Calculating One’s Carbon Footprint


You’ve no doubt been hearing more and more about carbon emissions and may wonder what it means. You may even be curious about how to measure your contribution to the humanity’s collective carbon footprint. Simply stated, carbon emissions is the sum of carbon dioxide (CO2) generated by society’s use of fossil fuels like oil, coal, and natural gas which, when combusted to produce energy, ends up in the atmosphere.

Carbon dioxide in the atmosphere is absolutely essential for life. All plants depend on it. They take it in through respiration and use it to produce the carbohydrates that we eat either directly or indirectly as part of our diets. The plants also convert CO2 to produce the oxygen we breathe. The problem with CO2 is that its atmospheric concentration has been increasing steadily starting around the time we began using fossil fuels that previously had been sitting there buried for millions of years. The atmospheric concentration of CO2 has gone up by about 25% in the past 50 years and since CO2 acts as a greenhouse gas by trapping heat, there is a legitimate concern that it may change the climate to a point where the earth may undergo a positive runaway effect, triggering another mass extinction event. I know that there is some debate about this issue, but the consequences are so dire it’s really not worth arguing about anymore. Combined with the inevitable depletion of fossil fuels, it means we must find alternatives to replace the energy society uses on a daily basis and the sooner we can do it, the better off we’ll be.

There are a number of websites with carbon calculators that try to accurately compute an individual’s carbon footprint. An individual carbon footprint is the amount of CO2 a person’s activities generate on an annual basis. The easiest way to estimate an individual carbon footprint is to look up your country’s per capita number on the Internet and assume yours is in that range. But if you’re curious how to you might modify your own personal carbon footprint, the calculator may be helpful, particularly if it explains the methods it uses to compute its values.

In the case of U.S. residents, our country’s annual CO2 emissions are 6 billion metric tons (13.2 trillion pounds). Divide that by our population of around 300 million and you get 20 metric tons per capita per year (about 44,000 lbs. per capita). I began to wonder where all that CO2 allocation was coming from and if there’s anything I could do to reduce my contribution to it. By the way, Americans have a much higher carbon footprint than most other countries and significantly more than third world countries. There are some who feel that we should be profoundly ashamed of our lifestyles and that we need to get our carbon footprint aligned with that of the rest of the world. In the meantime, some of the countries with the lowest per capita carbon footprints are desperately trying to get their lifestyles to more closely match ours which will have the effect of increasing world carbon emissions overall.

The largest personal carbon contributors are due to household energy use and transportation. Depending on the website you use, it will ask questions such as the state where you live, the size and age of your home, the type of vehicle(s) you drive, your annual mileage, and many other questions. I wanted to know more about the assumptions that were made and so after doing some of my own research, I came up some simple rules of thumb. They involve converting one’s energy consumption into BTUs and then converting BTUs into CO2 emissions depending on their carbon source. Otherwise, you may not know how clever the calculator is trying to be with all the questions it’s asking. For example, if you live in a state that gets most of its electricity from hydropower like Idaho does, you may get a reduction on your electrical carbon contribution, which isn’t quite right since electricity works a bit like a shared resource whose carbon emissions should be recognized by the combined sources of electricity on a national basis.

I like to use British Thermal Units (BTUs) to measure thermal energy. BTUs have more meaning to me than the metric units for energy like calories or Joules. A BTU is defined as the amount of energy required to raise one pound of water 1 degree Fahrenheit. The equivalent unit in the metric system is the calorie, which is defined as the amount of energy required to raise one gram of water 1 degree Celcius. The calorie such a small unit of energy that almost no one uses it. You may think that it’s used to measure food energy, but it’s not. A food calorie is actually 1000 thermal calories. In fact, it is really a kilocalorie (kcal). But for simplicity’s sake, everyone just calls it a ‘calorie’. Can you see a problem there? I sure can. Getting calculations wrong is very easy when you use the same name to refer to vastly different amounts of energy. For example, I once was reading a book called Games for the Super Intelligent and in the book the author calculated that the ice in each Scotch and soda he drank required his body to raise the temperature of the drink, presumably at 32F, up to his body’s internal temperature. He reasoned the energy his body needed to do that would more than offset the calories in the Scotch and soda. He should have been able to lose weight like mad with each and every drink. He tried it and concluded it didn’t work. It would have, but only if he had increased the ice water by a factor of 1000. Imagine drinking 50 gallons of ice water with each 7 oz. Scotch and soda! That would certainly require your body to work overtime to maintain a normal core temperature. So even people who are super intelligent can get tripped up by the food calorie vs. thermal calorie confusion. I find it best to stick with BTUs, because they only have one meaning.

