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Why carbometrix does not count your emails

Nov 29, 2021

Carbon emissions related to digital activities are notoriously difficult to estimate, so when actual figures are published on the topic, it is very tempting to take them at face value. Over the years, the same figure has emerged in the headlines, time and time again: the simple fact of sending an average email would allegedly generate 4 grams of CO2, and up to 50 grams for a very heavy one(1).

However, after careful consideration, we think that this number has become misleading. This estimation was made ten years ago by Mike Berners-Lee in his 2010 book, How bad are bananas? The figure was truthful when it was first published, but a lot has changed since then in terms of networking efficiency and device mix.

What does an email really emit?

The footprint of an email depends on the energy that is spent during each step of the process: the device on which the email was written, the data network, the data centre where it is stored, and the recipient’s device while the email is being read.

The user’s device is the first major source of variation. Phones tend to be very efficient, so they don’t consume a lot of energy (around one watt per hour seems a reasonable estimate), while laptops consume quite a bit more. The second source of variation comes from how you are connected to the network: to send the same quantity of data, cellular networks may consume up to ten times as much electricity as wired and Wi-Fi networks(2).

With these distinctions in mind, here are our carbon footprint estimations based on two different scenarios(3).

The takeaway is that you should probably wait until you’re home before sending a big attachment by mail, and avoid writing long emails (your recipient will probably not read them, anyway). As a rule of thumb, there are really two big ways that sending an email will emit CO2: writing a very long email on your laptop or desktop computer, or trying to upload or download a heavy attachment on the cellular network. The first case is already being accounted for in your total electricity consumption; and, depending on your data plan, the second one may not be such a great idea anyway.

With that in mind, assuming you write and receive 100 light emails per day on your laptop (and read all of them), your email carbon footprint for the year will be around 6.2kgCO2, which represents 0.1% of the emissions of an average European person(4). Overall, your emails have a very low impact on climate: surely, there must be other action plans to take first.

What to do, then?

Any means are good to become aware of your digital carbon footprint, but when it comes to emails, it is also important to keep a sense of scale. Buying a smartphone every three years instead of two can help you reduce your carbon footprint by 20kg of CO2 per year(5), which is the equivalent of receiving 1,095 light emails per day (assuming you spend all your time reading them, too). Taking the train instead of a plane for a 1,000km journey can help prevent the emission of 180 kg of CO2. 

This is why carbometrix won’t count your emails when computing the carbon footprint of your company. The CO2 emissions they generate are very small when compared to any productive activity; and we already count most of them when we estimate the electricity consumption of your offices.

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(1) Here are some examples of articles that were written on the topic: https://carbonliteracy.com/the-carbon-cost-of-an-email/, https://www.dailymail.co.uk/sciencetech/article-7224849/Annual-emails-office-worker-creates-CO2-flying-London-Bruges.html, https://nowthisnews.com/news/5-tips-that-make-your-inbox-work-for-you-and-for-the-planet )

(2) French Senate, Empreinte carbone du numérique en France, page 66. http://www.senat.fr/rap/r19-555/r19-5551.pdf 

(3) By “light email”, we mean an email that weighs 100 kB, that took a minute to write, and a minute to read. Likewise, a “heavy email” is an e-mail with an attachment that weighs 2 MB, that took ten minutes to write, and a minute to read. Phones are assumed to be connected to a cellular network. The electricity intensity factor used is 475 gCO2e/kWh, which is a worldwide estimation made by the IEA in the Global Energy & CO2 Status Report 2019 (https://www.iea.org/reports/global-energy-co2-status-report-2019/emissions). Finally, the computation assumes that the email will be stored on the email service provider’s data centres for ten years, with a replication factor of 3.

(4) According to Eurostat, the carbon footprint of EU-27 was 6.7 tons of CO2 per person in 2019. Source: https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Greenhouse_gas_emission_statistics_-_carbon_footprints 

(5) We assume that the production of a new smartphone generates around 60 kg of CO2, based on the iPhone 12 sustainability report (https://www.apple.com/environment/pdf/products/iphone/iPhone_12_PER_Oct2020.pdf). This figure should probably be thought of as a lower bound