Past climates: Ice ages

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Ice ages can be caused by a chain reaction of positive feedbacks triggered by periodic changes in the Earth’s orbit around the Sun. These feedbacks, involving the spread of ice and the release of greenhouse gases work in reverse to warm the Earth up again when the orbital cycle shifts back. The last ice age ended about 12,000 years ago and in the absence of other factors the next cooling cycle would be expected to start about 30,000 years or more into the future.