Carbon Footprint

‘A carbon footprint is the total greenhouse gas (GHG) emissions caused directly and indirectly by an individual, organization, event or product.’ It is calculated by summing the emissions resulting from every stage of a product or service's lifetime (material production, manufacturing, use, and end-of-life). Throughout a product's lifetime, or life cycle, different greenhouse gases GHGs may be emitted, such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), each with a greater or lesser ability to trap heat in the atmosphere. These differences are accounted for by calculating the global warming potential (GWP) of each gas in units of carbon dioxide equivalents (CO₂e), giving carbon footprints a single unit for easy comparison.

Sources of Emissions

FOOD

• Food accounts for 10–30% of a household's carbon footprint, typically a higher portion in lower-income households. Production accounts for 68% of food emissions, while transportation accounts for 5%.
• Food production emissions consist mainly of CO₂, N₂O and CH₄, which result primarily from agricultural practices.
• Meat products have larger carbon footprints per calorie than grain or vegetable products because of the inefficient transformation of plant energy to animal energy, and are due to the methane released from manure management and enteric fermentation in ruminants.
• Ruminants such as cattle, sheep and goats produced 178 million metric tons CO₂e of enteric methane in the US in 2018.
• Eliminating the transport of food for one year could save the GHG equivalent of driving 1,000 miles, while shifting to a vegetarian meal one day a week could save the equivalent of driving 1,160 miles.
• A vegetarian diet greatly reduces an individual's carbon footprint, but switching to less carbon intensive meats can have a major impact as well. For example, beef's GHG emissions per kilogram are 7.2 times greater than those of chicken.

Greenhouse gases contribution by food type in average diet pounds of CO₂e per serving

PERSONAL TRANSPORTATION

• US fuel economy (mpg) declined by 12% from 1987 to 2004, then improved by 30% from 2004 to 2018, reaching an average of 25.1 mpg in 2018. Annual per capita miles driven increased 9% since 1995 to 9,919 miles in 2018.
• Cars and light trucks emitted 1.1 billion metric tonnes of CO₂e or 17% of the total US GHG emissions in 2018.
• Of the roughly 66,000 pounds of CO₂e emitted over the lifetime of an internal combustion engine car (assuming 93,000 miles driven), 84% come from the use phase.
• Gasoline releases 19.6 pounds of CO₂ per gallon when burned, compared to 22.4 pounds per gallon for diesel. However, diesel has 11% more BTU per gallon, which improves its fuel economy.
• The average passenger car emits 0.78 pounds of CO₂ per mile driven.
• Automobile fuel economy can improve by 7–14% by simply observing the speed limit. Every 5 mph increase in vehicle speed over 50 mph is equivalent to paying an extra $0.13–$0.25 per gallon.
• Commercial aircraft GHG emissions vary according to aircraft type, trip length, occupancy rates, and passenger and cargo weight, and totalled 130.8 million metric tonnes of CO₂e in 2018. In 2018, the average domestic commercial flight emitted 0.39 pounds of CO₂e per passenger mile.
• Domestic air travel fuel efficiency (passenger miles per gallon) rose by 118% from 1990 to 2018, largely due to increased occupancy. Emissions per domestic passenger mile decreased 44% from 1990 to 2018, due to increased occupancy and fuel efficiency.
• In 2018, rail transportation emitted 42.9 million metric tonnes of CO₂e, accounting for 2% of transportation emissions in the US.

SOURCE: University of Michigan (2021).