Aviation has long been blamed for its share of anthropogenic global warming. Indeed, some travellers now ask themselves whether their flight is strictly necessary and, if they decide it is, salve their consciences by paying for the planting of trees. These, so they hope, will absorb the equivalent of their sinful emissions. But you, dear reader, are indulging right now in activity that is equally as polluting as air travel: using a computer.

According to a report published by the Climate Group, a think-tank based in London, computers, printers, mobile phones and the widgets that accompany them account for the emission of 830m tonnes of carbon dioxide around the world in 2007. That is about 2% of the estimated total of emissions from human activity. And that is the same as the aviation industry's contribution. According to the report, about a quarter of the emissions in question are generated by the manufacture of computers and so forth. The rest come from their use.