Personal carbon offsets

I’ve been hearing about this personal greenhouse gas emissions offsetting lately. Basically, the idea is that, while you may be concerned about climate change, it’s hard to stop emitting CO2 and thereby contributing to it: you’ve got to drive, heat your home, buy products that were transported in trucks, and so forth. But you can reduce or eliminate your personal contribution to climate climate change by paying to offset your emissions, that is, stop the same quantity of greenhouse gases from being emitted somewhere else. Salon has a thoughtful article on the phenomenon, and the author identifies a half-dozen companies who will sell you emissions offsets, noting that the price varies from $5.50 to $30 per ton of CO21. That’s pretty cheap, compared to what most people expect the industrial price of carbon emissions will be in the long term. Incidentally, I do research on technology the we estimate will offset emissions for something in the neighborhood of $140/ton-CO2.

So I’ve been thinking, why not join the some tens of thousands of offsetters and atone for my own emissions? There are a few interesting caveats to personal emissions offsetting, but I at least want to see how much I would need to purchase. Most of the company websites have calculators to help you estimate what your emissions are. You put in things like how much you drive and the mileage of your car, how high is your utility bill and in which state you live. Of the few that I tried, all were missing something big. One neglected air travel, one neglected home heating, and none accounted for the embodied emissions of consumer goods. Many of the companies focus just on car emissions or car + plane emissions. I guess the rationale here is that people feel guilty about driving and flying because it is something they have control over, on the margin, and so they are particularly willing to pay to relieve that guilt.

My emissions came out as little as a tenth of the average, largely because I don’t drive (I put in some miles for the rides I bum) and share an apartment. But I don’t think that is really capturing everything, so I’m going to try my own calculation. Here goes.


This is a wild guess for how much driving I’m responsible for. Most of it is probably when I’m home on break. I’m assuming that when there are multiple people in the car, the emissions get divided evenly by the number of passengers.

[tex]\left( \frac{500 \,miles}{year} \right) \left( \frac{gallon \, gas}{22\, miles} \right) \left( \frac{.011 \, ton \, CO_2}{gallon \, gas} \right) = 0.25 \frac{ton\, CO_2}{year}[/tex]


Here’s the category I’m probably highest on compared with other Americans. I routinely fly across the country to visit family, and lately I’ve been flying similarly far for research trips to Canada and to conferences. Airplanes have about the same emissions per passenger-mile as cars, coincidentally — equivalent to about 34 miles/gallon driving alone. I figured two trips home and two conference trips to get the miles per year. Sources: Energy Information Agency here and here.

[tex]\left( \frac{17000 \,miles}{year} \right) \left( \frac{3600 \, Btu}{passenger \cdot mile} \right) \left( \frac{7.64 \times 10^{-8} \, ton \, CO_2}{Btu \, jet \, fuel} \right) = 4.7 \frac{ton\, CO_2}{year}[/tex]


I’m making a rough estimate at the per-person gas bill since I don’t have all the month’s prices.

[tex]\left( \frac{\$600}{year} \right) \left( \frac{1000 cubic feet \, }{\$16.82} \right) \left( \frac{million \, Btu}{1160 \, cubic\, feet} \right) \left( \frac{0.015 \, ton \, CO_2}{million \, Btu} \right)= 0.45 \frac{ton\, CO_2}{year}[/tex]


This is a straightforward calculation using the Pennsylvania electricity price and emissions factor. Pennsylvania electricity is really dirty.

[tex]\left( \frac{\$120}{year} \right) \left( \frac{kW \cdot hr}{\$0.098} \right) \left( \frac{5.74 \times 10^{-4} \, ton \, CO_2}{kW \cdot hr} \right) = 0.70 \frac{ton\, CO_2}{year}[/tex]


I’m drawing my calculation from the article I discussed earlier about diet and greenhouse gas emissions. It’s a little tough because the authors do everything in terms of differences between diets, not absolute emissions. I think I’m in the neighborhood of 1 ton/year, compared with the average American diet which is about 2.3 tons/yr. Then again I regularly eat produce trucked from California, so it could be higher.

Consumer goods

I don’t have the patience to do them all, so I’m trying to think of the few most important categories of consumer goods I buy. I’m using the Economic Input-Output Life Cycle Analysis website to get emissions factors for various categories of goods. I’m making rough guesses at the dollar values, too.

