Americans in the upper Midwest have been dealing with brutally cold temperatures as low as 35 degrees (Fahrenheit) below zero and wind chills near 60 below this week. At the same time, the North Pole is in a heat wave with temperatures 25 degrees warmer than normal, and the culprit for this upside-down weather pattern is both the polar vortex and climate change.

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Just because millions of Americans are dealing with this potentially life-threatening cold blast doesn’t mean global warming is a hoax. There are far more people in North America and across the globe who are experiencing warmer than normal temperatures, and the symptoms of climate change can be difficult to understand.

Related: Climate change is killing reindeer in the Arctic

There are two polar vortices in the Northern Hemisphere that are stacked on top of one another. The lower one is known as the jet stream, which has strong westerly winds about 7 miles above the Earth. It exists year-round and is responsible for the daily high- and low-pressure systems that affect the fluctuation between warm and cold spells.

About 30 miles above the Earth, there is the stratospheric polar vortex, which is a circular river of wind that rings the North Pole and usually forms during the winter. The Earth’s rotation and the large temperature difference between the Arctic and mid-latitudes (areas located farther south) are what causes both of these winds to exist.

However, over the last half-century, greenhouse gas emissions from human activity have caused the Arctic to warm twice as much as the rest of the planet. This rapid Arctic warming has caused the north/south temperature difference to shrink.

The temperature change has reduced the pressure difference between the Arctic and mid-latitudes and weakened the jet stream. A slower-flowing jet stream takes more of a winding path instead of straight, and according to The Huffington Post, the “energy can travel upward and disrupt the stratospheric polar vortex.”

Via Huffington Post

Image via Edward Stojakovic