supply? Glad you asked! In 2023 the EIA stated that: "Based on U.S. coal production in 2022, of about 0.594 billion short tons, the recoverable coal reserves would last about 422 years, and recoverable reserves at producing mines would last about 20 years." (FYI, the year before they said that there were 357 years, which demonstrates that these calculations are scientific approximations. The increase in 2020 was because we were mining less coal, as you will see below). How do they get this number (357 years)? Hint: it is based on the total production and the Estimated Recoverable Reserves (ERR).

So the years of supplies remaining went from 261 years to 325 years to 332 years to 357 years to 422 years, all in the span of ten years. The moral of this short little story: All predictions of remaining resources on a large scale should be considered scientific estimates. They provide a sense of remaining supplies, but that can change quickly as supply and/or demand change.

Note that this assumes that coal production rates will remain the same and that technology will not change. And this, of course, assumes that this it is reasonable to mine all remaining U.S. resources, given environmental and social impacts, but it is a good starting point for the U.S. The same set of assumptions (with different numbers) are used to estimate how long the world will have coal - the recoverable reserves and current levels of coal production. According to the World Coal Association, there are between 110 years and 121 years of reserves available worldwide.

Feasibility

Coal has been used en masse as an energy source since near the beginning of the Industrial Revolution in the late 1700s. The infrastructure for coal mining, transportation, and use (mostly in power plants) is well-established and if it were not for the environmental and social impacts, coal would be a good source of energy. It is energy-dense, and we know how to use it. (I think there's a ZZ Top song about that.) It turns out that it is also pretty cheap to use (ignoring externalities, of course!).

Figure 4.3: Average Cost of Fossil Fuels at U.S. Power Plants in $/million Btu’s, 2004 – 2019
As you can see from the chart, coal is the cheapest fossil fuel for generating electricity on a dollar per million Btu basis. This, of course, does not include external costs.

Click for a text description of Figure 4.3.

Average Cost of Fossil Fuels at U.S. Power Plants in $/million Btu’s, 2004 – 2016

Year	Coal	Natural Gas	Petroleum
2004 (approximate numbers)	1.5	6.0	4.1
2006 (approximate numbers)	1.9	7.0	6.0
2008	2.07	4.11	10.87
2010	2.27	3.26	9.54
2012	2.38	2.83	12.48
2014	2.37	3.31	11.60
2016	2.11	2.47	5.24
2017	2.06	7.10	3.37
2018	2.06	9.68	3.55
2019	2.02	9.07	2.89
2020	1.92	2.40	5.98
2021	1.98	5.20	10.08
2022	2.36	7.21	16.53
2023	2.51	3.36	15.98

Credit: D. Kasper. Data downloaded June 2025 from U.S. EIA. Click to view the raw data.

Despite the relatively low cost of fuel, coal is rapidly being replaced by natural gas and to a lesser extent, renewable energy. This is partially due to the lower emissions of natural gas, but mostly due to basic economics (see for example these articles from the St. Louis Fed in 2017 and Energy Innovation in 2025). Energy generators want to make a profit like everyone else, and right now, natural gas and some renewables are simply more profitable, particularly in the U.S. In addition, investors and banks are less likely to invest in coal and insurance companies are increasingly likly to refuse to insure new power plants due to the risks involved with climate change and future regulation. See the suggested reading below for some insight into some of these issues on domestic and international scales.

It is no exaggeration to say that coal has played a starring role in delivering the energy that was used for the development of the U.S. and many other countries (especially Western countries) in the past 200+ years. It also currently provides over 40% of global electricity (according to the World Coal Association), and is the primary source of electricity for many "developing" countries like China and India  Coal is relatively cheap (again, as long as you don't include external costs), abundant, and relatively easy to use. There is a reason we've been using it at such a high rate for so long! So far, so good. So what's the catch?

Sustainability Impacts

Now the bad news: coal has a lot of negative environmental and social impacts.

Figure 4.4: This chart, which was created using Carbon Dioxide Information Analysis Center (part of Oak Ridge National Lab) data, shows the global anthropogenic emissions of carbon. As you can see, coal is the largest single source of carbon globally, overtaking oil in the early 2000s.

Credit: Borvan53, CC BY-SA 3.0

Probably the most important sustainability issue with coal is that it is so carbon-intensive. It emits about twice the carbon dioxide per Btu as natural gas and is responsible for more carbon dioxide emissions than any other energy source, and the energy sector is the largest source of carbon dioxide emissions worldwide.

Figure 4.6: This chart shows the global anthropogenic greenhouse gas emissions by percent. This is the main reason why CO₂ emissions are such a major focus instead of other emissions that have a greater impact on a per ton basis.

Click for a text description of Figure 4.6.

This image is a pie chart that illustrates the percentage contributions of various greenhouse gases to total global emissions. The chart is divided into five color-coded segments, each representing a different gas or group of gases and their share of total emissions.

• The largest segment, colored blue, represents carbon dioxide (CO₂), which accounts for 79.7% of total greenhouse gas emissions. This highlights CO₂ as the dominant contributor to global warming, primarily from fossil fuel combustion and industrial processes.
• The second largest segment, in red, represents methane (CH₄), contributing 11.1%. Methane is a potent greenhouse gas released from agriculture (especially livestock), landfills, and fossil fuel extraction.
• The third segment, colored teal, represents nitrous oxide (N₂O), which makes up 6.1% of emissions. N₂O is mainly emitted from agricultural activities, such as fertilizer use.
• The fourth segment, in green, represents a group of synthetic gases—hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆), and nitrogen trifluoride (NF₃)—which together account for 3.1% of emissions. These gases are used in refrigeration, electronics, and industrial applications and have high global warming potentials despite their smaller quantities.

This pie chart provides a clear and concise visual summary of the relative impact of different greenhouse gases, emphasizing the overwhelming role of CO₂ while also acknowledging the significant contributions of other gases, particularly methane and nitrous oxide.

Credit: U.S EPA

One possible solution to this is carbon capture and sequestration (CCS), which is a process that can capture CO₂ and bury it (i.e., sequester it) in underground rock formations. Under ideal circumstances, up to 90% of the carbon dioxide will turn into solid rock and thus not pose a leakage threat, though these "ideal" circumstances have proven to be elusive. (This is usually what is referred to as "clean coal" technology, though it is notable that only the carbon emissions are reduced in "clean coal" plants. Mining waste and particulates and other emissions still make this a relatively "dirty" source of energy, which causes "clean coal" to have higher mortaility rates than other sources.) While promising, there is some indication that CCS might not be as effective as once hoped. It is only beginning to be demonstrated on a commercial scale, and "clean" coal does not remove other harmful emissions, so the jury's still out.

One interesting irony is that carbon dioxide can be injected into oil wells to increase output, and has been since 1972. In addition to this, as pointed out in the EIA article above, about 11% of the methane emissions in the U.S. are due to venting of methane gas from underground coal mines. Recall that methane is about 30 times as powerful as carbon dioxide with regards to climate change.

While relatively inexpensive in simple terms (not including externalities), the external costs are likely quite high due in particular to negative health impacts (as you read in Lesson 1). But as you saw from the EIA, there other environmental concerns, such as mercury pollution, acid rain (which has mostly been mitigated through technology/policy), and remnants of power generation like fly ash. Coal mining can be a risky business, as you may remember from the Upper Big Branch Mine disaster that killed 29 miners in West Virginia in 2010. There have been many other accidents in the U.S. as well, as indicated above. China is the world leader in coal mining fatalities, according to the Wall Street Journal, including over 1,000 killed in 2013 and 2012, with more than 33,000 deaths in the past decade. There is also environmental damage that often results from mining and mining waste. Coal is a major source of particulate pollution, and contributes to the 1.1 million deaths in China from air pollution in 2016 and 1.2 million deaths in India in 2017

In short, coal is a reliable energy source, and is generally a relatively cheap source of energy as long as externalities are not included. If externalities were to be included, the price would undoubtedly increase, especially if the social cost of carbon and negative health impacts were included. CCS provides some hope for reducing the carbon dioxide emissions of coal use, but other significant sustainability problems will persist even with carbon capture.

