evidence: These ice ages are triggered and ended by slow changes in the Earth’s orbit. But changing atmospheric concentrations of CO2 also plays a key role in driving both cooling during the onset of ice ages and warming at their end. [...] The effect of the orbital features on the total sunlight reaching the Earth is almost zero; the sunshine is just moved between areas and seasons. But, the whole world cools into an ice age, and the whole world warms coming out of an ice age, despite half the world getting less sun when the other half gets more. And, so far, all the explanations of this require the effects of the CO2, which beautifully explain it. [...] In some ways, however, this is exactly what we would expect; there were no humans burning fossil fuels during the end of the last ice age, so CO2 served more as a feedback to orbital changes rather than the climate forcing that it is today.