Venice - Rialto Bridge on Grand Canal
The media and other sources have largely focused, among possible causes of the gradual subsidence, or sinking, of Venice, on "global warming" and consequent sea level rise because it's the most orthodox, fashionable and, frankly, simplistic explanation. In reality the causes are considerably more complex than that, and require an understading of the present geography, geological history, and human history of this unique, both enchanting and very difficult habitat: the Venetian Lagoon.
VENETIAN LAGOON GEOGRAPHY AND FORMATION
The Venetian Lagoon is a large, shallow body of water in the Veneto region of Italy at the norhern end of the the Adriatic Sea, from which it is separated by a narrow strip of sand, the barrier beach called "Lido", and other embankments. The Italian word "laguna", from which the English "lagoon" is derived, has given the world the name for an enclosed, shallow inlet of salt water.
The Venice Lagoon is the largest wetland in the Mediterranean. It extends over an area of 550 square kilometres, 8% of which is occupied by land, including Venice and many smaller islands.
Lagoons, formed by rivers and sea, are fragile, unstable interactions of salt water and fresh water, high and low tide, settling of matter and inflow of liquid.
The rivers Piave, Brenta, Sile, Bacchiglione, Dese, Musone, Marzenego for centuries have been carrying enormous amounts of sand and rubble from the mountains and depositing them into the sea, eventually giving rise to the Lido and other sandbars parallel to the coast that now separate the Venetian Lagoon from the Adriatic, with the 3 inlets of Lido-San Nicoḷ, Malamocco and Chioggia, called "bocche di porto", connecting it to the open sea.
A lagoon isn't the most practical, suitable place to found a city. Venice has been variably described as sitting on a "pudding penetrated by watery channels" by architect and Venice expert Wolfdietrich Elbert and on a "giant sponge filled with water".
The Venetian Lagoon was formed about 6,000 years ago, in the mid Holocene. At the acme of the Last Glacial Maximum the Veneto region, where Venice is, was very far from the Adriatic shoreline. The area was then made up of a vast river floodplain, part of what is today the Pianura Padana (River Po Valley), thus making the latter much larger. When, at the end of that glacial period, the climate became warmer causing the melting of the continental glaciers, globally the sea level started to rise fast. The north Adriatic coastal plain got flooded and became the Lagoon we know today.
That was a real global warming. Throughout its history, the earth has repeatedly experienced major cycles of glacial periods followed by interglacial periods, which are intervals of global higher average temperature lasting thousands of years and separating consecutive glacial periods. We are now in the Holocene interglacial, following the Pleistocene, the last glacial period, that ended circa 11,700 years ago and began about 2,588,000 to 1,800,000 years ago.
Correspondingly, during glacial periods ice-sheets grow and during interglacial periods they retreat: this has happened many times in the past. Our understanding of the earth's climate is still limited, but is growing and it offers us a long geological background against which to compare the present global climatic conditions. The claim that the 1990s were the warmest decade of the past millennium is highly problematic in the context of these historical temperatures. This claim was supported by the infamous "Hockey Stick" graph created in 1998-1999 by Michael Mann, that was exposed and discredited for using critically flawed statistical methods. The famous graph which became a symbol of the threat posed by global warming exaggerated the rise in temperature because, according to the head of the Royal Statistical Society, it was produced by employing "inappropriate" methods.
Marcott et al, in an essay published in 2013 in the leading scientific journal Science, said: "Our results indicate that global mean temperature for the decade 2000–2009 has not yet exceeded the warmest temperatures of the early Holocene (5000 to 10,000 years ago)." In plain words, in ancient Egypt's time the world was warmer than now.
Since its beginning, the Venetian Lagoon has continuously been circulating and mixing with the sea due to twice-daily tidal fluxes. This has produced a very dynamic environment of deposition with a constant fluctuation in the distribution of the main sedimentary components that make up the upper 5 metres: salt marshes, mudflats, lagoon sediments, and meandering tidal channels.
Under Venice are several layers. Those just mentioned are the uppermost and thinnest layers, on which Venice was built. Below them is a 1-to-2-kilometre-thick layer of river sediments. These fluvial deposits are slowly compacting, therefore getting denser and decreasing in volume. Due to the twice-daily tidal flows, the channels' movement erodes and re-works the sediments. While the shallower channels re-work the upper lagoon salt marsh and mudflat sediments, the deeper channels erode the pre-lagoonal floodplain that existed before the arrival of the rising Adriatic Sea to the Veneto region, thus increasing the water at the expense of the solid matter. Therefore, what lies in the depths below is particularly pernicious to Venice.