A greenhouse gas inventory estimates how much climate-warming gas is released by a school, city, company, or country. This cheat sheet helps students organize the main gases, units, and calculation steps used in environmental science. It is useful because inventories turn real-world activities, such as driving, heating, and electricity use, into comparable emissions data.
Students can use it to check formulas, understand reports, and avoid common unit errors.
The core idea is that emissions are calculated from activity data and an emission factor. Different gases are compared using global warming potential, or GWP, which converts each gas into carbon dioxide equivalent. Inventories often separate emissions into Scope 1, Scope 2, and Scope 3 categories to show where emissions come from.
Good inventories clearly state boundaries, data sources, assumptions, units, and uncertainty.
Key Facts
- Total emissions are commonly estimated with the formula emissions = activity data x emission factor.
- Carbon dioxide equivalent is calculated with the formula CO2e = mass of gas x GWP.
- GWP compares how much heat a greenhouse gas traps relative to CO2 over a chosen time period, often 100 years.
- The main Kyoto greenhouse gases are CO2, CH4, N2O, HFCs, PFCs, SF6, and NF3.
- Scope 1 emissions come directly from sources an organization owns or controls, such as fuel burned in its boilers or vehicles.
- Scope 2 emissions come from purchased energy, such as electricity, steam, heating, or cooling used by an organization.
- Scope 3 emissions are other indirect emissions in the value chain, such as commuting, waste disposal, purchased goods, or product use.
- An inventory boundary defines which locations, activities, gases, and time period are included in the emissions total.
Vocabulary
- Greenhouse gas inventory
- A greenhouse gas inventory is a measured or estimated list of emissions from defined sources over a specific time period.
- Activity data
- Activity data is the measurable amount of an activity that causes emissions, such as liters of fuel used or kilowatt-hours of electricity consumed.
- Emission factor
- An emission factor is the amount of greenhouse gas released per unit of activity, such as kg CO2 per gallon of gasoline.
- Carbon dioxide equivalent
- Carbon dioxide equivalent, or CO2e, is a common unit that expresses different greenhouse gases as the amount of CO2 with the same warming effect.
- Global warming potential
- Global warming potential, or GWP, is a number that compares a gas's heat-trapping effect to carbon dioxide over a set time period.
- Inventory boundary
- An inventory boundary states what sources, places, gases, operations, and dates are included in the greenhouse gas inventory.
Common Mistakes to Avoid
- Mixing mass units, such as kg and metric tons, gives totals that are off by factors of 1,000. Convert all emissions to the same unit before adding them.
- Adding different gases without converting to CO2e is wrong because CH4, N2O, and fluorinated gases do not have the same warming effect as CO2. Use CO2e = mass of gas x GWP before combining gases.
- Using the wrong emission factor for a fuel or region can make the inventory inaccurate. Match the factor to the activity, fuel type, location, and year whenever possible.
- Counting the same emissions twice can inflate the total. Check whether a source belongs in Scope 1, Scope 2, or Scope 3 and keep categories clearly labeled.
- Leaving out the inventory boundary makes the result hard to interpret. Always state the time period, included sites, activities, gases, and major exclusions.
Practice Questions
- 1 A school burns 2,000 therms of natural gas in a year. If the emission factor is 5.3 kg CO2 per therm, what are the annual CO2 emissions in kg?
- 2 A lab releases 4 kg of CH4. If the 100-year GWP of CH4 is 28, what is the release in kg CO2e?
- 3 A building uses 150,000 kWh of electricity. If the electricity emission factor is 0.38 kg CO2 per kWh, what are the Scope 2 emissions in metric tons CO2?
- 4 Why is CO2e needed when comparing emissions from carbon dioxide, methane, and nitrous oxide in one inventory?
Understanding Greenhouse Gas Inventory Reference
Inventory work begins by deciding what the reported total is meant to represent. A school district may include every building it operates during one calendar year. A business may report only facilities where it has financial control.
These choices affect the result before any numbers are entered. A useful inventory keeps a list of sites, fuel types, vehicles, refrigerants, purchased services, and travel categories. It records the start and end dates for each data set.
This makes later comparisons fair. If a new building opens halfway through the year, its energy use should not be treated as if it occurred for a full year. Changes in boundaries should be explained instead of hidden in a trend graph.
Some emissions are easy to miss because they do not come from a smokestack. Refrigerant leaks from air conditioners, refrigerators, and heat pumps can matter greatly. Many refrigerant gases have a high warming effect per kilogram, so a small leak can produce a large CO2e value.
Methane needs careful treatment too. It lasts for less time in the atmosphere than carbon dioxide, yet it traps heat strongly while present. The chosen time period changes its comparison value.
A one hundred year value is common for reporting, but it does not show every effect of short lived gases. Students should learn that CO2e is useful for comparison, not a complete picture of climate impact.
Good data collection is often the hardest part of an inventory. Utility bills may show electricity in kilowatt hours, natural gas in therms, or fuel in litres. Vehicle records may give distance travelled instead of fuel purchased.
Each type of record needs a matching conversion factor from a reliable source. Electricity factors can differ by region because power grids use different mixtures of coal, gas, nuclear energy, hydroelectric power, wind, and solar power. They can change from year to year as well.
Estimates are sometimes necessary, especially for employee commuting or purchased goods. An honest report labels estimates clearly and states how much confidence the writer has in them.
The final total is not the only useful result. Breaking emissions into categories helps people find the biggest practical opportunities for reduction. A school may find that heating dominates in a cold climate, while another may find that electricity use or bus travel is larger.
This prevents attention from going only to small visible actions, such as reducing a little paper use, when major fuel use remains unchanged. Scope categories should be read carefully because one organization’s indirect emission may be another organization’s direct emission.
This is normal, but totals from different organizations should not simply be added without checking for double counting. The strongest inventories keep their calculations organized so another person can trace each result back to a bill, meter reading, survey, or stated estimate.