Natural Gas

Unless you've been hiding under a rock for the past 20 years or so, you have heard about natural gas in the news. If you have heard about it, it was most likely in relation to hydraulic fracturing, or simply "fracking." This is a VERY controversial topic at the moment, and with good reason (as we'll see below). Because of this, you have to be careful where you get your information (good thing you are taking this course!). Our old friend Hank provides a pretty clear and unbiased description of fracking in the video below (4:32 minutes).

One popular misconception is that fracking has only been around since the early 2000's or so. As Hank explains, this is simply not the case. Hydraulic fracturing has been known to increase the output of gas (and oil!) wells since the mid-1900s. The main innovation that has caused the recent fracking boom is directional drilling (sometimes called horizontal drilling). Until relatively recently, oil and gas wells were generally drilled in a straight line. But directional drilling allows operators to change the direction of the drill bits so that they can trace the path of underground rock layers (which are rarely straight up and down). This allows for significantly more gas output per well and is what mainly facilitated the fracking boom.

Figure 4.7: Directional Drilling Illustration. The well on the left utilizes directional drilling once it reaches the shale layer. The bore can follow rock layers like this for thousands of feet! Think about how little could be accessed if the well simply went straight down like the well on the right.

Credit: JustJordan, public domain.

Supply

Like coal, it is impossible to determine the amount of natural gas reserves available in the U.S. or worldwide. First of all, it's underground, so we cannot directly measure it, though reasonable estimates can be made. But more importantly, as technology changes, the proved natural gas reserves change as well. Most of the data you will see are based on "proved reserves," which the EIA defines as "estimated volumes of hydrocarbon resources that analysis of geologic and engineering data demonstrates with reasonable certainty are recoverable under existing economic and operating conditions." (Source: US EIA). Basically, proved reserves are a reasonable estimate of the amount of natural gas that can be recovered given current technology, and for a profit.

The upshot to this is that 1) technology is changing rapidly, as evidenced by the boom in natural gas in the past 10 years or so, which is due entirely to fracking, and 2) as more test (exploratory) wells are drilled, more natural gas is discovered. See the chart below for the result of this moving target in the U.S.

Figure 4.8: Proved natural gas reserves in the U.S. since 1985. Note the sharp increase since approximately the year 2000, and the temporary decrease from 2011 to 2012, and 2014 to 2015 then went back up in 2016 and 2017, then leveled off until 2018, when it dropped. 2020-2024 saw a sharp increase.

Credit: US EIA (public domain), retrieved September 2024. Raw data available at the link provided. See the link provided for a clickable version of the chart.

What is particularly interesting about this chart is that the proved reserves have mostly increased even as we have continued to produce and use more natural gas. This can seem counterintuitive because it seems logical that as we take more of the gas out of the ground, less would be left. This is technically correct, but at the moment, the industry is less concerned with how much is left than how much is available. For the reasons indicated above - primarily technological advance - more is available even though less is left. The increase in production in the U.S., as well as the projected increase, can be seen in the chart below.

I'm sure you noticed the dramatic drop in proved reserves from 2011 to 2012 and 2014 to 2015. 2015 has a somewhat simple explanation: "Declines in natural gas prices in 2012 and 2015 contributed to reductions in proved reserves estimates in those years", according to the EIA. Again, this is a quirk of how we define proved reserves. Since proved reserves refer to the natural gas that is "economically recoverable," if prices are down and/or projected to continue, the proved reserves go down with them because it is more difficult to make a profit. (For a more in-depth discussion of these drops, see the optional reading below.)

Figure 4.9: It is clear from this image that natural gas production has increased dramatically beginning around 2005, and that the VAST majority of this increase is due to shale gas. Further, the EIA projects that this increase will continue.

Click for a text description of Figure 4.9.

This image is a line graph titled "U.S. dry natural gas production by type, 2010–2050", which presents both historical data and future projections of natural gas production in the United States. The x-axis spans from 2010 to 2050, with actual production data shown from 2010 to 2022 and projected estimates from 2023 to 2050. The y-axis measures production in trillion cubic feet, ranging from 0 to 50.

The graph is divided into four color-coded categories representing different sources of natural gas:

• Tight/shale gas (blue): This is the dominant source of production throughout the timeline, showing a steady increase and continuing to lead in projected output through 2050.
• Other lower 48 onshore (red): This category includes coalbed methane and excludes shale/tight gas. It contributes significantly but remains well below tight/shale gas levels.
• Lower 48 offshore (brown): This includes production from federal and state waters and shows relatively stable but modest output over time.
• Alaska (green): This is the smallest contributor, with minimal production that remains nearly flat across the entire period.

The graph shows a clear trend of growing total natural gas production, driven primarily by the expansion of tight/shale gas extraction. The data source is the U.S. Energy Information Administration (EIA), specifically from the Annual Energy Outlook 2023 Reference case, published in March 2023. A note clarifies that the offshore category includes both federal and state waters, and that the "other lower 48 onshore" category includes coalbed methane but excludes shale and tight gas.

Credit: U.S. EIA (Public Domain), retrieved September 2024. Raw data available at the link provided. See the link provided for a clickable version of the chart.

In the chart above, shale gas refers to gas that is locked up in the pores of shale in underground layers, as described in the fracking video above. It is clear that this is the biggest source of natural gas in the U.S. and is only projected to grow. (Seriously - look at that giant blue blob in the figure above! That's mostly shale gas.) Tight gas refers to gas that is locked up in other formations like low-permeability sandstone. For a full explanation of the terms, see the EIA website: Natural Gas Explained. One important thing to point out is that unlike oil wells, fracked gas wells rapidly lose production over a very short period of time. The table below shows the reduction in the production of wells in various parts of the U.S.

Figure 4.10: These data from the U.S. EIA's Annual Energy Outlook 2012 show the rapid decline in production from shale gas (fracked) wells.

Click for a text description of Figure 4.10.

This image is a dual-line graph titled "Figure 54. Average production profiles for shale gas wells in major U.S. shale plays by years of operation (million cubic feet per year)." It presents data on how natural gas production from shale wells declines over time across five major U.S. shale plays: Haynesville, Eagle Ford, Woodford, Marcellus, and Fayetteville.

The main graph plots the average annual production of wells over a 20-year operational period. The x-axis represents the year of operation (from 0 to 20), and the y-axis shows production in million cubic feet per year, ranging from 0 to 1,750. Each shale play is represented by a distinct colored line:

• Haynesville (blue) starts with the highest production, around 1,750 million cubic feet per year, but experiences a steep decline in the first few years.
• Eagle Ford (red) begins just below Haynesville and follows a similar sharp downward trend.
• Woodford (orange), Marcellus (green), and Fayetteville (brown) start at lower production levels and also show rapid early declines, though less steep than Haynesville and Eagle Ford.

The image also includes an inset graph titled "Percent of total EUR, cumulative", which shows the cumulative percentage of Estimated Ultimate Recovery (EUR) over time for each shale play. The x-axis again spans 0 to 20 years, while the y-axis ranges from 0% to 100%. This inset reveals that most shale wells recover the majority of their total expected gas output within the first 10 years, with all five plays reaching 80% or more of their cumulative EUR by that point.

Together, these graphs illustrate the rapid decline in production typical of shale gas wells and emphasize the front-loaded nature of gas recovery in these formations. The data highlights the importance of early-year output in determining the economic viability of shale gas development.

Credit: U.S. EIA

So, if the well output declines, how do companies keep up production? Drill more wells! In order to maintain supply, wells must be drilled at a very high rate.

Optional-What the heck happened to natural gas reserves in 2012 and 2015?

