Consider the Earth to be a giant recording device. All actions that take place - from continental movement, to volcanic eruptions, to a Triceratops being eaten by a T-Rex, to a man leaving footprints in the mud - can become a part of the earth’s ‘historical’ record at that point in time. Not surprisingly this record can also register climate - from signals as small as a hurricane or as big as a global ice age. This climatic information can be found in unconsolidated sediments (for example, mud at the bottom of a pond), in rocks, in glacial ice sheets, or even in something living like a tree or coral. Each of these systems record something about the world that they developed/grew in.