A statistical analysis of average global temperatures over the past two or so decades suggested the global warming "pause" is consistent with natural variations in temperature.
In a paper published in this month's Geophysical Research Letters, McGill University physics Professor Shaun Lovejoy concluded "natural cooling fluctuations" between 1998 and 2013 masked the warming effects caused by man-made emissions.
The researcher made his findings by employing statistical methodology that he had developed in an earlier study published in April in the journal Climate Dynamics. This study used pre-industrial temperature proxies to look at historical climate patterns. It ruled out with 99 percent certainty that global warming in the industrial era is associated with a fluctuation in the Earth's climate.
In the more recent paper Lovejoy used the same approach to look at the period after 1998 in which global average temperatures remained relatively high but were below what had been predicted by past computer models based on greenhouse gas emissions.
Over the past 15 years greenhouse gas emissions have continued to rise, but there has been a deceleration in rising temperatures in what has been called a global warming "pause" or "hiatus." Some have argued this pause debunks the idea that global warming is primarily caused by man-made emissions and the burning of fossil fuels.
"We find many examples of these variations in pre-industrial temperature reconstructions" based on proxies such as tree rings, ice cores, and lake sediment, Lovejoy said. "Being based on climate records, this approach avoids any biases that might affect the sophisticated computer models that are commonly used for understanding global warming."
The new study showed a natural cooling fluctuation of 0.28 to 0.37 degrees Celsius since 1998.
"[The cooling during this period] exactly follows a slightly larger pre-pause warming event, from 1992 to 1998," so that the natural cooling during the "pause" is no more than a return to the longer term natural variability, Lovejoy concluded. "The pause thus has a convincing statistical explanation."