The metric system can be pretty convenient when you don’t have a calculator because everything is expressed in powers of 10. However, once you bring in calculations that involve time or energy, much of the mathematical elegance falls apart, and you’re not much better off than any other system. So that’s why I don’t mind using the antiquated, yet convenient, British system when discussing energy. You’ll also notice that the world uses other non-metric units when discussing energy such as barrels for oil (which are 42 gallons each) and cubic feet for natural gas. So it’s pretty hard to get away from having to know all the conversion factors when dealing with energy. Fortunately, in the U.S., we are multi-lingual when it comes to measurement systems out of necessity. I’m just thankful I don’t have to know 4 languages like many of our friends in Asia and Europe need just to get through the day.

It’s pretty easy to get the values for BTUs since most fuels have pretty well-established BTU ratings. In fact, in the U.S., items like fuels and furnaces are more likely to be specified by their British system ratings than their metric equivalents.

Here are some common BTU ratings for typical fuels:

Ethanol 84,000 BTU per gallon
Gasoline 124,000 BTU per gallon
Jet Fuel 134,000 BTU per gallon
Natural Gas ~ 1,000 BTU per cu. ft.
Wood 7,000 BTU per lb. (average)
Coal 10,000 BTU per lb. (average)
Garbage 5,000 BTU per lb. (average)

I then did some calculations based on this data that allowed me to convert BTUs into lbs of CO2 emissions using these conversion factors:

Coal/Wood ~ 1 lb of CO2 generated per 5000 BTU
Oil/Gasoline ~ 1 lb of CO2 generated per 6500 BTU
Natural Gas ~ 1 per lb of CO2 generated per 8700 BTU

Once you know how much of each type of energy you use, you can more accurately determine its effect on the overall carbon footprint. Incidentally, I included wood only for comparison’s sake in the list above. If you use wood for heating, you get a pass on CO2 emissions because it’s only ancient carbon we’re worried about. However, if your wood burner is stinking up the neighborhood and stinging your neighbors’ eyes, you might want to keep your smug factor down to a minimum. 🙂

The best way to compute gasoline consumption is by knowing how many miles you drive each vehicle per year and the fuel economy of the vehicle. The U.S. average distance per vehicle per year is around 12,000 miles and the average fuel economy is around 25 mpg. So knowing those two numbers, an average car’s carbon footprint can be computed with this equation:

12,000 miles / 25 mpg x 124,000 BTU/gal x 1 lb CO2/6500 BTU = 9,156 lbs of CO2/yr

Similarly, a typical 2100 sq. ft. Colorado home uses about 90 MBTU for home heating per year and so dividing that by 8700 BTU/lb, assuming natural gas, which is the most common heating fuel in Colorado, gives 10,344 lbs of CO2/yr. for home heating.

Flying can also contribute significantly to one’s carbon footprint. A commercial jet effectively gets about 42 passenger miles per gallon so you can figure out if you fly 4 trips totaling 12,000 miles per year, the CO2 generated due to that activity would be 5890 lbs of CO2. Please note that the maximum theoretical values for fuel economy with public transportation are rarely reached due to capacity factors. Modern airliners can get 60 passenger miles per gallon if they are 100% full and fly only long routes, but 42 passenger mpg is closer to the average fuel economy.

