Historical Analysis of Climate Change Supports Man-Made Warming with 99.9% Certainty

New historical climate analysis by a McGill University researcher indicates that climate change in the industrial era is man-made with "greater than 99.9 percent" certainty.

Writing in the journal Climate Dynamics, McGill physicist Shaun Lovejoy reports his analysis of temperature data since the year 1500, concluding that global warming over the last century is not a result of natural long-term variations in temperature, but instead an anthropogenic effect.

"This study will be a blow to any remaining climate-change deniers," Lovejoy said in a statement. "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's approach did not rely on climate-simulating computer models, but a statistical analysis based on historical temperature records. The research employed a technique called "multi-proxy climate reconstructions" which are used to estimate historical temperatures. These climate reconstructions take into account data derived from tree rings, ice cores and lake sediments.

The likelihood that global warming since 1880 is due to natural variability can be ruled out "with confidence levels greater than 99 percent, and most likely greater than 99.9 percent," Lovejoy said.

To analyze industrial-era climate change, Lovejoy used carbon dioxide from the burning of fossil fuels as a proxy for all man-made climate influences. This simplification is justified, Lovejoy said, because of the "tight relationship 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," Lovejoy added.

Lovejoy's research falls in line with the recently released IPCC report on climate change. A doubling of carbon dioxide levels in the atmosphere would cause warming between 2.5 and 4.2 degrees Celsius. The IPCC report predicted 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 said. "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," he said.

An open-access copy of Lovejoy's study is availble here.