Objection: Current warming is just part of a natural cycle.
Answer: While it is undoubtedly true that there are natural cycles and variations in global climate, those who insist that current warming is purely natural — or even mostly natural — have two challenges.
First, they need to identify the mechanism behind this alleged natural cycle. Absent a forcing of some sort, there will be no change in global energy balance. The balance is changing, so natural or otherwise, we need to find this mysterious cause.
Second, they need to come up with an explanation for why a 35% increase in the second most important greenhouse gas does not affect the global temperature. Theory predicts temperature will rise given an enhanced greenhouse effect, so how or why is it not happening?
The mainstream climate science community has provided a well-developed, internally consistent theory that accounts for the effects we are now observing. It provides explanations and makes predictions. Where is the skeptic community’s model or theory whereby CO2 does not affect the temperature? Where is the evidence of some other natural forcing, like the Milankovich cycles that controlled the ice ages (a fine historical example of a dramatic and regular climate cycle that can be read in the ice core records taken both in Greenland and in the Antarctic)?
Is this graph a candidate for explaining today’s warming? A naive reading of this cycle indicates we should be experiencing a cooling trend now — and indeed we were gradually cooling over the length of the pre-industrial Holocene, around .5C averaged over 8,000 years.
Not only is the direction of the change wrong, but compare the speed of those fluctuations to today’s changes. Leaving aside the descents into glaciation, which were much more gradual, the sudden (geologically speaking) jumps up in temperature every ~100,000 years represent a rate of change roughly ten times slower what we are currently witnessing.
So could current changes be part of a natural cycle? Well, no natural cause has been identified. There is no climatological theory in which CO2 does not drive temperature. And natural cycle precedents do not exhibit the same extreme changes we’re now witnessing.
In short: No.