We take it for granted that email is a lot more efficient than the post, and therefore it must be significantly better for the environment too. But faced with headlines about the internet accounting for up to 20% of global electricity consumption and 5.5% of carbon emissions by 2025, we thought we ought to put this assumption to the test.
It turns out that answering this question is not as straightforward as you might first imagine. To measure the carbon footprint of an email we clearly need to consider the energy use from storing the email, transferring the email, and operating the user devices. But there is more, to do this properly we need to include the cost of building and maintaining the data centers, the cables and wireless infrastructure, and manufacturing the user devices. With user devices and data center equipment typically being replaced every three or four years or so, these costs are substantial.
The same logic applies to the calculation for the postal service. We not only need to look at the carbon footprint of manufacturing paper and transporting letters, but also need to consider the costs of building and maintaining the whole postal service infrastructure, including buildings and vehicles.
Given this level of complexity, it is perhaps no surprise that different studies produce very different results. A research paper from PitneyBowes on the environmental impact of traditional mail references fourteen different studies from around the world, with results ranging from 7.6g to 36g of CO2 (carbon dioxide) emissions per letter. [To give an idea of how much this equates to, the average human breathes out around 40g of CO2 per hour.] Some of these differences are down to regional variations in the way the postal service operates – for example, some countries have already prioritized reducing the carbon footprint of their post. But much of them are due to differences in approach.
As for email, an internet search will yield an even wider range of estimates. But many of these calculations are years out of date and are therefore likely to be significant over-estimates, as they do not take into account continuing advances in computing and data centre technology. Internet traffic is growing exponentially (at around 25% per year), but the energy use of the internet is increasing at a much lower rate.
We can estimate the carbon footprint of an email by combining the cost of sending the email with the cost of storing the email after it is sent and delivered. To estimate the cost of sending an email we will use figures for overall internet activity and make some assumptions about the average email size to estimate the contribution from one email. And to calculate the extra carbon used in storage we will simply compare with off-the-shelf storage solutions.
According to the BBC, the internet generated around 1.7bn tonnes of CO2 in 2020. And according to figures from Cisco, the total amount of internet traffic during the same year was around 2.6 zettabytes (zetta is 1 with 21 zeros after it).
We will make the heroic assumption that an average email is very roughly around 100KB (kilobytes) in size. Most emails are much smaller, but there are also plenty of emails with large attachments which are much larger. We will also assume that the typical email is transferred on average five times as it travels from sender to server to recipient, meaning that the average email generates approximately 500KB of internet traffic. Based on these assumptions and the figures above for the total internet, we estimate that sending the ‘average’ email generates around 0.3g of CO2:
1.7bn tonnes * 500KB / 2.6ZB = 0.3g
It is worth putting this into context with video streaming. HD streaming requires roughly 500KB per second – which happens to be the same amount of data as we estimated for the ‘average’ email. This means that for a ‘typical’ email account, receiving and sending, say, 60 emails a day, one day’s entire email activity emits the same amount of carbon as 1 minute of video streaming. Or alternatively, all the emails for a year are equivalent to watching six hours on Netflix.
But emails are different from video streaming in that they need to be stored, often for years. Assuming the average email is retained for ten years, we can estimate the additional energy cost of cloud storage by comparing with the energy usage of storage solutions available in the shops. In this example, approximately 7 watts of power is required to support 8TB (terabytes) of storage. Scaling this over ten years and for one email, we arrive at a total usage of around 0.008Wh (watt hours).
7W * 100KB/8TB * 10 * 365 * 24h = 0.008Wh
Using the general US electricity grid as a benchmark this equates to approximately 3mg of CO2, meaning that the carbon cost of storage is significantly less than the cost of data transfer (which was 0.3g, or 300mg).
So, to answer the original question, how much better for the environment is email compared to normal mail? An email generates around 0.3g of CO2, compared with something around 30g for a normal mail, meaning that, according to these calculations, a reasonable answer is that email is roughly 100 times better for the environment than the post.