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.
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
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.
Figure 4.2: Annual carbon dioxide emissions by world region, 1751 - 2023. (Data are available for download.)
This image is a stacked area graph titled "Annual CO₂ emissions by world region", which illustrates the total carbon dioxide (CO₂) emissions from fossil fuels and industrial processes—excluding land-use change—across different global regions and sectors from 1750 to 2023. The x-axis spans the years from 1750 to 2023, while the y-axis measures emissions in billion tonnes of CO₂, ranging from 0 to 40 billion tonnes.
Each region or sector is represented by a distinct color:
- International aviation – light pink
- International shipping – pink
- Oceania – light green
- Asia (excluding China and India) – green
- China – red
- India – orange
- Africa – purple
- South America – dark purple
- North America (excluding USA) – blue-green
- United States – blue
- European Union (27 countries) – yellow-orange
- Europe (excluding EU-27) – brown
The graph shows that global CO₂ emissions have increased dramatically since the mid-20th century, with a particularly steep rise beginning around 1950. Early emissions were dominated by Europe and the United States, but in recent decades, China has emerged as the largest single contributor, followed by significant growth in emissions from India and other parts of Asia. International aviation and shipping also appear as growing contributors in the modern era.
The data is sourced from the Global Carbon Budget 2024, and the chart is published by OurWorldInData.org. A footnote clarifies that the emissions shown include CO₂ from coal, oil, gas flaring, cement, steel, and other industrial processes, but exclude emissions from land use change, deforestation, soils, or vegetation.
This visualization provides a clear and comprehensive view of how regional and sectoral contributions to global CO₂ emissions have evolved over nearly three centuries, highlighting the shifting dynamics of industrialization, globalization, and energy use.