Ice everywhere : but how did it get there?

The first recorded glaciation in the northern hemisphere occurred in Greenland between 10 and 6 million years ago. From this point, the global climate gradually cooled. About 2.7 million years ago, the ice ages suddenly intensified which resulted in ice sheets growing to cover much of the northernmost areas in both North America and Eurasia. This climate transition is known as the onset of major Northern Hemisphere Glaciation. Scientists believe this cooling was caused by long-term changes in the positions of the Earth’s tectonic plates (such as the closure of the Panama ocean gateway between North and South America), together with decreases in the amount of solar radiation received by the Earth. The latter was caused by natural changes in the tilt of the Earth’s axis, and the distance from the Earth to the sun during the summer months. What remains unknown about this period is where all the water and snow came from to build the huge expanses of ice in North America.