Objection: Climate has always changed. Why are we worried now, and why does it have to be humans’ fault?
Answer: Yes, climate has varied in the past, for many different reasons, some better understood than others. Present-day climate change is well understood, and different. Noting that something happened before without humans does not demonstrate that humans are not causing it today.
For example, we see in ice core records from Antarctica and Greenland that the world cycled in and out of glacial periods over 120Kyr cycles. That climate cycle’s timing is fairly well understood to be caused by changes in the orbit of the earth, though the mechanism behind the response has not been conclusively established. These orbital cycles are regular and predictable and they are definitely not the cause of today’s warming. The other important difference between the glacial-interglacial cycles and today is the rapidity of the current change. The rate of warming is on the order of 10 times faster today than in the ice cores.
Such rapid warming on a global scale is quite rare in the geological record, and while it may not be entirely unprecedented, there is strong evidence that whenever such a change has happened, whatever the cause, it was a catastrophic event for the biosphere.