Hopefully, by this point, you have a reasonably good grasp of overarching and specific topics within sustainability. Please keep in mind that all of these concepts are intertwined and overlapped. What can I say? The real world is a messy, complicated place. But, hopefully, you will keep these in mind as you move forward in this class and in your life. These will pop up in the readings, book, and other materials in this course, and I hope that you will recognize the terms and concepts as you encounter them. Feel free to look back at these lessons if you need a refresher.
In this lesson, you will take a relatively deep dive into various sources of energy, including the fossil fuels, nuclear, and renewables. These were all introduced in Lesson 1; but in this lesson, we will look at these sources through the lens of sustainability. Specifically, we will investigate how much of each source is probably left (supply), how feasible continued use of the source is (feasibility), and some (not all, mind you!) sustainability implications of each source.
By the end of this lesson, you should be able to:
Please note that the quiz can only be taken once. You have unlimited time to complete it prior to the deadline, and can save your progress and pick up where you left off at a later time. See the Assignments and Grading section of the syllabus [1] for tips on how to do this. Once you submit the quiz, you cannot change answers. All saved answers will automatically be submitted at the deadline if you have not submitted them.
Requirement | Submission Location |
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Lesson 4 Quiz | Canvas - Modules tab > Lesson 4 |
Continue posting to the Yellowdig discussion board. | Canvas - Modules tab > Lesson 4 |
(Optional) Lesson 4 Extra credit quiz | Canvas - Modules tab > Lesson 4 |
If you have any general course questions, please post them to our HAVE A QUESTION? discussion forum. I will check that discussion forum regularly to respond as appropriate. While you are there, feel free to post your own responses and comments if you are able to help out a classmate. If you have a question but would like to remain anonymous to the other students, email me..
If you have something related to the material that you'd like to share, feel free to post to the Coffee Shop forum, also under the Discussions tab.
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.
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 then 2 C. (It is about 2 F warmer already!) The following is a quote from the latest report [2] from the IPCC, which was published in 2023. This is from the Summary for Policymakers [3]. (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 if we want the best chance of preventing the worst impacts of climate change. The UN published a report in 2018 that stated that to reach this, we need to cut global emissions by 40% - 50% by 2030 (that's really not long from now!) (See the NPR summary [4] if you are interested). 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 [5] 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, which is 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.
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself.
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.
Most energy sources have an "industry organization" or "industry association" associated with it. These organizations are funded by companies in the industry (e.g., the World Coal Association [9] is funded mostly by coal producers, energy companies that rely on coal, and others [10]) and promote policies that will increase the use of the energy source. They often fund and perform research, devise ad campaigns, lobby various levels of government, write press releases, and more. You should not view them as impartial - they exist solely to promote the energy source. However, they are generally a reliable source of data, e.g., how much coal was used in a year, the portion of GDP from coal, etc. They are also good sources of information regarding industry technology and trends.
But please keep this in mind when looking for energy data: if at all possible, you should use information (including data) from the EIA (Energy Information Administration), IEA (International Energy Agency), World Energy Council [11] (WEC), or some other reliable, impartial source. Note that these industry organizations will usually use data from one of these impartial sources, but it's always best to go to the original source.
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.
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.
There are many benefits to living in the United States, but having easy to understand energy units is not one of them. We use a mixture of Imperial and English units, with the system usually referred to as U.S. Customary units. Most of the rest of the world uses metric units, which are also considered SI units (Systéme international d'unités). Got all that? Good. (Here is an explanation of how convoluted the non-metric units are [17], if you are so inclined.)
Coal in the U.S. is usually measured in tons, which is a unit I'm sure you have heard of, and likely used, before. A U.S. ton is equivalent to 2,000 pounds. However, to prevent confusion with an Imperial ton, the U.S. ton should be referred to as a short ton. A long ton, on the other hand, weighs 2,240 pounds. Finally, the metric ton, which is also known as the tonne, is equivalent to 1,000 kg, or about 2204.6 lbs. To summarize:
Credit: Encyclopaedia Britannica [18] and U.S. EIA [19]
"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 2020, of about 0.535 billion short tons, the recoverable coal reserves would last about 470 years, and recoverable reserves at producing mines would last about 25 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 348 years to 325 years to 332 years to 357 years, all in the span of four 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.
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!).
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 [22]. 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.
