TROPICAL RAINFORESTS: Rainforest Diversity
ICE AGESRoughly 2.5 million years ago we entered the Pleistocene Epoch, better known as the "Ice Ages." Essentially the planet cooled as glaciers expanded to cover much of North America and Europe, while climates worldwide were dramatically altered.
What causes Ice Ages? No one actually knows for sure. Some argue that the sun's energy output is diminished, while others have suggested that the somewhat cyclical pattern (roughly one event every 100,000 years or 25 events in the past 2.5 million years) points to the changing distance between the Earth and the sun as the culprit. As the Earth travels around the sun in an elliptical orbit, the tilt of our planet's axis "wobbles" so the severity of summers and winters varies over time (the Milankovitch effect).
The movement of continents has also surely impacted global temperatures. For example, before the union of South America and North America (roughly 3 million years ago), the waters of the Pacific and the Atlantic intermixed allowing warm tropical waters to move poleward and cold polar waters to head toward the equator and keeping global temperature relatively balmy. The situation changed with the formation of the Panamanian Isthmus and retreating ice margins on the North American continent. Waters from the two oceans could no longer mix and the Arctic was deprived of warm water ocean currents by the formation of the strong circular Gulf Stream of the Atlantic. Other continental movement contributed to the effect and the Arctic Ocean became covered by a reflective ice pack that further cooled Earth.
Continued: Rainforest diversity