Venice has been flooded twice so far this week, the worst floods for 22 years. And the problem is growing worse as sea levels rise and the foundations of Venice gradually sink.

Exactly how much the city has sunk over the years is not clear, though, because measurements of the local tides only began in 1871.

However, some intriguing clues were left by the 18th-century painter Canaletto in his work.

Canaletto made a good living painting highly realistic pictures of Venice for wealthy tourists. His almost photographic paintings relied on a portable camera obscura, which projected an image on to a sheet of paper, or canvas, from which he drew the contours with a pen. This gave not only pinpoint accuracy but also allowed him to crank out roughly one picture every three days during the 1730s and 1740s.

The pictures were so detailed that they capture a greenish-brown slime of laminaria algae lining the canals. Because laminaria float on the water’s surface they show the average high tide marks. In an ingenious study, more than a hundred of Canaletto’s pictures were closely examined for the algae to measure the sea rise. The calculations showed the tides rose about 2.7mm per year.

This fits with official tide measurements in the 19th century, which showed a rise of 2.4mm per year. Knowing this history helps to predict future sea-level change.

For Venice, however, the only hope of avoiding a catastrophic flood is the completion of a colossal underwater flood barrier, using massive hinged barriers attached to the seabed, which is due to be completed by 2011.