Coal use has been on the decline in the U.S. for the past 15 - 20 years. Feel free to read the following article for some insight into what is causing this, but also some of the issues with coal on an international scale. Note the prominent role that insurance companies (and re-insurance companies) are playing.
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?
Now the bad news: coal has a lot of negative environmental and social impacts.
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. [29]
One possible solution to this is carbon capture and sequestration (CCS) [30], 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 [31] 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 rate [31]s than other sources.) While promising, there is some indication [32] that CCS might not be as effective as once hoped. It is only beginning to be demonstrated on a commercial scale [33], and some plants have had major issues [34], 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 [35]. In addition to this, as pointed out in the EIA article above [24], 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 [36] 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 [37], 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 [38] 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.
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself.
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.
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 [42]). 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.
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 [44]. 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.)
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 [46]. 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.
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.
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).
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 [50]" webpage:
The U.S. Energy Information Administration estimates in the Annual Energy Outlook 2021 that as of January 1, 2019, there were about 2,867 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 2019 of nearly 34 Tcf, the United States has enough dry natural gas to last about 84 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, 2019, the United States had about 475 Tcf of proved reserves and 2,392 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.
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 40% as of 2021. During the same period, coal's share has dropped from 45% to about 22%. This is a major change in just over a decade!
As budding energy and environmental experts, you should be familiar with industry terminology. The percent of electricity that a country (or other area) gets from various sources is referred to as "electricity fuel mix." Figure 4.12 is thus a chart that details electricity fuel mix in the U.S. The total energy by source (e.g. the Sankey chart we looked at in lesson 1) is the "energy fuel mix."
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.
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 [56] 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 [57] 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.
As noted above, natural gas is a very controversial issue, specifically with regard to fracking. Some of the issues involved are outlined in the articles below. To say that this only scratches the surface of information on this topic is a massive understatement! I encourage you to research this issue further.
Some key points from these articles include:
There are many other sustainability concerns regarding fracking, including:
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 [66]. 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."
Natural gas is really a mixed bag of sustainability implications, especially with regards to hydraulic fracturing:
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!
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself.
The oil business is not for the faint of heart - it has always been a boom-and-bust [67] 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:
(All of this information is publicly available from the EIA [68], 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.)
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:
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 [71]. 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.
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 [72]). 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. James Conca provides a very good explanation of the somewhat complex workings of the global oil market in the article below. As you will see, the price of oil and the economic feasibility of technology is not as simple as supply and demand. He also throws in a nice lesson on how fossil fuels are formed for good measure. Also, if, like me, you have found yourself wondering whether oil deposits are more like a jelly donut or tiramisu, he'll help you out with that as well.
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 [77] 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 [78]. In 2016, McKinsey and Company [79], 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.
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 [83], and possibly millions more are "supported" by oil and gas [84]. 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 [85] 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 [86] 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 [87]. (Translation: That's a lot of CO2!) This is being addressed but is still a major problem.
There are a number of other emissions [89] 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 [89]). Exposure to automobile exhaust has been found to increase hospital admissions [90] 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 [91] come from oil sands, and they have such a large reserve that they are second only to Saudi Arabia and Venezuela [91] 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 [92] in 2021), almost all of which is from oil sands.
Encyclopedia Britannica provides a short explanation of the environmental impacts of Canadian tar sands, also know as oil sands.
As you will see in the article below, oil is often associated with the so-called "resource curse [94]" when it is controlled by corrupt governments. This problem has historically been especially acute in African countries like Nigeria [95], 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 [96]. 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.
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself.
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.
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 [101]). 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 [102], but they state that at current rates of consumption, as of the fall of 2021 [103] the world has enough reserves to last about 90 years. The Nuclear Energy Agency [104] (NEA), like the WNA, [104]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 [105] that as of 2018, the world had about a 130 year supply of uranium.
The author of the article below provides a number of reasons why nuclear energy will not play a large role in the global energy future.
The first nuclear power plant came online in 1954 in Russia [107] (then the Soviet Union), and according to the World Nuclear Association, there are 436 reactors worldwide and another 59 [108] 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 [109]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.
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) [110], a U.S. National Lab, it has approximately the same lifecycle emissions as renewable energy sources.
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 [112] 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 [113]. There is not an easy answer here, as there are reasonable and strong pros and cons.
