Why are we required to prove that the 150 years of sporadic warming following the end of the Little Ice Age (1850) is not caused by increased atmospheric carbon dioxide from human activities when the eight periods of similar (often greater) warming during the past 20,000 years obviously were not?
(1.) There is no evidence of historic temperature increases or temperature levels similar to what we have observed in the past 40 years that could arise from natural causes.
(2.) The climate models upon which the IPCC reports rely fully incorporate the influences of water vapor, the El Nino southern, the Pacific decadal oscillation, the Atlantic multi-decadal oscillation, the Arctic oscillation, and the causes of long-term (1,500, 5,000 and 20,000 year) climate variation, thus eliminating the potential to mistake a natural cause in climate variation with a man-made cause of variation.
What perverse logic demands that we declare the oscillating warming and cooling of the past 60 years to be proof that mankind is causing warming, and further that this warming is imminently catastrophic? Was the Medieval Warm Period (850 to 1300 AD) catastrophic? Was the Little Ice Age (1300 to 1850 AD) beneficial?
For the historically challenged, the answers to both questions are "No!"