Global warming may be behind an increase in the frequency and intensity of cold spells

Global warming caused by increased concentrations of greenhouse gases is already affecting our lives. Scorching summers, more intense heat waves, longer drought periods, more extended floods, and wilder wildfires are consequences linked to this warming.

One less obvious consequence of global warming is also getting growing attention from scientists: a potential increase in the intensity and frequency of winter cold snaps in the northern hemisphere.

Weather phenomena like the Beast from the East in winter 2018, the cold spell of Arctic air that reached as far South as Texas in February 2021, or the storm that left Madrid and Athens unusually covered in snow for days in early 2021 are becoming more common.

Some of the mechanisms that lead to their occurrence are strengthened by global warming. Key climate mechanisms, like exchanges of energy and air masses between different altitude ranges in the atmosphere, are evolving in ways expected to cause an increase in both the intensity and duration of cold snaps. These link to the behavior of a region in the high atmosphere called the stratosphere.

Winter cold snaps have major societal impacts, from direct effects on health and loss of life, to effects on transport and infrastructure, surges in energy demand and damage to agricultural resources.

This winter, we have seen these effects over large parts of Europe and the US, with flight cancellations, airport closures, road queues and drivers trapped in extreme cold temperatures. There have also been sharp increases in energy demand to cope with indoor heating, an increase in cold-related hospital admissions and the activation of services needed to assist the most vulnerable.

We need to develop forecasting tools that can predict these events further in advance.

Polar vortex

Some of these cold snaps are linked to disruptions in a seasonal atmospheric phenomenon called the stratospheric polar vortex (SPV).

In the northern hemisphere, this vortex consists of masses of cold air centered over the north pole, surrounded by a jet of very strong westerly winds between 15-50km above ground. These spinning winds act as a wall and keep cold air confined to the Arctic region, stopping it from traveling to lower latitudes.

Something that can disrupt the vortex is a sudden stratospheric warming (SSW), when the stratosphere experiences an abrupt increase in temperature due to energy and momentum being transferred from lower to higher altitudes.

When a major SSW occurs, the wall of strong winds around the polar stratosphere can break, allowing cold air to escape the polar vortex and travel down to lower atmospheric altitudes and lower latitudes. When that air approaches the Earth's surface, significant cold spells can occur.

Even when SSWs are not strong enough to break the vortex, they can weaken it. This can cause polar air circulation patterns to meander further south into lower latitudes, reaching populated areas of North America and Eurasia, instead of staying nearer the north pole. Those areas can then experience temperatures tens of degrees lower than their winter average.

Under climate change, the transfer of energy from the lowest layers of the Earth's atmosphere to the higher stratospheric layer is changing and seems to be disrupting the polar vortex to a greater degree. A study has shown that the strength and duration of SSWs in the stratosphere have increased over the last 40 years. This increase is also expected to result in stronger winter cold snaps at surface levels.