What is a carbon footprint?

Of course you have. Over the past decade there has been an ever increasing focus on climate change and the ways in which we can prevent it. Because of this, we have all had to learn a lot about our impact on the planet in a short space of time. Knowing your carbon footprint is critical to understanding your impact on the planet and yet there still seems to be some confusion over what it actually means. The term crops up all over the place so think of this as your briefing: What is a carbon footprint? How can we measure it? And importantly: How do we

reduce it

?

Mike Berners-Lee, author of

There is No Planet B and How Bad are Bananas

, is a leading expert on carbon footprinting, and so we thought his definition of it would be a  good place to start. He suggests that a

carbon footprint

“is the best estimate that we can get of the full climate change impact of something”

. It’s useful to note from the outset that any carbon footprint, no matter how accurate or detailed the calculations are, is not always going to be 100% correct, but it will be the best estimation. We know that, and we also know that nobody is going to have a perfect carbon footprint, but everybody can reach a climate positive lifestyle through nature based solutions.

There has been much discussion over the correct way to measure carbon footprints, with disputes ranging from the time period of consideration to the units of measurement. Thomas Wiedmann (one of the professors who examined our own carbon footprint calculator) and Jan Minx noted in their 2008 [1] paper, that there seemed to be “

no clear definition

” of a carbon footprint and “

still some confusion over what it actually means and measures and what unit is to be used

”. However, at

Earthly

, we have developed a

carbon footprint calculator

that brings together ideas, data and considerations from a range of different influences to create what we believe to be the

best calculator available

. By explaining how we measure a carbon footprint in the next section, we can then draw out its definition. 

Across the world everyday, humans produce greenhouse gases (GHG) in vast quantities, through activities such as transforming coal, oil, and gas into electricity, energy, clearing woodland and forests for urban and agricultural use, and burning fossil fuels for travel and transportation. 

Your individual GHG emissions are most likely made up from the energy you use personally for electricity and travel, in addition to the energy that’s required to produce your food and all the other stuff you buy. These are called ‘direct’ emissions however, the true carbon footprint of something would take into account all of the ‘indirect’ emissions of a product, such as the extraction and processing of oil used in manufacture and transport. This makes the true footprint much harder to measure, but we believe that the entire life-time’s worth of emissions of a product should be taken into account, tracking manufacturing and production ‘indirect’ emissions. 

The most common GHG released by the burning of fossil fuels and other human activities is carbon dioxide (CO2) – hence why it is called a

carbon footprint

. However, there are also a whole host of other GHGs that also contribute to global warming and need to be considered in the calculation of an individual’s footprint. 

For example, Methane (CH4) and Nitrous Oxide (N20), both of which are mainly produced by the agricultural sector, are 25 and 298 times respectively more potent than CO2, over a 100-year time horizon. The potency measure used by climate scientists is called the

Global warming potential (GWP)

and is a measure of how much heat a particular GHG traps in the atmosphere (up to a specific time horizon), relative to CO2.

Table 1. [3]

Most products and activities produce a

mixture of GHGs

along its supply chain. This is why at Earthly we opt for expressing carbon footprint in terms of

CO2e

. This means “

the total climate change impact of all greenhouse gases caused by an item or activity rolled into one and expressed in terms of the amount of carbon dioxide that would have the same impact

.” Or in other words, we convert all the different GHGs into an equivalent impact from carbon dioxide, and so show your footprint value in “tonnes of carbon dioxide equivalent” –

CO2e

.

From these reasonings we can draw our definition of a carbon footprint:

A carbon footprint is the best estimated measure of the total (direct and indirect) amount of greenhouse gas emissions by an item, activity or individual, as expressed in tonnes of carbon dioxide equivalent (CO2e).

As can be seen, working out a carbon footprint is not as easy as it seems – and often can become very complicated, very quickly. To make them clearer to understand, we’re introducing a categorisation system for our carbon footprints, which will make it easier to  do something about it. We’re splitting the footprints into low, average and high. Low carbon footprints would be 10 tCO2e or under, average would be 15 tCO2e, while high would be 20 tCO2e.

