We've broken down the calculation into four categories and aimed for simplicity. The most important element of the calculator, as far as we are concerned, is that you become more aware of how your choices effect your carbon footprint. Only by collectively understanding how we contribute to the problem can we collectively find the solutions.
We use publically available data sources and use your choices to come up with an approximation of your carbon footprint. When we first looked at how other calculators were done, we were a little astonished to see the wide range of values. We believe that the methodologies used by other calculators are valid, but there is no standard. [If you are a data head, we'd love your help in building an open source carbon calculator that could be plugged into other websites. Send an email to firstname.lastname@example.org with the header “open source carbon calc volunteer”.]
Moreover, we think that having a quick way of calculating a carbon footprint is more important than trying to get to a really precise number with about the same level of uncertainty. So, we've erred on the side of making it a 30 second experience.
The home is a place where you as an individual can make great strides in reducing your carbon footprint. This includes choices you make for products you buy, how many low energy appliances you have, the number of cold-water laudry washes you do, how often you remember to turn off lights and electronics, how good the insulation is, whether or not you have “pressure leaks” in your home, whether or not you have recent-vintage double paned widows, and how you heat your home (what fuel, what temperature). In fact, there are so many factors influencing the CO2 footprint of your home, that a new industry has sprung up around doing a household energy audit.
There are two main areas we care about for the CO2 footprint – the electricity used and home heating.
For electricity in our calculator, we use the US national average for the carbon intensity of the grid – which is 1.34 lbs of CO2 per kilowatt-hr. Emissions factors used are from the US Department of Energy: Updated State Level Greenhouse Gas Emission Coefficients for Electricity Generation 1998-2000. (as used by the WRI calculator methodology). We also use a simple ratio that relates your house size to the average home size. We then assume you are an average American and consume 10,800 kWhrs per year. If you buy green credits, which pays for low-carbon sources of energy on the grid, we assume that is a 25% reduction in your CO2 from electricity usage.
For heating, we are using natural gas as the average heating source and again use the number of rooms and create a ratio based on an average of 3.5 rooms per home. We then use the BTU (British Thermal Units) or therms to tonnes CO2 conversion. There are 0.0001304 lbs of CO2 per BTU. If you live in an apt or condo or dorm, your emissions are on average 40% less, and if in a townhouse 20% less. If you heat with wood pellets or biodiesel we don't currently have a good way of measuring your CO2 from heating.
Finally, we use the number of people in the household to calculate your footprint, that is we divide by the number of people in the household to get the emissions per capita.
Diet is often missing from carbon calculators despite the fact that diet is responsible for a large % of an individuals' carbon footprint. How large depends on your dietary habits. Although buying local can theoretically save CO2 from transport, changing to a low red meat diet definitely lowers your footprint. According to a study done by the University of Chicago in 2006 entitled "Diet, Energy, and Global Warming", and covered in a discovery.com article, http://planetgreen.discovery.com/food-health/vegetarian-diet-carbon-foot...., by changing your diet to a vegan, vegetarian or occasional vegetarian (weekday vegetarian), you can save the following amount of greenhouse gases (carbon dioxide) per year as compared to the average American, approximately:
- Vegan: 2 tons
- Vegetarian: 1 ton
- Weekday Vegan: 1.4 tons
- Weekday Vegetarian: 0.7 tons
- Switch 1 meal per day to animal-free: 0.6 tons
- Switch 1 meal per day to vegetarian: 0.33 tons
- A red meat diet averages 3.57 tons per year.
Buying local can save up to 30% if you are a vegetarian or vegan and if you buy things that are in season, otherwise it has little effect. Official sources are sparse on this so we are using the best available from farmer's markets and www.eatlowcarbon.org. There is counter viewpoint on this. http://www.thevillager.com/villager_290/talkingpoint.html
Air travel accounts for a large percentage of climate emissions relative to use and is one of the more heavily studied areas of carbon calculations.
Essentially, the atmospheric impact of flights include both the direct consumption of fuel and the indirect impact from exhaust at high altitudes. The latter is known as radiative forcing and it is the subject of much study and debate, including the research cited below.
According to the greenhouse gas protocol (IPCC), air travel consumes 0.18 kgs CO2 per km on short haul flights. Many calculators use an RF value between 1.3 and 2. According to the Stockholm Environmental Institute, a more accurate number for Radiative Forcing (RF) would be 2.7. (http://www.co2offsetresearch.org/aviation/index.html)
We use US EPA miles per gallon estimates for three categories of cars: Small/hybrid, Medium, SUV/Truck. These are 29mpg, 26mpg, and 21.5mpg respectively. When burning fuel, the average gasoline vehicle creates 8.87 kgs of CO2 per gallon.
We divide the number of miles travelled by the miles per gallon to get gallons and then by the conversion factor above to get kgs of CO2.
COMMONLY USED FACTORS
- 1.61 miles per kilometers,
- 2.2046 kilograms to pounds of CO2
- 0.2642 gallons per liter
- 2205 lbs per metric tonne
- 1000 kgs per metric tonne
We intend to continuously improve the calculator and would like your comments and feedback on how to improve the user experience and the calculations we use. Please send an email to email@example.com with the header “carbon calc methodology comment”.
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