Calculating Consumer Carbon Footprints
What is a carbon footprint?
From turning on the air conditioning to filling our gas tanks, our day-to-day choices all leave their mark on the environment. Carbon footprints are a simple way to express the greenhouse gas (GHG) emissions produced by a person, group, company, or country over a period of time. While carbon footprints account for numerous different gases - including carbon dioxide, methane, and nitrous oxide - they are often expressed in simplified units of carbon dioxide equivalent (CO2e).
Our personal footprints include two types of emissions: (1) those we have direct control over, like our home energy or gasoline use, and (2) indirect emissions linked to the supply chains of the products and services we use. Commons includes indirect emissions in users’ personal footprints, not to assign blame, but rather to reveal the full influence that their choices can have on global emissions. In this way, carbon footprints can guide us to make changes - both personal and systemic - that improve our impact on the planet.
Individual actions, collective impact
Research from the International Panel on Climate Change (IPCC) has made clear that as a global community, we must slash emissions in half by 2030 and achieve net zero emissions by 2050 to avoid the most disastrous impacts of climate change.1 Achieving these goals will require a tremendous effort across multiple fronts - from sweeping policy change to bold corporate action.
Where do consumers fit into this effort? Individuals do not bear the sole responsibility of addressing climate change. Yet collective action can be a powerful force for change. The Sixth Assessment report from the IPCC finds that demand-side efforts could account for 40-70% of emissions reductions by 2050.2 Direct action to reduce emissions can have indirect consequences as well, shifting patterns of demand towards more sustainable products and services, though these effects are more difficult to measure.
The Sixth Assessment report also highlights the fundamental issues of inequity that underpin the climate crisis. Those who contribute the least to the climate crisis will bear the brunt of its impacts, from sea-level rise to rising temperatures to increased natural disaster.
Meanwhile, wealthier households contribute a disproportionate amount of emissions, with about half of global emissions coming from the richest 10% of the global population. The richest 10% of the global population is disproportionately represented in the US and Europe. Only 5% of households in the European Union, for example, are living within 1.5ºC climate limits, with footprints driven by carbon-intensive travel, meat-intensive diets, higher energy demand, and embedded emissions in purchased products. Reducing the emissions generated by higher income earners is not only achievable, it is necessary.
Building carbon intuition
There are billions of products in the world; assessing the exact carbon footprint of each and every unique product is currently impossible. Most manufacturers are not legally required to report on the carbon impacts of their products, though in recent years there has been increasing consumer pressure to provide this information.
In light of this, Commons’ carbon footprinting approach intends to help people build carbon intuition - a relative understanding of the carbon intensity of different types of purchases, with the intent to inform decision-making. A person can build their carbon intuition by regularly measuring and managing their personal carbon footprint, and they can use it to make choices that matter. Developing a carbon intuition is the first step to taking control of one’s carbon footprint.
Commons' Carbon Footprint Calculations
Spending data is a uniquely powerful tool to inform carbon intuition, both because it shows a holistic picture of one’s emissions – everything from food to travel to shopping to energy use – and because it can guide purchasing choices.
However, spending data often does not provide detailed information about what was contained within a specific purchase, and even if it did, product-level emissions data is spotty. Thus, supplementing spending data with lifestyle information is critical to improving footprinting accuracy. In the following sections, we describe how Commons combines spending data, lifestyle information, and third-party datasets to help people build carbon intuition.
Commons Data Sources
A combination of three key data sources allows Commons to provide personalized, accurate, and actionable information on personal footprints.
(1) USER-INPUTTED INFORMATION
In the Commons mobile app, users can provide information about their lifestyles (via the Carbon Survey) and their purchases (via transaction-level details) to personalize their carbon estimates.
(2) AUTOMATED SPENDING DATA
Users can connect their spending history by linking their credit card, debit card, andbank accounts to Commons via Plaid, an API that offers bank-level security. Commons receives information on transaction vendors, amounts spent, and purchase categories. Instead of relying on self-reported estimates of purchases, this automated stream of data provides a timely, truthful picture of one’s actual consumption.
