You get a quick reply to a work email, and before you know it, you’ve typed “Thanks!” and hit send. For a while, that two-second habit was being described as a climate problem.
In November 2019, the British energy supplier OVO Energy released research claiming that Britons send more than 64 million unnecessary emails a day. If every adult sent just one fewer, it said, the UK would save 16,433 tonnes of carbon a year, equal to more than 81,000 flights from London to Madrid. The campaign was called “Think before you thank,” and the figure spread quickly through news sites and social media around the world.
Where the number came from
The calculation was based on work by Mike Berners-Lee, a carbon-footprint researcher at Lancaster University (and brother of World Wide Web inventor Tim Berners-Lee). In his 2010 book How Bad Are Bananas?, he had estimated the carbon cost of everyday activities, including sending an email.
His reasoning was straightforward. When you write an email, your computer uses electricity. When you send it, the message travels through networks that use electricity, then sits on servers in data centers that use even more. Add it all up across tens of millions of people, and you get a big-sounding number.
The story got a second wind in late 2020, when the Financial Times reported that UK officials planning the COP26 climate summit were considering encouraging people to send fewer emails. That’s when the experts started pushing back.
“Back-of-the-envelope” maths
Berners-Lee himself was among the first to cool things down. He told the Financial Times that the figures were based on rough “back-of-the-envelope” calculations from 2010, useful for starting a conversation but not much more. On social media, he went further, saying the government had the wrong end of the stick and that the carbon footprint of sending an email is trivial.
The age of the numbers matters a lot in tech. Researchers have found that data center energy efficiency has roughly doubled every couple of years, so an estimate from 2010 would already have been badly out of date by 2019. It would be even more so today.
Even taken at face value, the claimed saving was tiny. The UK’s total greenhouse gas emissions in 2019 were about 435 million tonnes. The 16,433-tonne figure works out to roughly 0.004 percent of that, and only if every single adult in the country changed their habits.
Why skipping the email saves almost nothing
The bigger problem is how the original estimate was calculated. It divided up the energy used by the whole email system, including your laptop, your Wi-Fi, the internet and the data centers, and assigned a slice of it to each message. That’s a reasonable way to describe email’s overall footprint. It’s a poor way to predict what happens when you send one fewer message.
Chris Preist, a professor of sustainability and computer systems at the University of Bristol, explained it to the BBC. Your laptop stays on whether you send the email or not, and so does your Wi-Fi and your home internet connection. The wider network uses roughly the same amount of energy even if traffic drops slightly. There might be a tiny saving at the data center that stores the message, but he said it would be far less than a gram of carbon per email.
Jonathan Koomey, a former Stanford researcher who studies the energy use of computing, put it even more bluntly. He told Quartz that the extra climate impact of sending one more email, or one fewer, is essentially zero.
Average versus marginal: the useful lesson
This gets at a distinction that’s worth knowing for any tech-and-climate claim. There’s the average footprint, which takes the total energy of a system and divides it by how much it gets used. Then there’s the marginal footprint, which is how much extra energy one more use actually adds.
For systems that are always on, like home broadband, mobile networks and big data centers, the two can be wildly different. The equipment draws power whether it’s busy or idle, so small changes in how much you use it barely move the total. Many viral “your daily habit is killing the planet” stories quietly mix the two up.
That doesn’t mean digital tech has no climate cost. Data centers and networks use a substantial amount of electricity worldwide. But that cost is mostly driven by building and running the infrastructure, not by the individual short messages flowing through it.
What actually makes a difference
So if not emails, what? Preist’s advice to the BBC was simple: buy less hardware and keep it for longer. Making a laptop, phone or tablet involves mining materials, manufacturing chips and shipping products around the world, and much of a device’s lifetime carbon footprint comes from that production stage, before you ever switch it on.
In practice, that means getting another year or two out of your current phone instead of upgrading the moment a new model comes out. It means fixing a slow laptop with a cleanup or a battery replacement rather than replacing it, and passing old devices on so they get a second life. Those choices make a real dent, unlike trimming your inbox.
Preist also argued that the bigger responsibility lies with the companies that run digital services, which should design their systems to be as energy efficient as possible. Most large providers already have strong financial reasons to do exactly that, since electricity is one of their biggest costs.
So, should you send the “thanks”?
On the climate question, the honest answer is that it doesn’t matter. Whether you send it or not, your devices and the network will use about the same energy.
The better question is whether the other person wants it. Preist’s own rule of thumb was to send an email if you think the recipient will value it, and skip it if you think they won’t. That’s good inbox etiquette, and it has nothing to do with saving the planet.