The first article below is a good example of why it pays to pay attention to citations and be well-informed on a topic, in regards to finding good information sources. The article is on a website that I've never heard of before, so at first, I was suspicious of the content. However, they provide legitimate sources for the information presented, and I have enough prior knowledge to know that the arguments they put forth are legitimate. Overall, it's a good summary of some of the pros and cons of nuclear energy, though I have a few minor issues with the content, as I'll describe below. (See if you can figure out what I take issue with.)
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 [118] 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.
Nuclear is a mixed bag. To summarize:
Nuclear is a very controversial source of energy. It is embraced by many as a key to the 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.
Why are we "locked in" to the use of nuclear energy once a plant is built?
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself.
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.
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 [119]). 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 2021, about 15% of the world's electricity came from hydroelectricity. According to the International Energy Agency [60], 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.
The feasibility is a mixed bag. An oft-cited paper by Mark Jacobsen and Mark Delucchi of Stanford University [120]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 [121] 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.
electricity source | low | high |
---|---|---|
rooftop solar PV | $117 | $282 |
community and commercial/industrial PV | $49 | $185 |
utility-scale PV | $24 | $96 |
utility-scale PV + storage | $46 | $102 |
geothermal | $61 | $102 |
onshore wind | $24 | $75 |
onshore wind + storage | $42 | $114 |
offshore wind | $72 | $140 |
peaking gas | $115 | $221 |
nuclear | $141 | $221 |
coal | $68 | $166 |
gas combined cycle | $39 | $101 |
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!
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.
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 [129], 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:
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 [136] 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 [137]. 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.
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.
Overall, the biggest advantages of renewable energy are:
The main disadvantages of solar, wind, and hydro are:
The last things I'd like to note are the following:
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.
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:
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:
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!).
Complete all activities in Lesson 4. The quiz may include a variety of question types, such as multiple-choice, multiple select, ordering, matching, true/false and "essay" (in some cases these require independent research and may be quantitative). Be sure to read each question carefully.
Unless specifically instructed otherwise, the answers to all questions come from the material presented in the course lesson. Do NOT go "Googling around" to find an answer. To complete the Activity successfully, you will need to read the lesson, and all required readings, fully and carefully.
Each week, a few questions may involve research beyond the material presented in the course lesson. This "research" requirement will be made clear in the question instructions. Be sure to allow yourself time for this! You will be graded on the correctness and quality of your answers. Make your answers as orderly and clear as possible. Help me understand what you are thinking and include data where relevant.
For any other assignments (e.g., journal or discussion board), it will be helpful to look at the rubric before answering. You will see a button that allows you to view it below the assignment.
These activities are to be done individually and are to represent YOUR OWN WORK. (See Academic Integrity and Research Ethics [141] for a full description of the College's policy related to Academic Integrity and penalties for violation.)
The activities are not timed, but do close at 11:59 pm EST on the due date as shown on the Course Calendar.
If you have questions about the assignment, please post them to the "HAVE A QUESTION?" Discussion Forum. I am happy to provide clarification and guidance to help you understand the material and questions. Of course, it is best to ask early.
