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Carbon Assumptions

Carbon Assumptions

Carbon Footprint Calculations

We understand that everyone lives very busy lives, which is why we have kept our lifestyle questionnaire short and sweet. Our questionnaire covers 5 main areas to get an estimate of your carbon footprint: day-to-day travel, flights, diet, products and home.

We have listed below all of the assumptions and sources for data used in our calculations.

Day-to-day travel

Instead of asking you to figure out the number of miles you travel across various modes of transport in an average a week, we ask you how much time you spend traveling. We then work out your distance with average speeds for each mode of transport which is multiplied by the appropriate 2020 UK government emissions factors .

An emissions factor tells us how much carbon is emitted by traveling one mile using each type of transport. These emissions factors vary depending on things like the size of car, type of bus and type of train.

We have assumed an average car size for emissions factors relating to cars. Travel by train is assumed to be national and travel by bus is assumed to be a local bus. For ease, we have assumed travelling by Uber means you are traveling in a hybrid car.

Flights

In the same way that we calculate day-to-day travel, we can use the 2020 UK government emissions factors and the average speed of a plane (900Km/hour) to understand the carbon emissions from flights.

Again, to make things simple we provide you with four options on flight times and take the median time per category: short haul (2 hours), medium haul (6 hours), long haul (10 hours) and extra long haul (14 hours).

We have assumed that you will fly economy class and have opted for emissions factors that include radiative forcing. Radiative forcing includes emissions of nitrous oxides and water vapour when emitted at high altitude.

Diet

A study from the University of Oxford, Poore & Nemecek (2018) , analysed the carbon footprints of various foods in our diet. We ask you about your monthly eating habits to find a monthly footprint and then extrapolate to a year.

We have assumed that everyone eats their vegetables so have added the emissions for a standard vegan diet onto your footprint if you didn’t select a vegan diet.

Products

Everything we consume has a carbon footprint and the organisation Carbon Footprint has provided estimates for how much carbon is emitted from spending on various products and services.

Whilst the list is large of all the possible things you can buy, we have kept it simple with fashion, toiletries and electronics.

Home

We have included spending on furniture and appliances as an emission from your home, with data being provided by Carbon Footprint .

Waste at home can either go to landfill or to recycling and the carbon emissions are different depending on where your waste ends up. DEFRA estimates that the average home recycles only around 45% of their waste. We know from UK statistics on waste that the average home produces 0.975 tonnes waste each year. Using 2020 UK government emissions factors we can then calculate the average emissions per home from waste going to landfill and to recycling plants. This number is then divided by the number of people living in your household to find your share of the household emissions.

Emissions also arise from the supply and treatment of water for your home. Energy Savings Trust estimates the average UK home uses 349 litres of water every day. We multiply this by the 2020 UK government emissions factors to work out the emissions from water supply and treatment in the average UK home. This is then divided by the number of people living in your home to find your personal share of the household emissions.

If you said that you are on a renewable energy tariff or if your gas is carbon neutral then you won’t have any emissions from home gas or electricity.

If you aren't on a renewable tariff or have carbon neutral gas, we use Ofgem data for average energy usage per UK home, which is 2,900 kWh electricity and 12,000 kWh gas each year. We multiply this by the 2020 UK government emissions factors for the UK average fuel mix. This number is then divided by the number of people living in your household to find your share of the household emissions.


Unavoidable Emissions

Carbon Independent calculates that everyone in the UK is responsible for 0.18 tonnes per year per person of food emissions they can't avoid. This is down to things like methane emissions from soil management, and carbon emissions from operation of retail stores and farming.

Carbon Independent also estimates that everyone in the UK is responsible for 1.1 tonnes of emissions over which they have no direct control. These unavoidable emissions are things like the NHS, armed forces, social services and schooling.


Your Carbon Equivalent Metrics

Flights & Car Trips

We use 2020 UK government emissions factors to understand the emissions from a medium sized car and an economy flight. We then multiply the relevant emissions factor by the distance from London to Manchester as (208 miles) for car trips and distance from London to Rome (1440 km) for flights.

We divide the total carbon offsetted by the carbon emissions from each journey, which gives us the number of flights / car trips your carbon offset is equivalent to.

Please note, we round these numbers to the nearest 0.5 as the emissions from these journeys are significant.

Beef Steaks

We use data from the University of Oxford paper Poore & Nemecek (2018) , which calculated that a 75g serving of beef generates the equivalent of 7.73 kg (0.00773 t) of CO2 emissions. This includes the emissions caused by all farm-to-fork aspects including methane generation, meat production and logistics. Of course, 75g is a small steak, so we build the emissions figures up to a standard steak serving of 225g, which is 23.19 kg CO2 (0.02319 t).

We divide the total carbon offsetted by the carbon emissions from a standard steak, which gives us the number of steaks your carbon offset is equivalent to.

Please note, we round this number to the nearest whole integer as the CO2 emissions from one beef steak is relatively small in comparison to our other offsetting figures.

Parcel Deliveries

ASOS (who use DPD and Hermes as their primary delivery companies) report that each medium sized parcel delivery generates approximately 3.68kg of carbon. We divide the total carbon offsetted by the carbon emissions from a medium sized parcel, which gives us the number of medium sized parcels your carbon offset is equivalent to.

Please note, we round this number to the nearest whole integer as the CO2 emissions from one delivery is relatively small in comparison to our other offsetting figures.

Metres Squared of Sea Ice Saved

Three square metres of Arctic summer sea ice disappears for every tonne of carbon dioxide a person emits, wherever they are on the planet, according to UCL research . We then assume that for every tonne of carbon dioxide someone offsets, three square metres of sea ice has been saved.

Please note, we round these numbers to the nearest 0.5 as the emissions to save a square meter of sea ice is significant in comparison to other carbon equivalent metrics..

Days Online Streaming

The central IEA estimate for one hour of streaming video, 36g of Carbon Dioxide is emitted into the atmosphere. Obviously different streaming services will have different emissions per hour but we assume the average with IEA’s estimate.

We then calculate the total number of hours of online streaming your total carbon offset is equivalent to, before dividing by 24 to reach the total number of days.

Please note, we round this number to the nearest whole integer as the CO2 emissions from one delivery is relatively small in comparison to our other offsetting figures.

Gallons of Diesel

In the preamble to the joint EPA/Department of Transportation rulemaking on May 7, 2010 that established the initial National Program fuel economy standards for model years 2012-2016, the agencies stated that they had agreed to use a common conversion factor of 10,180 grams of CO2 emissions per gallon of diesel consumed (Federal Register 2010). For reference, to obtain the number of grams of CO2 emitted per gallon of diesel combusted, the heat content of the fuel per gallon can be multiplied by the kg CO2 per heat content of the fuel.
This value assumes that all the carbon in the diesel is converted to CO2 (IPCC 2006).
Calculation
10,180 grams of CO2/gallon of diesel = 10.180 × 10-3 metric tons CO2/gallon of diesel

Sources
Federal Register (2010). Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards; Final Rule, page 25,330 (PDF) (407 pp, 5.7MB, About PDF).
IPCC (2006). 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Volume 2 (Energy). Intergovernmental Panel on Climate Change, Geneva, Switzerland.

Other Assumptions

We're constantly communicating emissions reduction impacts in different ways. Whilst we update this page bimonthly, please just let us know if you see any other metric you'd like us to give you the data assumptions for.