The Intergovernmental Panel on Climate Change (IPCC) claim of human-caused global warming (AGW) is built on the assumption that an increase in atmospheric CO2 causes an increase in global temperature. The IPCC claim is what science calls a theory, a hypothesis, or in simple English, a speculation. Every theory is based on a set of assumptions. The standard scientific method is to challenge the theory by trying to disprove it. Karl Popper wrote about this approach in a 1963 article, Science as Falsification. Douglas Yates said,
"No scientific theory achieves public acceptance until it has been thoroughly discredited."
Thomas Huxley made a similar observation.
"The improver of natural knowledge absolutely refuses to acknowledge authority, as such. For him, skepticism is the highest of duties; blind faith the one unpardonable sin."
In other words, all scientists must be skeptics, which makes a mockery out of the charge that those who questioned AGW, were global warming skeptics. Michael Shermer provides a likely explanation for the effectiveness of the charge.