Over geological time, those landscape proportions have changed in response to shifting tectonic plates. This has changed how efficient silicate weathering has been at removing the CO2 emitted by volcanoes, thereby allowing high CO2 levels and warm climates at times, like in the Cretaceous, or lower CO2 levels and a cool climate when plate tectonics was building lots of “kinetic-limited” mountainous landscapes, like over the last few million years.
Wow, so when there are more mountains, more CO2 is drawn down as there's more terrain to expose the rock for weathering