Computers and parts, sort of amortizing the equipment I already have:
[tex]\left( \frac{\$500}{year} \right) \left( \frac{5.0 \times 10^{-4} ton \, CO_2}{\$\, purchased} \right) = 0.25 \frac{ton\, CO_2}{year}[/tex]

Audio/visual equipment:
[tex]\left( \frac{\$100}{year} \right) \left( \frac{6.5 \times 10^{-4} ton \, CO_2}{\$\, purchased} \right) = 0.07 \frac{ton\, CO_2}{year}[/tex]

Furniture (I don’t really buy furniture, but I’m trying to account for my roommates’ Ikea stuff that I use):
[tex]\left( \frac{\$100}{year} \right) \left( \frac{8.7 \times 10^{-4} ton \, CO_2}{\$\, purchased} \right) = 0.09 \frac{ton\, CO_2}{year}[/tex]


If I add that all up, I get about 7.5 tons CO2 per year. I clearly left stuff out. I didn’t account for energy use or anything else happening at school, and it’s not clear whether I should. Other than that, I think I got all the big factors. But it’s a messy calculation, anyway, so let’s say 10 tons roughly accounts for my climate impact. I could offset that for $300/yr, which, in the scheme of things is not bad. $300 to completely solve the climate problem? I could do that. Of course it wouldn’t be that simple for everyone to go carbon-neutral because the economy would have to dramatically change, but it wouldn’t necessarily be more costly in the economic sense.

P.S.: It’s nice to finally use the LaTeX plugin for my blog to write equations here. Heheh.

  1. For a sense of scale, United States CO2 emissions are about 25 tons per person per year. []
  1. #1 written by Cortney June 6th, 2006 at 21:22

    I’m not sure if you check out this link, but Green Tags has a calculator and allows you to purchase credits. My airfare was the biggest source for me — in a way it negates what little travel in a car I have.

    RE Q
  2. #2 written by joshuah June 8th, 2006 at 18:11

    I tried putting in the same numbers to the Green Tags website. The electricity result is the same, but their natural gas figure (2 tons CO2 is 4 times what I calculated. I got my emissions factor from this DOE document, which admittedly doesn’t account for upstream emissions, but I’m pretty sure that wouldn’t account for the whole difference. Their driving figure is about the same, but then my air travel comes in at a whopping 10.5 tons! They appear to be using an emissions factor for air travel that’s twice as dirty.

    RE Q
  3. #3 written by Cortney June 21st, 2006 at 08:00

    Tom and I were talking about the estimated flying emissions from Green Tags the other day — he was shocked that his one international flight a year can account for so much CO2. But your statement that it’s similar to driving in terms of CO2/passenger mile helped settle the debate.

    The green tags site takes into account other greenhouse gases besides CO2:
    The average passenger miles per gallon for a domestic or international trip originating in the U.S. equals 33.4, according to Transportation, Energy, and the Environment, Section A – U.S. Energy Consumption and Transportation Sector Energy Consumption, Table 4-21.

    Burning a gallon of jet fuel produces 21.095 lbs of CO2 according to the U.S. Department of Energy and the Energy Information Administration, Instructions for Form EIA 1605B, Voluntary Reporting of Greenhouse Gas Emissions, Appendix B.

    The result is that each passenger mile creates 0.63 lbs. of CO2 (21.095 lbs. per gallon divided by 33.4 passenger miles per gallon).

    Air travel also creates significant non-CO2 greenhouse gas emissions. These are expressed as CO2 equivalents, or CO2e. The Climate Neutral Network (CN), has done significant research on this topic, and estimates that the non-CO2 greenhouse gas emissions are at least as significant as the CO2 impacts. After considerable discussion, BEF and CN have agreed that the appropriate approach is to double the 0.63 lbs. of CO2, with a result of 1.26 lbs. of total CO2e per passenger mile (CO2 + non-CO2 greenhouse gasses).

    An additional 8 percent is added to cover the emissions associated with the upstream refining of jet fuel according to the Climate Neutral Network. The result is that 1.36 lbs. of CO2e are created for each passenger mile traveled (1.26 + 8% = 1.36).
    (from their FAQ)

    RE Q

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