Calculation of Greenhouse Gas Emissions on the Internet are Not Equivalent
By Wanda K. Hartmann
Recently, my husband asked me how greenhouse gases (GHG) emissions are calculated. My husband’s argument is, how accurate are these numbers and what are the approximations and estimations? What can we believe? Plato once said that “Science is nothing but perception.” In the case of measuring GHGs this is probably true. GHGs seem like a back of the envelope calculation, but measuring GHGs are really about measuring human activity. In contrast to plants that rely on photosynthesis and taking CO2 and turning it in to O2, we humans rely on respiration; our activity is based on producing CO2 and related gases from byproducts.
As a chemist, my first guess on measuring GHGs is that they are calculated from the amount of energy and fuel that is used by a person or a company. When you consider that companies and individuals invest money in carbon offsets, a highly accountable system of checks and balances should be in place, as well as a system and method of standards. The Nature Conservancy is one company that offers a plan where carbon offsets can be purchased.
I set out to answer this question and found the EPA, Nature Conservancy and AOL sites
- http://www.epa.gov/climatechange/emissions/ind_calculator.html
- http://www.nature.org/initiatives/climatechange/calculator/
- http://reference.aol.com/planet-earth/global-warming/calculator
The EPA site considers mainly fuel and energy for calculating GHGs. However, the Nature Conservancy site also includes things such as air travel, composting waste and diet. A meat eater’s diet contributes 3.6 tons of CO2/yr vs. vegetarianism at -6.4 tons of CO2/yr. Eating organic and composing unused food can help a bit. 20% of GHG production according to the Nature Conservancy is from food and eating (12 tons). The sum total of my house hold activity on the Nature Conservancy site was 43 tons compared to 20 tons (40089 pounds) on the EPA site. If you subtract out food the Nature Conservancy value is still 31 tons compared to the EPA. There is another 10 tons of difference to be accounted for. In looking at the individual numbers, the Natural Conservancy differs from EPA by twofold larger numbers on waste and recycling, 10 times larger estimates on home energy (heat and electricity) and is 40% less on the estimate for the car emissions.
Table 1. Household emissions for two people
| | EPA Pounds of CO2/yr | Nature Conservancy | AOL |
| Household emissions | Pounds | Pounds (tons) | Pounds |
| Car | 13519 | 5403(2.7) | 15145 |
| Natural Gas – heat | 6802 | 64000(32) | 8469 |
| Electricity | 18577 | | 20573 |
| Waste | 2036 | 4800(2.4) | |
| Recycling | -875 | -2000 (-1) | -940 |
| Composting | | -400 (-0.2) | |
| Air Travel (4 short flights) | | 2800(1.4) | |
| Food | | 16200(8.1) | |
| Eating Meat | | 7200(3.6) | |
| Total | 40089 | 86-98000 | 40282 |
When it comes to recycling EPA gives a 845 pound CO2 credit for recycling. The Nature Conservancy gives -1 ton or -2000 of CO2. The credit given to by the Nature Conservancy is double.
EPA Results
What I found from fueleconomy.com is that my Honda Accord 1997 puts out about 7.1 tons of CO2 in a year. My husband’s Passat emits 8.0 tons of CO2 in a year. For comparison the 2008 Prius puts out about 4.0 tons of CO2 a year. Our home energy bill breaks down into heat and gas. GHG are estimated from the average monthly bills. The final aspect is waste. Recycling can offset GHG emissions by as much as 50% if you recycle. Our total emissions are 40,089 pounds of carbon a year (1 ton = 2000 pounds,) which is close to the average amount.
We can improve on emission most by buying an energy efficient water heater and replacing the windows in our house. If we do this we will save nearly 15-20% of our household emissions. The most interesting fact I found is that most of our GHG emissions come from using electricity by the EPA calcuations. If we had photovoltaic solar panels we might be able to improve upon that. In addition we could buy a car with better gas mileage and save another 4 tones of CO2 a year or 8000 pounds a year.
Summary
When it comes to estimating automobile emissions EPA seems to be the most accurate as it considers the year of the vehicle. Of the three GHG online calculations EPA and AOL are in close agreement but still off by about 2000 pounds from each other. The Nature Conservancy numbers seem to be way off from the EPA and AOL numbers (conversion 1 ton = 2000 pounds). The home energy number of 32 tons or 64,000 from the Nature Conservancy seems to be far off from the other EPA and AOL numbers, although the Nature Conservancy brings up a good point with food and eating meat as a part of the GHG gas equation. If I considered buying carbon offsets I would serious reconsider after looking at these numbers. Some numbers are way off. The EPA bases the emissions on monthly energy bills. The Nature Conservancy bases these numbers data from geographical regions, which in my opinion may have more error. The next part in this series will discuss corporation based GHG calculations.
