When Gnarls Barkley’s Crazy reached number one in April, it made history as the first song to top the UK singles chart on download sales alone. Downloads now account for 78% of all single sales, up from 23% in 2004; there are now about 1m digital tracks bought legally each week in the UK - and an unknown amount, no doubt much larger in total, illegally downloaded, too.

This, in theory, is great news for the environment (less so, perhaps, for the copyright holders). Instead of all those CDs - thin discs of polycarbonate plastic, aluminium, gold, lacquer, and dye - being produced and shipped around the world, we are purchasing “virtual” tracks, each taking up just a few megabytes of disc space and being “transported” down copper wires or across the ether. For those strolling the streets nodding to the beats of their MP3 player, there has, for some at least, been the added satisfaction that this is the more eco-friendly way to listen to music.

In reality, there has been precious little research into this subject. But what does exist suggests that downloading tracks isn’t quite as environmentally pure as it might at first seem. It all hangs, it seems, on how exactly we use our MP3 players. In 2003, Digital Europe, a research project looking at the sustainability of our new “networked world” and conducted by three institutions in Germany, Italy and the UK (here it was Forum for the Future), published its findings. Working with EMI, it looked specifically at the environmental impact of digital music, by analysing three methods for acquiring 56 minutes of music (the average length of an album).

The research used a concept called the “ecological backpack”. Similar in thinking to a person’s ecological footprint, it is a measure used to calculate the amount of resources - fuel, minerals, water etc - that must “be moved” throughout the full lifespan of a product. For example, a 10-gramme wedding ring has an ecological backpack of five tonnes, whereas a 3kg laptop has a backpack of about 400kg.

The first purchasing route the study looked at was buying 56 minutes of music on a CD at a high-street store. It then looked at buying the same CD online, and then finally at downloading all the music. Buying a CD at a shop produced a backpack of 1.6kg, said the study, whereas buying it online reduced the impact to 1.3kg. But by downloading the music, the backpack fell to 0.7kg. In other words, a clear advantage - although hardly a “zero-impact” approach. The need to have a computer and an MP3 player, both of which need producing then powering, increased the weight of the backpack considerably.

But the study also noted some other important factors. It based its weight for downloading on the assumption that a broadband connection was used and that the music was never burned onto a CD at a later date. If this is the case, and a slower narrowband connection is used, the backpack leaps up to a whopping 5.5kg. In other words, “rematerialising” your downloads into a CD at home not only completely negates any environmental savings, but is actually about three times as damaging as just buying the music on a CD in the first place.

Of course, after an initial push to “rip” all their current CD collection into a digital format, most people probably do a mixture of all three to keep their MP3 players full to the brim with music. But it would seem that the ideal scenario would be to never buy a CD again and to always download music (a rather bleak, anodyne world that many musos are not keen to step into, it would appear).