You probably noticed a sharp drop in proved reserves from 2011 to 2012 and 2014 to 2015 in the chart above. It should jump off the page at you. So what happened that year? Did the technology all of a sudden decline? Did we pull out a record amount of natural gas? Actually, this was an adjustment known as a "revision." As explained by the EIA: "Revisions primarily occur when operators change their estimates of what they will be able to produce from the properties they operate in response to changing prices or improvements in technology." Recall that proved reserves depend upon financial feasibility and the state of the technology. This is an inexact science, and the natural gas industry is constantly adjusting expectations based on those changing factors. The energy industry is nothing if not dynamic!

At any rate, you can see in the chart below that the proved reserves had MAJOR downward "revisions" in 2012 and 2015. As noted above, this was primarily the result of the price of natural gas dropping, causing companies to revise the estimate of economically recoverable natural gas downward.

You might also notice that the most consistent negative impact on proved reserves is production, i.e., what is being extracted (represented as yellow columns). But in most years, operators make up for production with increased "extensions" which are "additions to reserves that result from additional drilling and exploration in previously discovered reservoirs." So basically, drillers are usually able to find ways to get more gas out of the same wells faster than they actually extract gas (at least according to their estimates).

Figure 4.11: Components of U.S. natural gas proved reserve changes from 2004 - 2014. Note that extensions usually more than counteract the reduction from gas production.

Credit: U.S. EIA.

As you can see, when it comes to determining how much natural gas is left, well, it's complicated. (Sorry if you are tired of reading this phrase by now!) But hopefully, at this point, you have a better understanding of how the remaining amount is quantified.

While knowing the (approximate) amount of accessible natural gas is helpful, it is perhaps more useful to know how long these supplies will last. I would now like you to think about how, using proved reserves as a starting point, you could calculate the number of years of supplies remaining. (Hint: You also need to know the rate at which supplies are used.) The EIA provides the following analysis and explanation on their "How much natural gas is left and how long will it last" webpage: 

The U.S. Energy Information Administration estimates in the Annual Energy Outlook 2023 that as of January 1, 2021, there were about 2,973 trillion cubic feet (Tcf) of technically recoverable resources (TRR) of dry natural gas in the United States. Assuming the same annual rate of U.S. dry natural gas production in 2021 of about 34.52 Tcf, the United States has enough dry natural gas to last about 86 years. The actual number of years the TRR will last depends on the actual amount of dry natural gas produced and on changes in natural gas TRR in future years.

Technically recoverable reserves include proved reserves and unproved resources. Proved reserves of crude oil and natural gas are the estimated volumes expected to be produced, with reasonable certainty, under existing economic and operating conditions. Unproved resources of crude oil and natural gas are additional volumes estimated to be technically recoverable without consideration of economics or operating conditions, based on the application of current technology. EIA estimates that as of January 1, 2021, the United States had about 445 Tcf of proved reserves and about 2,528 Tcf of unproved reserves of dry natural gas...

As with coal, to determine the approximate number of years left, you just divide the estimated reserves by the annual use. (Interestingly, the EIA calculated that we would only have about 80 years left two years ago and 400 trillion fewer cubic feet.) It is notable that the EIA's number includes unproved reserves, and thus should be seen as a high-end estimate.

Feasibility and Sustainability Issues

Like coal, the natural gas infrastructure is well-established, including wells, pipelines, and power plants. As you saw in the figure on the previous page, natural gas is relatively cheap. The recent boom in natural gas production has provided a lot of high-paying relatively low-skilled jobs and has generated millions of dollars in royalties for landowners. Increased use and cheaper (upfront) cost of natural gas has allowed the widespread replacement of coal-fired power plants, which has resulted in natural gas increasing its share of U.S. electricity production from 24% in 2010 to about 33% in 2015 (when it was about even with coal), to nearly 42% as of 2023. During the same period, coal's share has dropped from 45% to about 16%. This is a major change in just over a decade!

One major benefit of this is that it has contributed to reduced CO₂ emissions that come from electricity generation in the U.S. These emissions are at their lowest level since 1993. The EIA explains that: "A shift in the electricity generation mix, with generation from natural gas and renewables displacing coal-fired power, drove the reductions in (CO₂) emissions." This is a major benefit of natural gas (and renewable energy of course!). As indicated previously, burning natural gas results in approximately half of the emissions from an equal amount of coal energy.

Figure 4.13: Carbon dioxide emissions from electricity generation in the U.S. 1990 - 2015. The emissions in 2015 were the lowest since 1993, due to natural gas and renewables replacing coal. (see the EIA website for more details about this graph)

Click for a text description of Figure 4.13.

This image is a stacked bar graph that illustrates carbon dioxide (CO₂) emissions from the U.S. electric power sector over the period from 1990 to 2015. The x-axis represents the years, starting from 1990 and ending in 2015, while the y-axis measures emissions in million metric tons, ranging from 0 to 2,500.

Each vertical bar represents the total annual CO₂ emissions for a given year and is divided into three color-coded segments based on the source of emissions:

• Dark blue represents emissions from coal, which is the dominant source throughout the entire period.
• Medium blue represents emissions from natural gas, which gradually increases over time.
• Light blue represents emissions from other sources, which contribute the smallest share.

The graph shows a clear trend: total emissions rose steadily from 1990 until around 2007, peaking just below 2,500 million metric tons. After 2007, emissions began to decline, reflecting a shift in the energy mix and possibly improvements in energy efficiency and environmental regulations. Despite the overall decline, coal remained the largest contributor to emissions throughout the entire period, although its share began to shrink in the later years as natural gas use increased.

This visualization highlights the historical reliance on coal for electricity generation in the U.S. and the gradual transition toward cleaner energy sources, particularly after the mid-2000s.

Credit: U.S. EIA.

But this is not the whole story regarding emissions. Remember that while natural gas emits about half of the CO₂ as an equivalent amount of coal when burned, natural gas itself is about 30 times as powerful as carbon dioxide in terms of greenhouse effect impact over a 100 year period and about 80 times as powerful over a 20 year period. One result of this is that methane leaks throughout the natural gas supply chain (from the well to the end-user) counteract some of the positive impacts of natural gas being a relatively clean-burning fuel. How much of an impact is open to debate. Though some research has indicated that the emissions from leaks are vastly underestimated and may be worse for climate change than coal, a recent report by the International Energy Agency found that the best scientific estimates indicate that "on average, gas generates far fewer greenhouse-gas emissions than coal when generating heat or electricity, regardless of the timeframe considered." In other words, from a climate change perspective, the IEA believes that it is better to use natural gas than coal. But that is up for debate.

So that solves the debate, right? Not so fast! The IEA makes it clear in the same report that: "The environmental case for gas does not depend on beating the emissions performance of the most carbon-intensive fuel, but in ensuring that its emission intensity is as low as practicable" (my emphasis added). In other words, based on what we know about the GHG-climate change connection, we should not just use the "lesser of two evils" (those are my words, not theirs), but seek to reduce emissions as much as possible, regardless of the source. They also point out that about half of global leakage-based emissions could be stopped with no additional cost, and in many instances, it would actually save money to reduce emissions. And even where it would cost money to prevent the leaks, in all regions it is at least as cheap or cheaper to stop methane leaks than to reduce emissions in other ways.