High carbon footprints are footprints we categorise which are well above the UK average and are 20 tCO2e or above. These footprints are typical of high-polluting developed nations such as the USA and Australia but can be just as common in the UK.

It is easier than you think to have a high carbon lifestyle. There are a few habits which contribute disproportionately to a carbon footprint which might be pushing yours higher than you would want. 

Air travel 

Firstly, air travel is usually one of the largest component parts contributing to a high carbon footprint. Aviation is responsible for around 2% of the world’s global carbon emissions, and if you fly regularly it will most likely be making up a large percentage of your personal footprint. 

Domestic flights emissions per passenger per km amount to 254g, meaning a short flight from London to Edinburgh would emit 136kg of CO2e. If you fly frequently on domestic flights, this figure will soon add up far beyond a low (10 tonne) or average (15 tonne), carbon footprint. 

For long haul flights, the efficiency per km is slightly better, however overall due to the length of the flights the CO2e emissions are usually still much higher than shorter flights. Unfortunately, there doesn’t seem to be a magic, scientific solution coming anytime soon; although some

experiments with biofuels

are being piloted. In fact, ideas such as the the potential for a ‘zero-emission flight’ which were proposed at the turn of the century, have been more recently labelled as ‘technology myths’, and the real aviation fuel efficiency gains likely to be seen throughout the 2020s will be as low as <1% per year [3]. 

Meat-consumption 

One of the biggest factors affecting carbon footprints and GHG emissions that has come to light more recently is the impact of meat consumption within diets. Meat-lover’s diets can have up to double the impact of a vegan diet (3.3t compared to 1.5t).  (See figure 2. – source Shrink That Footprint)

However – not all meats are equal! Well, not in terms of carbon emissions anyway. Even being aware of the impacts of the different types of meat, can help you dramatically reduce your carbon footprint. For a start, minimizing consumption of beef can dramatically change your footprint. Per 1000 calories of food beef has the highest average CO2e. Beef production results in 9.6kg worth of CO2e per 1000 calories –

more than four times

the next highest (pork with 2.0kg per 1000 calories). 

Figure 3. [6]

A later section of this explainer will go into much more detail about food carbon footprints and the diets that can help reduce them.

Other travel habits

If you drive regularly or for long distances, this may be increasing your carbon footprint by a lot. According to Chris Goodall, author of The Green Guide for Business and How To Live a Low-Carbon Life, “

reducing the mileage of the average new car from 15,000 to 10,000 miles a year will save more than a tonne of CO2, about 15% of the average person’s footprint

.” If you regularly drive a lot, then it may be worth considering switching to an electric car which can lead to dramatically lower carbon emissions however, there are some additional considerations to be made. For example, the source of the electricity used to fuel these vehicles is often from the burning of fossil fuels so the impacts aren’t as dramatic. 

Another consideration in the manufacturing of these vehicles, there are contrasting opinions but many suggest that the manufacturing of the lithium charge batteries that go into most electric vehicles are both carbon costly to manufacture, but also harder to recycle.

Volvo research suggested that the production of an electric car produces

70% more greenhouse gas emissions

than the production of a petrol car.

Therefore, a ‘typical’ high carbon footprints lifestyle might include, high levels of meat consumption, regular travel by car and aeroplane, as well as a high rate of general goods consumption. However, because these factors are the greatest cause of GHGs, if they are changed or removed from a lifestyle they will have the most significant reduction on your footprint.

At Earthly, low carbon footprints are categorised as being 10 CO2e or below. However if your footprint is already at the 10 tonne mark, this doesn’t mean you shouldn’t stop lowering your footprint. In his book

How Bad Are Bananas: The Carbon Footprint of Everything

, Berners-Lee, proposes that people should strive for a 10-tonne lifestyle. This 10-tonne carbon footprint, which is by no means small as compared to the global average (which sits at around 4 tCO2e), would be a very positive first step towards a carbon positive future.

Living a 10-tonne lifestyle for a year works out to a limit of

833kg CO2e per month

or 24.7kg CO2e per day. 

