Winter is not coming: man-made climate change is wiping out seasonal temperature differences
Mankind may be responsible for something we long thought out of our control: the changing of the seasons, according to a new study by Chinese and Canadian scientists.
For the first time, researchers have found evidence that man-made climate change has suppressed the natural fluctuation of seasonal temperatures in the northern hemisphere, removing the distinction between summer and winter.
In the most affected regions, the gap between the mean temperature in summer and winter has been narrowing at a rate of more than 0.1 degrees celsius per decade since the 1950s, according to the study published in the American Meteorological Society's Journal of Climate.
The news comes ahead of crucial global climate change talks in Paris in December, where more than 120 nations are expected to reach a deal to cut emissions from 2020.
If the trend persists, winter in Beijing – which currently drops below freezing - could be as warm as the summer in 3,000 years.
Such a scenario was long thought impossible. Seasons, as defined in textbooks, are caused by the tilt of the earth as it orbits around the sun, with different amounts of heat and light falling on different regions throughout the year.
But that is not the whole story. The orbit and tilt of the planet do not explain why Antarctica, which has always received the least amount of sunlight, was once warm and covered in thick forest.
To establish whether human beings were a factor in the contracting seasonal temperature gap, Qiang Cheng of the Chinese Academy of Sciences and Zhang Xuebin of the Canadian government's Climate Research Division compared observed records with various climate models. Only the model which factored in human influence matched up with the factual data.
The further north, the more obvious the human impact was, so much so that its effect was comparable to that of the angle of the sun, long accepted as a factor in seasonal temperature fluctuation.
Potential human influence on the seasons has gone unnoticed before as measuring minute temperature fluctuation year-on-year is exceptionally difficult. Only by using the latest climate models from the World Climate Research Programme were the researchers able to detect with unprecedented precision the shift in temperatures.
The scientists suggested a number of ways in which humans may have affected the seasons, including greenhouse gas emission, massive changes in landscape, and the widespread use of winter heating in urban areas.
The researchers did not discuss the potential impact of the "disappearance" of the seasons, both negative and positive, but they emphasised that the narrowing in the temperature gap was more down to warmer winters than a drop in temperature in summer.
Some intriguing questions remain however, especially that the diminishing seasonal difference was a phenomenon wholly confined to the northern hemisphere.
In the Mediterranean and other southern regions, the winter-summer temperature gap has actually increased over the last few decades.