As Fred Pearce noted in his greenwash column last week, Dell is banning the export of electronic waste to developing countries. The move will be welcomed in many quarters. But an even more positive step would surely be to keep our computers in circulation longer by passing them on to someone else – or so we are often told.

Besides bridging the digital divide, the rationale for donating computers for reuse is that it is supposed to be greener. Even before a computer is switched on for the first time, as much as 75% of its lifecycle fossil fuels have already been consumed in manufacturing it, not to mention around 1.7 tonnes of raw materials and water. With half a million PCs disposed of globally every day and nearly half a billion mobile phones discarded each year, it stands to reason that giving your old laptop to someone who needs it is one sure-fire way to curb emissions, save precious resources and help the developing world in the process.

But in passing on our PCs, we need to also take into account an additional cost that is often overlooked or ignored.

Besides the emissions an individual PC is responsible for, there are the subsequent e-missions – the greenhouse gases incurred by the internet itself. In 2007 the world's computers, monitors, telecoms networks, routers and the data centres that keeps the internet running carried a carbon mouse-click of 830m tonnes of CO2. Even by the industry's own conservative estimates, that's the equivalent of 2% of all global greenhouse gas emissions for that year, putting it on a par with the aviation industry.

At the very least this is set to grow by 6% a year, with the biggest growth coming from the developing world. At the moment just one in 10 people in China own a PC. By 2020 this is expected to rise to seven out of 10, the same as the US. It's a similar story in India. And on a global scale we will go from one in 50 people owning a computer to one in three in the same timeframe.

Meanwhile the number of internet servers required to keep these computers connected will rise from around 18m to 122m. Even with energy-efficiency improvements it will result in a tripling of e-missions on 2002 levels taking them to a whopping 259m tonnes of CO2 in 2020.

And all this assumes that we maintain the current trend of chucking out our laptops roughly every three to five years. Yet with shops now offering free computers in exchange for mobile broadband subscriptions, there is a real danger that pretty soon we'll be following the mobile phone model and upgrading every 12 to 18 months.

This is not to say you shouldn't donate your old laptops and desktops to charities – there are lots of charitable and humanitarian reasons to do so. But to claim it is the greenest option is just wrong. As benevolent and laudable as it may seem donating your PC to someone else, it's worth bearing in mind that it is also helping to increase the user base of this technology, a user base that will require power, resources and produce its own e-missions.

Ultimately the greenest option is to just hang on to the computer, use thin client services to access the latest software and when the machine eventually packs up try to recycle it in the most responsible way.