"A study out of McGill University sought to examine historical temperature data going back 500 years in order to determine the likelihood that global warming was caused by natural fluctuations in the earth's climate. The study concluded there was less than a 1% chance the warming could be attributed to simple fluctuations.
'The climate reconstructions take into account a variety of gauges found in nature, such as tree rings, ice cores, and lake sediments. And the fluctuation-analysis techniques make it possible to understand the temperature variations over wide ranges of time scales. For the industrial era, Lovejoy's analysis uses carbon-dioxide from the burning of fossil fuels as a proxy for all man-made climate influences a simplification justified by the tight relationship between global economic activity and the emission of greenhouse gases and particulate pollution, he says.
His study [also] predicts, with 95% confidence, that a doubling of carbon-dioxide levels in the atmosphere would cause the climate to warm by between 2.5 and 4.2 degrees Celsius. That range is more precise than but in line with the IPCC's prediction that temperatures would rise by 1.5 to 4.5 degrees Celsius if CO2 concentrations double.'"