Miocene
How would new mountain ranges lower global temperature? This has several possible causes. A falling CO2 would cause the temperature to drop, so the real question is, what would cause the CO2 to drop? One type of cause is CO2 being sequestered (hidden away) by, for instance, organic remains not being recycled. A quite different type of cause points to changes in the Earth's orbit or changes in the heat given out by the Sun (called "insolation").