(3) THIRD-PARTY DATASETS
Commons’ calculations pull in data from academia, government, and other trusted national, regional, and local datasets to improve the quality and relevance of estimates.
Commons’ carbon calculations currently use data localized for the United States and Canada. If a user chooses to use Commons without inputting any personal information, the app uses top-down assumptions about the average person in that country as a proxy for their carbon footprint. As Commons expands to other geographies, our calculations will incorporate datasets appropriate for those regions.
Commons Calculation Approach
Commons’ starts with a top-down, spend-based approach to emissions accounting. Spend-based methods take the financial value of a purchase and multiply it by an emissions intensity factor (kg CO2e/$) to estimate carbon impact. Commons has mapped a rigorous dataset of emissions intensity factors to 180+ subcategories of consumer spending, which are nested under six high-level categories: Shopping, Travel, Home, Food & Drink, Finance, and Offsets.
Transactions are automatically categorizes into the appropriate subcategory, and Commons is constantly improving categorization. If a user notices an incorrect categorization, they can recategorize it, which will improve estimates not only for them but for all users with related transactions.
Instead of identifying the individual items that a user has purchased, this approach takes national-level data on industry emissions and determines the user’s share of those emissions based on the amount of money they have spent. For example, suppose a user purchased a chair for $50. If the average emissions for a furniture purchase is 0.44 kgCO2e/$, Commons will multiply this intensity by $50 to calculate the estimated emissions associated with the chair (22 kgCO2e).
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This approach is most accurate in categories with relatively homogeneous items (for instance, in the case of a utility, which sells very few items), and is particularly useful in building carbon intuition for the relative impacts of spending across different sub-categories -- for example, weighing the carbon footprint of this $20 furniture purchase versus a $20 rideshare purchase.
Where available, Commons also integrates bottom-up data to increase the localization and granularity of high-impact spending categories, such as flights, home utilities, gasoline, and food purchases. Bottom-up approaches identify and sum up carbon emissions on a per-product or per-activity level. For example, by telling Commons how many red meat meals you eat in a week, Commons can calculate and sum a user’s per-meal carbon impacts to generate a personalized food footprint. These methods are more data-intensive, but can generate more accurate estimates.
System-specific calculations
For card-connected users, Commons uses the top-down approach outlined above as the core calculation for estimating emissions in most of the six Systems: Shopping, Food & Drink, Home, Travel, Finance, and Offsets. Where possible, however, more detailed calculations are used to either enhance or replace this method.
SHOPPING
$18 trillion | Influences 15% of global emissions
A person’s carbon footprint from shopping is driven by the emissions generated from the production of a product or service – for example, during raw material extraction, manufacturing – through its delivery, including transport, distribution, and packaging that occurs before the point of purchase. Items in this category include goods, like clothing, furniture, digital and streaming services, and e-commerce purchases, as well as services, like personal care, healthcare, and education.
Shopping emissions are largely estimated using the top-down, spend-based method outlined in the section above. One exception is education-related subcategories, including child day care, elementary and secondary schools, and university, to which Commons currently assigns a carbon weight of 0 kgCO2e per dollar. The top-down approach assumes a linear relationship between dollars spent and carbon emitted. In the case of education, there are a number of other factors that disrupt this relationship, including scholarships, financial aid, and varied tuition levels. Commons has flagged these subcategories for future research.
If a user only inputs information manually via the Carbon Survey, the app generates a static shopping footprint based on the user’s estimated weekly spend on Goods & Services.
Capturing the impact of sustainable choices:
- Shopping secondhand? Categorize shopping purchases as Vintage, Secondhand, or Thrift' to receive a 0 kgCO2e footprint for that purchase.
- Repairing goods instead of buying new ones? Buying fewer items will translate to fewer dollars spent, which will reflect a lower carbon footprint.
- Buying experiences instead of things? Physical goods tend to have higher emissions associated with them, due the carbon emitted throughout the supply chain required to produce them. Going to a museum, park, or zoo will often generate a lower footprint than buying a piece of art or another physical item.
