Time Series

Disappearing lake?

Lake Chad was once the world’s sixth largest freshwater lake; today it is less than 10% of its former size.

Lake Chad in Africa in 1972 and then later in October 2001, showing the changes in water area and land cover conditions that have occurred between these two dates.
Source: UNEP

Changing sea level

The left hand picture of Venice was painted in 1732 by the artist Canaletto (1697 − 1768) who used a camera obscura (a pinhole camera), so that he has been able to maintain the correct geometric relationships in his paintings, much like the picture taken by a normal camera, as shown on the right.

View of the entrance to the Arsenal in Venice, painted in 1732 by Canaletto. Source: Wikimedia Commons

View of the entrance to the Arsenal in Venice, photograph taken in August 2006. The brown−green front left by an alga living between the high and low tide, indicates the average level of the high tide. Source: Wikimedia Commons

The two pictures cannot tell us if there was a gradual change in the level of water, if there was a sudden change, or even if the level of water has increased and decreased several times between 1732 and 2006.

Pairs of images like these highlight the changes that have taken place, but usually do not help in understanding what is causing those changes.

However, a sequence of many images recorded on a regular basis over time can often help in identifying the driving causes for the change.