Why does the climate change over time?

Throughout the history of the Earth, the climate has always been changing. This is clear from evidence of cycles of glacial advance and retreat. However the last ice age ended abruptly and this can be associated with the emergence of human life. Global warming is happening at a significant rate due to human impacts. Human activities such as burning fossil fuels have lead to there being much more carbon dioxide in the atmosphere. Carbon dioxide is a 'greenhouse gas', that has an insulating effect on the planet. Methane is another greenhouse gas, which is being produced at an alarming rate by the animal farming industry as there is more demand for food. Deforestation is the large scale clearing of trees to allow for another industry to take place, this has also led to global warming as trees normally absorb carbon dioxide (CO₂), however because there are less trees, more CO₂ is remaining in the atmosphere.