FOOD & DRINK
$6 trillion | Influences 21% of global emissions
Globally, emissions from the food industry are driven by land use and farming. In the Food category, production accounts for X-Y% of total emissions, making the highest-impact choices to be around which food one eats, namely the relative amounts of meat, dairy, and vegetables in one’s diet.
As the basis of our food calculation, Commons uses a top-down approach similar to that outlined in the "Our Approach" section. We then adjust these estimates up or down using a personalized food multiplier, constructed based on a user’s self-reported weekly number of red meat, white meat, dairy, and vegan meals.
If a user only answers the Carbon Survey, Commons creates a bottom-up estimate of their food footprint based on their self-reported dietary habits.
Capturing the impact of sustainable choices:
- Eating more plant-based? Update the Carbon Survey to reflect your new mix of meat, dairy, and vegetable meals per week.
- Cooking in instead of dining out? Connect your spending data to differentiate among the relative carbon intensities of groceries, restaurants, bars, etc. Recategorize a purchase if it doesn’t look right to receive the correct estimate.
TRAVEL
$9 trillion | Influences 11% of global emissions
Since flights and car rides often dominate consumer travel footprints, due to the fossil fuels required to power most of these vehicles, Commons uses more granular calculations for these two sub-categories.
For gasoline, since prices vary widely, Commons uses daily data on the average price of gasoline in a user’s zip code to estimate the number of gallons of gasoline they purchased. We combine this information with data on supply chain impacts and combustion impacts to calculate the footprint of their fuel usage.
The prices of flights also vary widely, and do not accurately reflect carbon impact because airfare prices depend on other factors. To address this, Commons has created a Flight Calculator3 through which users can input details, such as origin, destination, and ticket class, to generate a better estimate of their flight’s impact.
For other modes of transit - including public transit, trains, and bike shares - Commons uses a top-down approach. We are currently working to integrate more granular data related to EV charging and public transit systems into our calculations.
If you only provide information to Commons through the Carbon Survey, we will generate a static footprint based on your main commute mode and estimated daily commute in miles.
Capturing the impact of sustainable choices:
- Flying less? Taking fewer flights will lead to fewer flight transactions, resulting in a lower carbon footprint.
- Choosing more sustainable flights? Get a more accurate estimate of flight impact by clicking into a flight transaction and inputting key details into Commons’ flight emissions calculator.
- Walking and biking more? Driving less? Filling your tank less often will result in a lower footprint.
- Switching to an EV? Replacing their gas purchases with increased home electricity consumption for EV charging will lower travel emissions.
- Shifting to a bikeshare, public transit, and/or trains? Replacing flights or car trips with more sustainable modes of transit will displace more carbon-intense transactions for lower impact transactions.
UTILITIES
$64 billion | Influences 29% of global emissions
Residential energy use is a major contributor to global emissions, and home decarbonization is a powerful lever for influence. Globally, consumer home footprints are driven by electricity and natural gas consumption. Because of this, Commons has adopted more granular calculations for these categories.
When a person turns on a lamp or runs their dishwasher, they use electricity from their local grid. This electricity can come from a blend of different sources, including natural gas, coal, nuclear, or renewables. The specific blend of electricity sources, and thus the emissions associated with it, varies by a person’s location. To estimate the footprint of electric utilities bills, Commons combines monthly data on the average price of electricity in a user’s state with data on supply chain impacts and local grid emissions factors.
When natural gas is combusted to heat a user’s home boiler or furnace, it emits a number of gases, including carbon dioxide and methane. To estimate the footprint of natural gas utilities bills, Commons uses monthly data on the average price of residential natural gas in a user’s state with data on supply chain impacts and combustion factors.
For other types of utilities - including water, sewage, telecommunications, and internet - Commons uses a top-down approach.
If a user only answers the Carbon Survey, Commons combines data on average residential energy consumption with self-reported information on home size, household size, and energy sources.
Capturing the impact of sustainable choices:
- Using less energy? Less energy consumption will result in a lower utility bill, and thus a lower estimated carbon footprint.
