Cultural Impacts: Venice in Peril, a Case Study
Cultural Impacts: Venice in Peril, a Case Study
The sea continuously laps at the foundations of Venice, Italy. The initial trade-off for peril by the sea for the city founded in the fifth century, was found in great profits reaped from its strategically important port. By the Renaissance, Venice was one of the world's richest cities, a vital link in trade between the East and West. In the modern era, Venetian wealth is preserved and conveyed through beauty in art and architecture.
The sea is relentless and has time and tide as its allies. Numbers tell a tale of a battle slowly lost. With some degree of flooding along the lagoon now evident as many as 200 days per year, Venice endures almost thirty times the number of flood days experienced just a century ago. San Marco Square regularly floods and when flooding is anticipated, elevated wooden walkways are hurriedly set up in the square. The sight of local residents carrying galoshes on the vaporettos (waterbuses) is now as common as Londoners carrying umbrellas.
Historical Background and Scientific Foundations
Fluctuations of absolute global sea level result from climate change—including the normal cyclical growth and decay of Earth's polar ice caps (eustasy). Such cycles do not alone determine sea level relative to a specific coastal segment. Rates of sediment supply and transport along with patterns of deposition, erosion, crustal subsidence, and uplift also influence elevation at a particular location.
On a much larger geological scale, the Venetian region of Italy is very slowly dipping downward, at a rate that ensures that Venice will continue to sink at least a few centimeters a century.
The Consorzio Venezia Nuova, the public authority responsible for coordinating efforts to protect Venice from flooding, has inherited the task from a long line of agencies that date back as far as the fourteenth century Venetian Magistry of the Waters. For now the Consorzio uses stop-gap measures such as raising sidewalks and extending temporary flood walkways to protect the city, These measures, however, are merely life-support designed to keep Venice going while longer-term remedies are found and implemented.
Some prior stop-gap measures may have done long-term harm. For example, excess pumping of ground-water resulted in soil compaction and sinking. The practice was stopped in the 1980s and the city stabilized with regard to the rate of subsoil subsidence. Many hydro-geologists assert that the acceleration in the sink rate of the city during the twentieth century is a result of the combination of the over-extraction of groundwater by industry combined with ill-tested digging projects in the canal and lagoon. In addition to being a treasured cultural attraction, Venice still remains an industrially important city. Oil tankers course through deep-water channels, many cut in the 1950s, to supply nearby refineries. The effect of such digging also made Venice more vulnerable to flooding.
Impacts and Issues
Forecasts call for Venice to continue to sink.
To slow the sinking, construction is underway on an extended mobile flood barrier (MOSE) that will feature closable gates to protect Venice from sea surges during extreme high tides or other flooding conditions. MOSE is projected to be ready for operation by 2011, and its initial costs are projected to approach $4 billion dollars. Given the complexity of the engineering tasks and fragility of the local ecosystem that will require many downstream project modifications, many political officials are bracing for the potential that the ultimate costs of the project will soar well beyond initial projections.
If sea levels continue to rise, however, not even MOSE is projected to be able to hold back the seas for more than a century. Proponents of other countermeasures contend that this is all the more reason to reexamine other measures to counter subsidence (sinking).
For example, in a recent issue of Nature a group of geomechanical engineers from the University of Padua proposed that a restoration of the underground fluids via modern injection technology could actually raise the level of Venice. Optimistic projections indicate that Venice could be raised by 8 to 12 in (20 to 30 cm) by a large-scale injection project, a level that would counter the settling and subsidence of more than 8 in (20 cm) since 1950.
The group's plan called upon technology already used in the oil and gas industry to counter Venice's subsidence. Fluid pumping was initially considered in the 1970s, but the proposal was set aside because the technology to accomplish the task at that time was not as economically feasible—and because some engineers feared that uneven rising could do more harm than good.
The newest proposals for underground injections call for injections of seawater (the cheapest solution) and/or carbon dioxide (a byproduct of local power generation) into sandy layers located approximately 2,300 ft (700 m) below the lagoon. This would be one of the first potential practical engineering applications for carbon sequestration.
WORDS TO KNOW
CARBON SEQUESTRATION: Storage or fixation of carbon in such a way that it is isolated from the atmosphere and cannot contribute to climate change. Sequestration may occur naturally (e.g., forest growth) or artificially (e.g., injection of CO2 into underground reservoirs).
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.
