I took a break from blogging for the last few weeks to take a vacation with the family in Europe. We fell in love with Venice, a city full of charming alleyways and canals, with no wheeled transport of any kind. Part of the charm is the dilapidated, medieval feel to the place – the buildings are subsiding, their facias are crumbing, and most of the city’s infrastructure doesn’t work very well. In fact, given what a sorry state the whole city is in, I’m surprised how much I fell in love with the place.

But one thing we didn’t expect was that Venice flooded while we were there. It turns out that several times a year, particularly during the high spring and autumn tides, the meteorological conditions are such that more water than usual is driven into the lagoon, and the high tide washes over the canal sides, across the sidewalks, and into the houses and shops:

The locals all take this in their stride, don their long boots, and get to work pumping it all out the buildings again. The tourists stand there looking bewildered. But the kids loved it:

High tide in Venice, Oct 5th, 2010

In Venice, it’s just another Acqua Alta. I’d heard about Venice sinking, given that the buildings sit on wooden rafts, which in turn are supported by wooden pillars driven deep into the soft mud on the lagoon bottom. And of course, I know that sea level rise due to global warming threatens many of the world’s coastal cities. But I didn’t realise just how low Venice really is, and the flooding we saw got me thinking again about whether the future is already here.

The last IPCC report forecasts a rise of up to 59cm in sea level rise by the end of this century, due to thermal expansion and melting glaciers. And as we know, the IPCC numbers exclude the contribution of the Greenland and Antarctic ice sheets, which together could be considerably more, and it also fudges the point that sea level rise won’t magically stop in 2100. Which means that Venice, a city that’s around 1500 years old, is very unlikely to survive into the twenty-second century.

But like all attempts to pin down the impacts of climate change, it gets complicated. It turns out that Acqua Alta isn’t a recent thing – it has occurred throughout Venice’s history. Technically, Aqua Alta occurs when the high tide is more than 90cm above the average sea level (actually, the average as was measured in the year 1897, according to wikipedia). In the foreign media, floods in Venice are typically portrayed in breathless terms as a disaster (see HuffPost for some dramatic photos from last winter). The locals don’t see it that way at all, and get furious at these media reports as they damage the tourist trade on which Venice depends almost entirely.

One problem is that the media reports confuse the measures. As I said, the floods are measured in terms of height above a 1897 sea level average. These days, even low tides are often above this baseline too. Here’s the forecast for the next 48 hours:

Venice tides for Oct 29-31, 2010, from http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/1748

A you can see, the sea level is expected to vary from low tides around 0cm, and high tides in the range 50-75cm. Which is classified as normal high tides for Venice. A high tide of  up to 90cm causes almost no flooding, while one of +150cm floods about 2/3 of the city – this happens once every few years. The confusion in the media is that +150cm is about 5 feet; so the papers duly report Venice as being under 5 feet of water. But really the water is rarely more than ankle deep, as the flooding is only the difference between the canal sides and the high water. On the day we took these photos (5th Oct 2010), the high tide reached about 107cm, enough to flood about 14% of the city, but as you can see, the actual flood is only a few centimeters deep.

But a sea level rise of +50cm due to climate change shifts things so that every high tide will flood a significant proportion of the city. Flooding twice a day throughout the year is a very different proposition from a little light flooding a few times in the spring and fall.

Can Venice be saved? MOSE, a large and controversial flood barrier project, has been under construction for the last few years, and is anticipated to be ready by 2012. It aims to protect Venice with automatic flood barriers around the entrance to the lagoon. The project has been severely criticized both for high cost, for it’s impact on the lagoon ecosystems, and because it doesn’t provide an incremental solution – if sea levels continue to rise, they will overwhelm the barriers, and there’s no obvious way to extend them. The design for the barriers is based on the IPCC projections of up to 60cm sea level rise (although I haven’t been able to find any detailed specifications of exactly what height of tide they will work for). The problem is, if the IPCC reports underestimate sea level rise (and increasingly it looks like they do), then a vast multi-billion dollar project will only buy Venice a few more decades. The techno-optimism of the engineers who designed MOSE seems to be symptomatic a broader mindset when it comes to climate change, which says we can just invent our way out of the problem. It would be nice if it’s correct, but based on the science, I wouldn’t bet on it.