To get the contribution for electricity, I used the fact that 30% of the electricity produced in the U.S. is from non-CO2 generating sources, primarily nuclear and hydroelectric and a small, but growing, amount from renewables like wind, solar, and biomass. The other 70% comes from burning fossil fuels.

A coal plant has about a 33% thermal efficiency and coal accounts for 50% of the electricity in the U.S.. One kWh of electricity is equivalent to 3412 BTUs and dividing that by .33, I get 10,236 BTU of coal to generate a kWh of electricity. Using my conversion factors above, it means a typical coal plant generates about 2 lbs. of CO2 per kWh of electricity generated. Since coal accounts for 50% of total U.S. electricity production, I will use 1 lb. of CO2 per kWh as the average carbon contribution due to U.S. electricity generation from coal. Likewise, natural gas is responsible for about 20% of the electrical generation in the U.S., but it has better efficiency due to the fact that most of the gas-generated electricity uses combined cycle and it also generates less CO2 per BTU and so its overall contribution is about .2 lbs of CO2 per kWh. Some electricity is also generated by oil but it’s down to less than 2% of the total and declining, so I’ll ignore that contribution because it doesn’t affect the overall picture very much. This means that the fossil fuel contribution in the U.S. to electrical generation is about 1.2 lbs per kWh, on average. Since the average household energy consumption is about 8760 kWh per year, this adds another 10,512 lbs. of CO2 per household.

So, now I will add the major contributors to an individual’s CO2 footprint:

2 cars = 18,312 lbs
home heating = 10,344 lbs
air travel = 5,890 lbs
electricity = 10,512 lbs

Total = 45,058 lbs

CO2 per capita 11,264 lbs (assuming a family of 4) for electricity, heat, and transportation.

This only appears to add up to about 1/4 of the per capita number you get when you divide total U.S. CO2 emissions by its population. But I left off contributions due to food and other consumer items as well as energy used by commercial and industrial customers.

Food and consumer goods all include some amount of embodied fossil fuel energy. What I mean by that is that if I eat an apple, there is fossil fuel energy attributable to the apple grower, the trucking firm that delivered it, and the supermarket that sold it that is embodied in that apple and thus that energy should be attributed to my personal carbon footprint.

The food to sustain a human for a year, based on a diet of 2400 kcal/day is 876,000 kcal per year. There are 4.2 BTUs per food calorie so this is equal to 3.7 MBTU. However, the carbon content of food is based on recent CO2 absorbed into plants. So this CO2 would not count toward CO2 emissions from ancient carbon in fossil fuels, which is what we’re trying to measure here. However, some studies have found that every food calorie produced in the U.S. requires about 10 kcal (42 BTUs) of fossil fuel energy. So if you look at it like that, then you would get another 37 MBTU of embodied fossil fuels due to food consumed per person per year. This would add about 5500 lbs of CO2 per capita due to food. For that calculation I used an average of 1 lb. of CO2 per 6700 BTUs to give the embodied fossil fuel energy in food an average somewhere between coal, oil, and natural gas. This was a real eye-opener for me. I had assumed that my car consumed significantly more fossil fuel that my body did, but the number is only a factor of 2 higher for my car or my furnace compared to my body, if you take into consideration that the 10 x multiplier effect of food on fossil fuel energy. Not all food calories embody the same about of fossil fuel. It ranges from about 3 x for some foods to 35 x for meat according to that study. This is why many of the on-line CO2 calculators will ask how often you eat meat or whether you buy organic foods. I will reserve judgment on whether this embodied fossil fuel energy in food is accurate since many people unknowingly quote a study by Pimentel that corn ethanol takes more fossil fuel energy to produce than it provides as a fuel, and that study has been contradicted by every other researcher on that topic. Not surprisingly, the studies that allocate 10 kcal of fossil fuel per kcal of food extensively quote Pimentel’s publications.

