Why winter cold spells are getting milder

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The Beast from the East ravaged much of the U.K. and Europe in February and March 2018, bringing severe storms, snow showers and bitter winds. But as the climate changes, rising average temperatures will affect the severity of extremely cold weather like this.

In our latest study, my colleagues and I considered how cold spells—defined as extended periods of extremely low temperatures between November and March—have changed since 1980. The work is published in the journal Geophysical Research Letters. We found that cold spells such as the Beast from the East are becoming less frequent and intense across the northern hemisphere.

We also found that certain regions are experiencing less persistent or more localized cold spells. Across the entire northern hemisphere, cold-spell days have decreased by about 20 days per decade. In Europe, regional cold spells are declining in frequency by nearly one day every year.

Large-scale cold spells in the northern hemisphere occur as a result of cold air intrusions caused by the meandering of the polar jet stream. Characterized by a flow of strong winds in the upper troposphere, this jet stream acts as a barrier containing cold air within the polar regions. However, when the polar jet stream meanders, the barrier is disrupted, allowing cold air to move toward the equator.

This causes surface temperatures to drop. Because the Arctic is more sensitive to climate change, with temperatures in this region rising at a faster rate than the rest of the world, many early theories suggested that this could lead to more frequent and intense cold spells. But our study shows the reverse effect.

Across the northern hemisphere, cold spells are becoming milder as overall temperatures rise, with their intensity decreasing by 0.3°C per decade. Similarly, cold spells in Europe are becoming 0.6 to 0.9°C warmer per decade. Similar warming rates are found in North America and East Asia.