Some key points from these articles include:

• One of the researchers interviewed in the New York Times article states that: "Absolutely the biggest trend is the decline in coal use...Coal use dropped a further 20 percent from 2014 to 2016, to be overtaken by natural gas in 2016. Natural gas is now the number one fuel for electricity generation in the U.S." So one undeniable impact of the fracking boom (as indicated above) is that natural gas has increasingly taken the place of coal in the energy landscape (systems thinking alert!).
• Regarding fracking and contamination of water: "The overall peer-reviewed, final verdict was: 'These activities can impact drinking water resources under some circumstances. Impacts can range in frequency and severity, depending on the combination of hydraulic fracturing water cycle activities and local- or regional-scale factors.'" Widespread water contamination was not found, but there are verified cases of water supplies being tainted. On the flip side, we may not know the full impacts of fracking on water supplies for years or decades due to complex geology, especially on the U.S. East Coast.
• Further: "In Pennsylvania, there continue to be complaints and documented small incidents, researchers say, but concerns over surface activities and well integrity remain more common than those involving deep fracking; problems arise, for example, when companies leave thousands of feet of uncemented wells. Adhering to industry best practices’ and guidelines appears to eliminate most issues with deep fracking itself." The article goes on to point out that shallow wells (within 2,000 feet or so of the surface is, believe it or not, considered shallow!) pose the greatest risk in general. However, any well that is not properly sealed and cemented poses risks.
• Regarding earthquakes: "There is no longer serious doubt that activities associated with energy extraction can trigger earthquakes. Leading researchers have stated in a 2015 policy article published in Science that, to a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production. Evidence on that point involving the mechanics of these impacts has become clearer and more specific: Wastewater disposal, rather than the hydraulic fracturing itself per se, clearly causes most of the earthquakes." They go on to say that fracking itself may cause earthquakes, but that appears to very much be the exception. So to recap: the disposal of fracking wastewater by injecting into the underground formations, including oil wells, is causing earthquakes. Usually, this is not serious, but some fracking operations have been shut down due to earthquake risk.
• Regarding methane leaks: "Methane is a highly potent greenhouse gas that, if leaked in sufficient quantities, undermines at least some and potentially much of the purported emissions benefits of natural gas...Authors of another new study, published in the Proceedings of the National Academy of Sciences, PNAS, find that a 'small proportion of high-emitting wells, most of them no longer in active use, can account for most of the problem. Monitoring old wells, then, is a crucial aspect of the solution to stopping leaks, but it’s no easy task. As the PNAS study notes, the 'number of abandoned wells may be as high as 750,000 in Pennsylvania alone.'...authors of a 2017 study found that methane leaks were incredibly high across fracking operations in northwestern Canada." Overall, there is no clear verdict on this one. It is certain that there are fugitive emissions coming from oil and gas operations, but how much is up for debate. It does appear that most of the total leakage is from a few major emitters, but overall it can be difficult to monitor all leaks because of the huge number of wells in the U.S. 

There are many other sustainability concerns regarding fracking, including:

• up to 5-7 million gallons of water are used per well, much of which is unrecoverable - this is a particular problem in dry areas of the world (e.g., Colorado) where water is scarce;
• the water that is recovered is often contaminated with hazardous chemicals and substances;
• heavy truck traffic and noise are often associated with fracking, which is particularly burdensome in rural areas of the country;
• nearby landowners who receive little of the economic benefit from fracking share burdens of those who receive royalties;
• and more.

All that said, the recent fracking boom has revived the U.S. oil and natural gas industry and created or supported millions of jobs. Also, natural gas-fired power plants can also be energy to supplement renewable energy like wind. Natural gas-fired power plants can increase and decrease output quickly, much more so than coal or nuclear. So, if energy generation from solar or wind drops suddenly, natural gas can make up the difference through increased output. However, these "peaker" plants are very inefficient, and so are not good from an emissions perspective. Until widespread storage is available through batteries or other means, natural gas is under most circumstances the most reliable way to "balance the grid."

Summary

Natural gas is really a mixed bag of sustainability implications, especially with regards to hydraulic fracturing:

• The primary benefit from a sustainability perspective is that it has reduced CO₂ emissions relative to using coal.
• However, to what extent natural gas leaks have counteracted that is in question. It is possible that natural gas leaks have completely erased all emissions benefits of replacing coal.
• Fracking has created an economic boom, at least in the short term. Again, the overall benefit of this boom is dependent upon whether or not externalities are considered.
• There are many downsides, particularly with regards to environmental damage (water, air, land), but also with regards to the quality of life for some people near wells.
• Please keep in mind that (as indicated on the first page of this lesson) any energy source that emits GHGs is not sustainable. Also, consider that natural gas is non-renewable.

There has been some recent movement toward more regulation of the fracking industry, but that has lessened under the Trump Administration. Regardless, natural gas use is only predicted to increase, so the more we know about all of its impacts - good and bad - the better off we will be. Stay tuned!

The oil business is not for the faint of heart - it has always been a boom-and-bust industry since the first oil well was drilled in 1859 by Edwin Drake in Titusville, PA. Witness, for example, the changing price of oil since 1970 illustrated in the chart below, compared to the average price of electricity and natural gas in the U.S. over approximately the same period. A few things to note:

• The charts, from top to bottom, illustrate the price of electricity, natural gas, and oil, respectively.
• The oil price is its price on the international market, while the electricity and gas reflect the average cost for residential customers in the U.S.
• Also worth noting is that the nominal price (the bottom line on each chart) is how much it cost at the time, while the real price (the top line on each chart) is the price in inflation-adjusted dollars. The real price is a more accurate indication of how much it cost.
• It's very important to keep in mind that the scales of these charts are different, even though they each show the past 50 years of prices. The real price of electricity has ranged from about 25 cents to 15 cents per kWh a (40% swing) nd the real price of natural gas has varied from about $9 to about $20 per thousand cubic feet (a 120% change). However, the price of oil has swung wildly from 20 USD to over 100 USD per barrel (a 400% change).

(All of this information is publicly available from the EIA, and the charts are easy to create and interactive.) It may be a little difficult to see, but the key is to note the overall trends in real prices since 1970. (Remember - real prices are represented by the blue lines.)

Figure 4.14: These charts show a comparison between the average price of retail electricity in the U.S. (top chart), the retail price of natural gas (middle chart), and international price of oil (which the U.S. pays) since 1970 The most recent data point is from the last quarter of 2020.

Click for a text description of Figure 4.14.

Residential Electricity Prices (cents per kilowatt hour)

Year	Nominal Price of Electricity	Real Price of Retail Electricity
1968	2.30¢	15.90¢
1971	2.30¢	13.66¢
1974	3.20¢	15.12¢
1977	4.09¢	16.22¢
1980	5.36¢	15.64¢
1983	7.19¢	17.36¢
1986	7.41¢	16.24¢
1989	7.64¢	14.83¢
1992	8.23¢	14.11¢
1995	8.40¢	13.26¢
1998	8.26¢	12.19¢
2001	8.58¢	11.66¢
2004	8.95¢	11.39¢
2007	10.65¢	12.35¢
2010	11.54¢	12.72¢
2013	12.13¢	12.52¢
2016	12.60¢	12.64¢

Residential Natural Gas Prices (dollars per thousand cubic feet)

Year	Nominal Price of Natural Gas	Real Price of natural gas
1968	$1.04	$7.19
1971	$1.15	$6.83
1974	$1.43	$6.98
1977	$2.35	$9.32
1980	$3.68	$10.74
1983	$6.04	$14.58
1986	$5.83	$12.78
1989	$5.64	$10.94
1992	$5.89	$10.10
1995	$6.06	$9.57
1998	$6.83	$10.07
2001	$9.63	$13.08
2004	$10.75	$13.69
2007	$13.08	$15.18
2010	$11.39	$12.56
2013	$10.29	$10.63
2016	$10.06	$10.09

Imported Crude Oil Prices (dollars per barrel)

Year	Nominal Price of Electricity	Real Price of Retail Electricity
1968	$2.90	$20.04
1971	$3.17	$18.82
1974	$12.52	$61.08
1977	$14.53	$57.63
1980	$33.86	$98.85
1983	$29.31	$70.80
1986	$13.93	$30.55
1989	$18.07	$35.07
1992	$18.21	$31.21
1995	$17.14	$27.05
1998	$12.07	$17.80
2001	$21.99	$29.88
2004	$35.89	$45.70
2007	$67.19	$77.93
2010	$75.83	$83.62
2013	$98.12	$101.30
2016	$37.61	$37.74

Credit: U.S. EIA (Public Domain)

As you can see from the charts, the price of oil can be quite volatile, even on a year-to-year basis. The price reflects a complicated mixture of international supply and demand, and international events can (and do) severely impact the price. Note the following sudden changes in prices:

• spikes in 1973 and 1979, both of which were due to conflict in the Middle East (the so-called Oil Shocks),
• the rapid increase in prices starting around 2001 as demand outstripped supply,
• the downward spike during the Great Recession starting in 2008,
• the increase as the Recession faded, then
• the recent collapse of the oil market as supply outstripped demand due to a variety of factors, including fracking for oil
• the even more recent collapse of oil with the supply wars in Russia and Saudi Arabia, combined with collapsing demand due to Covid-19.
• the even more recent spike in price due both supply limitations (partial embargoes on Russian oil) and fear of energy market chaos to Russia's war in Ukraine

Only ~50 years of history is enough to make your head spin! Here's a good summary of these, and other oil trends in history. But this is the nature of the beast that is the international oil market. Compare that to the retail price of electricity, which has had only minor fluctuations, and mostly been in decline in terms of real prices the whole time. Natural gas prices are smoother than oil but more volatile than electricity.

