The climate has warmed and cooled naturally over its entire history, so 1) the present-day warming must therefore be natural, and 2) we shouldn't worry about it.Point 1 I debunked here. The science strongly supports the conclusion that humans are contributing significantly to today's warming. To understand why I object to point 2, let's consider the following image:
Following point 2 above, we should tell the pilot of that plane not to worry ... after all, the plane is in a descent, but this plane has descended lots of times before. Why should this descent cause any concern???
The answer should be obvious. This descent is quite different from previous descents, and one that the pilot will not survive. Similarly, the evidence we have is that the present-day warming is far more rapid than most past climate variations. Before people jump on me, I know there is some evidence that some past changes have been rapid (e.g., the cooling during the Younger Dryas), but these were all associated with reorganizations of the circulation of the atmosphere-ocean system that are not now occurring. Thus, it might be that today's warming has no precedent in the entire history of the Earth. Because of the quality and sparseness of data, there is significant uncertainty in this conclusion, but speaking as a citizen, just the possibility that this is true causes me grave concern about the state of the climate system.