We hear repeatedly that climate change is a huge problem that we need to solve.  Easier said than done!  Where to start?  Professional problem solvers, such as engineers, typically start with carefully and methodically defining the problem.  In our experience, many who are concerned about climate change do not fully appreciate the scale, magnitude and nature of the challenges we face.  A basic level of energy literacy is required.

Anthropogenic Global Warming: The Cause

The scientific consensus is that anthropogenic global warming (AGW) is a real phenomenon.1  Humanity’s industrial and agricultural activities have increased the amount of carbon dioxide in the atmosphere by 45% (from a prior high of 280 ppm to 405 ppm) and increased the methane content by 125% (from a prior high of 790 ppb to over 1770 ppb), vastly increasing the heat-trapping characteristic of the atmosphere, and as a result the energy balance of the planet has changed significantly.  More heat is retained than before, causing the globe incremental warming.  To be sure, there are many factors affecting this energy balance including natural methane releases from cows and melting permafrost, leaking methane gas from broken pipes and sinkholes, as wells as a much wider range of industrial gases and combustion exhaust fumes which include particulate pollution, water vapor and other more harmful gases which all have an effect.  However, mankind’s increasing combustion of fossil fuels has been identified as the dominant contributor and is something we directly control, so it makes sense to focus efforts there.  While all mitigation efforts are important, it stands to reason that if we fail to control the dominant cause, we are unlikely to successfully mitigate climate change.

The Carbon Cycle

We know that carbon and carbon dioxide are natural chemicals that abound within nearly all organic cycles on the planet.  Plants and trees, for example, take in carbon dioxide out of the air in order to grow and release oxygen. Thus forests and plants everywhere help protect the planet by sequestering carbon for us.  When a tree is cut down, however, and its wood burned, this releases that carbon dioxide again back into the atmosphere, where it can do damage.  Increasing forests and improved forestry and agricultural practices have the potential to sequester more carbon from the atmosphere.  Unfortunately, mankind is simultaneously increasing carbon emissions from energy usage while decreasing the planet’s forests, rainforests and vegetation cover, which could have been helping to reduce carbon.  Atmospheric carbon dioxide dissolves in seawater and can be sequestered by being incorporated into the shells of phytoplankton and other crustaceans.  Unfortunately, too much carbon dioxide being absorbed by the oceans causes ocean acidification, which inhibits the formation of phytoplankton, which reduces the ocean’s ability to properly sequester carbon over the long term.

Important Carbon Cycle Facts

  • Atmospheric carbon dioxide is the key gas that controls the temperature of the earth.
  • The rate at which fossil carbon is being added to the atmosphere greatly exceeds the rate at which carbon can be geologically sequestered; hence the balance problems with earth’s heat flux and ocean acidity.
  • Current carbon dioxide and methane concentrations in the atmosphere are still rising at increasing rates.
  • About half of the carbon dioxide released from burning of fossil fuels is not absorbed by the oceans and land vegetation, and remains in the atmosphere for millennia.
  • Increased absorption of atmospheric carbon dioxide by oceans is causing them to acidify, which is detrimental to corals and other marine life with carbonaceous shells.1,2

Climate Mitigation Targets

There is good agreement in the scientific community that to avoid catastrophic global warming and climate disruption, the world needs to essentially be carbon-neutral in emissions by mid-century, then be increasingly carbon-negative by year 2100.  To appreciate the magnitude of this challenge, we need to have a sense of how much total energy we use, what the energy is used for, what the various energy sources are, their levels of greenhouse gas emissions, and what are the best, quickest pathways to follow for meaningful climate mitigation.

Energy Breakdown

Although we hear reports that renewables are growing faster than any other type of energy, the fact remains that this growth is starting from very small bases, so in absolute terms, renewables continue to be a small portion of the total energy picture in the U.S., even if they are becoming more prevalent in a few areas, including California and Texas.

What often gets omitted from the positive reports about the growth of renewables, is that a large portion of what are considered “renewable” forms of energy — biomass waste, and biofuels  — actually emit carbon dioxide when they are burned.  It is only by re-capturing that carbon by regrowing trees (which takes decades) and biofuel crops — which is not a requirement but just a potential — are these polluting forms of energy rationalized within a climate-aware framework.

Hydropower, until recently, was considered a form of clean energy. More recently, studies have shown that hydropower systems, which include reservoirs, dams and large channeled conduits, emit large amounts of methane from decaying organic matter.  Thus, the carbon intensity of hydropower is being recalculated based upon new findings about methane releases.

As the chart on the right shows, not only do fossil fuels emit carbon but almost 75% of sources of energy we consider “renewable” also emit carbon (and/or methane equivalents).  As recently as 2015, only 12% of our total energy came from clean energy sources that do not emit carbon, and of that nuclear power provided more than two-thirds.

Energy Sectors

When we think about greenhouse gas emissions, we often only think about electricity generation.  Indeed, electricity is over a third of global primary energy demand, so it’s a good place to start.  But we also have other energy sectors that are non-electric, such as transportation, industrial process heat, and building space heat (see graphic at top of page).  Some of these other sectors can be partially electrified–electric cars and mass transit, for example.  However, many applications are not well suited for electricity.  It is very improbable that aviation as we know it today could be powered by batteries and electric motors, as batteries have far less energy density than liquid hydrocarbon fuels.