SEA LEVEL: The datum against which land elevation and sea depth are measured. Mean sea level is the average of high and low tides.
Layers of sand at a depth of 2,300 ft (700 m) are geologically squeezed between impermeable layers of clay and rock. The depth is significant because attempts to raise Venice from this level should be more uniform than the projections associated with earlier generations' injection proposals that called for injections into layers only about 160 ft (50 m) below the floor of the lagoon. Engineers feared that injections at the more superficial depth might result in uneven leveling that might prove disastrous to building integrity. Costs of such an injection project are now comparable to the proposed budgets of other proposed countermeasures that are technologically more ambitious, but also more risky in terms of outcome.
The idea to pump fluids underground is not intended to replace the construction of an extended mobile flood barrier (MOSE), but given that MOSE may offer protection for only another 100 to 150 years, any elevation increase resulting from other anti-subsidence countermeasures such as injection would increase MOSE's effectiveness and extend its projected useful life and help partially, but not fully, offset the anticipated accelerated sea level rise due to global warming.
IN CONTEXT: SEA LEVEL
Because tides raise and lower the actual sea level daily, and by different amounts in different parts of the world, scientists refer not to the actual level of water at any given time, but to the sea level datum plane, a reference height used in measuring land elevation and water depths. It refers to the vertical distance from the surface of the ocean to some fixed point on land, or a reference point defined by people. Sea level became a standardized measure in 1929. Mean sea level is the average of the changes in the level of the ocean over time, and it is to this measure that we refer when we use the term sea level.
Constant motion of water in the oceans causes sea levels to vary. Sea level in Maine is about 10 in (25 cm) higher than it is in Florida. The Pacific coasts' sea level is approximately 20 in (50 cm) higher than the Atlantic.
Mean sea level can also be influenced by air pressure. If the air pressure is high in one area of the ocean and low in another, water will flow to the low pressure area. Higher pressure exerts more force against the water, causing the surface level to be lower than it is under low pressure. That is why a storm surge (sea level rise) occurs when a hurricane reaches land. Air pressure is unusually low in the eye of a hurricane, and so water is forced toward the eye, creating coastal flooding.
Increases in temperature can cause sea level to rise. Warmer air will increase the water temperature, which causes water molecules to expand and increase the volume of the water. The increase in volume causes the water level to become higher.
Mean sea level has risen about 4 in (10 cm) during the last hundred years. Several studies indicate that this is due to an average increase of 1.8°F (1°C) in worldwide surface temperatures. Most climate scientists believe that rising sea levels will create environmental, social, and economic problems, including the submerging of coastal lands, higher water tables, salt water invasion of freshwater supplies, and increased rates of coastal erosion.
Sea level can be raised or lowered by tectonic processes, which are movements of Earth's crustal plates. Major changes in sea level can occur over geologic time due to land movements, ice loading from glaciers, or an increase and decrease in the volume of water trapped in ice caps.
About 30,000 years ago, sea level was nearly the same as it is today. During the Ice Age 15,000 years ago, it dropped and has been rising ever since.
The degree of peril to Venice caused by global warming can be generalized, but precise projections are difficult, especially given the controversy of the expected rise on sea levels associated with climate change. If by 2100, sea levels rise between 7 to 23 in (18 to 59 cm) as predicted by the Intergovernmental Panel on Climate Change (IPCC), projections of the protections imposed by MOSE will prove insufficient. If more dire scenarios are realized, such as might be caused by increased contributions to sea-level rise from accelerated glacial movement in the Greenland and West Antarctic ice sheets, the peril for Venice, and the economic cost of protection, must likewise escalate.
“Case Studies on Climate Change and World Heritage.” United Nations Educational, Scientific, and Cultural Organization. World Heritage Centre, 2007. < http://whc.unesco.org/documents/publi_climatechange.pdf> (accessed November 16, 2007).
“Save Venice” Media Center for Art History, Columbia University. <http://www.savevenice.org/site/pp.asp?c=9eIHKWMHF&b=67611> (accessed December 7, 2007).
“Venice in Peril.” British Committee for the Preservation of Venice, 2003. < http://www.veniceinperil.org/news_articles/newsarticles.htm> (accessed December 7, 2007).
“Venice's 1,500-year Battle with the Waves.” BBC News, July 17, 2003. < http://news.bbc.co.uk/1/hi/world/europe/3069305.stm> (accessed December 07, 2007).
K. Lee Lerner