Dozens,​ hundreds, perhaps even thousands of online ads flash before your eyes every day, so many that you probably don’t even notice most of them. Generating the electricity to get just one ad to appear on your screen can produce a puff of carbon dioxide sufficiently large that, if it were cigarette smoke, you would be able to see it. Showing a single digital ad to a single user involves, on average, emitting between roughly a tenth and a whole pint of carbon dioxide. And the digital ad business puffs on quite a scale. No one knows exactly how many ads are shown online across the world, but informed estimates collected by the researcher Mikko Kotila suggest as many as 400 billion a day.

Using your laptop or phone feels very different from taking a flight. Whenever I fly, I am queasily aware that I’m adding significantly to my carbon footprint. That’s harder to keep in mind when I am streaming video, transferring large files or storing my photos in the ‘cloud’ – a misleadingly ethereal metaphor for huge datacentres packed with energy-consuming computer servers. The carbon emissions produced by information and communication technologies (ICT) are difficult to measure, but probably account for around 2 or 3 per cent of total global emissions, perhaps as much as 4 per cent. That’s comparable to aviation’s contribution to climate change.

Online advertising accounts for around a tenth of ICT’s energy consumption, according to estimates in an article from 2018 in the Environmental Impact Assessment Review by Matti Pärssinen, Mikko Kotila and colleagues. The screens on which ads appear are of course switched on anyway (although some streaming services continue to send ads to TVs whose screens are off). But transmitting the electronic files that generate onscreen ads consumes additional electricity, especially when the ads involve video or elaborate graphics and animations. Rendering sophisticated ads of this kind onscreen is also energy-intensive. The files containing ads generally arrive separately from the content that you want to see or interact with – perhaps a second or two later. So, for example, your phone may be kept for longer in the high-energy state in which there is a dedicated radio channel open between it and a mobile phone mast.

Digital advertising firms also embed tracking code in websites and in the software packages they provide to developers of mobile apps and games, which means yet more energy-consuming to and fro between your device and external datacentres. And further to and fro can be triggered by each and every individual opportunity to show you an ad. If you are using a mobile app, the way it often works is that the opportunity will be offered first to one advertiser, and if they won’t pay for it then another advertiser, then another, and another.* If you are browsing the web, using Facebook, or searching on Google for something of commercial interest, the opportunities to show you ads are electronically auctioned in real time, with algorithms acting on behalf of various advertisers, all of whom are able to submit bids to show you an ad. The whole process is completed in less than a second. Publishers unhappy with Google’s centralised way of organising the bidding have adopted decentralised auctions. These can increase their revenues, but having multiple auctions going on simultaneously means more computing and more electronic messaging, and thus demands more electricity. The entrepreneur Brian O’Kelley, who was a pioneer of near instantaneous automated ad auctions, tells me that sometimes an opportunity to show a single ad can trigger as many as a thousand auctions.

Many large advertisers are keen to trumpet their commitment to sustainability and the carbon neutrality of their products. Emitting lots of carbon dioxide while doing so is an obvious contradiction, and advertisers’ awareness of that has surged in the past year or so. One response can be to keep puffing away and buy an equivalent amount of carbon credits associated with emissions-reduction projects elsewhere, usually in the Global South. But directly cutting advertising’s emissions must be a better path. There’s a lot of scope for doing this: research by O’Kelley’s carbon-measurement firm Scope3 and the consultancy Ebiquity suggests that the carbon footprint of ads varies wildly, with some nearly a hundred times bigger than others.

A good way of starting to reduce emissions would be to cut out waste. Many ads (nobody knows quite how many) are never seen by human beings. Sometimes that’s because of fraud: advertisers can be duped into paying to show ads on websites that few human viewers will ever see, on which computerised bots are clicking away instead. But there is plenty of waste even without fraud. Ads, for instance, can be transmitted to the user’s device but placed ‘below the fold’ (on a section of a page that is never seen unless the user scrolls down), or are in view for only a tiny fraction of a second as the user scrolls.

Those who work in digital advertising are fully aware of such issues, and the sector has created an extensive technical apparatus to detect fraud, measure the viewability of ads, and monitor the content alongside which ads will appear, preventing them from being shown if that content is judged controversial, upsetting or offensive. An advertiser can, however, still easily waste money and burn a lot of unnecessary carbon even when using this apparatus. Many ‘free’ mobile games, for example, make their users do things such as view a video ad or navigate to an external ad-bearing website before allowing them to move to the next level. This isn’t fraud, but having ads forced on you like this is unlikely to make you enthusiastic about the companies or products being advertised.

Another form of waste is showing ads on ‘made for advertising’ sites. These host lots of trackers and many of them sell each and every ad slot via multiple auctions. Their sketchy content (sometimes recycled or AI-generated) doesn’t usually trigger monitoring systems, and the sites’ metrics can look good to advertisers: clicks are generated, ads are fully viewable, video ads are played right to the end, and so on. But ‘it’s just clickbait,’ Ruben Schreurs of Ebiquity says. ‘“You won’t believe how much weight these five celebrities have gained.” And then the person clicks on that and … goes to a page which isn’t about that at all, or it’s a fully loaded page with ads, and you have to click refresh so many times to get to some type of content’ – all with minimal benefit to the advertiser. As much as between 10 and 15 per cent of what’s spent on ‘programmatic advertising’ (automated real-time ad-buying in marketplaces in which multiple buyers and sellers compete) is wasted on ads on such sites, Ebiquity reports.

Eliminating misspending of this sort would help, but there’s also a need to reconsider the processes at the heart of digital advertising, such as the way ads are bought and sold. No one did more to create today’s real-time market mechanisms than O’Kelley, but he is now deeply ambivalent about some of their effects: ‘I’m not sure it was for the good that we did this,’ he told me. These mechanisms have often sucked advertising revenue from serious news publishers to low-quality outlets, and the multiple auctions involved in decentralised buying and selling increase carbon intensity.

In order to cut emissions, must we therefore return to centralised ad auctions, probably run by Google (which does make serious efforts to power its datacentres with electricity from renewable sources)? That would be unpalatable to many in advertising, who argue that it is unhealthy to have a market dominated by a single big player. An alternative would be for advertisers to focus their ads better and deliberately select appropriate outlets for them, create direct commercial relations with those outlets, and thus reduce the need for multiple auctions and multiple forms of tracking. There is no inherent conflict between advertising that is more effective (and less intrusive), healthy competition, and reduced emissions. They can work hand in hand.

Send Letters To:

The Editor
London Review of Books,
28 Little Russell Street
London, WC1A 2HN

Please include name, address, and a telephone number.

Read anywhere with the London Review of Books app, available now from the App Store for Apple devices, Google Play for Android devices and Amazon for your Kindle Fire.

Sign up to our newsletter

For highlights from the latest issue, our archive and the blog, as well as news, events and exclusive promotions.

Newsletter Preferences