Nuclear is the only large-scale zero emission power source that can be used for non-electric applications.  Advanced reactor designs are being developed for these purposes:

  • District heating–underground steam distribution to heat towns and cities.
  • Carbon-neutral liquid fuels–plentiful carbon-free nuclear heat drives known industrial chemical processes to synthesize liquid fuels (methanol, diesel, gasoline, etc.) from carbon extracted from seawater or air.  Useful for the transportation sector, to avoid creation of an entirely new energy infrastructure (batteries, hydrogen, fuel cells, etc.).
  • Industrial process heat–on-site, high-temperature heat to drive processes that normally use fossil fuels.
  • Transportation–in addition to liquid fuel synthesis, ocean shipping could be powered directly by on-board small modular reactors.
  • Desalination–potable water may be increasingly in short supply due to weather extremes and increasing population.  Nuclear can power large desalination facilities to help meet this need.

Carbon Intensity

Carbon intensity is a measure of the amount of CO2 and/or its equivalents in methane and other heat-trapping molecules emitted per unit of power produced (grams CO2-e/kWh). The graph on the right shows the relative amounts of direct and indirect emissions of CO2 equivalents per kWh of energy calculated for eight types of energy as of 2007.


  • Recent years have seen impressive growth of solar and wind installations. However, this growth is not keeping up with the increase in energy demand, much less replacing fossil fuel generation.
  • Most credible sources project double or triple present global energy usage by 2050. This does not include extra energy which may be necessary for biosphere repair (atmospheric carbon drawdown, ocean acidity stabilization, geoengineering, etc.).
  • Typically, solar and wind are poorly suited to service non-electric energy sectors.
  • Nuclear can service non-electric energy sectors.
  • Hydro emits a significant amount of methane from decomposition of organic matter in reservoirs. Methane is considered 100 times more heat-trapping than carbon dioxide as a greenhouse gas over a 20-year time frame.  Opportunities to expand hydro globally are limited.
  • Biomass is classified by some countries as “renewable,” yet burning biomass emits CO2.
  • Nuclear is the only highly scalable zero-emission energy source that does not depend on geographical location or weather for reliable performance and which has the lowest emissions of any energy source.


Limits to Zero Emission Energy Sources

Energy sources such as solar and wind are considered by some to be attractive for their fuel-less nature.  To be objective and rigorous in our decisions, we should consider all pros and cons to determine whether any energy technology is the best fit for a particular application.  How different energy sources may be combined in a system also impacts the emissions of power generated by that system.


There are estimates for accessible, known reserves of most elements.  Many are plentiful; others are not.  As massive expansions of energy infrastructure are contemplated, requirements for some materials may exceed known reserves, and/or compete with other needs of society at large.

Materials which are otherwise plentiful may be limited by the annual rate at which they can be extracted and/or processed.  In either case, supply and demand will affect the price of a material, which may render it effectively unavailable for proposed energy infrastructure.

A useful metric is tons/TWh, the material throughput for energy produced over a generating plant’s lifetime. While many people are quick to assert that nuclear power is limited because of limits in the supply of uranium (which is not the case), they tend to disregard information regarding the limits of critical materials required in the production of either solar panels or wind turbines.  Some technologies require large amounts of certain materials, such as concrete, steel, copper and aluminum.  This can increase the embedded energy in the technology itself, with the associated mining, processing, transportation and fabrication inputs.


Rare Earth Elements

Rare earth elements (REE) is a group of 17 elements also known as “technology metals.”  They have specific and unusual properties, and are critical in many of our high-technology products, such as electronics, batteries, solar cells, lighting, high-strength rare earth magnets for motors in electric cars and wind turbines, and more.  The quantities used are typically very small, but their properties are often unique, with no or few substitutes.  A notable example is wind turbines, which depend on significant quantities of REEs for their large, high-strength generator magnets.

Although REEs are not particularly rare in the earth’s crust, they are distributed unevenly in various mineral deposits, and are difficult to separate from each other and refine to useful purity.  Currently, due in large part to its aggressive industrial trade policies, China controls about 95% of the world’s supply of REEs and the associated processing supply chain industry.  Although adequate mineral deposits exist elsewhere, economic barriers to entry in REE mining and processing are high.  The geopolitics of rare earth elements may be a significant limiting factor for energy technologies that require significant amounts of these materials

Energy Density

Energy density is a concept somewhat related to material throughput.  When our ancestors learned to burn wood for heat and light, it gave great advantage over natural cycles and conditions.  Coal and petroleum are more dense forms of chemical energy than wood; hence their usefulness for electricity, heat production and transportation, especially air travel.  Nuclear is extremely energy-dense; atomic bonds release at least a million times more energy than the chemical bonds in fossil fuels.  This is the reason that nuclear has a very small material and land footprint per unit of power produced.  By contrast, solar and wind are “fuel-less,” and require large amounts of material and land to harvest the dilute energy of the wind and sun.

