Friday, December 4, 2009

The sources of CO2 Emissions
(The United States)

I had questions about the sources of Carbon Dioxide (CO2), and attempted to find an answer on the Internet. My first question was what are the sources of CO2 emissions by percentage. In addition I had heard that driving a car produces less CO2 than flying. I wanted to confirm that, and also get an idea of how much more CO2 flying produces.

While looking for answer I found very good in-depth studies, and I found single issue studies. What was hard to find was an easy to follow summary of CO2 emission sources. This is my attempt.

My second part will be a single issue look, comparing driving versus flying. It is not in-depth.

Sources of CO2(1)

The most common CO2 reduction targets I hear are 20% by 2020 and 40% by 2020. Because the goals are in percentages I wanted to know, in percent where our CO2 emissions come from. Also because the goals are for CO2 only I will only examine the sources of CO2. There are other emissions that contribute to the green house effect, with methane being the second greatest.

CO2 emissions are almost completely from human activity. 94.0% come from fossil fuel combustion. (Graph on left)

eaking down the number further electrical generation and transportation are the major producers of CO2. (Graph below)

This graph is of the direct sources of CO2 emissions. For example, non-electric household heat is included in residential sources as it is burned on site, but electrical household heat is included in electrical generation.

Similarly the electrical generation does not include the secondary CO2 sources such as the mining, processing, and transportation of the fossil fuel.

Our emissions of CO2 are still increasing. Between 1990 and 2007 the emissions from electrical generation and transportation have increased. All other sources have remained almost stable. Stated another way, the only reason our CO2 emissions are increasing is our increases from transportation and electrical generation.

Electrical generation produces 38% of the total CO2 emissions. If we only look at fossil fuels, our electrical generation accounts for 36% of our fossil fuel consumption, yet it produces 42% of the (fossil fuel) CO2 emissions.

This next graph shows that the vast majority of the CO2 released in electrical generation is from coal powered plants. Coal is very dirty, and very high in CO2 emissions when burnt.
By rebuilding our electrical infrastructure from the generators to the end user we could meet the 20% CO2 reduction goal and almost meet the 40% reduction goal. (Please see my very important correction added as a comment.)

Plane vs. Car (2)

The chart to the right provides the total CO2 generated by different classes of transportation. The largest classes are, in descending order of the CO2 emitted, passenger cars, light-duty trucks, other trucks, and commercial aircraft. I will examine two classes in more detail; passenger car and commercial aircraft.

Passenger cars produce 33% of the CO2, and commercial aircraft produces only 8%. That does not account for the number of people carried. Taking that into account plane travel emits 0.000260 Teragrams (Tg) of CO2 per passenger mile while a passenger car emits 0.000249 Tg of CO2 per passenger mile. So, on average it is better to drive.

Planes produce more CO2 on takeoff than for cruising. A more accurate measure would be to compare a particular trip. http:\\ compared 100 people chartering a plane to fly from
Philadelphia to Boston with all 100 people driving alone in a medium-sized car. The cars each produced 104 kilograms (kg) of CO2 for a total of 10,400 kg. The plane emitted 18,400 kg of CO2. It was better for each 100 people to drive alone.

Journalist Pablo Päster calculated if it would be better to drive a family of 4 in a medium size car from San Francisco to Boston, or take a non-stop flight. He included all greenhouse gases, and included the extra detrimental effect of releasing the gases at altitude. Driving turned out to be better. Flying produced over 8 tons of emissions and driving (including stops in national parks) produced less than 2 tons. Comparing a single driver, flying would produce more than 2 tons of emission, and driving would be less than 2 tons. So, even driving alone would have been better than flying.

There are many more studies, and every one I found confirmed that it is better to drive a medium sized car than to fly.

To put this in personal terms, the 1955 Chrysler gets about 16 mpg on the highway. Other owners of cars like the Mercedes 300SEL 6.3 have found that mileage can be in the high teens, but the mid teens would be a better highway mileage figure. A mid sized car gets about 30 mpg highway. So, for these comparisons driving these classic cars with 2 people is about the same as driving a mid sized car alone. (The time to get there is not included in the calculations, nor the fun factor of driving.)

Foot notes:
(1) The data used for the first section came from the U.S. Greenhouse Gas Inventory - 2009 which covers 1990 to 2007. All graphs are my own except for the Emissions Allocated to Economic Sectors which came from the inventory. On Dec. 3, 2009 a report was released covering 2008, but I did not see the extra value in rewriting what I had already done.
(2) CO2 data comes from the Greenhouse Gas Inventory. Passenger mileage date comes from the Bureau of Transportation Statistics .


  1. I made a serious error in this post. I stated that rebuilding our electrical grid would meet or almost meet the goals. That is far from true.

    I used 2007 numbers. The CO2 reduction goals use the baseline of 1990. There is a huge difference. For example President Obama has talked about a 17% reduction from 2005 numbers while the rest of the world is talking about 20%, 25%, 40%, 45% ... reduction from 1990 numbers. 17% from 2005 baseline is about the same as 4% from the 1990 baseline.

  2. This is an excellent study. Thanks for calling it to my attention on facebook.