Natural climate change happens on such a long time scale, it can be attributed to changes in the Earth's cycle around the Sun and solar activity. When you look at graphs for the past 100k of years, changes occur very slowly and regularly.
Assuming the Earth was at an equilibrium in terms of temperature and climate, if the Earth's cycle changes slightly and winter is a little bit longer/colder year after year, then the Earth can go into an ice age. If the winter is shorter, summer is longer, glaciers retreat and the Earth warms up. This can also happen if the tilt of the Earth changes, since the tilt is what determines the seasons on Earth.
Changes in solar activity can also cause climate change, if they are sustained over long periods of time. If the sun goes through a period of less activity, then the amount of energy reaching Earth goes down, which results in lower temperatures.
Theory: I think also that warmer temperatures (in the past, before humans) have led to increases in the amount of life on Earth as glaciers retreat and open up land to plants and animals. This leads to increases in CO2 levels, which helps to keep warming the planet up through the greenhouse effect. Colder temperatures leads to the reverse.
(I thought the first answer described human changes really well already) :)