Where do we get the Numbers From?

Overview

We strive do get our emissions numbers from the most reputable sources available. When we feel that there may be some uncertainty in our calculations, we'll let you know. Please keep in mind that the site (and this form in particular) are works in progress. So - if you find that a number on the site that is not explained here that's most likely because we just haven't finished the form yet. When we do find better methodologies for our calculations, we will update this form and apply the new calculations to all of your existing records. If you have suggestions or would like further information regarding our methodology, please let us know. We're particularly interested in hearing about new methodologies and improved sources.

Units

A common source of confusion in interpreting emissions reporting numbers, is the difference between reports of carbon vs. carbon dioxide emissions. All our emissions numbers are reported in lbs. of carbon dioxide equivalent (or CO₂e). Certain emissions reports are quantified in terms of lbs. of carbon (C) as opposed to carbon dioxide (CO₂. To convert between the two:

• if your report is in terms of carbon emissions, multiply by 44/12 (approximately 4) to get the comparable weight of carbon dioxide emissions
• if your report is in terms of carbon dioxide emissions, multiply by 12/44 (approximatley one-quarter) to get the comparable weight of carbon dioxide emissions

Also, note that we generally report in terms of lbs. of carbon dioxide equivalent. Although carbon dioxide is the major component of the greenhouse gases (GHGs) that cause climate change, many emitting sources emitt other gases that also contribute to climate change. Rather than quantifying each of the gases individually we lump them into a single number that reports the total GHG emissions in terms of the equivalent carbon dioxide emissions. So, for example, if a unit of fuel produces 10 lbs. of carbon dioxide and 0.3 lbs. of another, equally deleterious gas, we would generally report this as 10.3 lbs of carbon dioxide equivalent. Since not all our sources are clear on whether they are reporting in terms of carbon dioxide or carbon dioxide equivalent, this is a minor source of uncertainty in our reports.

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Electricity

We calculate emissions due to electricity usage by taking the number of killowat hours (kWh) that you report and multiplying by a conversion factor. The conversion factor is taken from tables produced by the Energy Information Administration (EIA). The EIA reports conversion factors by state and we use your zip code to determine which conversion factor to use for determining your emissions. The variation from state to state is due to the varying methods by which electricity is generated. Generally, one kWh generated in the Pacific Northwest (where electricity is generated hydro-electrically) produces significantly lower emissions than a kWh generated in the Northeast or Midwest (where coal plants are often used to generate elctricity).

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Natural Gas, Heating Oil and Propane

For these fuels, we use the Energy Information Administration (EIA)tables titled Fuel Emission Factors:

• Natural Gas

  We use the weighted national average figure (under the heading Natural Gas) of 5.306 kg CO₂ per therm. We multiply by 2.2 to convert from kg to lbs. Thus, one therm of natural gas generates approximately 12 lbs of CO₂. Note that, although we ask you in your utility profile, to specify whether you measure your gas consumption in therms or CCF, we currently use the figures for therms, regardless of which units you specified. In practice, there is little difference between the two. In the future, we may use different conversion factors depending on whether you report usage in therms or in CCF.

• Heating Oil

  We use the figure for home heating oil (under the heading Petroleum Fuels) of 73.15 kg CO₂ per MMBtu. We multiply by 2.2 to convert from kg to lbs of emissions per MMBtu. We then use the conversion factor of 139,000 BTU per gallon of home heating oil (from the EIA's online calculator) and multiply by 0.139 to get lbs of emissions per gallon of fuel consumed.

• Propane

  We use the figure for propane (under the heading Petroleum Fuels) of 63.07 kg CO₂ per MMBtu. We multiply by 2.2 to convert from kg to lbs of emissions per MMBtu. We then use the conversion factor of 91,547 BTU per gallon of propane (from the National Propane Gas Association) and multiply by 0.0915 to get lbs of emissions per gallon of propane consumed.

Auto Fuel

For auto fuels, we use the Energy Information Administration (EIA) tables titled Fuel Emission Factors, subsection titled Carbon Dioxide Emission Factors for Transportation Fuels:

• Motor Gasoline

  We use the conversion factor of 19.54 lbs. of CO₂ per gallon of Motor Gasoline.

• Other fuel types

  Although we ask you to specify your fuel type in the vehicle profile section, we do not yet differentiate between the different fuel types for the purpose of calculating emissions. We recognize that in the case of various bio-fuels, this may cause significant error. However, due to the controversy surrounding the life-cycle emissions associated with certain bio-fuels, we are still working to find sources for conversion factors that are both accurate and realistic. This is a high priority item for us.

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Air Travel

For air travel, we base emissions calculations on distance flown. Since take off and landing account for a disproportionate amount of a flight's emissions, we take the common approach of dividing flights into three categories: short-haul (less than 300 miles), medium-haul (between 300 and 1,000 miles) and long-haul (over 1,000 miles). For these categories, we use conversion factors of 0.64 lbs/mile flown, 0.45 lbs/mile flown and .39 lbs/mile flown, respectively. These figures are taken from Terrapass's online emissions calculator. but are confirmed by numerous sources including the The World Resources Institute, The Clean Air Conservancy and The Greenhouse Gas Protocol Initiative. For the purpose of calculating miles flown based on the origin and destination airports that you specify, we calculate the great circle distance between the origin and destination airports based on their latitude and longitude.

The following sources of error must be considered when evaluating these calculations:

• Variations due to radiative forcing (RFI)

  Radiative forcing is the phenomenon by which greenhouse gases emitted at high altitudes are more delterious than the equivalent amount of gases emitted at sea level. There is controversy regarding the correct multiplier to use for this effect, but it is significant. In 1999, the International Panel on Climate Change (IPCC) estimated the RFI multiplier for aviation to be in the range of 1.9 - 4, most likely 2.7. We do not currently consider RFI but plan to do so in the near future.

• Variations due to aircraft type, aircraft load and similar

  The conversion factors that we currently use are averaged across all types of aircraft and do not consider the aircraft load. In reality, there are significant differences in the fuel efficiency of different types of aircraft. Emissions for different flights vary also due to the varying loads at which a specific aircraft might fly (both passenger load and freight load). In the near future, we plan to be using TRX's carbon calculator to provide our air travel emission's data. TRX's database provides high quality emissions data because it is based on actual types of aircraft flown on different routes and actual fuel consumed by these different aircraft.

• Variations due to class of service

  Passengers flying in business or first class occupy a significantly higher proportion of the aircraft's carrying capacity. As a consequence, the emissions attributable to these passengers is significantly higher than those attributable to coach class passengers. While we do not yet take class of service into account, TRX's calculator does.

• Deviation from great circle route

  Aircraft rarely fly exactly the great circle distance between their origin and destination airports. Deviations from this route result from the need to fly around no-fly zones, route adjustment in response to weather systems and other sources. Although these deviations are relatively minor, they are a source of error.

We commonly hear the argument that "The plane would have flown without me anyway, so - why should my presence on the flight be counted towards my emissions?". To this, we respond: When sufficiently large numbers of people change their flying habits, airlines respond. When less people fly, the airlines reduce the number of aircraft in service. We saw this most notably, after 9/11 and during the oil price spikes of 2008. Thus each flyer must take responsibility for their proportion of the emissions associated with their flight."

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