ON-LINE DEBATE

23 April 1998

Professor Edmund Penning-Rowsell


Talking to The Millennium Debate

I’ve read about your concern over Venice flooding, but before we get to that, could you tell me briefly how convinced you are that climate change is being caused by human activity?

Oh, that is a very big question. I go by the Inter-governmental Panel on Climate Change, which has reported over several years that there has been a significant shift in climate in the last ten, twenty, twenty-five to fifty years, and correlated that with increases in greenhouse type gas emissions. Now, there is huge controversy about that, because there is natural variation in climate anyway. But there is growing scientific opinion, based on the facts that have been collected, that the range of variation currently being experienced is getting outside the limits of what might normally be expected as a pace of natural change in climate induced by normal variations without human intervention. That is a rather long-winded way of saying yes, I do believe that climate change is now almost definitely a fact, and that it is induced being by human activities creating great concentrations of gases, resulting in global warming.

You were not convinced by the sunspot theory that was widely publicised last week?

Well, again, natural variation in climate can be related to sunspots. I am not a meteorologist or climatologist. I have to rely on the people who I consider to be experts, and all the people I have met and discussed this matter with in the meteorology field, who I think are serious scientists, not simply hypers of media attention, people have convinced me that yes, there is something to do with climate change and it is something to do with human intervention.

Given that we are going to get this climate change, and we seem to be getting it already, some areas are very vulnerable. What are your concerns and remedies for Venice?

It is not just climate change that has created problems for Venice. Over the last eighty or ninety years, since about 1900, Venice has suffered an adverse sea level change with respect to the Adriatic and its lagoon, of 23 centimetres, and that is well established and well corroborated by everyone who has worked on this. Quite a lot of that is to do with the fact that the Venetians have pumped ground water out of the sediments on which Venice is located, and these are very loose unconsolidated deltaic sediments, and as a result of that they suffered subsidence of the whole urban structure.

Add to that the fact that there is some seculous and long-term subsidence on these deltaic sediments anyway, and you have that 23 centimetres sea level rise – or actually the other way round. It is a 23 centimetres drop in the level of the city, in relation to the sea over that period of time. So the Venetians have contributed to their own downfall, as have many coastal cities who did not appreciate the problems of pumping ground water out of the sediments on which they sit. Now, add to that the projected 30 centimetre or so sea level rise between now and the year 2050, and that is a fairly conservative estimate, and I think there has been a convergence of scientific opinion around that sort of estimate – add that to the adverse sea level rise that Venice has already experienced of 23 centimetres, and you have a very serious situation. And that situation must be understood in relation to Venice’s location and geography.

The city was built at sea level on this coastal marsh, starting from the 7th century but certainly developing rapidly after the 14th century, and that site was chosen precisely because it gave access to the sea, and it gave a defensive position in the lagoon away from the enemies on the mainland. So all those factors contribute to create a very hazardous situation for Venice today. And all the projections are that I have seen are that the situation will get worse in the future.

And are there any remedies available for them?

Well, yes, there are lots of remedies. It is a question of what you want to spend your money on, and how long you think the remedies will last. The Consortio, which is a consortium of contractors working in Venice, have come up with a system of mobile gates across the entrance. But that solution was prescribed, unfortunately, and legislation passed in 1973, I think, following disastrous floods in 1966. The mobile gates are designed to allow navigation through them, so that navigation to the port can continue, because this is Italy’s fifth biggest port, and also so that the pollution which is inherent in the lagoon can be flushed out on every tide. So they’re responding to those needs which I believe are relatively short-term, the pollution and the port, short-term in the history of Venice, which one thinks of in centuries rather than a few years.

So that solution has been put forward. I believe they are extremely extravagant, wildly extravagant I might call it, and only designed effectively to last for fifty years. Not because it is not a good engineering structure, which might last much longer than that, but after fifty years, given the kind of sea level rise that we are currently experiencing, the gates will have to be closed every day, not once a month, as currently projected. And that will mean that the pollution within the lagoon will build up, the navigation to the port will be almost impossible, and therefore you have to solve the problem and you have to solve the problem of the port anyway.

So let us begin to solve the problem of the pollution now, and let us think about moving the port away from Venice now, so that you do not just spend a huge amount of money developing a solution that will only last fifty years, but develop a solution that will last for a hundred and fifty years.

That solution must be a multi-faceted one, whereby you simultaneously attack the pollution – the erosion within the lagoon – the flooding that results from the fact that the entrances between the lagoon and the sea were hugely widened and deepened in the 19th century, and then begin to reconvert the lagoon in which Venice sits back to something more closely resembling a coastal marsh rather than an inland, rather stormy sea. It is that stormy sea which Venicians have that creates some of the flood problems, coupled with the fact that the entrances now allow the storm surges which are induced by cyclone-type conditions in the Adriatic to inflict their damage upon the historic core of Venice.

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