Supply and Feasibility

In terms of feasibility, oil is so ingrained in modern society and its infrastructure is so well-established that there is no risk of not being able to integrate oil supplies into the economy and society. However, oil supply projections have a very interesting history, and like the price, projections of supply have been volatile. First of all, like natural gas, the calculation of proved reserves is subject to limitations of using current technology, economics, and known reserves, each of which can change from year to year. Like natural gas, for oil, proved reserves refer to "those quantities of petroleum which, by analysis of geological and engineering data, can be estimated with a high degree of confidence to be commercially recoverable from a given date forward, from known reservoirs and under current economic conditions" (Source: CIA Factbook). The result (again, like natural gas) is that even though oil use is increasing globally every year, there are paradoxically more proved reserves. Please note that the chart below represents global proved reserves.

How is it possible that we can continue to use more oil each year, yet the estimated remaining supplies keep increasing? The primary reason is improving technology. We have so far been able to exploit new resources as the market demands more oil. The most recent increase in proved reserves, especially in the U.S., is from shale oil that can be extracted through hydraulic fracturing (aka fracking). There has been an oil boom that has come in lock-step with the recent natural gas boom, all due to fracking. Access to additional "unconventional" reserves via tar sands in Canada has also contributed to the increase in proved reserves and supply.

Dr. Conca makes it clear that despite dire warnings of "peak oil" since the 1970s: "For every barrel of oil consumed over the past 35 years, two new barrels have been discovered." In other words, technology has increased the available oil despite the fact that humans have been using it at an increasing rate for over a century. For the past 15 or so years, fracking (and directional drilling) is the main reason that proved reserves have increased. He also provides some insight into the global nature of the oil industry when he notes that Saudi Arabia and other OPEC countries purposefully decreased the price of oil by refusing to cut the output of oil in an attempt to starve out American competition. In short, peak oil will not come any time soon, but Dr. Conca notes that: "Unfortunately, the environmental cost of unconventionals is even greater than for conventional sources." This is important to keep in mind, as fracked oil has the same negative impacts as fracked natural gas.

So, how much oil is left, and how long will it last? Unfortunately, that is an impossible question to answer with certainty. In 2022 BP released its well-regarded annual Statistical Review of World Energy and determined that there is enough oil to satisfy global needs for 53.5 years, but only if we continue on our current trajectory. (This includes the recent boom in proved reserves.) This is not a very long time if you think about how important oil is to society.

Also, keep in mind that as we approach this point of exhaustion, the price of oil - and all of the goods that depend on it, which is basically, you know, everything - will increase. Yet, there are people like energy reporter Jude Clemente of Forbes magazine stating that oil will basically never be economically unavailable. In 2016, McKinsey and Company, a highly respected global research firm, reported that the world may actually reach peak demand (not peak supply, as is usually referred to) for oil by around 2025. This was unheard of only a few years ago, but the combination of oil extraction technology, energy efficiency, renewable energy, and energy policy may make the era of oil over before oil becomes scarce. (Note that I wrote MAY, not WILL!) The video below from Bloomberg illustrates how this might occur (3:40 minutes).

It is impossible to know who is right, that is until the future happens. There is a risk associated with this, as you will see below (especially if we keep getting oil through particularly damaging methods such as oil sands). But in terms of raw physical resources, the future is difficult to predict. We may run out of oil at some point, given that it is a finite resource. It is almost certain that before we reach the physical end we will reach a point where other issues (e.g., sustainability impacts, economics, or even reduced demand) cause the collapse of the oil industry. You've heard it before, so this should be no surprise: when it comes to predicting the future of oil, folks, it's complicated.

Sustainability Issues

Oil is extremely important to the functioning of modern society, as noted in a previous lesson. A little under 40% of all of the energy used in the U.S. is from oil (the biggest primary energy source in the U.S., you may recall from a previous lesson), and in addition to that, oil is used in the manufacture of common things like plastic, car tires, and asphalt. It is energy-dense, and relatively easy to transport. Around 150,000 people in the U.S. work in the oil and gas extraction industry, and possibly millions more are supported by oil and gas. Oil is intertwined with every industry in the U.S. It has allowed food to become cheaper and made international and other long-distance travel more accessible. Do you think you could get two-day shipping from Amazon without readily available oil? Electricity and other alternatives can be used to substitute for many of these functions, but for now, it is oil that is the dominant force. A lot of this helps provide some quality of life improvements, and even some equity advantages (e.g. cheaper food). But it does come at the expense of other sustainability aspects, particularly the environment.

One of the problems with not knowing how much oil is left is that it makes it easier to justify not planning for its eventual unavailability. As discussed above, energy (and oil) is deeply ingrained in modern society. When oil shocks happen, they have a severe negative impact on the economy. If we knew exactly how much oil we had left, and how much we were using, society would be able to prepare for its demise. But because we do not know this with certainty, very little has been done to prepare for it. This is a sustainability issue for many reasons. Primary among them is that if we do not reduce our dependence on oil, there will be a lot of suffering when the next oil shock happens. This is an economic and equity issue primarily, as oil scarcity will hit us economically, and the poor will be most affected, especially at the beginning. I'll leave it to you to think about what those that practice the precautionary principle would advise!

But there are a lot of reasons to be concerned about the current use of oil. First of all, recall from the chart on the Sustainability of Coal page from this lesson that oil is second only to coal in global carbon emissions. There is no practical way to prevent the emission of carbon dioxide when an oil product like gas is burned. Given the gravity of the issue of climate change, this is an essential consideration.

Yet another climate change implication is the use of gas flaring. Frequently, natural gas is found (and hence extracted) along with oil because they often form together underground. When a facility is designed to handle oil and not natural gas, the gas is "flared." Flaring entails separating the gas from the oil, then burning it off and not using any of it. This seems wasteful, right? So how much gas is flared each year? According to the World Bank 141 billion cubic meters of natural gas was flared around the world in 2017, which was actually down a bit from 2016. This is about twice the annual total usage of natural gas in the U.S. each year! In terms of emissions, it results in about 350 million tons of carbon dioxide, which according to the World Bank is equivalent to the emissions from about 77 million cars. That is about 1% of total annual emissions worldwide, or about 7% of U.S. energy-related emissions. (Translation: That's a lot of CO₂!) This is being addressed but is still a major problem.

Figure 4.16: Gas flare from atop oil rig in the North Sea. Gas flares worldwide account for nearly 1% of all carbon dioxide emissions.

Credit: Varodrig, CC SA-BY 3.0

There are a number of other emissions associated with the burning of oil products like diesel and gasoline, including nitrogen oxides and volatile organic compounds (which cause lung damage), sulfur dioxide (acid rain and some health impacts), particulate matter (asthma, bronchitis, visual pollution, possibly lung cancer), and others (source: U.S. EIA). Exposure to automobile exhaust has been found to increase hospital admissions for people with lung disorders (asthma, bronchitis, pneumonia, etc.). Nearly all of these impacts are externalities because they are not included in the price of oil, it should be noted.

