Can art save Venice?

Nowhere, not even in the heart of the dampest English seaside town, had I witnessed such an impressive selection of wellies in one shop.

They came in a variety of styles - ankle bootees, regular full-length through to waders - and in colours ranging from stylish buck-cream and flashy scarlet to sensible olive green. There were rainproofs and sou'westers to match, plus a pile of lifeguard-issue buoyancy rings. From the prices, though, it was clear that this was no Portsmouth shopfront: at e 94 (£60) for a pretty ordinary-looking pair of waders, this could only be Venice.

It is, of course, no secret that Venice has a serious water problem. St Mark's Square - the city's biggest tourist magnet, once described by Napoleon as "the drawing room of Europe" - is flooded on average one day in three. The city is not only sinking due to subsidence, but the sea level is gradually rising because of global warming. And for at least three decades, policymakers and scientists have wrangled not only over the rate of the city's disappearance under the waves, but also what to do about it.

This year, however, Venice's future looks brighter than it has for a very long time. Not only have the city authorities finally agreed to push ahead with the long-touted plan to develop flood barriers, but scientists at the Climate Change Institute in nearby Padua are shortly to publish research that promises once and for all to end debate over the rate at which Venice is being submerged.

Credit for the idea - as simple as it is brilliant - belongs to Dr Dario Camuffo, the institute's director of research and a noted climate-change expert. Sea levels in Venice, as he explains, have been systematically measured only from 1872; before that, reports were sporadic.

It was while reading one day about the Venetian painter Canaletto that his idea struck him: thanks to the artist's use of a camera obscura, the accuracy of Canaletto's images can be trusted absolutely. Why not therefore compare Canaletto's views from 250 years ago with those of today? Camuffo decided to use the level of algae - as visible now as it is in Canaletto's work - as the line to be measured. "Simple," he smiles.

Although Camuffo refuses to divulge the exact findings of his research - for fear it will jeopardise publication in the science jour nal Nature - he gives me photocopies of a selection of the paintings he used. Before marking on a map the best sites to make my own comparisons, he makes one last plea to the politicians. His research, he says, has real practical potential. "It will help make precise decisions about the barriers - how big they should be, how they should work. Venice cannot wait any longer. Something has to be done now."

It strikes me the next day as I stand on the edge of the Grand Canal clutching my Canaletto copies that there are also strong touristic possibilities to Camuffo's work. How about a "Canalettour"? There's a real spot-the-difference satisfaction here, involving not only close observation but also a measure of deduction. Enough to make Aurelio Zen, Michael Dibdin's Venetian sleuth, mightily proud.

Looking north across the Grand Canal from what - with its furcollared clientele and ranks of polished king prawns, scallops and monkfish - must be the world's smartest fish market, the genius of Camuffo's idea is immediately clear. In the painting Il Canal Grande fra campo San Sofia e il ponte di Rialto - the windows of the closest building are a good 1.5 times above the algae. Now, that space seems at least a metre shorter.

Elsewhere it is clear that past city planners, facing certain destruction of their houses and monuments, made protective alterations. In the painting by Canaletto's nephew Bellotto of San Stae church, there are four entrance steps to the building. Now, raising of the pavement against the rising water has meant that only two steps remain. Had this work not been done, water would be lapping at San Stae all year round.

Paintings of bridges, in particular, reveal vast discrepancies: Canaletto's view of the Rialto Bridge, painted from the north, shows an arch at least a metre higher out of the water than today; the curve back then was more rounded, complete. The contrasts - as befits the dramatic nature of the city itself - are seldom less than astonishing.

Yet it can be a melancholy exercise, too, this calibration of Venice's stately drowning - in many of the houses along these damp-walled alleys, where families once lived, there are now only abandoned cellars. The population today is around 70,000, almost half what it was in the 1960s. And just occasionally, holding a Canaletto up against the scene as it is today, there are tangible human reminders. In L'Ingresso a Canareggio, con la chiesa di San Geremia, a figure stands in the doorway of the house on the far right of the picture. Looking across the water now, the foot of the same door is under water. Behind this door would have been elegant merchant's reception rooms; now the same space is home only to fish.

Way to go

Edward Marriott travelled to Venice with Citalia (020 8686 5533). Three nights at the three-star Hotel do Pozzi from £297pp B&B, including British Airways flights.