I had done calculations many times in my head and had been ignoring the value due to food because I knew intuitively that the daily consumption of 9600 BTU of food is equivalent to about a pound of coal, and that is only about 1 kWh ($.10) of electricity which is tiny compared to the amount of energy we use for household electricity and heating. I hadn’t really thought that food could have impacted my carbon footprint very much until I considered the embodied fossil fuel energy in it. Similarly, you can imagine that the energy required to produce other items we use daily such as clothing, furniture, appliances, etc., all have some contribution from fossil fuels.

It’s also important to realize that after the kids are grown and move out of the house, it becomes a household of two adults, and the per capita energy consumption goes up proportionally. The tendency for fewer people to live in ever-larger houses is also causing per capita carbon emissions to rise as well.

To figure out where the rest of the per capita energy comes from, there are other energy consumers such as retail stores, private industry, schools, government offices, etc., whose energy consum
ption all gets allocated to each U.S. citizen.

So what did this exercise show me? It showed me that to significantly reduce my carbon footprint to a level where I should no longer feel shame and guilt, I should grow my own organic food, live in an unheated hut (probably closer to the equator), use wood and dung for cooking, and get rid of my car. In other words, I would need to swap my lifestyle with someone who lives in a third world country. Actually, “swap” isn’t really a good term to use, since if I simply swapped lifestyles with someone else in the third world, the person who got my house would likely want to keep it heated and use the electric appliances, cars, etc., so that wouldn’t work. Instead it would be better if I dismantled my house and buried it or recycled it along with the cars, and let the prairie re-grow in its place. It sounds a little absurd, but there are some who think that is the correct approach.

I’ve already written about the difficulty of trying to substantively cut carbon emissions with minor steps like replacing incandescent bulbs with CF bulbs. To make a real difference in a carbon footprint, we’ll need to cut consumption by significant amounts and at the same time start generating more of our energy with renewable sources. Can it be done? I think it can. But I’ll leave that for a future posting.

2 thoughts on “Calculating One’s Carbon Footprint

  1. Very interesting Lee, I find that many of the carbon footprint calculators online vary drastically and don’t allow for enough detail my footprint has varied from 3 tonnes to 5.5 metric dependant on which you use. I have only calculated my car by your calculations so far at 4.6 imperial tons or 2.115 metric tonnes. Thats based on a similar milage but due a smaller engine in the UK. Out of interest is the number affected much by which octane fuel you use (95,97,98) or whether a catalytic converter is fitted or does that only filter out particulates?

  2. Hi Matthew,

    Yes, I found the on-line carbon calculators to be confusing and inconsistent. A gallon of gasoline weighs about 6 lbs and generates 19 lbs of CO2 when burned. This confuses a lot of people because they don’t understand that combustion requires oxygen from the atmosphere which combines with the carbon atoms in gasoline at the rate of 1:2 to form CO2, and thus more CO2 is produced per lb of fuel than the fuel itself weighs. The hydrogen in gasoline also combines with oxygen to form water.

    Octane rating makes little difference when computing carbon generated from burning gasoline. The BTU rating of fuels vary a bit depending on additives, but the octane rating primarily influences the rate of combustion, not necessarily the energy content of the fuel. In the U.S. where ethanol is a common additive, about 10% by volume, the BTU content of the fuel is reduced accordingly, by about 3% along with adding oxygen to the fuel, so it would reduce carbon emissions per gallon, but not per mile, since the fuel economy is reduced when the energy content per gallon is reduced.

    The catalytic converter has 3 purposes: to burn any unburned hydrocarbons, to convert any nitrous oxides to N2 and O2, and to convert carbon monoxide to carbon dioxide. These pollutants are present in small percentages prior to entering the catalytic converter and reduced to parts per million as the catalytic converter does its job. Thus the catalytic converter has a very minimal impact on carbon emissions. In theory, an engine that had perfect combustion would not need a catalytic converter.

    I’m not sure about the large difference you show between imperial tons and metric tons, they should only be different by less than 2%. A U.S. ton (short ton) is 2000 lbs. A metric ton is 1000 kg. A long ton, i.e., imperial ton is 2240 lbs., which is 1016 kg.

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