The recent series of cold winters in North America was not a by-product of human-caused global warming after all, researchers have established, but what appears to be good news comes with an unsettling caveat.
Bitterly cold winters in central North America in recent years have encouraged political commentators critical of the notion of climate change, especially when compared to the higher temperatures projected in climate models. A phenomenon known as the ‘polar vortex’—a large mass of cold, dense air that sits atop the Arctic—when weakened can send this very chilly air toward the equator, driving temperatures down sharply and rapidly.
In 2014, a study suggested that the extra cold, polar-vortex driven winter of 2013-14 was a product of atypical warming in the western tropical Pacific as a result of anthropogenic global warming. Rather than proof that climate change wasn’t real, the polar vortex winters appeared to be driven by the same process.
This week however, that idea has been challenged by new modelling by researchers with Environment and Climate Change Canada, the federal government ministry, whose paper detailing their findings was published in the journal Nature Climate Change. After looking at some 200 simulations of the Earth’s atmosphere, they found that such cold winters, as exceptional as they are, are actually within the normal range for central North America. These winters resulted instead from a relatively rare fluctuation in air circulation in this region that was unrelated to the tropical Pacific. The good news here is that because these super-cold winters are not linked to climate change, they are not expected to become the new normal.
Canadian Centre for Climate Modelling and Analysis researcher John Fyfe, one of the two scientists behind the paper, explains how the public and policy-makers need to come to grips with these sort of effects to separate what is caused by normal variability and what is human-caused.
“As you move down from the global scale to smaller scales such as the North American continent, what is the result of this variability can actually be quite large and even mask the effects of anthropogenic global warming,” he told The Climate Examiner.
Fyfe, who is based at the University of Victoria, says the trouble is that during this masking, the public and policy-makers sometimes mistakenly think that global warming has gone away.
The new study reinforces the idea that natural variability is behind the so-called ‘hiatus’ in global warming – a smaller-than-expected increase in global average surface temperature since about the year 2000 due to natural factors including La Niña events in the Pacific.
“Already the reception we’ve seen on the web, in comments, is that climate sceptics are once again saying this is evidence that anthropogenic global warming isn’t happening. This is incorrect.”
He explains that in the future, there may be what we could call a ‘flipped phase’, where the same natural variability swings in the opposite direction producing higher temperatures atop the human-caused warming.
“We need to get used to a more waxing and waning of all this atop anthropogenic global warming,” he said.
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