Also, all of the issues associated with fracking, in particular, the heavy use of water (see the Natural Gas Sustainability page) are the same for shale oil. Another unconventional source of oil is Canada's oil sands (sometimes referred to as tar sands). 97% of Canada's known reserves come from oil sands, and they have such a large reserve that they are second only to Saudi Arabia and Venezuela in terms of proved reserves. Oil sand extraction is particularly damaging to the natural environment and has a very low EROI (see Lesson 2). Canada is the U.S.'s largest supplier of foreign oil (over 4 million barrels per day in 2023), almost all of which is from oil sands.

As you will see in the article below, oil is often associated with the so-called "resource curse" when it is controlled by corrupt governments. This problem has historically been especially acute in African countries like Nigeria, but oil revenues have propped up many undemocratic regimes elsewhere, e.g. Middle Eastern Countries (Iran, Iraq) and South American Countries (like Venezuela). Finally, oil spills are a common occurrence, some larger than others. Since 2000, hundreds of thousands of metric tons of oil have been spilled worldwide. Some of these spills are more damaging than others.

Oil is an extremely useful resource, and it is a very important aspect of the modern economy, and by extension, society. Considering that current projections assert that we only have about 50 years of supplies left, we should probably try to maintain our resources for as long as possible, and avoid an abrupt collapse. But we also should be conscious of the sustainability impacts of its extraction and use. Climate emissions are all but unavoidable when it comes to oil use, and there are many other sustainability impacts to consider as well. It is becoming increasingly likely that much of our automobile-based demand for oil will diminish, but recall that we only have about 10 - 15 years to significantly reduce our global carbon dioxide emissions. If we do not significantly reduce oil use soon we are unlikely to hit that target.

Nuclear energy has been a hot-button issue for a very long time, both domestically and internationally. It provides about 10% of the global electricity supply, as you can see in the image below.

Figure 4.17: As you can see, nuclear provides around 10.4% of global electricity, second only to hydroelectric in terms of carbon-free electricity. Note also that coal is a source of about 37% globally, but around 22% domestically. Click on the image for access to a larger, resizable image.

Text description of figure 4.17.

The image is an informative infographic from Our World in Data that compares the sources of global electricity generation with those of total energy consumption, highlighting the disparity in low-carbon energy usage between the two. Titled "More than one-third of global electricity comes from low-carbon sources; but a lot less of total energy does," the graphic is split into two sections: "Electricity only" and "Total energy (electricity, transport & heat)."

In the electricity-only section, fossil fuels dominate with coal (36.7%), gas (23.5%), and oil (3.1%), totaling 63.3%. Low-carbon sources make up the remaining 36.7%, led by hydropower (15.8%), nuclear (10.4%), wind (5.3%), solar (2.7%), and other renewables (2.5%).

In contrast, the total energy section—which includes electricity, transport, and heating—shows an even heavier reliance on fossil fuels: oil (33.1%), coal (27%), and gas (24.2%), summing up to 84.3%. Low-carbon sources contribute only 15.7%, with nuclear (4.3%), hydropower (6.4%), wind (2.2%), solar (1.1%), and biofuels and other renewables each contributing less than 0.9%.

Credit: Our World in Data, CC BY

Supply

As you (hopefully) recall from Lesson 1, nuclear energy is non-renewable. Uranium is by far the most-used nuclear fuel, though there are possible alternatives (such as thorium). As with other non-renewable fuels, all of the uranium that is on earth now is all that we will ever have, and estimates can be made of the remaining recoverable resources. As you will see in the article below, at current rates of consumption, we will not run out of uranium any time soon. But - at risk of sounding like a broken record - this highly depends on a number of variables, including keeping consumption at current levels, technology not advancing, estimates of reserves changing, and so forth. If, for example, we waved a magic wand and doubled the output of nuclear power tomorrow, the estimated reserves would last half as long.

The World Nuclear Association (WNA), an industry association, provides a very thorough explanation of possible complicating factors, but they state that at current rates of consumption, as of the summer of 2025 the world has enough reserves to last about 90 years. The Nuclear Energy Agency (NEA), like the WNA, is effectively an industry group and has a wealth of expertise at its disposal. It operates out of the OECD (remember them from Lesson 1?) in Paris. They are a pro-nuclear group, but are very good at providing technical data, as well as statistics. They indicate that as of 2018, the world had about a 130 year supply of uranium. Meanwhile, the International Atomic Energy Agency reported that as of 2025 supplies could run out by 2080.

Feasibility

The first nuclear power plant came online in 1954 in Russia (then the Soviet Union), and according to the World Nuclear Association, there are 436 reactors worldwide and another 59 under construction. The technology is well-known by now, and despite the extreme danger posed by nuclear meltdowns, there have been very few major incidents. You are probably familiar with the Fukushima Daichi meltdown that happened in 2011, and perhaps heard of Chernobyl in the Ukraine in 1986 (still the worst nuclear disaster to date), and maybe even Three Mile Island in the U.S. in 1978. Here is a partial list of nuclear accidents in history from the Union of Concerned Scientists (UCS).

But putting aside this risk at the moment, nuclear energy has shown itself to be a viable source of electricity, and likely will continue to be used for the foreseeable future. Among other things, nuclear power plants generally have a useful lifetime of around 40-60 years, so we are "locked in" until mid-century at least. That said, increasing the use of today's nuclear technology would likely pose some problems, for a variety of reasons. The article below sums up these and a few others reasons for and against nuclear energy.

Sustainability Issues

Okay, now for the fun part. Nuclear energy is a mixed bag in terms of the question of sustainability. The biggest dilemma for those concerned about anthropogenic climate change but skeptical of nuclear is that nuclear energy is considered a carbon-free source, and since it is responsible for a significant portion of non-fossil fuel based electricity production worldwide and is a proven and reliable source, it is seen by many as a good option. Note that despite being considered "carbon free," nuclear energy results in some lifecycle emissions because of the materials used in mining, building the power plant, and so forth. (Lifecycle emissions are all the emissions generated by all processes required to make an energy source, including things like mining of materials, manufacturing of equipment, and operating equipment.) But according to the National Renewable Energy Laboratory (NREL), a U.S. National Lab, it has approximately the same lifecycle emissions as renewable energy sources.

Figure 4.19: Lifecycle emissions of select energy sources. This chart indicates the average grams of CO₂ emitted per kWh of electricity over the life of each energy source. As you can see, solar photovoltaics, concentrated solar, wind, and nuclear have near-zero emissions (median of less than 50 g/kWh), while coal averages close to 1,000 g/kWh, which is over 20 times more emissions than most of the other sources in this chart. Oil is a close second at around 800 g/kWh, and natural gas closer to 500 g/kWh. Biopower has a median of less than 50 g/kWh, but can be as high as 1,300 g/kWh.

Credit: U.S. National Renewable Energy Laboratory, public domain

Nuclear energy is a very reliable source of electricity, and power plants can operate at near full capacity consistently. Once a plant is built, electricity is relatively inexpensive to generate. But nuclear energy is very expensive in terms of lifetime costs (as you'll see in the article below), and the waste from nuclear reactors can remain dangerous for thousands of years, which can result in large externalities. Since they are so expensive, there is an incentive to keep a plant online for as long as possible to recoup costs. Thus people are effectively "locked in" once a plant is built. There is, of course, the risk of another disaster, which however rare the possibility, could be catastrophic. There are also some issues with the equity impacts of uranium, particularly in terms of mining. There is not an easy answer here, as there are reasonable and strong pros and cons.

Did you guess the issues I have with the first article? First, the author calls nuclear a very "efficient" energy source. If you recall from previous lessons, the efficiency of a nuclear power plant hovers around 35%. It is, however, energy dense (a lot of energy by volume), which is what he describes as "efficient." (Though he also mentions energy density as well, confusingly.) The second - and more subtle - problem I have is with the assertion that nuclear is an "inexpensive" energy source. This was clearly indicated in the second article (if you read it) but is also asserted by the EIA. Nuclear plants are inexpensive to run once they are built, but they are extremely expensive to build. The author glosses over that part, but it is a really important consideration. Finally, he says that nuclear is "sustainable" but as you know by now, any energy source that is non-renewable is not sustainable.

