Transportation is by far the biggest portion of my carbon footprint. In the past five years, I ranged widely across the American Cordillera to study geology and to climb. I drove across the country headed east and west, in five different vehicles (two minivans, one cargo van, an SUV, and a compact hatchback). Three times, I flew a quarter of the way around the Earth to climb on faraway ice and snow.

Recently, I rode my iron pony west, entering Wyoming from the prairies of northern Nebraska. A railroad paralleled the highway; a train steamed in the opposite direction. Its buckets were overtopped with coal en route to midwestern power plants and destined for the atmosphere.

In a lot of the media I take in there’s a focus on the benefits of clean energy over coal. Common arguments are that clean-energy jobs are probably healthier, safer, and better than coal jobs and that clean energy technologies are better for the environment than coal could ever be. I won’t argue about what constitutes a ‘good job’ but I’ve long wondered about the extent of the environmental impacts of renewable energy versus fossil energy. Regardless of which energy source we develop, literally everything humans use today stems from our creative use of mineral resources. If we magically, totally switched away from fossil fuels we would still consume mind-boggling quantities of virgin minerals. There’s just no moving away from extraction.

Watching those buckets roll by, I wondered: how much CO2 do we produce when burning enough coal to supply one person energy for one year? How much do we produce to create enough solar capacity for a year (forgetting the intermittency and storage problems)? Let’s consider the impacts of producing coal versus the impacts of producing photovoltaic panels (solar pv).

Based on numbers for utility power generation in 2016, 30.4% of all electricity in the United States comes from coal, while 14.9% comes from renewable sources (0.9% of the total comes from solar). 68% of total carbon emissions come from coal power; less than 1% come from renewables. So, while in production, coal is at least 32 times more carbon-intensive than renewable sources in the United States.

However, this kind of analysis does not take into account the emissions associated with extraction and refinement of mineral resources, refinement of those resources into usable materials, and transport of these materials and products throughout their functional life. This type of overarching analysis is called a life-cycle analysis. The National Renewable Energy Laboratory led LCAs for every imaginable energy source to determine their impacts in terms of CO2 emissions. Solar pv systems produce 40 g of CO2 emissions per kWh of produced energy. Coal power produces 1,000 g of CO2 emissions per kWh of produced energy. Coal is 25 more carbon-intensive than solar pv.

Coal is literally stored energy, while energy produced from solar pv needs to be used right away or stored for later. That’s an issue with solar power because of the intermittency of sunshine (not a problem in the desert Southwest, a very big problem in the Pacific Northwest, a minor problem in the Northeast). Solar power creates other requirements – the need for batteries to store energy and infrastructure to produce power for when the sun isn’t shining. Those things should factor into a comprehensive life-cycle analysis for energy systems.

I like to think about fossil fuels in terms of digging. If you had to dig your fuel out of the ground, how big a hole would you leave? Well, the average U.S. residential utility customer uses 10,812 kilowatthours (kWh) each year and there are 2.6 people per household. So, if each person were responsible for her supply, she’d need to stockpile about 2075 kilograms—a solid block of high-grade coal the size of a very large refrigerator. What kind of hole would an equivalent amount of solar capacity create? It’s an interesting question to ponder.

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