“The speed and waviness of the northern jet stream, a river of wind across the Northern Hemisphere, is affected by the temperature difference between the Arctic and mid-latitudes.”
Increasing levels of greenhouse gases are causing increasing disruptions of the jet stream, a powerful river of winds that steers weather systems in the Northern Hemisphere. That's causing more frequent summer droughts, floods and wildfires, etcetera.
Summers like 2018, when the jet stream drove extreme weather on an unprecedented scale across the Northern Hemisphere, will be more frequent if emissions of carbon dioxide and other climate pollutants from industry, agriculture and the burning of fossil fuels continue at a high rate.
Note: Because our media is focused upon the northern hemisphere, we need to remind ourselves that the southern hemisphere’s jet stream is also affected: