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The carbon footprint, in carbon dioxide equivalent, of sending an email message, has been estimated at 0.3 gCO2e for a spam email, 4 gCO2e for a "proper email", and 50 gCO2e for an "email with long and tiresome attachment that you have to read". These figures have been repeated in the media many times, even recently in 2018, in 2017, etc., and implied to mean that it is climate-conscious to avoid sending too many emails. Indeed, with these figures, an average 121 emails per day would amount to 176 kgCO2e/year, which is certainly not negligible. My question is whether these figures are accurate (in 2020).

The figures seem to come from the 2010 book How Bad Are Bananas? The carbon footprint of everything by Mike Berners-Lee. I found them in the book, but no info about how they were computed, except a mention of research by McAfee without a specific source. I think it is probably this 2009 report, which does have the 0.3 gCO2e figure for a spam email, and a figure of 131 kgCO2/year in email-related emissions for the average business email user. But there is little information there about how the statistics were computed.

My reason to doubt this is that watching Netflix was more recently estimated (in 2020) to have a 28-47 gCO2e footprint for half an hour. Even taking the lower value, this would mean that a "proper email" would have the same emissions as 4 minutes of watching Netflix, and a spam would correspond to 20 seconds of Netflix. This is weird to me, as Netflix does seem to require much more bandwidth, computation power on the server and client, storage, etc. One explanation for why the email estimate seems high may be that the carbon footprint of network transfer has steadily decreased since 2009-2010: see this study Figure 3 for instance.

So I am wondering whether these estimations have been updated or refuted since 2009-2010. Looking online, I haven't been able to find anything that revisited or contradicted these statistics.

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    Does the claim perhaps include the computer + screen time spent reading the email? An hour of reading emails on a nice 32" 4k monitor vs. switching the computer off and working in the garden? – gerrit Jun 4 at 11:57
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    @Polygnome: for most display types, those that filter colored pixels from a backlight, background color doesn't matter. – dandavis Jun 4 at 19:51
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    IMO it's a nonsense statistic. My computer runs all day, and it makes no difference to the carbon cost what the length of the email is. Similarly the network server runs all the time. It's just an average divided by another average. Similarly in my UK doctor's surgery a notice informs that it costs the NHS £xx every time I consult a doctor, which again is an average of total costs divided by consultations, taking no account of the knock-on cost of me not visiting a doctor. It costs the surgery no less to run, whether or not I visit (although if many visit they need more staff). – Weather Vane Jun 5 at 21:43
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    @WeatherVane There cannot be no additional carbon cost. The transmission of an email necessitates that a lot of devices worldwide change a couple of bits in their internal state and that requires a certain growth of entropy, the resulting heat of which can only come from various electric power supplies. The theoretical physical minimum is perhaps tiny, but of course actual and typical (in particular: fast) devices do not even get close to such a minimum – Hagen von Eitzen Jun 5 at 21:54
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    @WeatherVane, there isn't a zero activity state when they're running, but there is a very low activity state. In the past decade or so, a great deal of effort has gone into designing devices so that the "waiting for something to happen" state consumes very little power. – Mark Jun 5 at 22:13

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