(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: It was warmer during the Holocene Climatic Optimum than it is today — without any human influence.
Answer: Though some temperatures during that period were in the same range as today, they were confined to the northern hemisphere and the summer months.
What’s more, the cause is understood (orbital forcing similar to what controlled the Ice Ages), just as today’s cause is understood (CO2 emissions), and these causes are very different. NOAA has a page on this that contains the following quote:
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere. More over, we clearly know the cause of this natural warming, and know without doubt that this proven “astronomical” climate forcing mechanism cannot be responsible for the warming over the last 100 years.
As an aside, it’s worth noting that even if the Holocene had been as warm as or warmer than today, it would do nothing to undermine the theories and data that indicate today’s warming is rapid and anthropogenic.