Regarding the cost of nuclear: The high up-front cost makes nuclear power one of the most expensive types of electricity available. For a technical discussion of this, feel free to read through this description of levelized cost of electricity from the EIA, which indicates that over the lifetime of the energy source, nuclear is more expensive than geothermal, onshore wind, solar, hydroelectric, and most types of natural gas plants. 

Summary

Nuclear is a mixed bag. To summarize:

• Nuclear is reliable and almost carbon-free, but is non-renewable.
• Nuclear is relatively inexpensive to operate after established, but the high up-front cost makes it one of the most expensive electricity sources.
• Because power plants are so expensive to build, once they are built they are generally used for as long as possible, as long as they can be operated profitably. (Gotta get that investment back!) We are effectively "locked in" once they are built.
• When accidents happen, they can be catastrophic, but they are extremely rare.
• The waste product from nuclear power plants is dangerous for thousands of years, and right now, we have no way of safely disposing of it - it is kept in storage, usually at the power plants themselves. This has not shown to be a major problem yet, but society will be dealing with the wastes for thousands of years.

Nuclear is a very controversial source of energy. It is embraced by many as a key to a carbon-free future, while many think we should move away from it because of its inherent danger and/or expense and/or general sustainability problems. There are arguments to be made on each side. Hopefully, you have a better handle on some of them after reading through this.

Given the scope of this lesson and course, I will limit the focus of this page to the three most prominent renewable electricity technologies: solar photovoltaic, wind, and hydroelectricity.

Supply and Feasibility

I've combined these two sections into one because the supply aspect is very straightforward for wind and solar: they are inexhaustible! As stated in Lesson 1, both of them get their energy from the sun, and if the sun stops shining we have more important issues to deal with than not having a source of renewable electricity. The amount of solar energy that hits the earth in about one hour is enough to power the world for an entire year (this is a commonly held fact, but here is one source from Sandia National Laboratories). There is no shortage of solar energy!

As for hydroelectric, though it also gets its energy from the sun, it is limited due to its dependence on the availability of flowing water. As of 2024, about 14% of the world's electricity came from hydroelectricity. According to the International Energy Agency, there is about 5 times as much technical potential for hydroelectric worldwide as is currently generated today. We certainly would not want to exploit all of it, given some of the environmental impacts of large hydroelectric facilities (see below), but this number does provide a frame of reference.

Figure 4.20: Global electricity and energy fuel mix. Renewable electricity accounts for around 25% of total electricity generation, but only about 15% of total energy use in 2021.

Click here for text description of Figure 4.20.

The feasibility is a mixed bag. An oft-cited paper by Mark Jacobsen and Mark Delucchi of Stanford University showed that through wind, water (hydroelectricity), and solar, all of the world's energy needs could be met by 2030, or in a less aggressive scenario, 2050 (note that this is all energy, not just electricity). This assumes that energy efficiency would increase worldwide by 5% - 15%. According to their research, this could be done using existing technologies and would require the use of about 1% of all dry land on earth. They assert that the barriers to accomplish this are "social and political, not technological and economic." They calculate that it would cost $100 trillion over a 20 year period. There are a lot of other details to this study - way too many to get into here.

There are no shortage of critiques to this study, including this critique from Ted Trainer of the University of New South Wales, who is an advocate of renewable energy. He cites possible underestimates in their cost calculation, underestimates of the amount of energy required to provide a high quality of life (the authors assume that the per person energy use in 2050 would be about 1/6th of the current per person energy use in Australia, for example), and probably overestimate the reasonableness of electric storage capacity, among other things. If nothing else, this plan would require an alteration to the global energy infrastructure at a pace and scale that has never been seen before. It is almost certainly possible with enough political and social will, but it would take a lot of both.

One sign that bodes well for renewables is that the cost has come down significantly in recent years. In the U.S. the cost of generating electricity from wind and hydroelectric - assuming they are sited and installed properly - is cost-competitive with fossil fuels, even without incentives. Residential-scale solar is still relatively expensive, but utility-scale (large arrays) are cost competitive today. This is all based (as you will see below) on the levelized cost of electricity (LCOE), which was noted in the nuclear lesson. The LCOE is the amount it costs to generate each unit of energy (usually measured in $/megawatt-hour) on average over the lifetime of an electricity source.

Levelized Cost of Energy (LCOE): the amount it costs to generate each unit of energy (usually measured in $/megawatt-hour) on average over the lifetime of an electricity source.

This includes everything from building the power plant (e.g. nuclear plant, solar array, wind turbine), to purchasing the energy source (e.g. coal, natural gas), to operating the plant, to decommissioning the plant at the end of its life. To calculate the LCOE, you take the total lifecycle costs and divide it by the total electricity output over the lifetime of the source. This is of course not including externalities, which would likely make renewable energy cheaper right now, especially if the social cost of carbon were to be considered.

Credit: Our World in Data, CC BY

The feasibility is a mixed bag. An oft-cited paper by Mark Jacobsen and Mark Delucchi of Stanford University showed that through wind, water (hydroelectricity), and solar, all of the world's energy needs could be met by 2030, or in a less aggressive scenario, 2050 (note that this is all energy, not just electricity). This assumes that energy efficiency would increase worldwide by 5% - 15%. According to their research, this could be done using existing technologies and would require the use of about 1% of all dry land on earth. They assert that the barriers to accomplish this are "social and political, not technological and economic." They calculate that it would cost $100 trillion over a 20 year period. There are a lot of other details to this study - way too many to get into here.

There are no shortage of critiques to this study, including this critique from Ted Trainer of the University of New South Wales, who is an advocate of renewable energy. He cites possible underestimates in their cost calculation, underestimates of the amount of energy required to provide a high quality of life (the authors assume that the per person energy use in 2050 would be about 1/6th of the current per person energy use in Australia, for example), and probably overestimate the reasonableness of electric storage capacity, among other things. If nothing else, this plan would require an alteration to the global energy infrastructure at a pace and scale that has never been seen before. It is almost certainly possible with enough political and social will, but it would take a lot of both.

One sign that bodes well for renewables is that the cost has come down significantly in recent years. In the U.S. the cost of generating electricity from wind and hydroelectric - assuming they are sited and installed properly - is cost-competitive with fossil fuels, even without incentives. Residential-scale solar is still relatively expensive, but utility-scale (large arrays) are cost competitive today. This is all based (as you will see below) on the levelized cost of electricity (LCOE), which was noted in the nuclear lesson. The LCOE is the amount it costs to generate each unit of energy (usually measured in $/megawatt-hour) on average over the lifetime of an electricity source.

Levelized Cost of Energy (LCOE): the amount it costs to generate each unit of energy (usually measured in $/megawatt-hour) on average over the lifetime of an electricity source.

This includes everything from building the power plant (e.g. nuclear plant, solar array, wind turbine), to purchasing the energy source (e.g. coal, natural gas), to operating the plant, to decommissioning the plant at the end of its life. To calculate the LCOE, you take the total lifecycle costs and divide it by the total electricity output over the lifetime of the source. This is of course not including externalities, which would likely make renewable energy cheaper right now, especially if the social cost of carbon were to be considered.

As you can see from this table, almost all renewables are cost-competetive with non-renewables and in most cases, cheaper, including wind and solar plus storage. Note that wind and utility-scale solar have the lowest low-end cost.

The bottom line in terms of cost is that right now, well-sited wind and utility scale solar ("utility scale" refers to large arrays, usually hundreds or thousands of panels in size) are the cheapest form of electricity available, other than only the least expensive natural gas power plants. (Please note that, as stated in a previous lesson and the Greentech Media article above, energy efficiency is cheaper than all energy sources!) Other renewable sources such as small hydroelectric, biomass, geothermal, solar thermal, and commercial-scale solar are very cost-competitive with coal and natural gas, and generally less expensive than nuclear. All of this does NOT include subsidies, by the way!

