The chance that global warming observed since the beginning of the Industrial Revolution is down to just natural fluctuations in the climate on Earth just doesn't compute, scientists say.
Researchers at McGill University say their analysis of world temperatures since the 1500s all but rules out the possibility.
With researchers trying to determine if global warming during the industrialized era is largely down to man-made emissions created by the burning of fossil fuels, the researchers analyzed the historical temperature data to see if the past century's warming trend could just be the result of long-term natural deviations in the world's temperatures.
The result, McGill physics Professor Shaun Lovejoy says, is that a natural-warming argument can be ruled out "with confidence levels great than 99 percent, and most likely greater than 99.9 percent."
The temperature survey results mirror some complex computer models that have been used to examine the climate effect of various greenhouse-gas emissions from human activity, he says.
"This study will be a blow to any remaining climate-change deniers," Lovejoy says of those people who still hold global warming tracked since 1880 is owing to natural climate variability. "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."
To compare natural variability before the human impacts of the industrial era to afterward, the researchers used "multi-proxy climate reconstructions" to estimate historical temperatures, including natural climate recorders such as ice cores, lake sediments and tree rings.
That allows for an understanding of temperature variation at a wide range of time scales, the researchers say.
Lovejoy says his study predicts a doubling of carbon-dioxide levels in the atmosphere from human activity would bring an increase in global warming of between 2.5 and 4.2 degrees Celsius.
"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.
Although not a definitive proof one way or the other, Lovejoy says his study should be taken as a strong indication of the likely causes of global warming in the past century.
"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 says.
