"Earth's Climate can be rather stable if nothing is causing it to change, but when the climate is 'pushed' or forced to change, it often jumps suddenly to very different conditions, rather than changing gradually. You might think of the climate as a drunk: When left alone, it sits; when forced to move, it staggers...
The 'pushes' that have caused climate changes in the past probably have included drifting continents, wiggles in the Earth's orbit, surges of great ice sheets, sudden reversals in ocean circulation, and others.
Small 'pushes have caused large changes because many processes in the Earth system amplify the pushes. Greenhouse gases (such as carbon dioxide) have probably been the most important amplifiers. (The bottom line is that) Humans can foul our own nest -- and we can clean it up."
Image source: clker.com
"Think of Earth's climate as a drunk: When left alone, it sits; when forced to move, it staggers." —Dr Richard Alley.