A good first definition of carbon footprint is the total greenhouse gas (GHG) emissions caused by an individual, event, organization, service, or product, expressed as carbon dioxide equivalent.
Let’s break that down:
Footprint is a metaphor for the total impact that something has.
Carbon in this case is a shorthand for all greenhouse gases.
When we talk about the carbon footprint or GHG emissions of an individual, event, organization, or service we usually express them as emissions over a specified time period. e.g. For an individual we might say annual footprint.
When we talk about consumer products, we mean life-time emissions, which include raw material extraction through materials processing, manufacture, distribution, use, repair and maintenance, and disposal. Another way to express it is “cradle to grave”.
When we talk about food, we include land use change, farming practices, processing, transport, retail, and packaging. The added impact of refrigeration and cooking in your home are usually not included but they are part of your home total energy use.
Greenhouse gas (GHG)
Any gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary GHGs in Earth’s atmosphere are water vapor (H 2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3).
Greenhouse Gas Emissions By Source Canada, Mt=1000kg
Carbon dioxide equivalent (CO2e)
CO2e is a standard unit for measuring greenhouse gases according to their global-warming potential (GWP). (That’s a whole lot of math we won’t get into here). However, excluding water vapor and clouds, when the GWP is used to calculate the effect of all GHGs added together, CO2 is responsible for about 80% of their effect.
As a gas rises in the atmosphere and atmospheric pressure decreases, the volume increases, and it rapidly disperses. In the atmosphere we generally use parts per million (ppm) to measure concentration of CO2. Remember 350.org.
Down on the ground we talk about the weight of CO2 or CO2e expressed in kilograms (kg) or metric tonnes (Mt=1000kg). We may express the carbon footprint of tomatoes for example as 1.4 kg of CO2e per kg.
If you’re wondering what a kilogram of CO2 might look like, it’s about the volume of two bathtubs, check out this short humorous video.
Reduce your footprint by 8% a year to reach 50% by 2030? – Show me the math!
Back of the envelope calculation.
If you start with a carbon footprint of 100 units a year at the beginning of 2021, in the first year you need to reduce it by 8.
Your footprint is now 92. To reduce your carbon by 8% in 2022 you take 8% of 92 = 7.36, subtract that from 92 and get 84.64.
Do the same math 7 more times and at the end of 2029 you have reduced your carbon footprint to 47.24 units.
Now you’re more than halfway to zero!
When talking about carbon footprints, we do a lot of estimating and rounding but you get the idea, right?
Return to Carbon 101 – here
Carbon 101 – Part3 “Drawdown” –here