An analysis of temperature data since 1500 A.D. all but rules out the possibility that global warming in the industrial era is a natural fluctuation of climate, according to a paper in Climate Dynamics.
The study examined historical data to assess the hypothesis that warming over the past century is due to natural long-term variations in temperature. These "multi-proxy climate reconstructions" have been developed to estimate historical temperatures, as well as fluctuation-analysis techniques from nonlinear geophysics.
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.
"This study will be a blow to any remaining climate-change deniers," says McGill University physics professor Shaun Lovejoy, firmly establishing where he sits on the political scale. "Their two most convincing arguments – that the warming is natural in origin, and that the computer models are wrong – are either directly contradicted by this analysis, or simply do not apply to it."
Lovejoy applied statistical methodology to determine the probability that global warming since 1880 is due to natural variability. His conclusion: the natural-warming hypothesis may be ruled out "with confidence levels great than 99%, and most likely greater than 99.9%."Statistics experts every are going to be annoyed by this assertion, especially because it uses proxies and then declares statistical significance.
I'm a statistician. My motto is 'I haven't read your paper yet but I'm virtually certain your methods are flawed&your results are wrong.'— Stephen John Senn (@stephensenn) April 9, 2014
For the industrial era, Lovejoy's analysis used carbon-dioxide from the burning of fossil fuels as a proxy for all man-made climate influences – a simplification but he justified it due to the link between global economic activity and the emission of greenhouse gases and particulate pollution. "This allows the new approach to implicitly include the cooling effects of particulate pollution that are still poorly quantified in computer models."
His finding made no use of climate models commonly used to estimate the magnitude of future climate change, but judging by his contempt for the unwashed masses unconvinced by a UN panel determined by gender and geographical quotas, it is no surprise his findings mirror those of the International Panel on Climate Change (IPCC). He 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 aggressive than the IPCC's prediction that temperatures would rise by 1.5 to 4.5 degrees Celsius if CO2 concentrations double.
"We've had a fluctuation in average temperature that's just huge since 1880 – on the order of about 0.9 degrees Celsius," Lovejoy says. "This study shows that the odds of that being caused by natural fluctuations are less than one in a hundred and are likely to be less than one in a thousand. "While the statistical rejection of a hypothesis can't generally be used to conclude the truth of any specific alternative, in many cases – including this one – the rejection of one greatly enhances the credibility of the other."