By using the time primitives in KML, you can create animated thematic maps showing how a variable change over time.
The prism maps above visualise the world population in four different years (1950, 1975, 2000 and 2050). This and this blog posts show how a prism map can be made with KML. Here, I've added country polygons for several years, each extruded and colourised according the the population value.
All polygons for one year are wrapped in a Folder with a TimeSpan element. Whenever time is specified in a KML file, a time slider appears in upper-right corner of Google Earth (I've not seen this functionality implemented in other geobrowsers). By dragging this slider, you'll see the human population explosion visualised:
You can download the KMZ file here and try yourself. The file is big (1 MB) because the complex country polygons have to be repeated for each year in the animation. It's not possible to change altitude values dynamically with KML. It'll be on my list of feature requests...
Population statistics from UNdata.