This past July, Earth experienced its hottest month on record, thanks in large measure to human-caused global warming. But don’t forget to blame climate change for some recent cold temperatures and weird weather patterns, too.
Arctic warming is likely responsible for increased stagnant weather systems, colder temperatures farther south, and recent record highs in northern U.S. states, according to Jim White, Director of the Institute of Arctic and Alpine Research.
Speaking to graduate students at the University of Colorado Boulder on Tuesday, Sept. 6, White said arctic warming and a loss of reflective ice at the poles has slowed down jet streams that carry currents of warm and cold air across the northern hemisphere. The result of this slowing is that the jet streams have become “loopy.”
As the arctic warms up, the difference in temperature between the equator and the north pole lessens. This lower temperature gradient also means a decrease in the energy which drives the jet stream through its movements. Less energy means less movement, and the jet stream slows down.
Air-flows act in this way the same as water. The more energy a river has, the straighter its path. But when jet streams slow down, they meander, like rivers on open plains.
“That’s energy being dissipated through these meanders,” White explained. “As you slow air down, it gets loopier.”
One of the effects of loopier jet streams is that they can curve far enough around and cut themselves off, which is also seen in river systems.
These cut-off lows can create unique weather events if they happen in specific locations. When cut-off lows occur in the southeastern region of Colorado, they are known for pumping moisture up against the Front Range. This causes large amounts of precipitation in the form of rain or snow, which can lead to 3-foot snowfalls in the fall or spring.
But the more prevalent effect of meandering jet streams, which has made national news in recent years, is how they pull colder air farther south and draw warmer air farther north than usual throughout the year.
And although arctic warming means that the air being pulled down from high latitudes is not as cold as it used to be, it’s the warm air finding its way north that is setting records. Compared with its history, the U.S. is now on a steep trajectory of breaking high temperature records, and has not been breaking many low temperature records.
Northern cities such as Minneapolis have seen unusually high temperatures in the last few years, setting records above 80 degrees Fahrenheit early in the spring and late in the fall. In contrast, Minneapolis rarely breaks daily record lows, White said.
Yet Midwestern, southern and eastern states now receive colder forecasts more often during the winter, setting record monthly average lows. On the other side of the jet stream, winters can end up being warmer for western states, as seen in this map by NOAA.
These abnormal seasonal temperature shifts cause all sorts of problems for states’ economies and their residents, who are not properly prepared to deal with intense heat or cold. Extreme weather events can shut down transportation, damage property, overtax public resources, or be fatal to those without adequate shelter.
So how much loopier will the jet stream become, and is it really all because of climate change?
Rutgers University researcher Jennifer Francis thinks she has the answer.
In an article she published in 2013 explaining her theory, Francis describes two main impacts on the jet stream caused by arctic warming.
First, she writes, there is a slowing of its west-to-east winds, “a phenomenon that already appears to be occurring.” This causes weather conditions to become more persistent, or seem more “stuck in place.”
Second is the “waviness” of the jet stream, which Francis explains will likely continue to increase. This phenomenon is illustrated in a model used during a talk Francis gave at Rutgers in 2013.
Some of these patterns have been noticed firsthand by the public, with terms for extreme southern incursions of cold weather now part of popular culture, such as “polar vortex,” acknowledged by Francis in a 2014 paper she co-authored.
While Francis’ work is not yet settled science, it is an active area of research and gives strong insight into the scientific reasons for weird weather in the 21st century.
And we are not currently deep enough into climate change, White said, “to see enough of these effects become screamingly obvious.” Yet as the planet continues past its one degree Celsius change in global temperature, wavier jet streams are likely to become more evident.
And so will all the atypical seasonal temperature changes and weather patterns that come with them.
Sorry, Minneapolis. The new (ab)normal looks to be here to stay.