The last ice age began to thaw about 15,000 years ago. Five thousand years later the great ice sheets that had covered much of North America and northern Europe had retreated towards the North Pole, and the Earth was about as warm as it is today. But this escape from the deep freeze was rudely interrupted by the Younger Dryas event, which began about 12,900 years ago. Changes in the shape, tilt and wobble of the Earth's orbit around the Sun are thought to have brought about the ice ages. Understanding the causes of such rapid climate change is crucial for calculating the possible ramifications of the human-induced greenhouse effect.
Greenhouse gases come from natural sources too.
The deep waters of the world's oceans circulate, carrying warm water from the tropics to the poles, and cold, dense water back towards the tropics. This is driven by the fact that water is denser when it is colder and saltier. Warm water from the tropical Atlantic, for example, sinks as it flows north and cools, and as it becomes saltier by the formation of salt-free sea ice. If thermohaline circulation were to shut down, the oceans would cease to bring heat from the tropics to the poles, and the North Atlantic region would become much colder. This suggests that the thermohaline circulation had shut down, so that the tropical oceans were retaining their heat.