Earth's climate is a complicated system, one which climatologists are still studying. Like in a car or computer, a minor change in one component can cause more major effects throughout. Although volcanoes can disrupt this delicate machinery, there are other factors which can plunge Earth into an ice age, some of which built in, others brought about by humans.
History
150 000 years ago, Earth orbited a little further from the Sun than normal. It went through a particularly severe ice age, where the human population dropped to as low as 2000 people.
