25 million years ago, a new type of plant evolved on earth. It started using a photosynthesis process called C4, which is distinct and more efficient than other plants. Examples of this type of plant include sugarcane (pictured above), maize, sorghum and switchgrass. These plants grow very quickly and are more efficient at using CO2 while being resistant to droughts and high temperatures. They are concentrated in the tropics (within latitudes of 45°). Many herbivores can not easily digest this new type of plant and it gradually spread throughout the earth. By about 5 million years ago, C4 plants became ecologically significant and started reducing CO2 levels.
Other things were happening at the same time. Ithmus of Panama formed and the Himilayian Mountians grew higher. Hard to say which was dominate, but the Earth gradually cooled and this ushered in a new epoch on earth with periodic ice ages. The ice ages have not been permanent as periodic variations of earths orbit around the sun have allowed the ice to melt and warm periods to return for brief (10K year) periods. At first, the ice ages occurred every 41K years. But as CO2 levels continued to fall, the ice ages lasted longer and the warm periods occurred less often.
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