The Arctic has borne the brunt of global warming, with temperatures three degrees higher than the pre-industrial era.
But linking the current mild conditions to climate change is premature, warned Natalie Hasell, meteorologist at the Canadian Department of the Environment, saying that scientists do not base their conclusions "on one anomaly".
The North Atlantic depression also brought eastern Canada an unusually warm holiday period, with temperatures hitting 15.9 degrees Celsius in Montreal on Christmas Eve, around 20 degrees above the seasonal average. This was followed by heavy snowfall that covered the country's eastern half.
In the United States, tornadoes and floods left at least 49 dead. The waters of the mighty Mississippi River have already exceeded overflow levels by four metres in some areas, claiming 13 lives. In the south, spectacular tornadoes devastated parts of Texas.
The common cause of the unusual events is a particularly strong El Nino system, amplified by global warming, according to scientists. El Ninos, which emerge every four to seven years on average and run from October through January, are triggered by a shift in trade winds across the Pacific around the Equator. However, the 2015 episode is "probably the strongest in the last 100 years," said Jerome Lecou, ​​forecaster at Meteo-France. "There is no simple answer" to explain the exceptional conditions, he added. - Strawberries at Christmas -
Although centred on the Pacific Ocean, El Nino "has an impact on a global scale," Jean Jouzel, former vice president of the Intergovernmental Panel on Climate Change (IPCC), told AFP. "Generally, El Nino years are warmer than average," he said.
In Australia, scorching heat has led to large forest fires and temperatures are forecast to soon hit 38 degrees Celsius in the south. In France, the unseasonably warm weather has baffled Mother Nature with Perigord strawberries, Alsatian asparagus and mimosas all flourishing on the French Riviera. In Spain, higher temperatures and low rainfall have led to an unusual outbreak of wildfires in the north.
Several Italian cities have introduced traffic restrictions to combat fine-particle pollution, which has been worsened by the dry and calm conditions. Record rainfall has brought unprecedented flooding to the north of England, leaving the historic city of York under feet of water and thousands evacuated over December. Stormy weather buffeting the North Sea has brought havoc to oilfields on New Year's Eve, with one man reported killed by a giant wave hitting a rig, and hundreds of workers evacuated after a huge barge broke its moorings and began drifting.
"There is no doubt in my mind that climate change is partly responsible for the flooding across the north of England," said Piers Forster, professor of climate science at Leeds University.
Britain's mean December temperature was a record-breaking eight degrees Celsius, smashing the previous high of 6.9 degrees set in 1934, according to the Met Office. It was also the wettest December on record for Scotland and Wales and one of the top 10 wettest in Britain as a whole.
Worldwide, temperatures between January and November have already broken records, suggesting that 2015 will be the hottest in modern history, according to the National Oceanic and Atmospheric Administration. "On a global level, we really are in an exceptional year as it is just over a tenth of a degree warmer than 2014, which itself was a record year," said Jean Jouzel.
"This is a real leap, one tenth of a degree from one year to another," he added. With El Nino showing little sign of running out of steam, he predicted that 2016 could be warmer still, but that following years may be less extreme.