Links
[1] https://www.e-education.psu.edu/emsc240/node/446
[2] https://www.ipcc.ch/report/sixth-assessment-report-cycle/
[3] https://report.ipcc.ch/ar6syr/pdf/IPCC_AR6_SYR_SPM.pdf
[4] http://www.npr.org/2018/10/08/655360909/grim-forecast-from-u-n-on-global-climate-change
[5] http://www.nytimes.com/2018/11/23/climate/us-climate-report.html
[6] http://ourworldindata.org/co2-and-other-greenhouse-gas-emissions
[7] https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions
[8] https://creativecommons.org/licenses/by-sa/4.0/
[9] http://www.worldcoal.org/
[10] https://www.worldcoal.org/about-us/our-mission-our-members/#our-members
[11] https://www.worldenergy.org/
[12] http://www.eia.gov/energyexplained/index.cfm?page=coal_reserves
[13] https://www.eia.gov/tools/glossary/index.php?id=Demonstrated%20reserve%20base%20(coal)
[14] https://www.eia.gov/tools/glossary/index.php?id=Estimated%20Recoverable%20Reserves%20(coal)
[15] https://www.eia.gov/tools/faqs/faq.php?id=70&t=2
[16] https://www.eia.gov/energyexplained/coal/how-much-coal-is-left.php
[17] http://www.britannica.com/science/British-Imperial-System
[18] https://www.britannica.com/science/British-Imperial-System
[19] https://www.eia.gov/tools/glossary/index.php?id=Metric%20ton
[20] http://www.eia.gov/electricity/data.cfm#avgcost
[21] http://www.eia.gov/electricity/annual/html/epa_07_01.html
[22] https://www.stlouisfed.org/on-the-economy/2017/december/coal-declining-due-economics-regulation
[23] https://e360.yale.edu/features/as-investors-and-insurers-back-away-the-economics-of-coal-turn-toxic
[24] http://www.eia.gov/energyexplained/index.cfm?page=coal_environment
[25] http://www.theatlantic.com/business/archive/2015/08/coals-externalities-medical-air-quality-financial-environmental/401075/
[26] http://www.cnn.com/2013/07/13/us/u-s-mine-disasters-fast-facts/
[27] https://en.wikipedia.org/wiki/Climate_change_mitigation#/media/File:Global_Carbon_Emissions.svg
[28] https://creativecommons.org/licenses/by/4.0/deed.en_US
[29] https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data
[30] http://web.archive.org/web/20170125065638/https://www3.epa.gov/climatechange/ccs/
[31] https://web.stanford.edu/group/efmh/jacobson/Articles/I/NatGasVsWWS&coal.pdf
[32] http://news.mit.edu/2015/carbon-dioxide-sequestration-doubts-0120
[33] http://insideenergy.org/2016/10/11/clean-coal-fact-or-fiction/
[34] https://www.theguardian.com/environment/2018/mar/02/clean-coal-america-kemper-power-plant
[35] http://energy.gov/fe/science-innovation/oil-gas-research/enhanced-oil-recovery
[36] http://www.npr.org/tags/129437792/upper-big-branch-mine-disaster
[37] http://www.wsj.com/articles/SB10001424052702304626304579509023076837450
[38] https://www.iisd.org/gsi/subsidy-watch-blog/air-pollution-and-health-cost-coal
[39] https://www.youtube.com/c/SciShow
[40] https://www.youtube.com/embed/51wOisfdIPo
[41] https://commons.wikimedia.org/wiki/File:Petroleum_drilling_onshore_system.svg
[42] https://www.eia.gov/naturalgas/crudeoilreserves/
[43] http://www.eia.gov/energyexplained/index.cfm?page=natural_gas_reserves
[44] https://www.eia.gov/energyexplained/natural-gas/how-much-gas-is-left.php
[45] http://www.eia.gov/energy_in_brief/article/shale_in_the_united_states.cfm
[46] https://www.eia.gov/energyexplained/natural-gas/where-our-natural-gas-comes-from.php
[47] http://www.eia.gov/forecasts/aeo/pdf/0383(2012).pdf
[48] https://www.e-education.psu.edu/emsc240/node/586
[49] http://www.eia.gov/naturalgas/crudeoilreserves/
[50] https://www.eia.gov/tools/faqs/faq.php?id=58&t=8
[51] http://www.forbes.com/sites/peterdetwiler/2012/12/17/just-how-long-will-us-gas-supplies-last/
[52] https://www.e-education.psu.edu/emsc240/node/588
[53] https://ourworldindata.org/electricity-mix
[54] https://www.eia.gov/todayinenergy/detail.php?id=26232
[55] http://www.eia.gov/todayinenergy/detail.cfm?id=26232
[56] https://www.nytimes.com/2015/08/05/science/methane-leaks-may-greatly-exceed-estimates-report-says.html
[57] https://www.iea.org/commentaries/the-environmental-case-for-natural-gas
[58] https://www.yaleclimateconnections.org/2017/06/pros-and-cons-of-fracking-research-updates/
[59] https://www.iea.org/search?q=Commentary%3A%20The%20environmental%20case%20for%20natural%20gas
[60] https://www.iea.org/
[61] http://www.nytimes.com/2015/06/05/us/epa-hydraulic-fracking-water-supply-contamination.html
[62] http://www.iea.org/newsroom/news/2017/october/commentary-the-environmental-case-for-natural-gas.html?utm_content=buffer77c83&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer
[63] http://www.economist.com/news/business/21702493-natural-gass-reputation-cleaner-fuel-coal-and-oil-risks-being-sullied-methane
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