Sustainability Issues

All three of these sources are considered carbon-free, so they are ideal with regards to anthropogenic climate change. Even after consideration of the embodied energy of these sources - hydroelectric dams require a LOT of concrete; solar panels are manufactured in an energy-intensive process, as are wind turbines; and all of them require mining - the lifecycle carbon footprint is minimal for renewables, as you can see in the chart below. In terms of climate change concern, there is really no debate: these renewables are great choices.

Figure 4.23: As you can see from this chart, solar PV, wind, and hydroelectric all have near-zero lifecycle emissions on a per kWh basis. Note that the next highest emission rates are natural gas, coal, and oil, respectively.

Click here for text description of Figure 4.23.

Electricity Generation Technologies Powered by Renewable Resources

Near-zero lifecycle emissions on a per kWh basis: Biopower, Photvoltaics, Concentrating Solar Power, Geothermal Energy, Hydropower, and Ocean Energy.

Electricity Generation Technologies Powered by Non-Renewable Resources

Near-zero lifecycle emissions on a per kWh basis: Wind Energy

Highest emission rates include: gas, coal, and oil.

Count of Estimates and References for Electricity Generation Technologies Powered by Renewable Resources

Count of Estimates	222(+4)	124	36	8	28	10	126
Count of References	52(+0)	26	10	6	11	4	49

Count of Estimates and References for Electricity Generation Technologies Powered by Non-Renewable Resources

Count of Estimates	125	83(+7)	24	169(+12)
Count of References	32	36(+4)	10	50(+10)

Credit: National Renewable Energy Laboratory, Public Domain.

However, there are some other considerations to make in terms of sustainability. First, large hydroelectric facilities are not very environmentally friendly. Depending on the location, there can be problems with flooding of habitats and even towns, compromising fish migration, altering stream content and temperature, impacting scenic areas, and other considerations. The articles below provide some insight into some of these potential problems. Note also that not all hydro has the same problems - by using different types of hydroelectric facilities such as run-of-the-river and microhydro, environmental and social impacts can be minimized.

In terms of social equity, there are a few important considerations to make. First of all, do people have access to energy, and can they afford it? This is a tricky question to answer, as it depends on a lot of factors, many of which were indicated above. Some equity and other considerations include:

• Solar panels used to be prohibitively expensive, but new business models are making them much more affordable, even effectively free to the customer. See for example community solar and power purchase agreements.
• Wind and hydro can usually be purchased through utilities at the same or lower rates than fossil fuels.
• One very important equity consideration is that fossil fuel power plants are often sited near low-income areas of the country and world, and thus the negative health impacts are unevenly distributed, with the least powerful bearing the brunt. (The term "environmental justice" hopefully comes to mind for you right about now!) Add to the previous point that the environmental destruction from coal mining is especially unevenly distributed.
• Wind turbines can be a nuisance, as indicated in the article above, but is relatively minor compared to power plants.
• Solar is usually unobtrusive, though some people do not like "the look" of the panels.
• Large hydro can have a major social cost, as you saw in the article above. Flooding of houses and even whole towns can result from dams being built, though this is more the exception than the rule in industrialized countries. As always, these impacts are disproportionately felt by low income and marginalized people.

One of the benefits of conventional energy generation is that the infrastructure is largely set up, at least in industrialized countries. In the U.S., for over 100 years, we have built an energy infrastructure based on large power plants and fossil-fuel based vehicles. This gives conventional energy sources an advantage in terms of providing access. That said, wind, hydro, and solar can all utilize the existing infrastructure. Hydroelectric dams provide the same service as fossil-fuel power plants, but usually on a slightly smaller scale, so they are a good fit. They also provide a very consistent stream of electricity as long as no droughts are occurring, and they can increase and decrease production pretty rapidly, unlike solar and wind.

Probably the biggest current problem with solar and wind is that they are intermittent - the sun does not always shine and the wind does not always blow. This is a major issue because we currently do not yet have widespread storage capabilities to provide the energy on demand, though improving battery technology and policy are rapidly changing that. As you can see in figre 4.21, solar and wind plus storage are now cost-competitive with fossil fuels, and costs continue to drop. This has resulted in some utilities agreeing to purchase or build "solar + storage" facilities because it makes economic sense. For the sam reasons, multiple "wind + storage" facilities were online as of 2020. The future of renewables is in storage! (Side note: If any of you discover an energy dense, cheap storage medium that is abundant and safe, feel free to give me a cut of your billions of dollars in wealth!)

One common problem with wind and solar are that they are often highest in areas with low population densities. In the U.S., for example, the greatest onshore wind resources are in the Great Plains in the Midwest, where the population density is very low. Because of this, a lot of infrastructure (high voltage power lines for example) will need to be added to fully tap into these resources. That said, as you can see in the map below, offhore wind resources match up pretty well with heavily populated areas. However, offshore wind is currently expensive.

Figure 4.24: Wind resources in the U.S. Note that the best onshore resources are located in the Midwest, which has a relatively low population density. The offshore resources are very good, and are close to many population centers. Offshore farms are expensive, and, ironically, because they are proposed near population centers, there is often significant public resistance to putting them there. There are currently no offshore farms in the U.S., but there are many worldwide. Click on the image for a larger version.

Credit: National Renewable Energy Laboratory, Public Domain.

One of the benefits of solar is that as long as there is not too much shading, many households can satisfy their energy needs using existing rooftop spaces. However, not every location is ideal for solar. 

Summary

Overall, the biggest advantages of renewable energy are:

• they are inexhaustible (hydro has limits);
• they are effectively carbon free;
• they have very minimal environmental impacts (notwithstanding large hydro);
• they also tend to be more democratic, in that many of them are suitable for scaling down to a personal level. This is not possible with current technology for fossil fuel-based electricity generation;
• most of them are also cheaper than almost all forms of non-renewable energy.

The main disadvantages of solar, wind, and hydro are:

• solar and wind are intermittent, and so without storage cannot be relied upon to deliver energy when needed;
• they are not able to be deployed in every location, e.g., shaded areas for solar, calm areas for wind, and dry areas for hydro;
• large-scale hydro has a lot of negative environmental impacts, and there are some environmental and social problems with wind.

The last things I'd like to note are the following:

1. Regardless of any other sustainability considerations, as noted in the beginning of this lesson, the world's energy (not just electricity!) must come from carbon free sources relatively soon if we are to avoid climate catastrophe. Remember - we must be carbon neutral by around 2050 is the goal to limit warming to 1.5 C!
2. The most sustainable energy is the energy that you don't use. Remember that energy efficiency is sometimes called the "fifth fuel?" That is very much applicable to these considerations. Also, as noted above, energy efficiency has been found to have a lower LCOE than any other energy source! The more we can reduce our energy use while getting the same benefits from the energy service, the better off we will be.

Summary

That's it for this week! Please make sure you complete the two required assignments listed at the beginning of this lesson. This week, we went over a lot of the supply and sustainability implications of a variety of modern energy sources. You should be able to do the following after completing the Lesson 4 activities:

• analyze current supply and feasibility of a variety of energy sources;
• differentiate between various projections of remaining energy supply and the impacts of technology and price considerations on them;
• describe the trends in U.S. electric power production with regards to fuel use and carbon dioxide emissions;
• describe the complexity of predicting oil supply and prices; and
• describe and analyze sustainability implications of contemporary energy use.

The Language of Sustainability

We went over a lot of fairly heavy concepts this week. Hopefully, this list will help spark some memories of the content, both now and as we move forward:

• demonstrated reserve base, estimated recoverable reserves, carbon capture and sequestration;
• hydraulic fracturing (fracking), directional drilling, proved natural gas reserves, shale gas;
• real price, nominal price, proved oil reserves, peak oil, peak demand, gas flaring, unconventional oil sources;
• lifecycle emissions;
• solar energy, wind energy, hydroelectric energy.

Reminder - Complete all of the lesson tasks!

You have finished Lesson 4. Check the list of requirements on the first page of this lesson and the syllabus to make sure you have completed all of the activities listed before the due date. Once you've ensured that you've completed everything, you can begin reviewing Lesson 5 (or take a break!).