It was cold enough in Chicago last Monday to freeze the deicing fluid (propylene glycol) used on aircraft at O’Hare. That said, this arctic outbreak isn’t as rare as the extensive media coverage would suggest – in fact, the low temperature of -16 at O’Hare broke a daily record, but didn’t even approach the all-time record of -27. Even so, the “polar vortex” is now a household term, and inevitably climate change has become involved – either as a joke or as a driver of all sorts of “extreme weather,” including cold. As always, it’s not that simple.
The northern hemisphere has two persistent jet streams – compact areas of powerful winds between 30 and 40 thousand feet – that drive much of our weather. One is centered in the mid-latitudes, or generally over us, and a second, the “polar jet”, blows in a west-to-east circle usually above 60 degrees north. These jet streams exist (more or less) due to the earth’s rotation and the surface temperature gradient between the equator and the arctic.
However, the position and speed of the jet stream is not at all constant. Both jets ungulate like snakes across the planet, sometimes bulging and slowing, and at other times becoming highly compact and extremely fast. These changes in the position of the jets are the result of global atmospheric disturbances called Rossby waves, which in turn are driven by changes in the atmosphere’s tangential rotation speed with latitude (see the Coriolis effect).
These jet streams can be thought of as dividing lines between the hemisphere’s three primary air masses – tropical, temperate (ours), and polar. When the mid-latitude jet deviates significantly southward from its normal path over the northern half of the country, cold air from Canada is able to rush south. This is what happened last week – a powerful section of the jet bulged as far south as Florida as a strong low pressure system moved southeast into the Midwest.
This low pressure system was (somewhat incorrectly) referred to as the “polar vortex.” The real polar vortex is a persistent area of low pressure centered over the arctic, and that low is more or less stationary. However, as Rossby waves and their associated weather systems move around the planet, parts of the polar vortex – or at least the cold air within it – can be pulled southward. This is what happened last week – just a normal, if slightly unusual, weather system.
Now, this somewhat extreme event was not caused by climate change, and it most certainly does not disprove it. As shown in Figure 1, winter low temperatures have been steadily warming over the last thirty years, and they will continue to do so. Accordingly, arctic outbreaks such as this one will (and already have) become less frequent and less extreme.
There has been recent research suggesting a potential link between the rapid decline in arctic sea ice volume and increasing variation in the path of the jet stream. If true, this would suggest that extreme jet stream deviations like what we saw last week could become more common in the future, leading to more extreme heat and cold events. However, there has been significant push-back from parts of the scientific community on this, and the result is far from being established. So, no, climate change did not cause this arctic outbreak any more than it caused the seasonably normal temperatures that occurred in California while we froze.
Somewhat anticlimactically, this arctic outbreak was just weather – rare perhaps, but normal. Arctic outbreaks will continue to happen, although they will get less and less severe over time. The attempts to attribute individual weather events to climate change illustrate the difficulty of coherently explaining how the climate influences us now and how it will change in the future – there is always a push for a simple, direct display of the effects of climate change on people, but that is very rarely possible. The reality is that as temperatures warm, cold events will become less likely and heat events will become more so, and in the process our agriculture, transportation, water, and energy infrastructure will be stressed by conditions it was not designed to handle.