Warm winter spells have increased in frequency and duration two- to three times over since 1878, according to scientists led by the University of Warwick.

In a new analysis of historical daily temperature data published in the Journal of Applied Meteorology and Climatology, scientists from the Department of Physics at the University of Warwick, the British Antarctic Survey, and at the London School of Economics and Political Science examined data from the Central England Temperature (CET) record, the longest available instrumental record of temperature in the world. They focused on warm spells during the winter months, defined as sustained periods of time above a fixed temperature threshold.

The conclusions do not rely on identifying and counting winter warm spells directly but instead use observations of daily temperatures to show how the likelihood of different temperatures has changed. By applying a method called crossing theory to these probabilities, the scientists have provided information on the changing relationship between frequency, duration and intensity of these warm spells.

The researchers focused on the maximum daily temperatures in December, January and February in observations from 1878. Week-long warm intervals that return on average every 5 years now consistently exceed 13 degrees C. In the 1850s, a winter warm spell lasting more than 5 days with a daily maximum temperature above 12-13°C would typically take at least 5 years to reoccur. Nowadays they occur more often, typically every 4 years or less.

Read more at University of Warwick