- Switching to green energy through your utility or a third-party? Update the Carbon Survey to let Commons know what percent of your electricity consumption is provided via Renewable Energy Credits, community solar, or other green energy programs.
- Installing solar? Update their Carbon Survey to let Commons know what percent of your electricity usage is provided by installed solar panels.
- Live in an area with a greener electricity grid? Share your zip code in the Carbon Survey to factor in your local electricity grid mix in the calculation.
- Live in a small home? Update the Carbon Survey to indicate if you live in a small home that requires less electricity and heating.
FINANCE
$25 trillion | Influences 6% of global emissions
Many financial transactions that show up in a user’s transaction history represent simply shifting money from one location to another. Commons currently assigns these types of transactions (e.g. transferring a lump sum from a checking to a savings account) a carbon value of 0 kgCO2e.
However, our money can create emissions inadvertently, even when we’re not spending it. For example, new research shows that Dirty Dozen banks participate in over 50% of global fossil fuel funding.4 Choosing to bank with a financial institution that doesn’t invest in fossil fuels is one way to leverage savings and investments to have a positive impact. Similarly, crypto-assets have been shown to contribute ~150 million tCO2e each year due to their high energy requirements.5
Commons is currently working on integrating carbon data related to banking and cryptocurrencies into our footprint calculations to better reflect these impacts.
Capturing the impact of sustainable choices:
- Switching to a green bank? Participate in the “Green Your Bank" action to receive guidance, log progress, and get credit for shifting money away from the Dirty Dozen.
OFFSETS
$272 billion | Influences 23% of global emissions
A carbon offset is created when a person or organization pays to avoid, reduce, or remove greenhouse gas emissions somewhere in the world to compensate for emissions that have been released elsewhere. The IPCC has made clear that carbon removal and protection of natural carbon sinks will be required to achieve net zero emissions by mid-century. While the majority of progress must come from carbon reduction, quality offset and removals projects can play an impactful role in an individual’s holistic climate practice.
As offset skeptics ourselves, the Commons team knows that not all offsets are created equal. There are many different types of offset projects, ranging from renewable energy installations to avoided deforestation to direct air capture. Projects vary substantially in quality, carbon integrity, and co-benefits. Through a rigorous evaluation process, Commons curates a portfolio of providers that not only reduce and remove carbon (carbon integrity), but also prioritize innovation, scalability, and environmental justice (transformative potential).
In the app, users can purchase offsets for flights, send an offset to someone else as a gift, or subscribe to offset a recurring proportion of their total emissions each month. If a person purchases offsets within the Commons app, those offsets are assigned negative carbon value, because they reverse the effects of emissions.
As Commons cannot verify the quality of all external offset purchases, the Carbonizer only assigns a negative carbon value to offsets procured through Commons. External offset purchases are recognized as Climate & Environmental Donations and receive a carbon estimate of 0 kgCO2e.
Capturing the impact of sustainable choices:
- Offsetting your footprint through Commons? Subscribe to monthly offsets from the Commons Carbon Portfolio or offset a flight one-off, and see your net emissions on the Footprint page of the Commons app.
- Purchasing offsets from other vendors? Categorize external offset purchases under Climate & Environmental Donations to receive a 0 kg impact.
- Making donations to other climate or charitable organizations? Similarly, categorize other climate-related donations under Climate & Environmental Donations to assign a 0 kg impact.
Sources & Notes
1 Climate Change 2021: The Physical Science Basis. Working Group I to AR6 of the IPCC.
2 Carbon footprints of 13,000 cities. Daniel Moran et al. 2018. Environ. Res. Lett. 13 064041.
3 Commons has based its flight calculator on the methodology developed by myclimate. This methodology takes into account factors including: flight distance, fuel consumption per aircraft kilometer, emission from fuel pre-production, aircraft and infrastructure emissions, cargo vs. passenger load, cabin class, and non-CO2 effects like contrail formation. Research shows non-CO2 effects can comprise the majority of the radiative forcing impact of a flight. These impacts are included so that consumers can consider the full global warming effects of flight choices.
4 Banking on Climate Chaos. 2022. Fossil Fuel Finance Report 2022.
