If all human knowledge is in fragile digital storage, vast chunks could be lost forever, Steve Bowbrick believes.

Almost 2000 years ago, in a fort in the shadow of Hadrian's Wall called Vindolanda, Roman bin men dumped thousands of small wooden tablets inscribed with the equivalent of today's Post-it notes or text messages: ephemeral scribbles of no intrinsic value but, for historians, hugely useful in drawing a picture of the period and its people. Today we're leaving the equivalent of these notes in immensely fragile digital repositories in every corner of our lives. Reading the Vindolanda tablets is an almost impossibly arduous task, involving hundreds of hours of human effort and the use of specialist scanners to yield the shreds of knowledge that each bears. In burning our lives and our creations on to disks and chips scattered around the net we're setting our descendants an even tougher challenge.

Knowledge is one thing but its persistence is quite another. We instinctively know that knowledge unrecorded, unremembered and untransmitted is an abomination: the ultimate waste of the human faculties. So we build stores for this knowledge and concern ourselves with its preservation. The library is one of the most potent symbols of this belief in the value of knowledge. We treasure libraries and mourn their loss - Ptolemy's great library of Alexandria was torn down by Roman know-nothings 1700 years ago but the thought of that destruction still causes a twinge of loss. How could they do that? We know that trashing libraries is the final sign of a culture's dissolution. When Nazi dimwits toasted marshmallows around piles of burning books onlookers knew this lot wouldn't amount to much - that their ignorance would find expression only in horror and turmoil.

Which brings me to computers. A fundamental component of every computer since the very first war-time behemoths has been something we blithely call "memory", which sounds sort of promising, doesn't it? A basic function of all computers is to make and store endless perfect copies of whatever you shove into them. So, presumably, computers represent the saviour of human culture: an infallible, permanent store for everything we make and say. The ultimate library. Brilliant.

But the dark and quite unhappy truth is that the computer era, which we think of as a kind of golden age of creativity and communication, is more likely to turn out to be a dark age from which hardly anything survives. The evidence is coming in. Looking back over the first 50 or so years of fairly widespread use of computers we see a series of discrete eras, layered one upon the other a bit like geological strata. And layer by layer, it's pretty clear that hardly any of the information created and squirrelled away over the decades is still available.

During the '50s and '60s, when computers were really catching on with corporations and appearing in movie thrillers as the ultimate high-tech backdrop (mini-skirts, whirring mag tapes, men in white coats), data was stored on huge magnetic drums and on hard drives the size of wheelie-bins. In the next period, in the PC era, people stored programs and information on cassette tapes and on floppy disks the size of LPs. Machinery to read these disks exists - you'll see it at car boot sales and on eBay. You could get at the information they contain but, seriously, who's bothering? That's another swathe of data lost to the information archaeologists of the future.

In the network era, if anything, things are worse: the manic profusion of gadgets, data formats and carriers - your phone, for instance or the computer that controls your car, or the servers in a datacentre in Basildon where you keep your diary - means that we are daily consigning terabytes of useful data to effective oblivion. That USB stick in your back pocket contains a viable trace, a thin slice of life as lived in the early 21st century. When the machine that knows how to read it is obsolete it's gone for good.

Relying on the web to keep everything of value doesn't seem particularly wise either. The billions of photographs attached to profiles at Facebook, for instance. Who will guard them for posterity? What will happen if Yahoo doesn't weather its current troubles and goes bust? Will the mine of photos and emails and videos stored on its servers survive or will a court-appointed goon just press delete? And what about your music? The thousands of tracks you've downloaded and ripped from CDs. Can this priceless record of the period's tastes and desires survive a dozen or more technology replacement cycles? In five or ten years there'll still be thousands of copies of the deathless Chico Time on accessible media around the world - but in 50 years or 500?

We're moving essentially all of human culture into digital storehouses whose fragility we're only slowly becoming aware of. We're laying the foundations for decades of blank pages in the middle of human history that people will call the second dark age. People will ask, "What were they up to for all those years between about 1950 and 2050? Were they just sitting around scratching their arses?"

Luckily for human culture, though, in a former army base called The Presidio right on top of the San Andreas fault in San Francisco, an internet millionaire called Brewster Kahle has set about archiving all of human knowledge. This sounds like the ultimate geek hubris until you realise he's made a pretty good start and his non-profit project - The Internet Archive, which is barely a decade old - has a good chunk of everything on disk already, although he still has a way to go. Kahle's world library is a conscious counter to the impermanence and fragility of our digital memories - and his library's perilous location shouldn't worry us too much either. Should the Pacific plate decide to slip below the North American and take the archive with it - as one day it surely will - its duplicates in the rebuilt library of Alexandria in Egypt or in Amsterdam will quietly take over.