This is what I was wondering today as I deleted old emails from my bulging inbox folders. Working from a vague sense of data centres that use a lot of CO2-emitting energy, I felt the halo glowing above my head. But was that halo deserved?

I have found a number of sources that quote the figure of 4 g of CO2 emitted for a simple email, up to 50 g for one with an attachment. The emissions come from the electricity used by the devices (such as your laptop and wifi) you use to send it, plus the energy use of the data centres the email goes through. However, these figures are over 10 years old, and according to the BBC’s Smart Guide to Climate Change, may have increased.

Data centres store and process data in large quantities. They are ‘the cloud’. And not only is ‘the cloud’ here on land, but transmission between countries takes place via cables laid on the ocean floors. The Conversation tracked down the physical location of some Sydney data centres in Alexandria, making their existence even more earth-bound.

Data centres use an enormous amount of energy to process and store data and for temperature control. In 2013 they consumed 7.3TWh (26.3 PJ) of electricity in Australia (3.9% of national consumption). More recent figures for worldwide use show data centres consumed at least 1% of global electricity. If a data centre uses green energy, this is a lot less polluting than fossil-fuel energy, but how can I know where my email is going, or what sort of energy it is being transmitted by? Once it leaves my house (powered by green energy) the answer is, I don’t know. Search as I might, burning about 0.2 g of CO2 with each search, I can find lists of Australian and NZ data centres, and information about Telstra’s data centre ownership, but what does that mean for me? My internet is provided by Telstra so presumably my emails and internet searches go through them, but are my photos stored there too, or in an Apple data centre? Following these assumptions, I can find no information about their power sources.

Of course a lot of internet use is creating efficiencies, letting us email rather than send a letter, or Zoom rather than travel, but at a time when we have to look at reducing our CO2 emissions, and quickly, every reduction counts.

So, did I deserve my halo? Yes! Deleting emails means they’re not stored in the data centre, requiring processing and cooling to survive. Better still though, would be to have fewer emails in the first place. I need to go through the emails I subscribe to and unsubscribe from the ones that I don’t read. Even those aspirational ones that someone I admire recommended but which I never seem to have the time for. And in the future, please don’t think me rude if I don’t send a ‘Thanks’ by email. I might send it by text instead, and just use 0.014 g of CO2.

I know I know! There are 1 million grams in a tonne, so you’d need to send 250,000 simple emails to generate 1 tonne of CO2. Trimming your inbox isn’t going to save the world, but it does remind you to do so.

PS If you’re willing to use an extra 0.2 g of CO2 (maybe you can economise elsewhere) here’s a good infographic to take you through the figures for all types of internet use.