Venice: Using two sets of satellite data, a team of scientists calculated how quickly Venice is sinking and how much of it can be blamed on nature, and how much on humans. The scientists used radar imagery data collected by four different satellites and a relatively new analysis technique known as Persistent Scatterer Interferometry to make a much higher resolution calculation of the sinking than was previously possible.
Two of the satellites are a bit older and produce somewhat lower resolution data, but they have been in orbit longer, so the data spans a longer period of time from 1992 to 2010. The other two satellites have higher-resolution data that only dates back to 2008. The scientists used the longer data set to find the average rate of sinking, which can be attributed to the natural sinking of Venice relative to sea level. This is caused by a combination of factors including deformation of the tectonic plate beneath Venice and compaction of the sediments under the city.
They then looked at the higher-resolution short-term data set and subtracted the average natural sinking. They argue that what was left, as shown in Figure 1, can be attributed to human activity such as reconstruction of historical buildings, restoration of canal edges and erosion along the canal banks by waves from the high volume of ship and boat traffic in the area.
They conclude that the average background sinking is around 1 millimeter per year. The more acute man-made sinking ranged up to 10 millimeters a year, but in some places human activity actually reduces the natural sinking.