How does climate change naturally over time?
The earth goes through alternating periods of global warming and cooling.
What causes these alternations of climate is not completely known.
The solar flares and storms affect the climate. Volcanic eruptions can cause major cooling periods. Increased glaciers reflect more light causing a spiral effect of global cooling.
There is substantial evidence of a global ice age where glaciers covered large areas of the earth, and people retreated to living in caves.
Around 1000- 1200 AD there was a warming trend. Vikings grew grapes in Greenland which was green at the time. The Vikings sailed across the arctic sea in the mythical Northwest passage, between North America and Russia. (Rus was a name for the north people and Russia is named after these Vikings.)
Even further back the age of the Dinosaurs was much warmer throughout the globe. Vast coal deposits are evidence of lush vegatation. Greater humidity and cloud cover are thought to be responsible for the jungle like climate of this era.
Climate naturally goes through alternating periods of warming and cooling. Lush tropical climates are followed by ice ages. What causes these fluctuations is not fully understood.