In this case study we will look at image compression. Typically when trying to optimize a website for network resource consumption and thus CO2 emission images are the lowest hanging fruit.

Modern image formats like AVIF typically achieve +50% compression ratio when compared to PNG in a similar perceived quality.
Try out on Squoosh.app for yourself to see the potential!

AVIF received many positive blogposts. However modern image formats like AVIF have a downside: They are not supported in older browsers like IE, Safari or old Chrome devices. Check support on CanIUse.com.

If you still want to support these old browsers, so that they do not see a blank image you typically would use a polyfill.

In this detailed article we will look at the potential backlash when using polyfills and how to watch out for the tipping points when a move to AVIF might actually get worse in terms of carbon emissions, although it might make no difference in performance for the users.

In this example moving to AVIF even with the needed polyfill is still worth regarding the carbon emissions. However the margin got quite diminished and if the users are visiting less pages or the browser ratio is different this might also easily reverse and become a rebound effect.

If you look at this example I hope we could convey that if you optimize for lesser CO2 emissions the backlash to consider are often different when optimizing just for performance, where a mere 300 kB polyfill will most likely go unnoticed.

When looking at our example one might also argue, that instead of including the polyfill we could also use a switch that would redirect to a different HTML page which includes only PNG images, or to use an image service, that only delivers the correct image based on detected browser capabilities.

All of these approaches have different problems and backlashes though, since they typically mean that you save on network bandwith, but therefore have to increase the storage, which is also powered by energy and thus emits CO2.

In a follow-up article we will further drill-down into the cost of storage in particular and how replicating your data manyfold to different CDNs also has a CO2 cost that has to be factored in.