Aiming for ten tonnes or below over the course of a year might seem more straightforward than it is. For instance one 24 hour long-haul return flight will account for 3.4 tonnes. That means making three of these trips will land you at the 10-tonne mark without any other emissions accounted for. Driving an average car (33miles/gallon) 8000km/5000 miles in a year-long time period would result in 3.59 tonnes of carbon equivalent emissions. Cycling, walking and public transport would likely be necessary alternatives in order to keep below a ten tonne limit. 

Similarly, your diet has a huge part to play as well, using the figures from the National Geographic bar chart shown earlier, getting 1000 calories from beef every day for a year would lead to 3.6 tonnes of CO2e. This would take up almost 300kg of your 833kg monthly CO2e limit. Eating less meat would lower this footprint. For example, eating a larger beef burger only two times per month would only lead to 60kg CO2e over the course of a year. These sort of changes can make a massive difference and are definitely worth considering as we strive towards a carbon positive global outlook.

The next section goes into more detail about the ways in which your foods have an impact on your footprint. 

Food carbon footprints can be some of the most influential on a person’s overall footprint, since there are a huge amount of GHG emissions produced along the course of its supply chain. Food and agriculture are responsible for 26% of global GHG emissions, in addition to having huge impacts on land use, freshwater use, eutrophication and biodiversity [4] (see graph below). On a more personal level, on average, food accounts for 10-30% of a household’s carbon footprint, but in lower income households this is typically an even higher proportion [5]. 

‘Eating local’ can radically reduce the amount of GHG emissions caused by the transportation of food (DEFRA, 2012). One of the best things you can look for when food shopping to reduce your carbon footprint is locally sourced produce. This doesn’t mean that you have to live off a diet exclusively from the area you live in – it isn’t actually the case that all foreign food produce has an incredibly high footprint. However, any foods which are transported by aeroplane (air freighted) have a very high carbon footprint. Don’t worry though – air freight is actually far less common than many believe it to be, it accounts for about 0.2% of food miles [4]. Although air-freighted goods only make up less than 1% of food miles they still contribute

11% of CO2 food distribution emissions

. This means that you should be especially careful not to pick up these items – air-freighted products emit 50 times more carbon dioxide equivalent per tonne per kilometre than boat freighted food. The goods that tend to be air-freighted are often  more perishable (and hence require quicker transportation). Fruits and vegetables such as asparagus, limes, grapes, green beans and blueberries are all culprits of being air-freighted and thus having a high carbon footprint. 

As a general rule, it is good to avoid foods that have a very short shelf-life and have travelled a long way to be on your supermarket shelf. Also consider trying to buy ‘in-season’ – this means buy produce that could have feasibly been grown locally for that time of year. For example, strawberries in the supermarket in December are unlikely to have been grown within the UK, and if they have – it would have been in carbon costly, high energy greenhouses.

Global food miles equates to

3.0 gigatonnes of carbon dioxide

with transport accounting for 19% of total food emissions.

The highest proportion of the carbon footprint of the food industry doesn’t come from the food miles or the packaging, although these are both important, but rather comes from agriculture, livestock farming and the associated usage of fertilisers, which often break down into Nitrous Oxide. At the moment, most research shows that diets which contain fewer animal sourced food, with a larger proportion of plant-based food, are not just notably better for the environment, but also have been shown to be healthier.

The EAT-Lancet Commission

, which is the first full scientific review of what constitutes a healthy diet from a sustainable food system suggests that a ‘planetary health diet’ (see below) should be created and implemented “to highlight the critical role that diets play in linking human health and environmental sustainability and the need to integrate these often-separate agendas into a common global agenda for food system transformation”.

Here are some suggestions of diet changes that if implemented could

lead to carbon footprint savings

:

Changes & Annual Savings (tonnes of CO2)

It can be easy to go about your daily business without realising that many of the things you do or consume have carbon footprints attached. Even if they’re small, the carbon footprints of often repeated things will add up over the course of the year. Lots of the information in this section comes from Mike Berners-Lee’s book: How Bad are Bananas: The Carbon Footprint of Everything. If you want a comprehensive review of the carbon footprints of everything you might use daily, then this book is a great resource to start. 