Land Area.   Solar and wind harvest dilute and intermittent energy and they do it with technologies capable of capturing only portions of the available energy from the sun or the wind.  This is one reason they require large amounts of land area — also known as an energy footprint.  The chart at the right shows the energy footprint for wind, solar and nuclear, with a range of existing capacity factors for wind and solar.  Compared with nuclear power, which requires 1.3 square miles for a 1,000 MW system, solar requires 45 to 75 square miles are land area and wind requires 260 to 360 square miles of land area to generate the equivalent of a 1,000 MW nuclear system.  This large land requirement is likely to become limiting at high deployment levels, and will vary widely with geographic location, locally available solar/wind resources, competing uses for land area, energy demand, and distance from load centers.

Economics. Economics can make or break an energy infrastructure project.  There are a number of ways to evaluate economic viability.  All should be considered to obtain a good, objective assessment:

  • Capital Cost—total cost to build a facility. Includes capital plus financing charges, discount rate, labor, materials, land cost, and licensing/administrative fees.  The discount rate considers the expected value/lifetime of the project in the future, and affects the cost of borrowing money.  Project build time also affects financing cost.
  • Operating Cost—fuel cost, labor, maintenance, other operating costs.
  • Levelised Cost of Electricity (LCOE)—comprehensive dollar costs per unit of power produced over the lifetime of the facility (e.g., $/megawatt-hour). Plant lifetime can have a large effect on this figure.
  • Energy Returned on Energy Invested (EROEI)—a cradle-to-grave measure of the total energy produced relative to the total amount of energy required to produce the facility, obtain and transport the fuel, decommission, etc. We want to see a better ratio than 1:1, and the higher the better!  EROEI ratios can vary considerably depending on the assumptions used, but can range from the single digits for solar photovoltaics, 30 for coal, and 75 and higher for nuclear.  In the chart on the right, EROEI is shown buffered and unbuffered.  Buffered includes the energy required to balance loads for intermittent generators (pumped hydro storage is used in this model).
  • Life Cycle Analysis (LCA)—estimated carbon emissions from the full life cycle of an energy producing technology.
  • System LCOE—if technologies are not interchangeable, comparing on the basis of LCOE alone may be misleading. Intermittent and variable generators such as solar and wind incur system Integration Costs to allow them to function while preserving grid reliability.  Such integration costs can include:  dependency on inefficient fast-ramping gas turbines to balance loads, inefficiencies from baseload generators (coal, gas, nuclear) being forced to curtail output to prioritize solar/wind, curtailment of solar/wind output to avoid grid disruption, energy and material costs of energy storage resources (batteries, pumped hydro, etc.), “smart grid” demand management technologies, and long distance high-voltage transmission lines.
  • Cost of Emissions—historically, fossil fuels have not been charged for the cost to society of their emissions. It is said that these costs are externalized.  Some jurisdictions are starting to place a price on carbon emissions.  In other areas, zero emission generators are given a credit for their zero emission energy.4

Other Energy-Related Terminology

  • Load—in an electric grid, the total amount of electricity used at any given time.
  • Load Balancing—electricity production must match consumption on a second-by-second basis. Failure to do so may result in poor quality power (voltage and/or frequency fluctuation), or grid failure (blackout).
  • Capacity Factor—the proportion of an electrical generation technology’s actual output over time, compared to its rated peak output. A plant that runs all year at peak output with almost no down time would have a capacity factor of almost 100%.  Intermittent or peak generators can have capacity factors from 10% to 50%.  Nuclear plants typically have a capacity factor of 90% or more.
  • Capacity Credit—for an intermittent generator, this is the percentage of its peak output that the grid can rely on at any given moment. For wind, this is typically 6-10%.  This number affects the portfolio of generators that may be needed to meet grid reliability standards.
  • Deaths per Kilowatt-hour—a (morbid) measure of safety; the number of deaths per unit of power actually produced. Nuclear is the safest of all major power sources.  Fossil fuels cause the most deaths, due to polluting emissions.
  • Kilowatt (kW)—a measurement of power; the rate of energy usage. For example, a 1000-watt (1 kW) drill requires 1kW to run.
  • Kilowatt-hour (kWh)—a measurement of energy; the total amount of electricity used over time. For example, a 1000-watt drill operated for one hour consumes 1 kWh.  U. S. homes typically consume about 1000 kWh per month.
  • Megawatt (MW)—1000 kW, or 106 watts. Used to describe power plant capacity; e.g., a 1200 MW nuclear plant.
  • Gigawatt (GW)—1000 MW, or 109 watts.
  • Terawatt (TW)—one million MW, or 1012 watts.4



  1. Wikipedia:  Carbon Dioxide in the Earth’s Atmosphere
  2. PBL Netherlands Environmental Assessment Agency: Greenhouse Gas Emissions Per Type of Gas and Source, Infographic | 27-09-2017
  3. Lawrence Livermore National Laboratory: World Energy Flow in 2011, with support from the U.S. Department of Energy.
  4. The Actinide Age: Electrical Energy Literacy Numeracy Primer

Banner Image: Credit to Terrestrial Energy: Clean Sustainable.