Why winter cold spells are getting milder
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.
The persistence of cold spells in northern North America and southern Europe is decreasing by about a tenth of a day each year. Likewise, the area covered by cold spells in northern North America is getting smaller every decade.
A change in temperature variability
We also explored how these cold spells would be affected if the long-term temperature rise related to climate change were removed. Under this scenario, we considered how the remaining changes in temperature variability—calculated after removing the long-term temperature rise—affect cold-spell behavior.
In most cases, the cold spells showed no detectable change when the background temperature rise was removed. However, in northern North America under this scenario, cold spells were still found to become less severe.
Our study reveals that in northern North America, cold spells are becoming significantly less frequent, intense and persistent even when the impact of climate change is removed from our calculations.
We suggest this is related to decreasing temperature variability in North America due to sea ice loss—an effect highlighted in previous studies . In other words, sea ice loss is potentially causing cold extremes to warm faster than hot extremes.
Our results show that as temperatures continue to rise, the cold spells experienced a few decades ago are becoming rarer across much of the northern hemisphere—and that cold, snowy winter days of the past are becoming increasingly uncommon for many people.