The internet is one of the greatest, and most useful advancements of society since the industrial revolution. It lets us share photos, download music and gives us social media, however amongst all of this our online habits do have an impact on our carbon footprint. According to some estimates, the yearly footprint from our gadgets, computers and internet usages is about

3.7% of our total emissions

. This is largely due to the emissions caused by the manufacture and running of digital technologies, which globally adds up to

1.7 billion tonnes CO2e per year

. However this doesn’t split evenly per capita, as many Western countries such as the UK and Australia have higher than proportional data usage. As well as this, the source of the power for data servers should also be considered – in many parts of the world servers are largely powered by the burning of fossil fuels. The carbon footprint of sending an email starts at about 4g CO2e per email but this can quickly increase up to 50g CO2e if the email is very long and/or carrying attachments. These days as people are using their phones and computers more and more often the individual digital footprint is most likely to increase. Especially since COVID-19, as people have been spending longer on their devices. However the positive is that if the technology is being used to replace real life travel (e.g. Skype instead of commuting for a meeting) then there will be significant savings on emissions.

The key thing to consider when learning about your laundry’s carbon footprint is how the clothes are getting dried. Tumble drying clothes is the biggest contributor to your laundry footprint – you could save almost half a tonne of CO2e a year by switching to a clothes rack (assuming 200 uses of the dryer per year). Another top-tip is washing your clothes on a lower heat, to save energy and thus lower your footprint. 

Even going to the pub isn’t without a carbon footprint of sometype. If you are wondering which tipple is going to be the best for your personal footprint then you can often use a similar metric as you would with your food: is it local? Any beer that has extensively travelled will have a much larger footprint than locally brewed beers. Additionally, anything that was made to be transported will have a higher footprint, so this will include any bottles and cans. However kegs and casks of beer are designed to be refilled and recycled so have lower packaging footprints. Therefore your best bet at the bar is a locally brewed cask ale which comes in at

300g CO2e each

. A locally bottled beer from a shop and a foreign pint at a pub are about equal to 500g CO2e each. Bottled beer that has been extensively transported has the worst carbon footprint at around 900g CO2e per bottle – drinking these bottles daily or an equivalent to this could equate to as much as a tonne CO2e over the course of a year. At home, cans are slightly better compared to bottles as long as you make sure they get recycled.

If you want to fully commit to a lower carbon footprint from your pint, then a pint of water is the way to go. Your taste buds might not like you for it, but a pint of UK tap water has a footprint of only 0.14kg CO2e per pint. In fact, a year’s supply of water, including washing, cooking and cleaning is equivalent to only a 20-mile car drive.

We measure carbon footprints using the direct and indirect impacts of the item or activity to take into account all of the emissions of the product. We also use the measure CO2e which incorporates all of the harmful GHGs as converted into carbon dioxide equivalent. At Earthly we categorise footprints into high, average and low – which are 20 tonnes CO2e, 15 tonnes CO2e and 10 tonnes CO2e. Even if you have a footprint of 10 tonnes or below, everyone should be striving to reduce emissions and lower their footprint. Some of the largest contributors to a high footprint are diet and frequent flying, however there are plenty of other contributors to a carbon footprint and many that you might not have ever considered before. It’s important to remember that everyone and everything has a carbon footprint – nobody will ever be perfect, but we can do the best we can to lower our impact on the environment.

[1] Wiedmann, T. and Minx, J., 2008. A definition of ‘carbon footprint’.

Ecological economics research trends

, 1, pp.1-11. 

[2] Nutongkaew, P., Waewsak, J., Chaichana, T. and Gagnon, Y., (2014). Greenhouse gases emission of refuse derived fuel-5 production from municipal waste and palm kernel.

Energy Procedia

, 52, pp.362-370.

[3] Peeters, P., Higham, J., Kutzner, D., Cohen, S. and Gössling, S., 2016. Are technology myths stalling aviation climate policy?. Transportation Research Part D: Transport and Environment, 44, pp.30-42.

[4] Poore, J. and Nemecek, T., (2018). Reducing food’s environmental impacts through producers and consumers. Science, 360(6392), pp.987-992.

[5] Jones C., Kammen D. (2011) “Quantifying Carbon Footprint Reduction Opportunities for U.S. Households and Communities.”