The jet stream blows around the Earth, moving from the west to the east. If you were to sit the world on the South Pole, this stream would run from Japan to the U.S. in a curvy fashion for part of its path. The effect of the jet stream works as a barrier fence, keeping the Arctic cold air contained closer to the North Pole.
Global warming changes the temperature of the planet, including the polar caps. One result of this change has caused ice shelves to melt and become part of the ocean, adding to the water mass. Another effect warms what should be much colder arctic air. That in turn shakes up the jet stream, allowing colder air to travel south instead of staying closer to the North Pole.
During the winter, when the planet tilts slightly away from the sun and creates a colder climate in the northern hemisphere, the cold air pushes south, bringing what we know as winter. With a destabilized jet stream failing as an air mass fence, more cold air gets to places that should be warmer in winter. As a result, severe cold in places such as Florida and stronger winters in the mid-U.S. or Europe become more common.
The planet's jet streams are extremely sensitive to temperature changes. So when cold air changes the climate of an area, this changes the temperature. The change in turn causes more changes to the jet stream, creating an aggregating cycle of jet stream and cold air modifications.
