December 2015 was off-the-charts warm – but not just in the eastern U.S. and Canada. In Europe, at least five countries had their warmest ever December, including Germany, France, and the U.K. In many cases, records fell by several degrees.

The early winter warmth was so persistent that cherry blossoms in Berlin, Germany, were blooming around Christmas. In the Alps, iconic ski resorts from France to Austria struggled to stay open from lack of snow. And in the northern U.K., the balmy weather was accompanied by record-setting rainfall and disastrous flooding.

The U.K. had its warmest December on record, with temperatures running 7.4 degrees (4.1°C) above long-term averages*. It was so bizarrely warm that the Met Office said temperatures “were closer to those normally experienced during April or May.”

It was a similar story all across Europe, where December was the grand finale in another year of heat records. Here’s a recap (note this is hardly an exhaustive list):

  • Germany had its warmest December since records began 1881. Across the country’s network of nearly 2,000 weather stations, last month averaged 9.3 degrees (5.2°C) above normal, breaking the previous December record by 1.4 degrees. The German weather service reports that 2015 was that country’s 2nd-warmest year on record, behind only 2014. Last year was noteworthy for setting a new all-time national heat record of 104.5 degrees (40.3°C) on both July 5 and Aug. 7, 2015.
  • Neighboring Switzerland also had its warmest ever December, ending 6.1 degrees (3.4°C) warmer than normal, reports MeteoSuisse. The unseasonable warmth was especially pronounced at high elevations, where some stations bested their previous December temperature record by nearly 2 degrees Celsius. For the year as a whole, 2015 was Switzerland’s warmest year since records began in 1864, bumping the previous record-holders (2014 and 2011) to 2nd and 3rd place, respectively.
  • The Netherlands had its warmest December in over three centuries of records, according to Dutch meteorological institute KNMI. The national monthly average temperature was 10.4 degrees (5.8°C) above normal, breaking the previous record by nearly 4 degrees (2.2°C). Most of the Netherlands had its warmest Christmas and first-ever frost-free December on record. KNMI also notes that December’s average temperature was even warmer than what’s considered normal during the month of April!

Behind the record-setting warmth

Several strong storms swept across the North Atlantic in December, including one that brought historic warmth into the Arctic last week.

Counterclockwise flow around these storms frequently interacted with a ridge of high pressure over central Europe, creating a southwesterly fetch of mild air off the Atlantic. The North Atlantic Oscillation (NAO) – an index of atmospheric pressure differences between Iceland and the Azores – was strongly positive in December, which helped reinforce the warm weather pattern.

But December’s warmth was also fueled by a powerful El Niño set against a long-term trend in rising global temperatures, driven by increases in heat-trapping greenhouse gases. Last month the World Meteorological Organization confirmed that for Europe as a whole, 2015 was the continent’s second-warmest year on record, behind only 2014. With El Niño expected to persist into 2016, additional records may well be in the pipeline.

*Long-term averages all refer to 1981-2010 climate normals.